ADS (Cites/AR query)

Operational space weather detection and tracking using small spacecraft

One of the greatest challenges facing current space weather monitoring operations is forecasting the arrival of coronal mass ejections (CMEs) and solar energetic particles (SEPs). This paper presents a mission concept for operational detection and monitoring of solar weather events as a means of forecasting the arrival of potentially hazardous CMEs and SEPs at Earth-like distances. Foregrounding the operational (rather than scientific) requirements of the system, this work proposes a high-level mission design that could provide detection of solar weather events by tracking associated solar radio bursts, enabling advanced warning of their arrival at Earth. This work concludes that 35 small spacecraft equipped with radio spectrometers positioned at the SunEarth Lagrange points and in Earth-leading/-trailing orbits could be used to provide this capability, with the L4 and L5 Lagrange points most advantageous for mission performance. While technical developments in CubeSat survivability would be required to enable the SURROUND mission, suitable launch, injection and communication options are identified, indicating its potential feasibility in the near future.

The POEMAS solar radio telescope evaluation without HPC

The increasing temporal resolution and structural diversity of modern solar instruments place growing demands on database systems used in observational astronomy. At the Center for Radio Astronomy and Astrophysics Mackenzie (CRAAM), this challenge is amplified by the need to consolidate heterogeneous data streams from multiple telescopes within a single virtual machine. With only 32GB of RAM available (16GB allocated to the database), a central design question emerged: when restricted to a single physical host, can a virtualized sharded cluster offer practical scalability advantages over a standalone deployment? To investigate this, we conducted an empirical evaluation of MongoDB using 10ms observations from the POEMAS radiotelescope, tested at volumes of 15M, 150M, and 500M documents. Results show that, although sharding introduces coordination overhead for selective queries, it provides substantial gains for global aggregations, achieving speedups above 600 while maintaining compression ratios near 85%. The analysis identifies an operational threshold of roughly 150 million documents per collection to sustain stable performance under the available resources. Based on these findings, the same single-node configuration used in the benchmarks was employed to process the full historical POEMAS dataset, totaling 3.3 billion records and producing approximately 50GB of consolidated FITS products. These products and their associated metadata are made available to the community through a cloud-hosted portal with reduced operational cost. This work documents practical scalability boundaries for astronomical time-series in resource-constrained environments and supports the deployment currently operating at CRAAM.

Dataset for Recognition and Detection Based on Solar Radio Spectrogram Data

With the increasing volume of solar radio spectrogram observation data, deep learning-based recognition and detection of solar radio bursts have become a key research direction. However, most studies on meter-wave solar radio spectrogram recognition and detection rely on proprietary datasets, and publicly available datasets remain scarce. To address this issue and facilitate the use of public datasets for deep learning model validation, thereby promoting the development of automatic detection methods for meter-wave solar radio spectrogram, we propose a new solar radio spectrogram dataset. The dataset is constructed from meter-wave solar spectrogram observation data from the Learmonth Observatory in Australia and the meter-wavelength observing system of the Chashan Solar radio Observatory (CSO) of Shandong University. Experimental results demonstrate that the proposed dataset effectively supports the recognition and detection of solar radio spectrograms, providing essential data resources for the intelligent recognition of solar radio burst features.

Large area and long-distance ionospheric D region virtual height determination method using lightning VLF electric field waveforms

This paper introduces a highly effective method for real-time determination of the large-scale ionospheric D region virtual height using lightning electromagnetic pulses (LEMP). Utilizing lightning location and very low frequency (VLF) electric field waveform data from the Asia-Pacific Lightning Location Network (APLLN), the method can cover most regions of China with just 15 LEMP detection sites. It analyzes the arrival times of ground wave, first-hop, and second-hop sky waves in the 330 kHz cloud-to-ground lightning return stroke electric field waveforms to derive ground wave propagation delay and D region virtual height results. Using these results during Mclass solar flare events, our analysis shows a linear dependence of D region virtual height on the logarithm of solar X-ray intensity (0.10.8 nm), with a height descent rate of 5.97 km/decade and a 12 min ionospheric response delay. It shows remarkable effectiveness in monitoring the impact of solar flares on D region virtual height. Its advantages lie in achieving extensive D region virtual height coverage with fewer lightning detection stations, offering richer data support for statistical analysis and potential applications in radio wave propagation studies.

From Blazar to Magnetar and Sun

Using Gaussian process methods, we analyzed the light curves of three extreme solar X-ray flares observed by the RHESSI satellite. Their variability characteristics were then compared with those of HXMT-HE X-ray burst (XRB; in SGR 1935+2154) associated with fast radio burst (FRB) 200428 and blazar -ray giant flares to investigate the origins of these extreme flaring events. The variability patterns of the solar X-ray flares follow the stochastically driven damped simple harmonic oscillator model. The derived timescales t_{B_steep} and t_{B_flat} (corresponding to power spectral density breaks) are in the range of 47 and 1653 s, respectively. The FRB-associated HXMT- HE burst has a Q value near 0.3, matching those of the solar flares that occurred on 2002 July 23 (flare 1) and 2003 November 3 (flare 2). By contrast, blazar -ray giant flares show Q > 0.3, similar to the solar flare that occurred on 2014 February 25 (flare 3). We proposed that the critically damped state of the system may be the condition triggering the association between the XRB in SGR 1935+2154 and the FRB. In this scenario, the critical damping Q value of the system is around 0.3, not the theoretical 0.5. The similarity in Q values might imply that the FRB-associated HXMT-HE XRB and solar X-ray flares 1 and 2 share comparable dynamic behavior, while blazar -ray flares and solar X-ray flare 3 exhibit another distinct but similar dynamic behavior. Like solar X-ray flares, these extreme flares may all be related to the magnetic reconnection process.

Dynamics of the Coronal Magnetic Field in the 2022 October 2 X-class Flare

Solar flares are driven by the release of free magnetic energy and are often associated with the restructurization of the magnetic field topology. Yet, observations of the evolving magnetic field in the flaring volume are limited to very few cases, including the 2017 September 10 X8.2 limb flare; thus, a verification of whether a similar evolution takes place in other solar flares is needed. Here, we report one more: the 2022 October 2 X1.1-class solar flare, seen on the disk, whose microwave data permit mapping the magnetic field over the flaring source and tracking the magnetic field evolution over the course of the flare. We find that the coronal magnetic field shows a prominent decay with a rate up to 10 G s¹ in several (above the) looptop locations. The magnetic field is also confidently measured at the loop legs and the bottom part of the erupting filament. Prominent acceleration of electrons is detected where the magnetic field decays. We develop 3D models of the flare, whose magnetic field shows resemblance to and also deviation from the magnetic field inferred from the microwave data. This study confirms that the coronal magnetic field decays during the rise phase of the solar flare. The amount of released magnetic energy is sufficient to support other components of the flare energy.

Signatures of Large-scale Magnetic Field Disturbances and Switchbacks in Interplanetary Type III Radio Bursts

Type III solar radio bursts are driven by nonthermal electron beams travelling along heliospheric magnetic fields, with the radio emission frequency drift rate determined by the beam speed and the plasma density profile. Analyzing beam kinematics inferred from the drift rate reveals behavior inconsistent with the emitter moving radially through smooth, monotonically decreasing density. We examine whether these features are driven by disturbances in the guiding magnetic field direction, such as switchbacks, rather than plasma inhomogeneities along the beam path. Using simulations and remote observations of 24 interplanetary type III bursts observed by Parker Solar Probe, we relate measured drift rate variations to local field deflections. In 50% of events, we identify disturbances above a 2 noise level that can be attributed to perpendicular deflections of the field between (0.7 and 1.7) R, over scales (1.86.4) R at heliocentric distances (930) R. The features correspond to either density changes of (10%30%), or deflections of the field direction by (2388). Further, beam transport simulations show field direction perturbations produce additional observational signatures in type III bursts: delayed emission, intensity breaks, and enhanced emission resembling stria fine structures. In addition, we identified four bursts where the observed variations are more plausibly explained by field deflections, possibly in the form of magnetic switchbacks than by unrealistically large density changes along the field line. The results show that variations in type III burst profiles can arise from magnetic as well as density fluctuations and demonstrate the value of type III bursts as remote probes of inner-heliospheric structure at kilometric wavelengths.

Herringbone Structures during an X-class Eruptive Flare

In this paper, we report quasiperiodic herringbone structures during the impulsive phase of an X-class flare, coinciding with the distinct acceleration phase of eruptive prominence ejection on 2023 December 31. The prominence propagates nonradially in the southeast direction with an inclination angle of 35.4. The fast coronal mass ejection (CME) at a speed of 2852 km s¹ drives a shock wave and a coronal extreme-ultraviolet wave. The herringbone structures lasting for 4 minutes take place at the initial stage of a group of type II radio bursts. The herringbones in the frequency range 2070 MHz are characterized by simultaneous forward-drift and reverse-drift bursts with average durations of 2.5 and 3.1 s. The frequency drift rates of these bursts fall in a range of 1.39.4 MHz s¹ with average values of 3.6 and 4.1 MHz s¹, respectively. The speeds of electron beams producing the herringbones are estimated to be 0.04c0.41c, with average values of 0.23c and 0.11c for forward- drifting and reverse-drift bursts, respectively. The heights of particle acceleration regions are estimated to be 0.640.78 R above the photosphere, which are consistent with the height of the CME front (0.75 R) when the shock forms. Quasiperiodic pulsations with periods of 17.521.3 s are found in the radio fluxes of herringbones, suggesting that electrons are accelerated by the CME- driven shock intermittently.

Complexity and scaling descriptors as diagnostic predictors of heliophysical indices across solar-cycle timescales

Heliophysical variability emerges from a coupled, multiscale system in which changes in the solar atmosphere and heliospheric plasma translate into measurable signatures in widely used activity indices. Operational space-weather workflows often summarize this variability through amplitudes and a small set of bulk solar-wind covariates, yet important dynamical information may also reside in the evolving morphology of the signals. We examine whether shape descriptors computed from heliophysical time series provide information beyond classical amplitude summaries and standard bulk solar-wind covariates. Using daily OMNIWeb- era records spanning 19642025, we compute ten sliding-window descriptors under a past-only convention, designed to capture complementary aspects of temporal morphology such as irregularity, roughness, and long-range dependence. The descriptor set combines entropy measures, fractal-dimension estimators, the Hurst exponent, and LempelZiv (LZ) complexity, yielding a compact representation of time- series structure that is not reducible to amplitude alone. The window length is treated as a methodological hyperparameter and selected through a target-specific sensitivity analysis that jointly favors competitive out-of-sample RMSE and stable permutation-importance rankings across neighboring windows. Two complementary learners, gradient boosting and a multilayer perceptron, are used as diagnostic probes to quantify permutation-based feature relevance under chronological splitting and training-only preprocessing. Across three targets (F10.7, Sunspot Number, and Dst), shape descriptors consistently rank among the most informative predictors, often matching or exceeding the relevance of standard solar-wind inputs. The most robust signals arise from LZ complexity and a compact subset of entropy/fractal measures, whose windowed trajectories track solar-cycle phases with characteristic leadlag behaviour. Correlation analyses on both levels and standardised first differences expose redundancy within descriptor families and reduce spurious associations driven by shared nonstationarity, motivating a family-level interpretation of relevance rather than causal attribution. Overall, the results indicate that heliophysical time-series morphology encodes dynamical information complementary to amplitude- and bulk-plasma descriptions, suggesting compact, instrument-light features for augmenting future space-weather modelling pipelines.

Statistical investigation of Langmuir waves in Type III and II sources

Type II and III solar radio bursts involve electron beams driving Langmuir waves that then couple wave energy into radio emission. These Langmuir waves can be driven to large enough amplitudes that they undergo electrostatic (ES) decay into a backward propagating Langmuir wave and a forward propagating ion acoustic wave. The forward and backwards propagating Langmuir waves can then "beat" together to produce characteristic Langmuir waveforms and spectra, plus radio emission. Stochastic growth theory (SGT) predicts that the probability distribution of the Langmuir wave electric field strength should be lognormal, with known modifications if nonlinear processes like ES decay are occurring at high fields. Other analyses suggest the more general set of Pearson distributions may be relevant. Here, previous work on Langmuir waves in type II and III source regions is generalized and tested by analysing the probability distributions of the waveforms of Langmuir waves observed by the STEREO spacecraft. The focus is on a set of published events identified using spectral analyses to have or not have spectral evidence for ES decay. For events for which spectral analyses provide evidence of ES decay, 92% of the probability distributions are consistent with the combination of SGT and a nonlinear process like ES decay, while 68% of those without spectral evidence for ES decay are better fitted by pure SGT. Better fits with stronger statistical significance are obtained for pure and nonlinear SGT than for Pearson distributions in the majority of events (90%). These results provide strong evidence for SGT and ES decay proceeding in type II and III source regions, particularly type III source regions.

Three-dimensional mapping of coronal magnetic field and plasma parameters in a solar flare

Aims. Diagnosing solar flare conditions is essential for understanding coronal energy release. Using combined microwave and X-ray data, we aim to reconstruct 3D maps of the magnetic fields and plasma parameters in the SOL2021-05-07 flare. Methods. We used imaging spectroscopy from the Expanded Owens Valley Solar Array (EOVSA) to derive spatial maps of the magnetic field strength, as well as the thermal and nonthermal electron densities, along with the power-law index of nonthermal electrons via gyrosynchrotron modeling. Simultaneous X-ray observations from Hinode/X-Ray Telescope (Hinode/XRT) and Solar Orbiter/Spectrometer Telescope for Imaging X-rays (SolO/STIX), taken from different vantage points, enable a stereoscopic reconstruction of the flaring loop. By correlating the positions of microwave and thermal X-ray sources, we associated the 3D coordinates with the microwave-derived plasma parameters. Results. We derived observational 3D maps of magnetic field strength, Alfvn speed, and plasma beta in a flaring volume, revealing a magnetically dominated environment. These spatially resolved diagnostics provide valuable constraints for models of magnetic reconnection and flare dynamics, representing a step toward a realistic 3D characterization of energy release in solar eruptive events.

How Good Are GOES XRS Temperatures and Emission Measures?

The soft X-ray (SXR) measurements made by NOAA's GOES weather satellites are an important resource for solar physics and space weather. In particular, they are extensively used to study the energetics of solar flares via temperatures and emission measures derived from the SXR data. However, the SXR instruments measure just two channels, 0.5 4 and 1 8 : with just two data points, it is only possible to represent the flare plasma with a single temperature component, whereas it is well known that a flare can display a wide range of temperatures at any given time. In order to assess how representative the GOES SXR temperatures and emission measures are, we compare GOES measurements with EUV data for six spectral lines of Fe that cover the typical temperature range of flares, 10 20 MK. From a sample of 23 large flares, we find that the GOES temperatures match the emission-measure-weighted EUV temperatures surprisingly well, but (assuming photospheric abundances) the GOES emission measures are smaller than the EUV emission measures by up to 50%, with the discrepancy larger at higher temperatures.

Fast variability and circular polarization of the 6.7 GHz methanol maser in G33.641-0.228

The 6.7 GHz methanol maser in a high-mass star-forming region G33.6410.228 is known to exhibit burst-like flux variability due to an unknown mechanism. To investigate the burst mechanism, we conducted high-cadence flux and circular polarization monitoring observations, simultaneously using left- and right-hand circular polarizations. We found that the flux density increased and decreased on a short timescale of 0.3 d during a burst. We also found strong circular polarization, reaching up to 20% in the component exhibiting the bursts. Circular polarization of 0%-20% was continuously observed from 2009 to 2016, even in the quiescent period. The polarization also varied on timescales of less than one day. When a burst occurred and the flux density increased, the circular polarization decreased to zero. To explain the observational properties of the flux variability and circular polarization, we propose a model in which an explosive event similar to a solar radio burst occurs on the line of sight behind the maser cloud, producing circularly polarized continuum emission due to gyro- synchrotron or gyro-resonance radiation, which is then amplified by the maser.

Daily Predictions of F10.7 and F30 Solar Indices With Deep Learning

The F10.7 and F30 solar indices are the solar radio fluxes measured at wavelengths of 10.7 and 30 cm, respectively, which are key indicators of solar activity. F10.7 is valuable for explaining the impact of solar ultraviolet (UV) radiation on the upper atmosphere of Earth, while F30 is more sensitive and could improve the reaction of thermospheric density to solar stimulation. In this study, we present a new deep learning model, named the Solar Index Network, or SINet for short, to predict daily values of the F10.7 and F30 solar indices. The SINet model is designed to make medium-term predictions of the index values (160 days in advance). The observed data used for SINet training were taken from the National Oceanic and Atmospheric Administration as well as Toyokawa and Nobeyama facilities. Our experimental results show that SINet performs better than five closely related statistical and deep learning methods for the prediction of F10.7. Furthermore, to our knowledge, this is the first time deep learning has been used to predict the F30 solar index.

Solar radio bursts and their applications in space weather forecasting

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Modeling Metric Gyrosynchrotron Radio Emission From the Quiet Solar Corona

The radio emission of the quiet Sun in the metric and decametric bands has not been well studied historically due to limitations of existing instruments. It is nominally dominated by thermal brehmsstrahlung of the solar corona, but may also include significant gyrosynchrotron emission, usually assumed to be weak under quiet conditions. In this work, we investigate the expected gyrosynchrotron contribution to solar radio emission in the lowest radio frequencies observable by ground instruments, for different regions of the low and middle corona. We approximate the coronal conditions by a synoptic magnetohydrodynamic (MHD) model. The thermal emission is estimated from a forward model based on the simulated corona. We calculate the expected gyrosynchrotron emission with the Fast Gyrosynchrotron Codes framework by \cite{Fleishman:2010}. The model emissions of different coronal regions are compared with quiet-time observations between 20-90 MHz by the LOw Frequency ARray (LOFAR;\cite{vanHaarlem:2013}) radio telescope. The contribution of gyrosynchrotron radiation to low frequency solar radio emission may shed light on effects such as the hitherto unexplained brightness variation observed in decametric coronal hole emission, and help constrain measurements of the coronal magnetic fields. It can also improve our understanding of electron populations in the middle corona and their relation to the formation of the solar wind.

A TimesNet-based Real-time Forecasting System for the F10.7 Index Using DRAO and Chinese Langfang Datasets

In this paper, we construct the DRAO univariate dataset, the DRAO multivariate dataset, and the Chinese Langfang dataset. We develop and compare five deep learning models (TimesNet, iTransformer, PatchTST, N-Beats, BiGRU) and a benchmark artificial neural network (ANN) model to predict the F10.7 index. We study the impact of different feature combinations on the performance of the recommended TimesNet model. Furthermore, we develop a real-time forecasting system for the F10.7 index, incorporating both univariate and multivariate TimesNet models. During the same period, we compare F10.7 prediction performance between our system and that of four foreign institutions (British Geological Survey (BGS), SWPC, Collecte Localisation Satellites (CLS), DRAO). We conduct daily averaged and hourly resolution forecasting using the Langfang dataset. To our knowledge, we establish the first TimesNet- based framework for F10.7 prediction, advancing hourly resolution F10.7 forecasting for the first time. The main results are as follows. (1) The univariate TimesNet model achieves superior prediction performance on the first to the 27th day of forecasting, outperforming both four deep learning models and the ANN model. With the increase in the prediction days, the prediction performance of the six models all shows a downward trend. (2) The multivariate TimesNet-FIAC model, using optimal feature combinations, outperforms the univariate TimesNet-F model. (3) In short- term prediction, TimesNet-FIAC within our system surpasses four foreign institutions. On the first forecasting day, its root mean square error, mean absolute error, and mean absolute percentage error decrease by 15.06%, 18.54%, and 20.90% compared to BGS, and by 3.54%, 10.21%, and 14.94% compared to CLS. (4) On the Langfang dataset, TimesNet-F demonstrates superior generalization in daily averaged short-term forecasting, and maintains good and stable performance in hourly resolution short-term prediction.

First Detailed MeerKAT Imaging Spectroscopy of a Solar Flare

Radio observations provide powerful diagnostics of energy release, particle acceleration, and transport processes in solar flares. However, despite recent progress in radio interferometric imaging spectroscopy, current instruments still face limitations in image fidelity and resolution, restricting detailed spectroscopic studies of flaring regions. Here we present high-fidelity imaging spectroscopy of an M1.3 GOES class flare with MeerKAT, a precursor to the future-generation array SKA-Mid. Radio emissions at the observed frequencies typically originate in the low corona, offering valuable insights into magnetic reconnection and primary energy-release sites. The obtained images achieve an unprecedented dynamic range exceeding 10³, enabling simultaneous analysis of bright coherent bursts and faint incoherent emission from the active region. Multiple spatially distinct coherent sources are identified, implying contributions from different populations of accelerated electrons. The incoherent emission extends beyond Atmospheric Imaging Assembly structures, highlighting MeerKAT's ability to detect dilute but hot plasma invisible to extreme-ultraviolet instruments. Combined with cotemporal hard X-ray images and magnetic field extrapolations, the radio sources are located within distinct magnetic structures, further revealing their association with different populations of accelerated electrons. These results demonstrate MeerKAT imaging spectroscopy as a powerful diagnostic of solar flares and pave the way for future solar flare studies with SKA-Mid.

The Spectral Features of Single-band Type III-like Radio Bursts Observed by PSP during Its First Nine Orbital Encounters

The spectral features of 1122 single-band type III-like radio bursts were analyzed statistically using data from the Parker Solar Probe (PSP) during the first nine orbital encounters. After excluding overlapping events, 923 bursts were classified into 706 complete-spectrum (CS) bursts and 217 incomplete-spectrum (IS) bursts depending on radiation completeness. The distribution of start frequency f_st for IS events (2054 MHz) was predicted by a lognormal fit. The higher cutoff frequency (f_lo 1.21.6 MHz) than previous results may be attributed to the limitation of exciting conditions or the radial expansion of guiding magnetic flux tubes. Short durations (14 minutes) and low peak fluxes (10³ sfu) indicate that most events are short-time weak bursts. The spectral index of duration versus frequency is near 1, which reflects the influence of velocity dispersion and radio-wave scattering. For the denoised spectra of CS events, the peak frequency of maximal flux (4.5 MHz) is obviously higher than those from STEREO (1 MHz) and WIND ( 2 MHz). This implies that the radiation may be the second or third harmonic wave rather than the fundamental wave. Statistical frequency drift rates follow the classical power-law relation (D_abs f^1.84) and are consistent with simulation results of typical type III radio bursts. CS and IS bursts show similarities in f_lo, duration, and drift features, but obviously differ in the peak frequency of maximal flux. These results can provide a useful reference for further exploring the emission mechanisms of single-band type III-like radio bursts and the associated energetic electron acceleration processes.

Comparisons of Intervals with and without Type III Storms

Solar p-modes are 5 minute acoustic waves, which can be used as helioseismological diagnostics of the Sun's subsurface. Recent studies relate them to quasiperiodic pulsations (QPPs) identified in X-rays, radio waves, and extreme ultraviolet (EUV) emission. QPPs with 5 minute periods have been simultaneously observed in Solar Dynamics Observatory (SDO) EUV measurements and Parker Solar Probe (PSP) observations of Type III radio storms, suggesting a link between p-modes and electron acceleration. Using examples when potential field source surface mapping indicated that PSP and SDO were magnetically connected, we compared periodicities in intervals with ("loud") or without ("quiet") coincident Type III radio storms, and looked for small jets (jetlets), as indicators of open field lines enabling electrons to escape and produce radio waves. QPPs of 310 minutes occurred in EUV and Helioseismic and Magnetic Imager (HMI) data in all intervals. Whereas the p-mode amplitudes in photospheric EUV and HMI data were similar in "quiet" and "loud" events, amplitudes in coronal EUV waves were approximately an order of magnitude larger during "loud" intervals. The jetlet rate was comparably higher during "loud" times, consistent with the low corona as the source of electron beams producing Type III waves. The larger EUV amplitudes and higher jetlet rates during "loud" intervals indicate that the presence of electron acceleration along open field lines depends strongly on QPP magnitude and the associated magnetic field configuration. These findings provide new insights into the conditions under which p-mode energy can leak from the photosphere into the corona.

Longitudinal variation in the ionospheric responses to two successive geomagnetic storms in early Solar Cycle 25

This study examines the ionospheric response over different longitude sectors to two consecutive minor to moderate storms of Solar Cycle 25 that occurred on 34 November 2022 and 78 November 2022, using Total Electron Content (TEC) measurements from the SWARM-A satellite. The first storm, occurring on 34 November, was driven by a Corotating Interaction Region (CIR) and characterized by a gradual commencement, while the second storm on 78 November was driven by a Coronal Mass Ejection (CME). On 7 November 2022 at 00.11 UT, a powerful impulsive M5-class solar flare erupted from Sunspot AR3141, resulting in significant ionization of the upper atmosphere and triggering a shortwave radio blackout across the South Pacific region, including parts of Australia and all of New Zealand. The initial storm led to widespread positive ionospheric storm effects globally during both dawn- side and dusk-side hours. The subsequent storm exhibited cumulative effects, producing pronounced positive storm conditions, particularly over the East Pacific, Asia-Pacific, and Australian sectors. Over the American sector, the second storm also produced a notable hemispheric asymmetry in the SWARM-TEC distribution during the morning hours, with enhanced TEC in the Southern Hemisphere and a flattened profile in the Northern Hemisphere. This asymmetry is attributed to storm-time equatorward winds and composition-driven differences in electron density loss. Compositional changes during these geomagnetic storms were investigated using O/N₂ ratio observations from TIMED/GUVI, highlighting thermospheric responses to solar and geomagnetic disturbances. Additionally, temporal variations in the Equatorial Electric Field (EEF), derived from real-time modelling, were analysed to evaluate fluctuations in the equatorial eastward electric field across the studied longitude sectors, providing insights into the electrodynamic processes driving the observed ionospheric variability. The occurrence of two closely spaced geomagnetic storms allowed us to uniquely quantify the cumulative impact of successive solar disturbances on global TEC variability, revealing pronounced longitudinal and hemispheric asymmetries not previously characterized in detail for Solar Cycle 25.

CLEAN and multiscale CLEAN for STIX in Solar Orbiter

CLEAN is a well-established deconvolution approach to Fourier imaging at radio wavelengths and hard X-ray energies. One of the main limitations of CLEAN for hard X-ray imaging is that it requires a final convolution step by means of a convolution kernel whose width is strongly user dependent, and moreover, under-resolution effects are often introduced. This paper describes a multiscale version of CLEAN that is specifically tailored to the reconstruction of images from measurements observed by the Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter. Using synthetic STIX data, this study shows that multiscale CLEAN might represent a reliable solution to the two CLEAN limitations described above. Further, we show the performances of CLEAN and its multiscale release in reconstructing experimental real scenarios characterized by complex emission morphologies.

A solar jet-induced perturbation propagating through coronal loops and in-loop electron beam transport as indicated by type II and type N radio bursts

Aims. Solar type II radio bursts are commonly attributed to coronal shocks driven by coronal mass ejections (CMEs). However, some metric type II bursts have occasionally been reported to occur in the absence of a CME and to be associated with weak solar activity. The aim of this study is to identify the driver of the coronal shock in this kind of type II event. Methods. We investigated a high-frequency metric type II burst with clear band splitting, observed simultaneously by the Chashan Broadband Solar radio spectrograph and the Nanay Radioheliograph. It is associated with a C3.1-class flare and a small-scale jet, but without a detectable CME in the coronagraphs. Results. The type II burst is preceded by multiple type III bursts, one of which exhibits characteristics of a type N burst. The type II burst source is associated with the jet-induced perturbation front propagating through nearby closed loops at a speed of 880 km s¹, rather than the much slower jet front. This suggests that the disturbance initiated by the jet can convert to a shock wave within low Alfvnic coronal loops, providing the necessary conditions for electron acceleration and subsequent radio emission. Our findings offer new insights into the formation mechanism of high-frequency type II bursts associated with weak flares and jets.

Radio, X-ray, and extreme ultraviolet signatures of internal and external reconnection of an erupting flux rope

Aims. We aim to interpret and relate features observed in gigahertz radio, extreme UV (EUV), and X-ray emissions during a filament eruption at the start of a solar flare on 2 April 2022. Methods. We analysed imaging (EUV, X-ray) and spectral (radio, X-ray) data obtained by ground-based and space instruments on board space missions both on Earth (Fermi, Hinode, Solar Dynamics Observatory) and in solar orbits (Solar Orbiter, STEREO-A), thus providing a multi-directional view on the same event. Results. The combination of EUV and X-ray images and X-ray spectra allowed us to identify hot loops in the vicinity of the filament before its eruption. We interpreted their interaction with the rising filament as a signature of an arcade-to-rope reconnection geometry. The subsequent EUV brightening within the filament revealed a helical structure of the erupting rope. We explain co-temporal radio slowly positively drifting bursts as being a result of beam acceleration within the magnetic rope and propagation along the helical structure. The corresponding X-ray spectra are consistent with a thermal origin. The filament rising was accompanied by co-temporal normal and reverse drift type III radio bursts. We interpret them as a signature of a reconnection event and estimated the electron density at the reconnection site. Further untwisting of the helical structure led to formation of a quasi-circular EUV structure seen from Earth and STEREO-A. Its occurrence was co-temporal with a unique tangle of radio U- and inverse U-bursts. We propose that several accelerated beams propagate within that complex structure and generate the burst tangle. During the start of the flare, hard X-ray emission was concentrated near the filament leg only, suggesting predominant propagation of the beams towards its rooting. Conclusions. We collected multi-wavelength observations indicating interaction of the erupting magnetic flux rope with the overlying arcade and internal magnetic reconnection inside the rising flux rope.

The Chinese Radio Telescope Array for Interplanetary Scintillation Monitoring

Solar flares, coronal mass ejections (CMEs) and enegertic particles, etc., are the driving sources that may cause catastrophic space weathers. It is desirable to obtain information of solar eruptions like flares and CMEs, etc., propagating from the Sun to the near-Earth space. The Chinese Meridian Project includes the interplanetary scintillation (IPS) telescopes to investigate the structures and properties of the solar wind throughout the inner heliosphere. From IPS observations one can obtain disturbance information on CME speeds and thus on CME arrival times well off the Sun-Earth line. When combined with modeling techniques and/or in situ data, other parameters such as CME masses can also be obtained, along with CME propagation directions and arrival times. Therefore, a radio telescope array with three 140 m ${\times} $ 40 m parabolic cylinder antennas at the main site and two ${\Phi }$30 m antennas at two subsites about 200 km away from each other, featuring a multi-site array with the highest sensitivity dedicated to IPS observations in the world, has been supported as a major facility of the Chinese Meridian Project. The detailed description of the final optimized design and implementation of this IPS radio telescope array is introduced. The antennas and array configuration, the analog and digital receiving systems for the main site and subsites, the calibration of the IPS telescope array and data processing are described. Finally the overall performance of the IPS telescope array is provided. The detailed information on the IPS radio telescope array will facilitate the use of its data serving space weather research and applications.

Shock Signatures of the Successive Type-II Solar Radio Bursts at Meter Wavelength

The successive type-II solar radio bursts observed on 31 July 2012 by the Bruny Island Radio Spectrometer (BIRS) in the frequency range between 62 6 MHz is reported and analyzed. The first type-II radio burst shows clear fundamental and harmonic band structures, while only one band is observed for the second type-II radio burst and is considered as the harmonic band. The first type-II radio burst is observed in the frequency range of 57 27 MHz between 00:03 00:09 UT at the harmonic band. The second type-II burst is observed between 00:18 00:27 UT in the frequency range of 43 17 MHz. The type-II radio bursts are associated with a C6 class flare located at the southeastern limb (S24E87) and a CME observed from STEREO and LASCO observations. The EUVI signatures of the CME are observed in the STB EUVI FOV between 23:56 (on 30 July 2012) to 00:06 UT (on 31 July 2012), and in the ST-B COR1 FOV between 00:10 00:35 UT moving within an average speed of 725 101 km s¹. The CME is observed in the LASCO C2 FOV after 00:12 UT as a partial halo CME moving with an average speed of 486 km s¹. The height-time plot shows that the first type-II radio burst was formed by the CME-shock along the shock front and the second type-II radio burst along the shock-dip structure, probably the dip structure results from the shock transiting across the high dense streamer structure. The successive type-II bursts are most likely produced by the single CME shock and their interactions with the streamer structures. The first type-II radio burst by the CME shock and the second type-II radio burst by the CME shockstreamer interactions.

Investigating the interplanetary causes of severe geomagnetic storms during solar cycle 23 using SYM-H and OMNI data

During Solar Cycle 23, a detailed analysis was conducted to identify the interplanetary drivers of severe geomagnetic storms (Dst - 150 nT) and to investigate their dependence on solar cycle phases. All such storms were associated with prolonged intervals of southward interplanetary magnetic field (IMF Bz), emphasizing magnetic reconnection as the dominant triggering mechanism. Using OMNI data, key interplanetary parameters (B, Bz, Vsw, Dsw, Tsw) and geomagnetic indices (Kp, Ap, SYM-H) were examined, along with solar proxies (Sunspot Number, SSN; and solar radio flux, F10.7). The results indicate that most severe storms were driven by four primary interplanetary structures: magnetic clouds with shocks (sMC, 30.4%), sheath fields (Sh, 30.4%), combined sheath- magnetic cloud events (Sh + MC, 17.4%), and non-magnetic cloud ICMEs (non-MC, 17.4%), with complex structures contributing only 4.3%. Together, these accounted for roughly 75% of all severe storms during Solar Cycle 23. In terms of solar cycle dependence, sMCs and sheaths were the most frequent drivers during the rising phase, sheath fields dominated during solar maximum, and sMCs, sheaths, and CIRs were the primary contributors during the declining phase. The analysis further confirms that interplanetary magnetic field strength (B, Bz) and solar wind conditions (Vsw, Dsw, Tsw) are more directly responsible for storm intensity than solar proxies (SSN, F10.7), reinforcing the importance of interplanetary monitoring for space weather forecasting.

Design of a New Solar Radio Observing System Working in 36 GHz

Solar radio bursts from decimeter to centimeter wavelength originate from radiations from coherent to incoherent mechanism. This spectral domain hosts a rich variety of fine structures that encode critical information about energetic electron beams, coronal magnetic fields, and ambient plasma conditionskey elements for understanding particle acceleration and energy release during solar flares. To exploit this diagnostic potential, a broadband observing system with high time and frequency resolution is essential. In this study, we designed and developed the Chashan Broadband Solar Decimetric Spectrometer (CBSdm), which enables high-precision observation of solar radio bursts in the 36 GHz frequency band. The system employs a 6 m parabolic antenna with a log-periodic dipole array feed, and utilizes nanosecond-level microwave switching technology to construct a dual-channel analog receiving chain (3.14.4 GHz/4.65.9 GHz). Real-time spectral processing is achieved via a 14-bit 3 GSPS analog-to-digital converter and an Field Programmable Gate Array-based digital backend. Key performance metrics include a noise figure of 2 dB, sensitivity of 77.9 dBm@2.6 GHz, and system linearity better than 0.998. Relative calibration was performed and cross-validated with data from the Learmonth Observatory. Since its deployment in 2023 October, the system has successfully captured X-class flare events, with dynamic spectra exhibiting typical continuum features and multi-frequency synchronous evolution. The coordinated observations of CBSdm (36 GHz) and CBScm (615 GHz) provide continuous gyrosynchrotron spectra covering the optically thick to thin transition, enabling diagnostics of flare magnetic fields and nonthermal electron distributions.

A real-time triggering framework using Yamagawa solar spectrograph for active solar emission observations with the MWA

Some of the most interesting insights into solar physics and space weather come from studying radio emissions associated with solar activity, which remain inherently unpredictable. Hence, a real-time triggering system is needed for solar observations with the versatile new-generation radio telescopes to efficiently capture these episodes of solar activity with the precious and limited solar observing time. We have developed such a system, Solar Triggered Observations of Radio bursts using MWA and Yamagawa (STORMY) for the Murchison Widefield Array (MWA), the precursor for the low frequency telescope of upcoming Square Kilometre Array Observatory (SKAO). It is based on near-real-time data from the Yamagawa solar spectrograph, located at a similar longitude to the MWA. We have devised, implemented, and tested algorithms to perform an effective denoising of the data to identify signatures of solar activity in the Yamagawa data in near real time. End-to-end tests of triggered observations have been successfully carried out at the MWA. STORMY is operational at the MWA for the routine solar observations, a timely development in the view of the ongoing solar maximum. We present this new observing framework and discuss how it can enable efficient capturing of event-rich solar data with existing instruments, like the LOw Frequency ARray (LOFAR), Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA), etc., and pave the way for triggered observing with the SKAO, especially the SKA-Low.

Correlation between active regions' spectra at high radio frequencies and solar flare occurrences

High radio frequencies observations with the Italian network of large single-dish radio telescopes resulted in 450 solar images between 2018 and 2023 in K-band frequency range (1826 GHz). Solar radio mapping at these frequencies allows the probing of the Active Regions (ARs) chromospheric magnetic field close to the Transition Region, where strong flares and coronal mass ejection events occur. Enhanced magnetic fields up to 15002000 G determine anomalous spectra in the ARs brightness compared to pure free-free emission, due to the addition of a steeper gyro-resonance component also associated with circular polarisation up to 40%. When a significant AR spectral flattening is detected, the probability of a strong flare occurrence within 30 hours is high (89% in terms of statistical precision). Despite an approximate weekly cadence of our observations, only 12% of strong flares are missed/unpredicted within this time interval. Through a correlation analysis, we assess the trade-off on the sensitivity and the robustness of this physics-based flare forecast method.

Automatic detection of equatorial plasma bubbles using deep neural networks

The equatorial plasma bubble (EPB) is one of the most important phenomena in the Earth's ionosphere. In this paper, we propose a method for the EPB image data processing and automatic labeling, utilizing the Global-scale Observations of the Limb and Disk (GOLD) 135.6 nm nightglow data. The method extracts the central position of EPBs, significantly reducing the workload of manual labeling. Through manual analysis, a dataset of 1,380 image samples was established. Based on the unique features of EPB images, a deep learning model was developed to improve detection accuracy. After optimization and validation, the YOLO-LessHead model achieved a mean Average Precision (mAP@0.5) of 78.39 %, enabling automatic and accurate identification of EPB images. We used the developed model to identify and statistically analyze the GOLD airglow image data from October 2018 to December 2024. The results indicate that EPB occurrence rates show strong seasonal and longitudinal variability, with distinct seasonal patterns across different longitudes. The zonal drift velocities of EPBs increase with enhanced solar radio flux (F10.7) and decrease as geomagnetic activity (Ap index) intensifies. Drift speeds are generally higher and more variable at the magnetic equator, while the northern and southern EIA regions exhibit similar values and trends.

Comparative Analysis of Ground-level Enhancement (GLE) Events and Non-GLE Solar Energetic Particle Events

This study explores the acceleration origins of solar energetic particles (SEPs) by analyzing the energy spectral characteristics of particles in ground-level enhancement (GLE) events and common SEP (non- GLE) events, as well as the correlations of the peak intensities of energetic particles with flare and coronal mass ejection (CME) diagnostic parameters. By examining the peak energy spectra of protons, for protons with energies >66 MeV, the peak intensities in most GLE events are significantly higher than those in SEP events, establishing an energy threshold distinguishing GLE from non-GLE events. Given the strong correlation between energetic electrons and protons, radio bursts and soft X-rays (SXRs) produced by energetic electrons can be used to study acceleration regions of GLE. Based on data from solar cycles 23 and 24, multifrequency radio data (5, 8.8, and 15.4 GHz) and SXR intensity analysis reveal the heights of particle acceleration. The results indicate that protons with energies >66 MeV in GLE events are closely associated with flare-related acceleration in the low corona, although a contribution from CME-driven shock acceleration cannot be excluded. In contrast, lower-energy protons (<10 MeV) are primarily accelerated by CME-driven shocks. Moreover, SEPs at intermediate energies exhibit a mixed origin, which includes particles accelerated by both flares and CME-driven shocks. These findings provide key insights into the mechanisms of particle acceleration in solar events.

Improving Hard X-Ray and EUV Diagnostics

Solar flares effectively accelerate particles to nonthermal energies. These accelerated electrons are responsible for energy transport and subsequent emissions in hard X-ray (HXR), radio, and UV/EUV radiation. Due to the steeply decreasing electron spectrum, the electron population and consequently the overall flare energetics are predominantly influenced by low-energy nonthermal electrons. However, deducing the electron distribution in this energy-containing range remains a significant challenge. In this study, we apply the warm-target HXR emission model with kappa-form injected electrons to two well-observed GOES M-class flares. Moreover, we utilize EUV observations to constrain the flaring plasma properties, which enables us to determine the characteristics of accelerated electrons across a range from a few keV to tens of keV. We demonstrate that the warm-target model (WTM) reliably constrains the properties of flare-associated electrons, even accounting for the uncertainties that had previously been unaddressed. The application of a kappa distribution for the accelerated electrons allows for meaningful comparisons with electron distributions inferred from EUV observations, specifically for energy ranges below the detection threshold of RHESSI. Our results indicate that the accelerated electrons constitute only a small fraction of the total electron population within the flaring region. Moreover, the physical parameters, such as electron escape time and acceleration timescale, inferred from both the WTM and the EUV observations further support the scenario in which electrons undergo thermalization within the corona. This study highlights the effectiveness of integrating the WTM with EUV observations to accurately characterize energy-containing electrons and their associated acceleration and transport processes.

Microwave Bright Points with Multiple Periodic Oscillations in the Solar Polar Coronal Hole during Solar Cycle Minimum

During solar cycle minimum, polar coronal holes show a prominent radio brightening cap. The analysis of polar microwave enhanced radiation in polar coronal holes is helpful for understanding the magnetic field characteristics and the origin of the solar wind. Using the Koshix synthesis method on Nobeyama Radioheliograph data, we identified microwave bright points (BPs) superposed on the microwave brightening cap of the northern polar coronal hole. These microwave BPs manifested intermittently at fixed locations, with their radiation intensities displaying multiperiodic oscillations (20 s, 50 s, and 2.5 minutes). Multiwavelength analysis revealed that nearly all BPs were adjacent to or coincide with 171 open structures, indicating a strong correlation between microwave BPs and solar wind, propagating along the open field lines. At lower layers, BPs were associated with bright ribbons in the chromosphere and local enhanced magnetic structures in photosphere. The 20 s lifetime of microwave BP enhancement corresponds to their shortest oscillation period, which, in the temporal characteristics of Alfvn waves, implies a localized heating or a small-scale magnetic reconnection process modulated by Alfvn waves at the root of open magnetic field lines. The microwave 2.5 minute and 171 5 minute oscillations likely stem from chromospheric and photospheric oscillation leakage/propagation, respectively. And longer-period (12 minute) oscillations in 171 open structures may link to large-scale coronal hole evolution. The multiperiod oscillatory processes in the polar coronal hole imply complex plasma dynamics inside, which contributes to our understanding of the origin and propagation of solar wind along the open structures in the coronal hole.

Siberian Radioheliograph image classification using ensemble of CLIP, EfficientNet and CatBoost models

The Siberian Radioheliograph (SRH) is a ground-based radio interferometer in Irkutsk, Russia, designed for high-resolution solar observations in the microwave range. It can observe dynamic solar events with spatial resolutions of 730 arcseconds and temporal resolution up to 0.1 s. Generating solar radio images from the Siberian Radioheliograph (SRH) is a multi-step calibration process that corrects instrumental and atmospheric distortions, using redundancy-based calibration with both adjacent and non-adjacent antenna pairs to address phase and amplitude errors in visibility data. The CLEAN algorithm is then applied to deconvolve the point spread function, reduce sidelobes, and enhance the visibility of solar features, resulting in higher quality and more reliable images. While the calibration process generally improves image quality, it can sometimes result in noisy or spatially shifted images that are not suitable for scientific use. We developed a deep learning approach for automatic image quality classification. The training dataset was prepared using a zero-shot CLIP model and further validated manually. We evaluated four different models: a fine-tuned EfficientNet, two CatBoost variants using embeddings from CLIP and EfficientNet, and an Ensemble model that combined predictions from all three individual models. The ensemble model delivered the highest performance and is now used for daily SRH image classification in the web service.

Performance evaluation of NTCM ionospheric model variants (NTCM-GL, NTCM-BC, NTCM-Klobpar and NTCM-GlAzpar) during the 25th solar cycle

In GNSS navigation and positioning applications, single-frequency users typically require additional ionospheric Total Electron Content (TEC) information to correct for signal propagation delays and mitigate ranging errors. The Global Neustrelitz TEC Model (NTCM-GL), an empirical model for estimating TEC, has been adapted into three variants through parameter reduction or driver parameter modification: the Neustrelitz TEC broadcast model (NTCM-BC), the NTCM driven by GPS Klobuchar parameters (NTCM-Klobpar), and the NTCM driven by Galileo broadcast parameters (NTCM-GlAzpar). This study evaluates the performance of these four NTCM variants both in the TEC domain (assessing the precision of their TEC calculations) and in the positioning domain (assessing their improvement in single-frequency, single-point positioning (SPP) during the 25th solar cycle. Using Global Ionospheric Map (GIM) product from the International GNSS Service (IGS) as a reference, the global ionospheric TEC correction percentage is 48.24 % for NTCM-BC, 71.16 % for NTCM-GlAzpar, 56.14 % for NTCM-Klobpar, and 62.93 % for NTCM-GL. Furthermore, SPP results from 32 globally distributed GNSS stations show percentage improvements in 3D positioning accuracy compared to the uncorrected solution: On low solar activity days in 2020, improvements reached 38.37 % (NTCM-BC), 46.15 % (NTCM-GlAzpar), 39.48 % (NTCM- Klobpar), and 45.16 % (NTCM-GL). For high solar activity days in 2023, the corresponding values were 42.42 %, 53.79 %, 36.98 %, and 52.33 %, respectively. Among the evaluated models, NTCM-GL and NTCM-Klobpar demonstrate the highest sensitivity to solar activity, with their root mean square (RMS) errors closely correlating with variations in the 10.7 cm solar radio flux (F10.7) index. Conversely, NTCM-GlAzpar exhibits the least sensitivity, maintaining stable performance across varying solar flux conditions.

Ionospheric topside sounding revival

Although satellite sounding of the upper ionosphere yielded impressive results and enabled numerous important discoveries across different nations, its application halted in 1999 following the deployment of the last topside sounder aboard the Russian space station Mir. During the last decades monitoring of the upper ionosphere traditionally relied on total electron content (TEC) measurements obtained from ground- and space-based Global Navigation Satellite Systems (GNSS) receivers. However, this methodology struggles to resolve the internal structure of the ionosphere effectively, particularly in scenarios involving steep horizontal gradients of plasma density. Alternative techniques, such as GNSS occultation experiments, although widely adopted, suffer from similar limitations when confronted with strong gradient conditions. Meanwhile, ground-based tomographic imaging, dependent on empirical models like IRI and NeQuick, fails to reconstruct sharply layered ionospheric structures reliably. Addressing these gaps, the pioneering launch of two LAERT topside sounders on November 5, 2024, aboard the Ionosfera-M1 and Ionosfera-M2 satellites marked a turning point to direct ionospheric sensing after a hiatus of 25 years. Deployed into a sun-synchronous orbit at an altitude of 820 km, the new-generation LAERT sounders introduced multifunctionality, combining traditional vertical sounding with advanced techniques such as HF radio spectrometry and relaxation sounding. Retaining the capability to deliver precise vertical profiles of plasma density, these innovations allow for a more nuanced exploration of plasma dynamics near the spacecraft, magnetospheric interactions, and extraterrestrial phenomena like solar radio bursts.

Band splitting in m-Type II radio bursts and their role in coronal parameter diagnostics

Type II radio bursts are signatures of shock waves generated by solar eruptions, observed at radio wavelengths. While metric (m) type II bursts originate in the lower corona, their longer-wavelength (up to kilometers) counterparts extend into interplanetary space. A rare but valuable feature observed in some type II bursts is band splitting in their dynamic spectra, which provides crucial insights into physical parameters such as shock speed, Alfvn Mach number, Alfvn speed, and coronal magnetic field strength (B). In this study, we investigate band- splitting in 44 m-type II radio bursts observed by the Radio Solar Telescope Network during solar cycle 24 (20092019). These events exhibit splitting in both fundamental and harmonic bands and are analyzed under both perpendicular and parallel shocks. All events are associated to solar flares and 41 (93 %) with the coronal mass ejections. Shock speeds, derived using a hybrid coronal density model proposed by Vrnak et al. (2004), range from 350 to 1727 km s¹. The relative bandwidth (BDW) of the split bands remains constant with frequency and height. Alfvn Mach numbers indicate moderate shock strength (1.063.38), while Alfvn speeds and B vary from 2301294 km s¹and 0.487.13 G, respectively. Power-law relationships are established as BDWfL-0.4 and BDWR1, while the coronal magnetic field decreases with height as BR-3. These results enhance our understanding of shock dynamics and magnetic field structures in the solar corona.

A Global TEC Spatiotemporal Prediction Model Based on Multisource Data

The accurate prediction of Total Electron Content (TEC) is crucial for ensuring the safety of space missions and enhancing the stability of satellite navigation. Combining the advantages of Convolutional Neural Networks (CNN) in local feature extraction with the Transformer self- attention mechanism's ability to capture global dependencies, this paper proposes a spatiotemporal prediction model based on an encoder-decoder structureED-ST-Transformer, aimed at improving TEC prediction accuracy. Given the close relationship between TEC, solar activity, and geomagnetic disturbances, this study incorporates auxiliary inputs including the disturbance storm time index (Dst), planetary K-index (Kp), 10.7 cm solar radio flux (F10.7), and sunspot number. Using multi- source data from the preceding 24 hr (with a temporal resolution of 2 hr), the model is constructed to forecast global TEC maps for the subsequent 24 hr. To evaluate the performance of the ED-ST-Transformer model, this study compares it with the C1PG, ED-ConvLSTM, and ED- BiConvLSTM models in terms of overall prediction accuracy, error distribution, visualization effects, and performance in different months and latitude regions under various solar activity years. Additionally, the prediction performance of the model during geomagnetic storms is also analyzed. This outcome demonstrates that ED-ST-Transformer provides the most stable and accurate predictions under all conditions, outperforming the other models.

Type II and Type III Solar Radio Burst Classification Using Transfer Learning

The Sun periodically emits intense bursts of radio emission known as solar radio bursts (SRBs). These bursts can disrupt radio communications and be indicative of large solar events that can disrupt technological infrastructure on Earth and in space. The risks posed by these events highlight the need for automated SRB classification, providing the potential to improve event detection and real-time monitoring. This would advance the techniques used to study space weather and related phenomena. A dataset containing images of radio spectra was created using data recorded by the Compound Astronomical Low frequency Low cost Instrument for Spectroscopy and Transportable Observatory (e-Callisto) network. This dataset comprises three categories: empty spectrograms; spectrograms containing Type II SRBs; and spectrograms containing Type III SRBs. These images were used to fine-tune several popular pre- trained deep-learning models for classifying Type II and Type III SRBs. The evaluated models included VGGnet-19, MobileNet, ResNet-152, DenseNet-201, and YOLOv8. Testing the models on the test set produced F1 scores ranging from 87% to 92%. YOLOv8 emerged as the best-performing model among them, demonstrating that using pre-trained models for event classification can provide an automated solution for SRB classification. This approach provides a practical solution to the limited number of data samples available for Type II SRBs.

Three-Dimensional Characterization of Coronal Loops Using Combined Radio and EUV Observations

Coronal arcade loops play a key role in our understanding of solar activity because they contain plasma within closed magnetic field lines. Although these structures have been extensively studied using various observational techniques, combining radio and extreme ultraviolet (EUV) observations provides a unique opportunity to analyse their properties more comprehensively. In this study, we present the first three- dimensional characterisation of coronal loops by analysing simultaneous observations of type J solar radio bursts and EUV imaging. Data were collected from the Observations Radiospectrographiques pour FEDOME et l'tude des ruptions Solaires, the Nanay Radioheliograph, and the Solar Dynamics Observatory/Atmospheric Imaging Assembly instruments during an event on 6 March 2014. Our results reveal a direct spatial correlation between the sources of type J bursts and visible coronal loops. Using a new methodology combining radio polarisation measurements and EUV-based three-dimensional loop reconstruction, we determined several key physical parameters: a temperature of approximately 0.82 MK, an electron density distribution ranging from around 109 cm3 at the foot of the loop to around 107 cm3 at the top, and a magnetic field strength varying from around 850 Gauss at the footpoint to around 5 Gauss at the top. Our results confirm the validity of hydrostatic equilibrium and dipole field models for coronal loops while providing unprecedented insights into their three-dimensional structure and physical properties. This research introduces a new diagnostic technique for studying coronal loop dynamics and their role in solar eruptions.

Relevance to PSP and other solar wind observations

Based on the linearized Maxwell-fluid equations, this paper investigates how an electronion plasma responds to a short-term current variation with respect to the generation of electromagnetic waves. It is shown that Langmuir waves can be triggered in the high-frequency range, the direct conversion of which is directly associated with the generation of type III radiation. In contrast to the classical approach of Ginzburg and Zhelezniakov ["On the possible mechanisms of sporadic solar radio emission (radiation in an isotropic plasma)," Sov. Astron. 2, 653 (1958)], after which beam-excited Langmuir waves in a two-step process are converted into electromagnetic radiation, the presented mechanism works without any parametric decay and wave coalescence. Rather, electric field oscillations at the electron plasma frequency can be triggered by different pulses of the driving current, e.g., by the sudden (uncompensated) net current onset of the strahl at t = 0 in a core-strahl plasma or by given temporal current variations, which may occur as transient phenomena if the solar wind is disturbed by shocks, magnetic switch-backs or other discontinuities. The application of current pulses that imitate the beamplasma instability may generate type III radiation with double-peak frequency spectra, often observed on satellites. Moreover, suitable current profiles enable the simultaneous excitation of type III emission and whistler waves. Measurements of Langmuir waves, type III radiation, and whistler waves on board various satellites in the solar wind, and in particular some of the recent results of the Parker Solar Probe, are interpreted in the light of the theoretical model. The fluid approach is confirmed by the results of fully kinetic particle-in-cell simulations, presented in the Appendix.

A type II solar radio burst without a coronal mass ejection association

Type II solar radio bursts are commonly associated with shocks generated by coronal mass ejections (CMEs), where plasma waves are excited by magnetohydrodynamic (MHD) processes and converted into radio waves at the local plasma frequency or its harmonics. However, there are instances where type II bursts occur in the absence of whitelight CMEs. We analysed one such metric type II radio burst observed on 2 November 2023, characterized by split band features and fundamental-harmonic lanes. Notably, no CME was detected with space-based coronagraphs during this event. However, an intense M1.6 class flare was observed just before the type II burst and an extreme ultraviolet (EUV) disturbance was observed expanding into surrounding regions. The absence of any whitelight CME seen in any coronagraph field of view, even though the EUV shock had a moderate speed of 500 km s-1, which was close to the shock speed derived from radio observations. These observations indicate that the shock in the inner corona was most likely driven by the EUV ejecta seen in the lower corona, but the ejecta did not survive as a CME in the coronagraph field of view.

Environmental and cosmic drivers

This study employs Lomb periodogram analysis to investigate the spectral characteristics of atmospheric gases (NH₃, O₃, NOx, NO₁, NO₂, NMHC), solar radio flux (F10.7), and cosmic rays (CR) in Riyadh from 1999 to 2007. Significant periodicities ranging from 10 days to 1.6 years were identified, with prominent cycles for gases at 454584 days, 291293 days, 215 days, 155171 days, 113 days, 97 days, 39 days, 1517 days, and 10 days; for F10.7 at 467 days, 255 days, 172 days, and 17 days; and for CR at 489 days, 273 days, 140 days, and 32 days. The 215-day semi-annual and 39-day synoptic cycles, prevalent across multiple gases, alongside the 155171-day cycle strongly linked to F10.7 and CR, highlight robust seasonal, synoptic, and heliospheric influences. Cross-spectral analyses, along with zero-order and partial correlation analyses, were conducted and confirmed that, while variations in atmospheric gases are subject to terrestrial factors such as meteorological conditions (temperature, humidity, wind), extraterrestrial factors, including solar activity and CR, play a significant role in their variations, with common periodicities validating these influences. Solar activity enhances NOx and NMHC photochemistry, while CR ionization significantly affects O₃ and reduces NOx. These extraterrestrial impacts, which warrant further investigation, are critically modulated by meteorological factors. These findings are vital for addressing Riyadh's environmental challenges, supporting sustainable urban development, and enhancing understanding of extraterrestrial influences on climate, atmospheric processes, and environmental sciences.

Comparative Assessment of Solar and Geophysical Parameters during the Initial Six Years of Solar Cycles 24 and 25

This study presents a comparative statistical analysis of solar activity during the first six years of Solar Cycles 24 (20082013) and 25 (20192024). The analysis focuses on key solar and geophysical parameters, including sunspot numbers, halo coronal mass ejections (CMEs), solar radio flux at 10.7 cm (F10.7), and geomagnetic storms, to assess differences in solar behavior between the two cycles. Sunspot numbers varied between 0 and 139.1 in Solar Cycle 24, whereas they ranged from 0.2 to 216 during the corresponding period of Solar Cycle 25. Similarly, the F10.7 cm radio flux fluctuated between 65.7 and 153.5 in solar flux unit (s.f.u.) during 20082013, and between 67.05 and 245.6 s.f.u. from 2019 to 2024, reflecting an overall increase in solar output. The study also includes an analysis of halo CMEs, with 192 events observed during Solar Cycle 24 and 227 during Solar Cycle 25, both characterized by an angular width of 360. Geomagnetic activity was assessed using 104 events from Cycle 24 and 179 from Cycle 25, with disturbance storm time (Dst) index values ranging from 50 to 350 nT. The results indicate a significant increase in solar activity during the early phase of Solar Cycle 25 compared to Solar Cycle 24. This suggests a more intense and dynamic space weather environment in the current solar cycle, which may have important implications for space weather forecasting and satellite operations.

The 27-Day Oscillation in Ionospheric Total Electron Content Observed by GNSS

The 27-day oscillation in total electron content (TEC) is analysed by means of world maps of TEC. The TEC maps are derived from measurements of the ground receiver network of the Global Navigation Satellite System (GNSS) and are provided by the International GNSS Service (IGS). The observed 27-day oscillation in TEC is mainly due to the 27-day solar rotation period, which induces a 27-day oscillation in extreme ultraviolet radiation (EUV) of the Sun. Analysing the time interval from 2003 to 2020, cross-correlation of the 27-day oscillation of the solar MgII-index of the Solar Radiation and Climate Experiment (SORCE) and the 27-day oscillation in TEC shows an average time delay of about 1.1 days for the ionospheric response with respect to the solar EUV variation. The average correlation coefficient of the solar and the ionospheric variation is 0.85. The cross-correlation of the 27-day oscillation in solar radio flux F10.7 and the 27-day oscillation in TEC gives a time lag of about 1.3 days and an average correlation coefficient of 0.78. The world maps of the amplitude of the 27-day oscillation in TEC are discussed for the TEC data from 1998 to 2024. Finally, TEC composites are derived for F10.7 enhancement events and geomagnetic storms.

Reconnection-driven Decaying Pulsations Modulated by Slow Magnetoacoustic Waves

Decaying pulsations have been simultaneously detected in the low-energy X-rays of solar/stellar flares, which are supposed to be associated with standing slow magnetoacoustic or kink-mode waves. The physical mechanism behind rapid decay remains unknown. We present the detection of quasiperiodic pulsations (QPPs) with rapid decay in high-energy emissions produced in two major flares on 2024 January 10 and May 14. Using empirical mode decomposition, decaying QPPs are identified in hard X-ray and microwave emissions during the flare-impulsive phase, suggesting a process of oscillatory magnetic reconnection. The quasi- periods and decay times are determined by a damped harmonic function, which are approximately 177 8 s (249 25 s) and 118 4 s (124 5 s), respectively. The restructured X-ray images reveal double footpoints connected by hot flare loops. Their phase speeds are estimated to be about 400 and 670 km s¹, both below the local sound speed in high-temperature plasmas, indicating the presence of slow-mode waves in hot flare loops. We perform coronal diagnostics based on standing slow- mode waves and derive key physical parameters, including the polytropic index, the thermal ratio, viscous ratio and radiation ratio, which are consistent with previous results. Our observations support the conclusion that decaying QPPs are triggered by oscillatory magnetic reconnection that is modulated by standing slow magnetoacoustic waves, with their rapid decay attributable to a coeffect of viscous damping and localized magnetic reconnection rate.

"Ab-initio General-relativistic Neutrino-radiation Hydrodynamics Simulations of Long-lived Neutron Star Merger Remnants to Neutrino Cooling Timescales" (2023, ApJ, 959, 46)

Not Available

Spatially Resolved Transport Parameters in Solar Flares

The time profile of solar flare radio emission is often modeled as an injection of energetic particles onto a closed magnetic loop, where they may be trapped by the pinching of field lines and remain for a time before decaying through loss of energy to the background or escaping to the solar surface through the bottom of the loop. These injection, trapping, and precipitation models for energetic particle transport have often been used to explain the characteristics of spatially integrated microwave emissions in solar flares. With the high-cadence imaging spectroscopy capabilities of modern radio instruments, these ideas can be probed with new depth. Radio imaging allows for the selection of particular regions of flares to spatially and temporally isolate individual injections and determine individual decay parameters that could be confused in spatially integrated spectra. Simultaneous spectroscopy allows the fitting of light curves versus frequency for insight into the evolution of the particle energy spectrum and a deeper physical understanding of the decay process. Using currently available time resolution and data quality, injections and decays can be fit simultaneously to the order of 1 s. These considerations motivate the creation of the Pulsed-Injection-Precipitation Decomposition Fitter (PIP_Decomp), which implements an automated method for fitting a series of light curves with injection functions convolved with exponential decays to produce spectrally resolved fit parameters. Herein, PIP_Decomp is introduced and tested by applying it to model flares. Then, PIP_Decomp is used to investigate two relatively simple flares observed by the Expanded Owens Valley Solar Array.

Enigmatic Centi-SFU and mSFU Nonthermal Radio Transients Detected in the Middle Corona

Decades of solar coronal observations have provided substantial evidence for accelerated particles in the corona. In most cases, the location of particle acceleration can be roughly identified by combining high spatial and temporal resolution data from multiple instruments across a broad frequency range. In almost all cases, these nonthermal particles are associated with quiescent active regions, flares, and coronal mass ejections (CMEs). Only recently, some evidence of the existence of nonthermal electrons at locations outside these well-accepted regions has been found. Here, we report for the first time multiple cases of transient nonthermal emissions, in the heliocentric range of 37R, which do not have any obvious counterparts in other wave bands, like white-light and extreme ultraviolet. These detections were made possible by the regular availability of high dynamic-range low-frequency radio images from the Owens Valley Radio Observatory's Long Wavelength Array. While earlier detections of nonthermal emissions at these high heliocentric distances often had comparable extensions in the plane of sky, they were primarily associated with radio CMEs, unlike the cases reported here. Thus, these results add on to the evidence that the middle corona is extremely dynamic and contains a population of nonthermal electrons, which is only becoming visible with high dynamic- range low-frequency radio images.

Solar Eruption Onset and Particle Acceleration in Nested-null Topologies

The magnetic breakout model explains a variety of solar eruptions, ranging from small-scale jets to large-scale coronal mass ejections (CMEs). Most of our previous studies are focused on jets and CMEs in single null-point topologies. Here, we investigate the initiation of CMEs and associated particle acceleration in a double null-point (or nested fan-spine) topology during multiple homologous M- and X-class flares from an active region. The initiation of the flare and associated eruption begins with inflow structures moving toward the inner null of the closed fan-spine topology. Simultaneous slow flare reconnection below a small filament formed a hot flux rope along with expansion of the overlying flux during slow breakout reconnection at the inner null. The first explosive breakout reconnection of the flux rope at the inner null produced a circular and a remote ribbon along with successful eruption of the flux rope and associated fast EUV (shock) wave. Simultaneous flare reconnection beneath the erupting flux rope produced a typical two-ribbon flare along with two hard X-ray footpoint sources. When the flux rope (with shock) reaches the outer null, a second explosive breakout reconnection produces another large-scale remote ribbon. The radio observations reveal quasiperiodic Type III bursts (period = 100 s) and a Type II burst during the breakout reconnection near the inner and outer nulls, along with gradual solar energetic particles observed at 1 au for magnetically connected events. This study highlight the importance of two successive breakout reconnections in the initiation of CMEs in nested-null topologies and associated particle acceleration and release into the interplanetary medium. The particles are accelerated by the shock ahead of the flux rope, which formed during the inner breakout reconnection. These findings have significant implications for particle acceleration and escape processes in multiscale null-point topologies that produce jets and CMEs.

Effects in the upper atmosphere and ionosphere in the subauroral region during Victory Day 2024 Geomagnetic Storm (May 1012, 2024)

This study provides a detailed analysis of the impact of the magnetic storm (referred to as the Victory Day Storm, as the solar flares occurred on May 89, 2024), which took place from May 10 to 12, 2024, and has become the most significant geomagnetic event of the 21st century to date, on the subauroral and mid-latitude ionosphere using a variety of instruments and observation methods. The research is based on data from satellite systems (Swarm, DMSP, TIMED), ground-based magnetometers, ionosondes, VLF (very low frequency) signals, GNSS (global navigation satellite system) receivers, and all-sky cameras. The solar flares and powerful coronal mass ejection (CME) from the Sun caused intense geomagnetic disturbances, leading to significant changes in the density, structure, and dynamics of the ionospheric plasma at subauroral and mid-latitudes. In particular, auroras were observed at these latitudes, which is a rare phenomenon for these regions. Satellite observations, SUSSI data (allowing for measurements of auroral emissions in the ionosphere) from the DMSP satellite, and all-sky cameras provided detailed information on the spatial development of storm phenomena in the ionosphere. The cooling flux of nitrogen oxide during geomagnetic activity was studied using TIMED/SABER data, as well as latitudinal profiles of the integral O/N2 ratio from TIMED/GUVI data. Measurements from magnetometers and ionosondes revealed significant changes in the magnetic field and ionospheric plasma density, while VLF (very low frequency) stations and GNSS receivers recorded substantial disruptions in radio communication and navigation during the Victory Day Geomagnetic Storm. The study emphasizes the need for integrating various data to deeply understand and predict the impact of extreme magnetic storms on the ionosphere, upper atmosphere, and related technological systems.

A case study of Mother's Day solar storm (1015 May 2024)

The strongest geomagnetic storm of solar cycle 25 (SYM-H < 518 nT), referred to as the "Mother's Day Storm", occurred between 10 and 15 May 2024. Its source traces back to a solar event on 8 May 2024, when the solar Active Region 13,664 produced multiple solar flares and launched several Earth-directed coronal mass ejections (CMEs). Classified as a G5 geomagnetic storm, it was the most intense one of the space age since March 1989 storm (SYM-H < 720 nT). The storm's impact on Earth's ionosphere-magnetosphere system, particularly the significant variations in electron density, degraded radio wave propagation, disrupted Global Navigation Satellite System (GNSS) operations and communication services. These disruptions pose challenges to the integrity and accuracy of GNSS-based high-precision positioning methods, such as Precise Point Positioning (PPP). This study evaluates the performance of multi-GNSS dual-frequency PPP (DF-PPP) during both the quiet pre-storm period (89 May 2024) and the Mother's Day storm (1011 May 2024). Using a robust dataset from over 1,800 globally distributed GNSS stations, the analysis investigates the correlation between PPP errors and the rate of Total Electron Content (TEC) index (ROTI), a widely used metric for characterizing ionospheric plasma irregularities. The results reveal three critical findings: first, positioning errors showed strong latitudinal dependence, with high-latitude stations suffering the most severe degradation median errors rising from 2.4 cm (quiet-time) to 9.0 cm (275 % increase) due to intense ionospheric irregularities within the auroral oval. Mid-latitude stations experienced moderate degradation (25 % median error increase) associated with expanded Equatorial Plasma Bubbles (EPBs), while low-latitude regions showed more stable but inherently higher positioning errors. Second, the elevation-weighted ROTI (RO TIele ) exhibited strong correlations with PPP errors (r = 0.78 at high latitudes), validating its utility for global storm-impact assessments. Third, multi-GNSS combinations (GPS + GLONASS + Galileo) reduced errors by 4152 % compared to GPS-only solutions, with Galileo's signals showing particular robustness. Collectively, these findings establish multi-GNSS PPP as a critical tool for space weather monitoring, thereby advancing global understanding of ionospheric impacts during extreme geomagnetic events.

Comparative assessment of GNSS ionospheric broadcast models during the May 10, 2024 geomagnetic storm event

The Single-Frequency GNSS ionospheric corrections models play a vital role in improving the positional accuracy of GNSS users, satellite orbit determination, Radio Occultation for atmospheric profiling, and remote sensing satellite data analysis. Ionospheric correction models such as GNSS Klobuchar, NeQuick G, BDGIM, and USRG can be investigated to understand space weather effects like geomagnetic storms and solar flares in low-latitude regions. In this paper, the performance of four ionospheric Total Electron Content models, BDGIM, USRG, NeQuick G, and Klobuchar, are evaluated for geomagnetic storm events that occurred on 10 May 2024 over the Indian region, a low-latitude area profoundly influenced by the Equatorial Ionization Anomaly (EIA). The results indicate that the BDIGM model outperformed the RMSE of 7.7TECU over the NeQuick G (11.16TECU) and Klobuchar (25.38TECU) models on main and geomagnetic storm days. The season variability of the BDGIM model is also well predicted as compared to other single-frequency ionospheric models. The BDGIM TEC models are closer to dual frequency GNSS TEC estimations than Klobuchar and NeQuick G at eight GNSS stations in the Indian region. Further, the positional accuracy analysis of single- frequency ionospheric correction models with the GLAB GNSS data processing tool for Bengaluru GNSS station TEC data. The BDGIM model static positioning errors are minimal compared to Klobuchar and NeQuick G over low-latitude GNSS stations under geomagnetic storm conditions. The outcome of this research work would be beneficial for identifying suitable single-frequency GNSS ionospheric corrections models over the low and equatorial latitude conditions for aviation, maritime, and remote sensing data applications.

a challenge to the coronal mass ejection origin of long-duration gamma-ray flares

We present a multi-spacecraft analysis of the 2024 July 16 long-duration gamma-ray flare (LDGRF) detected by the Large Area Telescope on the Fermi satellite. The measured > 100 MeV -ray emission persisted for over seven hours after the flare impulsive phase, and was characterized by photon energies exceeding 1 GeV and a remarkably hard parent-proton spectrum. In contrast, the phenomena related to the coronal mass ejection (CME)-driven shock linked to this eruption were modest, suggesting an inefficient proton acceleration unlikely to achieve energies well above the 300 MeV pion-production threshold to account for the observed -ray emission. Specifically, the CME was relatively slow (600 km/s) and the accompanying interplanetary type-II/III radio bursts were faint and short-lived, unlike those typically detected during large events. In particular, the type-II emission did not extend to kilohertz frequencies and disappeared 5.5 hours prior to the LDGRF end time. Furthermore, the associated solar energetic particle (SEP) event was very weak, short-duration, and limited to a few tens of MeV, even at magnetically well-connected spacecraft. These findings demonstrate that a very fast CME resulting in a high-energy SEP event is not a necessary condition for the occurrence of LDGRFs, challenging the idea that the high-energy -ray emission is produced by the back-precipitation of shock-accelerated ions into the solar surface. The alternative origin scenario based on local particle trapping and acceleration in large- scale coronal loops is instead favored by the observation of giant arch- like structures of hot plasma over the source region that persisted for the entire duration of this LDGRF.

Insights from radio and in situ observations, and EUHFORIA modeling

Context. The distribution of the coronal electron density at different distances from the Sun strongly influences the physical processes in the solar corona, and it is therefore a very important topic in solar physics. The majority of the methods used to estimate coronal electron density, including radio observations, were up to now not fully validated due to the absence of in situ observations closer to the Sun. Consequently, space weather forecasting models that simulate coronal density lacked proper validation. Newly available Parker Solar Probe (PSP) in situ observations at distances close to the Sun provide an opportunity to study the properties of plasma near the Sun and to compare observational and modeling results. Aims. The focus of this work is to study type III radio bursts, estimate their propagation path, and validate the coronal electron density obtained from in situ radio observations and modeling with the EUropean Heliospheric FORecasting Information Asset (EUHFORIA). Methods. In this study of type III radio bursts observed during the second PSP perihelion, we employ radio triangulation and modeling to analyze coronal electron density. Using the radio triangulation method, we determined the 3D positions of the radio sources. Additionally, we utilized the state-of-the-art EUHFORIA model to estimate electron densities at various locations. The electron densities derived from radio observations and EUHFORIA modeling were then inter-validated with in situ measurements from PSP. Results. We studied 11 type III radio bursts during the second PSP perihelion, with radio triangulation showing their propagation path in the southward direction from the solar ecliptic plane. The obtained radio source sizes ranged from 0.5 to 40 deg (0.525 R), showing no clear frequency dependence. This indicates that scattering of radio waves was not very significant for the studied events and in this frequency range. A comparison of electron densities derived from radio triangulation, in situ PSP data, and EUHFORIA modeling showed a large range of obtained values. This result is influenced by the different propagation paths across different coronal structures and model limitations. Despite these variations, EUHFORIA successfully identified high-density regions along type III burst paths, demonstrating its capability to capture large- scale density structures. Conclusions. Our study emphasizes that type III bursts do not always follow the Parker spiral but instead trace distinct magnetic field lines that can be very differently oriented. The study shows constant radio source sizes and confirms that small-scale density fluctuations in PSP data remain relatively low. These two characteristics indicate that scattering effects do not significantly change observed radio source positions within the studied distances.

Solar Radio Wide-Band Spectroscopy and Imaging Facilities of the Chinese Meridian Project Phase II

Solar eruptions, including flares and coronal mass ejections, are the most energetic phenomena in the solar system. These explosive events accelerate high-energy particles and generate electromagnetic radiation from radio to gamma-ray wavelengths, producing heliospheric disturbances and acting as primary drivers of space weather hazards. Wide-band solar radio observations, spanning decameter to centimeter wavelengths, constitute a key component of the Chinese Meridian Project (CMP) for tracking and monitoring solar eruptions from the Sun's atmosphere into interplanetary space. The technique of solar radio imaging spectroscopy is still challenging and new. The Mingantu Spectral Radioheliograph (MUSER) with three arrays at low (30400 MHz), intermediate (400 MHz2 GHz), and high (215 GHz) frequency bands images the solar atmosphere in 3D from the top-chromosphere up into the mid-corona. The solar radio spectrometers include a Metric Wavelength Solar Radio Spectrometer (90600 MHz) at Chashan in Shandong province, together with the three Decameter-Metric to Centimetric Wavelength Solar Radio Spectrometers at the MUSER site, offering spectrum monitoring ability across a super wide band from 30 MHz up to 15 GHz. The overall design, some technical details, calibration method, and performance with some preliminary data of these facilities are described.

Detection of Various Solar Radio Bursts Based on Stable Diffusion and Self-Supervised Pretraining

Accurate identification of solar radio bursts (SRBs) is of great significance for solar physics research and space-weather forecasting. Most existing studies focus on the mere detection of SRB occurrence or the identification of a single class (e.g., Type III bursts), which fails to meet the demand for precise detections of various solar radio bursts. Additionally, current mainstream SRBs detection models often employ complex architectures and redundant parameters, resulting in low computational efficiency. To address these limitations, we constructed a spectrogram dataset based on the e-CALLISTO platform, comprising Type II, Type III, Type IV, and Type V bursts. The dataset contains 8752 images with 10,822 annotated instances, where samples of types IV and V are incredibly scarce. To overcome the challenge of pretraining with few-shot classes, this paper proposes a pretraining method that integrates a stable diffusion generative model with a self-supervised learning strategy, effectively enhancing the model's learning capability for few-shot classes. Building on this, this paper presents a detection model for various solar radio bursts, VitDet-SRBs (Vision Transformer Detector for Solar Radio Bursts), which incorporates a channel attention mechanism into the feature fusion module to enhance performance while controlling model complexity. Experimental results show that VitDet-SRBs achieve an average precision at a single Intersection-over-Union threshold of 0.50 (AP@50, AP with IoU = 0.50) of 81.2% on the SRBs dataset, outperforming existing mainstream methods in both precision and recall. This study not only provides a novel approach for efficient detections of various solar radio bursts but also offers a feasible solution for other few-shot astronomical data processing problems, with broad application prospects.

Relative Strengths of Fundamental and Harmonic Emissions of Solar Radio Type II Bursts

Solar radio type II bursts are slow-drifting bursts that exhibit various distinct features such as Fundamental (F) and Harmonic (H) emissions, band-splitting, and discrete fine structures in the dynamic spectra. Observationally, it has been found that in some cases the F emission is stronger than the H emission, and vice versa. The reason for such behavior has not been thoroughly investigated. To investigate this, we studied 58 meter wave (20 500 MHz) type II solar radio bursts showing both F and H emissions, observed during the period from 13 June 2010 to 25 December 2024, using data obtained with the Compound Astronomical Low frequency Low cost Instrument for Spectroscopy and Transportable Observatory (CALLISTO) spectrometers at different locations and Gauribidanur LOw-frequency Solar Spectrograph (GLOSS). We examined the intensity ratios of the H (IH) and F (IF) emissions and analyzed their variation with heliographic longitude. We found that 14 out of 19 bursts originating from heliographic longitudes beyond 75 exhibited an IH/IF ratio greater than unity. In contrast, 32 out of 39 bursts originating from longitudes within 75 showed a intensity ratio less than unity. From these results, we conclude that the relative strength of the F and H emissions can be influenced by refraction due to density gradient in the solar corona, directivity and viewing angle of the bursts.

Sources of a Stationary Type II Radio Burst on the Associated Coronal Mass Ejection Driven-Shocks

Solar type II radio bursts are associated with shock waves driven by coronal mass ejection (CME). Their shapes in the solar radio dynamic spectrum depend on the shock velocity and the electron density traversed by their radio sources. This study examines a stationary type II radio burst. By analyzing observations from the Daocheng Solar Radio Telescope and the Solar Dynamics Observatory/Atmospheric Imaging Assembly, we aim to determine the spatial relationship between type II burst sources and shock fronts. Observations suggest that the radio sources are located in the interaction region between the CME shocks and the coronal streamer. Extensive analysis of their fine structures, particularly the type II herringbones, shows that the radio sources are generally distributed in a frequency order from downstream to upstream regions of the shocks, or only in the upstream region. The observations confirm the existence of type II bursts and associated energetic electrons in the regions upstream and downstream of a shock wave.

The Bimodal Solar Corona Revisited

During Solar Cycle 24, several groups independently noticed that a sharp transition in coronal activity occurred early in 2011. The transition took the form of a sudden jump in the intensity of ("hot") EUV lines formed at temperatures above about log(T) = 6.1, whereas ("cool") lines formed below log(T) = 6.0 showed little change. This led to the suggestion of bimodal behavior in the corona, and has been linked to the timing of the "terminator" of the previous solar cycle. An obvious question is whether this behavior is typical of solar cycle onsets in the corona. In this brief article we investigate whether the corona showed similar behavior at the onset of Solar Cycle 25, using data from the EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) satellite. Previous observations have shown that hot coronal lines vary by orders of magnitude over the solar cycle while cool lines show very little variation. EVE has measurements of a number of strong coronal lines, and here we compare the onsets to Cycles 24 and 25 in the hot Si XII 499 line and the cool Ne VIII 770 line. We find that, in contrast to Cycle 24, the onset of emission in the higher temperature lines during Cycle 25 is relatively gradual, with no clear indication of bimodal behavior, suggesting that sharp onsets of coronal activity are not a recurrent feature of the solar cycle.

Entropy transfer from solar radio bursts to energetic particles

Space plasma thermodynamics is thought to be affected by wave activity. Here, we show that solar radio bursts (SRBs) can transfer entropy to solar energetic protons (SEPs), affecting their thermodynamics. In particular, our analysis (i) detects the statistically significant SEP density fluctuations, associated with SRB activity that triggers a systematic increase in the thermodynamic kappa; (ii) estimates the polytropic index of SEPs, which is anticorrelated with kappa, serving as an independent measure to validate the increase in kappa; (iii) derives the entropy transfer by using its theoretical relationship with kappa; and (iv) compares SRB wave intensity with the entropy transferring to SEPs to demonstrate their wave-particle coupling. We lastly expose the thermodynamic association between type III SRB wave intensity and SEP entropy transfer as well as their respective coupling, thus developing a paradigm for further systematic investigations among other types of wave activity and particle populations.

Radio Stars in the Era of New Observatories

An international conference Radio Stars in the Era of New Observatories was held at the Massachusetts Institute of Technology Haystack Observatory on 2024 April 1719. The conference brought together more than 60 researchers from around the world, united by an interest in using radio wavelength observations to explore the physical processes that operate in stars (including the Sun), how stars evolve and interact with their environments, and the role of radio stars as probes of our Galaxy. Topics discussed at the meeting included radio emission from cool and ultracool dwarfs, extrasolar space weather, stellar masers, thermal radio emission from evolved stars, circumstellar chemistry, low frequency observations of the Sun, radio emission from hot stars, applications of very long baseline interferometry techniques to stellar astrophysics, stellar explosive events, the detection of radio stars in the latest generation of widefield sky surveys, the importance of radio stars for understanding the structure and evolution of the Milky Way, and the anticipated applications for stellar astrophysics of future radio observatories on the ground and in space. This article summarizes research topics and results featured at the conference, along with some background and contextual information. It also highlights key outstanding questions in stellar astrophysics where new insights are anticipated from the next generation of observational facilities operating at meter through submillimeter wavelengths.

Improvement Method for Solar Radio Burst Long-tailed Detection Using Probabilistic Diffusion Generative Models

Solar radio burst (SRB) detection plays a crucial role in solar physics research and space weather warning. However, multi-class SRB detection suffers from a long-tailed distribution, where the number of samples in most classes (long-tailed classes) is extremely small, while a few classes (head classes) dominate the dataset. To address this issue, we propose a diffusion model for SRBs detection (DiffuSRBsD) model, a generative augmentation method employing diffusion model for long-tailed class enhancement. DiffuSRBsD introduces a mask-conditioned generation strategy to control SRB morphology in spectrograms and employs textual inversion to embed multiple SRB features into a unified semantic representation. Furthermore, we develop a memory structural similarity index measure, a generative image filter specifically for SRBs. Extensive experiments on a benchmark Dataset of SRBs demonstrate that DiffuSRBsD significantly improves the performance of representative detection models, achieving gains of 9.9%, 41.4%, 31.7%, and 11.8% in mAP@50 for the most long-tailed Type IV SRBs. These results validate the effectiveness of our approach and offer a novel perspective for enhancing SRB detection, providing robust data processing support for space weather prediction. The dataset and code are publicly available at https://github.com/onewangqianqian/DiffuSRBsD.git.

Space Weather Impact of Three Solar Flares Observed by the POEMAS Telescope at 45 and 90 GHz

Solar flares are often associated with coronal mass ejections (CMEs) that, when directed toward Earth, can disrupt the magnetosphere and impact space weather. In this study, we investigate three long-duration solar flares observed in 2012 by the POlarization Emission of Millimeter Activity at the Sun (POEMAS) telescopes at 45 and 90 GHz. We analyze the correlation between their radio spectral properties, associated CMEs, and geomagnetic and ionospheric responses, to assess the possibility of space weather prediction. Spectral fitting of the radio data using a gyrosynchrotron model reveals accelerated electron populations with a hard energy distribution (spectral index $(\delta =2.8\pm 0.5)$) that hardens with time, and magnetic fields of $B=160\pm 50$ G. The most energetic flare involved a source region 36 times larger than the others and produced 10 times more energetic electrons. In all three events, CMEs were launched simultaneously with the flares. The July 12 X1.4 flare was associated with a fast (884 km/s) halo CME that led to a moderate geomagnetic storm (Dst = 139 nT, Kp = 6). VLF radio data show that all three flares increased ionospheric electron density, lowering its boundary from 70 km to 5867 km. These results suggest that long- duration flares with high flux at 45 GHz and hard radio spectra are associated with more energetic events characterized by higher GOES class, greater SXR energy, faster and more energetic CMEs, and stronger geomagnetic disturbances, highlighting the potential of high-frequency radio observations as a tool for space weather prediction.

Quantification of Perturbation to the Daytime Lower Ionosphere From a Gamma Ray Burst Using ELF Remote Sensing

On 9 October 2022, a powerful gamma ray burst (GRB), GRB221009A, caused significant changes in the electron density of the lower ionosphere, as evidenced by VLF (330 kHz) radio wave observations. However, GRB221009A did not yield any observable signatures at the Schumann resonances (8, 14 Hz), which are also sensitive to the lower ionosphere. We show that the effects of GRB221009A are observable in a decrease in propagation velocity of ELF (31,000 Hz) lightning impulses. Analytical and numerical analysis points to an ionospheric perturbation in which the GRB increased the daytime electron density uniformly over a wide altitude range from 50 to 90 km. The GRB perturbation is markedly different from perturbations from solar flares, which significantly increase the vertical gradient of the electron density. ELF propagation velocity is shown to be a technique that can identify the altitude range of ionospheric perturbations.

Signatures of Confined and Eruptive Solar Flares in Microwave Spectra

We describe how microwave spectra of confined flares differ from those of eruptive flares. All 29 confined M1.4 soft X-ray flares from NOAA Active Region 12192 in 2014 October that were observed by the Radio Solar Telescope Network in the >300 MHz microwave range (encompassing RSTN frequencies from 410 to 15,400 MHz) had low-frequency (410 MHz) cutoffs in their peak-flux-density spectra, with peak emission <10 solar flux units (sfu) at 410 MHz. Wind/Waves observations at 1 MHz for 20 of these cutoff microwave bursts suggest that few, if any, of the 29 flares were accompanied by escaping electrons. We find a marked difference between microwave spectra for samples of intense (M5) confined and eruptive flares from 2011 to 2016: 20 of 21 confined M5 flares had cutoff spectra, while 27 of 30 M5 eruptive flares had peak 410 MHz emission >10 sfu (with a median value of 431 sfu). For the subsets of these events with Wind/Waves observations, only one of 20 confined events was unambiguously accompanied by 1 MHz emission, while 25 of the 29 eruptive flares had peak 1 MHz fluxes >10³ sfu (above a background of 200400 sfu), with an overall median peak value of 10⁵ sfu. These results indicate that strong confined flares characteristically do not involve or affect open field lines, ruling out interchange reconnection as a confined-flare generation mechanism, leaving reconnection between closed loops as the likely alternative. The microwave spectral signatures of confined and eruptive flares have potential application for the determination of confinement/eruption for flares on solar-type stars.

Extreme-ultraviolet Wave and Quasiperiodic Pulsations during an Eruptive M-class Flare

In this paper, we report multiwavelength and multipoint observations of the prominence eruption originating from active region 11163, which generated an M3.5 class flare and a coronal mass ejection (CME) on 2011 February 24. The prominence lifts off and propagates nonradially in the southeast direction. Using the revised cone model, we carry out three- dimensional reconstructions of the ice-cream-like prominence. It is found that the latitudinal inclination angle decreases from 60 to 37, indicating that the prominence tends to propagate more radially. The longitudinal inclination angle almost keeps constant (6). The highly inclined prominence eruption and the related CME drive an extreme-ultraviolet (EUV) wave, which propagates southward at speeds of 381.60 and 398.59 km s¹ observed in 193 and 304 , respectively. The M3.5 class flare presents quasiperiodic pulsations (QPPs) in soft X-ray, hard X-ray, EUV, and radio wavelengths with periods of 80120 s. Contemporary with the flare QPPs, a thin current sheet and multiple plasmoids are observed following the eruptive prominence. Combining with the appearance of drifting pulsation structure, the QPPs are most probably generated by quasiperiodic magnetic reconnection and particle accelerations as a result of plasmoids in the current sheet.

SIMPL

Context. The LOw Frequency ARray (LOFAR) is capable of imaging spectroscopy of the Sun in the 10240 MHz frequency range, with high spectral, temporal, and spatial resolution. However, the complex and rapidly varying nature of solar radio emission spanning several orders of magnitude in brightness and further exacerbated by the strong ionospheric phase distortions during daytime observations, poses major challenges for calibration, imaging, and automation. Aims. We aim to develop a fully automated, high-fidelity imaging pipeline optimised for LOFAR solar observations, capable of handling the intrinsic variability of solar emission and producing science-ready images with minimal human intervention. Methods. We have built the Solar Imaging Pipeline for LOFAR (SIMPL), which integrates excision of radio-frequency interference (RFI) for the solar-specific scenarios, calibration strategies, and self-calibration. At present, SIMPL processes data from the LOFAR core stations and produces total-intensity solar images, with ongoing developments aimed at enabling full polarimetric imaging and incorporating increasingly distant antennas. The pipeline is designed to enable scalable and uniform processing of large archival datasets. Results. In terms of performance, SIMPL achieves more than an order of magnitude improvement in imaging dynamic range compared with previous efforts and reliably produces high-quality spectroscopic snapshot images. It has been tested across a wide range of solar conditions. It is currently being employed to process a decade of LOFAR solar observations, providing science-ready Flexible Image Transport System (FITS) images for the community and enabling both comprehensive and novel studies of solar radio phenomena, ranging from quiet Sun emission and faint non-thermal features to active regions and their associated dynamic events, such as transient bursts.

Source location and evolution of a multilane type II radio burst

Context. Shocks in the solar corona are capable of accelerating electrons that in turn generate radio emission known as type II radio bursts. The characteristics and morphology of these radio bursts in the dynamic spectrum reflect the evolution of the shock itself, together with the properties of the local corona where the shock propagates. Aims. In this work we study the evolution of a complex type II radio burst with a multilane structure to find the locations where the radio emission is produced and relate them to the properties of the local environment where the shock propagates. Methods. Using radio imaging, we were able to separately track each lane composing the type II burst and relate the position of the emission to the properties of the ambient medium, such as density, Alfvn speed, and magnetic field. Results. We show that the radio burst morphology in the dynamic spectrum changes with time and is related to the complexity of the local environment. The initial stage of the radio emission is characterized by a single broad lane in the spectrum, while the later stages of the radio signature evolve in a multilane scenario. The radio imaging reveals how the initial stage of the radio emission separates with time into different locations along the shock front as the density and orientation of the magnetic field change along the shock propagation. At the time when the spectrum shows a multilane shape, we find a clear separation of the imaged radio sources propagating in regions with different densities. Conclusions. By combining radio imaging with the properties of the local corona, we describe the evolution of a type II radio burst and, for the first time, identify three distinct radio emission regions above the coronal mass ejection front. Two regions were located at the flanks, producing earlier radio emission than the central position, in accordance with the complexity of density and Alfvn speed values in the regions where radio emission is generated. This unprecedented observation of a triple-source structure provides new insights into the nature of multilane type II bursts.

Effects of anisotropy of beam temperature on Langmuir and transverse wave spectra generated by beamplasma instability

Understanding the dynamics of beamplasma interaction is key to understanding solar radio bursts of types II and III. Most of the numerical work done on the topic assumes a beam that is isotropic in temperature. As we are not aware of any fundamental reason for that assumption, in this work, we analyze the effect that beam temperature anisotropy has on the dynamics of the beam-plasma system, assuming a two-dimensional geometry. The numerical results obtained show that the increase in the beam parallel temperature leads to a decrease in the Langmuir wave emission, and are corroborated by an analytical description of the relationship between beam parallel temperature and Langmuir wave intensity. We also investigate the effect of temperature anisotropy on electromagnetic waves generated by the nonlinear dynamics, and obtain that the change of the beam parallel temperature affects the intensity for both the fundamental and harmonic emissions, with a more pronounced effect for harmonic emission.

Multi-scale modelling of energetic particle dynamics and radio signatures in coronal and heliospheric plasmas

Solar Energetic Particles (SEPs) accelerated during space weather events such as solar flares, Coronal Mass Ejections (CMEs), and corotating interaction regions, can reach energies up to several GeV per nucleon for ions and several tens MeV for electrons, thereby posing significant risks to satellites, astronauts, and ground-based systems. Understanding SEP acceleration and transport, along with their associated radio signatures, is crucial for enhancing space weather forecasts and mitigation strategies. This thesis presents, in three stages, a novel, physics-based framework to simulate the acceleration, transport, and radio emission of SEPs in realistic solar wind and coronal environments. Building on an earlier implementation in which the MagnetoHydroDynamic (MHD) model EUHFORIA has been coupled with the focused transport code PARADISE, we first replace EUHFORIA with the more advanced inner heliospheric solar wind MHD code Icarus, which supports localised grid refinements and higher shock resolutions via adaptive mesh refinement. Next, to study SEP and CME dynamics below 0.1 au, the framework is extended into the corona using the COCONUT MHD model. Finally, we integrate the Ultimate Fast Gyrosynchrotron Codes to compute radio emission from energetic electrons trapped in a CME flux rope. Applications for each stage demonstrate significant advances in modelling particle dynamics in both the corona and heliosphere. The framework enables realistic simulations of particle acceleration at finely resolved shocks, investigation of particle confinement and escape in low-coronal magnetic flux ropes, and direct linkage of these processes to observable type IV radio emission, offering valuable diagnostic capabilities for CME magnetic fields and SEP properties near the Sun. Moreover, the integration of coronal and heliospheric domains represents a key step towards global-scale simulations of CME and SEP events from the solar surface to Earth's orbit and beyond, supporting future efforts to untangle the complex, interconnected processes governing space plasmas.

Observations of successive CMEs and their successive Type II solar radio bursts in the corona

This paper reports the observations of two coronal shocks from two Coronal Mass Ejections (CMEs) for the Successive type II Solar radio bursts observed on 02 May 2021 in the frequency range of 80 1 MHz with the time interval of 20 minutes between them. Both the bursts show clear band splitting features in the harmonic band. The estimated heights for the source of the first type II burst lies in the range of 2.06 2.93 with the average speeds of 601 76, 700 91 and 783 105 km for 2 X, 3.5 X and 5 X Saito electron density models, and the heights for the source of the second type II burst lies in the range of 2.24 3.83 with the average speeds of 1063 113, 1287 145 and 1478 172 km . The successive CMEs are observed by the twin Solar Terrestrial Relations Observatory STEREO-A between 11:20 12:21 UT in the Extreme Ultra Violet Imager (EUVI) and in the Internally Occulting Refractive Coronagraphs (COR1) FOV, the two coronal shocks are generated by the two successive CMEs observed at (11:30) 11:26 UT and (11:55:00) 11:56 UT according to ST-A (EUVI) COR1 observations and most likely released from the same active region. The average speeds of CMEs at COR1 FOV are about 574 64 km and 595 82 km . The simultaneous observations of the EUV structures and the radio bursts, their coinciding height-time further confirms that the successive CMEs are responsible for the successive shocks and their related radio bursts in the corona. The observed band-splitting in the successive type-II radio bursts provides the compression ratios of 1.26 and 1.45 respectively. Therefore, these observations confirms the presence of shock waves in the corona.

Reconnection and Viscous Control of Dayside Field Aligned Currents for Northward Interplanetary Magnetic Field

We study the morphology and magnitude of dayside field aligned currents (FACs) for northward interplanetary magnetic field (IMF) using observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). For near zero IMF clock angles the FACs form a quadrupolar pattern centered at local noon, which is associated with the reverse convection cells driven by lobe reconnection. The poleward pair of FACs, known as the northward ${B}_{Z}$ or NBZ FAC system, comprises upward and downward FAC pre and post noon; and the equatorward pair have the opposite polarity. The magnitude of the NBZ FACs is modulated by dipole tilt and phase of the solar cycle, the former controlling the solar zenith angle in the vicinity of the FACs, and the latter controlling the solar radio flux at 10.7 cm (F10.7), both of which contribute to the conductance of the ionosphere. The NBZ FACs are also modulated by the magnitude of the IMF (or the Z component of the IMF for near zero clock angle), which we presume controls the lobe reconnection rate. The NBZ FACs do not respond to the X component of the IMF, solar wind speed, nor solar wind density, so we presume that these do not affect the lobe reconnection rate. High solar wind speed leads to the appearance of region 1 and 2 FACs at auroral latitudes, which we suggest are associated with a viscous interaction between the solar wind and the magnetosphere.

Solar Flare Induced Gradient Drift Instability Observed by SuperDARN HF Radars

Solar flares are a rapid increase in solar irradiance, specifically in X ray and Extreme Ultraviolet spectra, which enhances the ionization in the dayside ionosphere and creates Sudden Ionospheric Disturbances (SIDs). SIDs are known to create space weather impacts on traveling high frequency (HF: 330 MHz) radio waves, by disrupting the communication channels. In this study, we examine ionospheric scatters at dawn terminator, which stems from a severe X9.3 flare on 6 September 2017 peaked at 12:02 UT, utilizing SuperDARN HF coherent scatter radars and Global Navigation Satellite System (GNSS) Total Electron Content (TEC) observations. Specifically, we are interested in the transients in the ionospheric electrodynamics at the sub auroral latitude near the terminator stemming from the flare effect. Observations suggest that flare induced density gradient likely favors the formation of gradient drift instability near the dawn terminator, leading to the irregularities observed by the SuperDARN radars with line of sight (LoS) Doppler velocity reaching nearly 300 m/s. The flare amplifies the eastward TEC gradient near the dawn terminator by approximately 23 times compared to a geomagnetically quiet and non flare day. The observed irregularities, attributed to flare driven instabilities, exhibit a velocity consistent with the equatorial return flow of ionospheric Hall convection. In contrast to prior studies indicating decreased cross polar cap potential and associated ionospheric convection flow, our findings show the flare is followed by an increase in localized electric field near the dawn terminator, as depicted in radar LoS velocity.

Research on the Impact of Differences in Solar Flare Backgrounds of the Same Class on Low Frequency Time Code Time Service Signal

This study systematically investigates the impact of solar flares on the strength and timing deviation of China's low frequency time code (BPC) time service signals under different occurrence background conditions based on same class M1.1 class solar flare event. Seven representative observation cases were selected for the study, with calm day data 24 days before and after the cases used for comparison. The duration of solar flares and whether they were accompanied by geomagnetic storms were studied as variables. The study also conducted an in depth analysis of the changes in BPC during solar flare occurrences using the SYM H index, Kp index, SuperMAG geomagnetic data, and X ray flux records from the GOES 16 satellite. The results show that when the X ray flux reaches its peak, the BPC signals exhibit a sharp decay in strength and fluctuations in timing deviation. The longer the duration of the flare, the greater the disturbance to the signal. In the daytime mid period, the response of BPC signals in the mid to low latitudes to geomagnetic storm background is not significant, as the dominant effect of solar radiation on exciting the D layer of the ionosphere masks the influence of geomagnetic disturbances. This study not only reveals the impact of background differences of same class solar flares on BPC time service signals and provides strong support for the current understanding of signal atmosphere interactions but also offers a theoretical basis for the anti interference design of BPC systems.

Research Progress on Human Body Energy Harvesting and Storage Systems for Wearable Electronic Devices

With the advancement of artificial intelligence, wearable technology is evolving toward multifunctionality, integration, flexibility, and intelligence. Wearable electronic products such as smartwatches, fitness bands, and medical assistive devices are increasingly prevalent in people's lives. This trend enables rich applications such as health monitoring, humanmachine interaction, and the Internet of Things. Energy availability remains a significant challenge for wearable devices, with energy harvesting emerging as an ideal alternative to batteries in wearable technology. The rapid development of energy harvesting creates new opportunities for wearable energy solutions. Various energy sources can power wearable devices, including mechanical, thermal, chemical, solar, radio frequency, wind, acoustic, vibration, and thermal energy. However, providing stable performance over extended periods remains highly challenging. Moreover, most wearable energy harvesters rely on specific materials or structures, which may create inconvenience when applied on the human body. To illustrate the power supply and storage issues of wearable electronic devices based on the human body, we review the latest advancements in self-charging power systems integrated with energy harvesting and storage devices. Key achievements in this field include the integration of various renewable energy sources such as mechanical, solar, and thermal energies, as well as multiple energy sources. Different integration schemes (integrated within device modules vs. separate units or electrode modules with shared electrodes) and various mechanisms for storing harvested energy using electrochemical batteries or supercapacitors demonstrate promising sustainable power systems for wearable/portable electronics or sensor environments. These self-charging power systems, utilizing diverse energy resources, exhibit respective advantages in the field of wearable energy harvesting.

Observations of a Faint Nonthermal Onset before a GOES C-class Flare

We present an analysis of a GOES C1-class flare from 2022 September 6, which was jointly observed as occulted by Nuclear Spectroscopic Telescope ARray (NuSTAR) and on-disk by Spectrometer/Telescope for Imaging X-rays (STIX). NuSTAR observed faint coronal nonthermal emission as well as plasma heating >10 MK, starting 7 minutes prior to the flare. This onset emission implies that during this time, there is a continuous electron acceleration in the corona, which could also be responsible for the observed heating. The nonthermal model parameters remained consistent throughout the entire onset, indicating that the electron acceleration process persisted during this time. Furthermore, the onset coincided with a series of type III radio bursts observed by Long Wavelength Array-1, further supporting the presence of electron acceleration before the flare began. We also performed spectral analysis of the impulsive flare emission with STIX (thermal and footpoint emission). STIX footpoints and the onset coronal source were found to have similar electron distribution power-law indices, but with increased low-energy cut-off during the flare time. This could suggest that the nonthermal onset is an early signature of the acceleration mechanism that occurs during the main phase of the flare.

Possible First Detection of Gyroresonance Emission from a Coronal Mass Ejection in the Middle Corona

Routine measurements of the magnetic field of coronal mass ejections (CMEs) have been a key challenge in solar physics. Making such measurements is important both from a space weather perspective and for understanding the detailed evolution of the CME. In spite of significant efforts and multiple proposed methods, achieving this goal has not been possible to date. Here we report the first possible detection of gyroresonance emission from a CME. Assuming that the emission is happening at the third harmonic, we estimate that the magnetic field strength ranges from 7.9 to 5.6 G between 4.9 and 7.5 R. We also demonstrate that this high magnetic field is not the average magnetic field inside the CME, but most probably is related to small magnetic islands, which are also being observed more frequently with the availability of high-resolution and high-quality white-light images.

Predicting Solar Magnetic Activity from S_ph and Seismic Parameters Using Random Forest Regression

We investigate the potential of using the photometric magnetic proxy S_ph and seismic parameters, such as the frequency of maximum power (max) and the large frequency separation (), derived from Solar and Heliospheric Observatory/Variability of Solar Irradiance and Gravity Oscillations observations to predict the 10.7 cm solar radio flux, a widely used index of solar magnetic activity. A random forest regression model is trained and tested on time series divided into multiple temporal subsets and input parameter combinations. The model achieves strong predictive performance (R² > 0.92) across configurations and significantly outperforms a classical linear regression model. Our results show that S_ph effectively captures long-term variations, while the seismic amplitude parameter Hmax is more responsive to short-term fluctuations. Combining S_ph with the full set of seismic parameters yields the highest accuracy and offers a promising approach for diagnosing activity in other solar-like stars where direct magnetic field measurements are infeasible.

Relevance to PSP and Other Space Observations

The aim of this paper is to demonstrate that electron current oscillations may generate electromagnetic waves as type III radiation and whistler waves without the involvement of the classical plasma emission via the coalescence of waves. Particle-in-cell (PIC) simulation results of a corestrahl plasma without initial current compensation are presented, which describe the conversion of current-driven Langmuir oscillations/waves into type III radiation, whereby simultaneously whistler waves are excited. In contrast to the classical approach of V. L. Ginzburg & V. V. Zhelezniakov, after which beam-excited Langmuir waves in a two-step process are converted into electromagnetic radiation, any instability is suppressed by selecting a low strahl velocity. Rather, electric field oscillations at the electron plasma frequency are triggered by the initially noncompensated current of the strahl. The arising electromagnetic fields exhibit amplitude oscillations, which are caused by the superposition of the two wave modes of mixed polarization at the point of mode coupling. This basic mechanism of wave generation and transformation has already been described in earlier papers using simple fluid models. It is also the topic of the companion paper. Besides the fundamental electromagnetic radiation, the second harmonic of nearly the same intensity has been obtained, which is an indication of its generation by nonlinear currents. Measurements of Langmuir waves, type III radiation, and whistler waves on board various satellites in the solar wind, in particular Parker Solar Probe observations, are analyzed in the light of our results. Interpretations of earlier PIC simulations are critically reviewed.

AIA and RHESSI Observations

We present a comprehensive multiwavelength analysis of a coronal mass ejection (CME) on 2014 January 8 from the active region (AR) NOAA 11947 by analyzing the data from Atmospheric Imaging Assembly (AIA) on board Solar Dynamics Observatory, RHESSI, and Hiraiso Radio Spectrograph. The CME is associated with an M3.6 flare and filament eruption. Observations from the AIA 171 images reveal the origin of pre-CME arcade 1 hr prior to the eruptive event. Before the onset of event, the hot AIA 131 and 94 images exhibit an existence of a hot flux rope, which on eruption results in the activation and eruptive expansion of the pre-CME coronal arcade. The eruptive process of pre-CME arcade consists of a slow rise evolution with a speed 3 km s¹, "arcade-to- bubble" transformation, and blowout expansion of the CME-bubble with a fast speed of 400 km s¹. A differential emission measure analysis suggests the presence of multithermal plasma in the bubble- structure and strong plasma heating at the core of the AR. A blowout expansion of CME is accompanied with multiple type III and a split-band type II radio bursts in association with X-ray emission up to 50 keV energies from its source region. Our observations reveal that the formation and expansion of the CME-bubble serve as the earliest signatures of the CME, capturing its development in the low corona. Concurrent X-ray and radio emissions further highlight the critical role of large-scale magnetic reconnection in powering both the flare emissions and the early evolution and dynamics of the CME.

Ion-scale Turbulence and Energy Cascade Rate in the Solar Corona and Inner Heliosphere

Plasma turbulence cascading from MHD to kinetic scales in the heliospheric plasma is believed to play a key role in coronal heating and fast solar wind acceleration, but the properties of the turbulence remain poorly constrained by observations. Here we compare the ion-scale density fluctuation levels inferred from the properties of solar radio bursts with the magnetic field fluctuation levels obtained through in situ measurements in the inner heliosphere. We find that the observed magnetic and density fluctuation amplitudes are consistent with excitation by kinetic Alfvn waves (KAWs) and/or KAW structures over a broad range of distances from the Sun. We then use the radio diagnostics and the KAW scenario to deduce the radial variation of magnetic fluctuation amplitudes in regions close to the Sun where in situ measurements cannot be obtained. Further, we calculate the energy cascade rate (plasma heating rate) profile over a region that extends from the low corona (0.1 R) into the heliosphere (out to 1 au), and compare it to the energy deposition rate required to drive the solar wind. The cascade rate agrees with the available in situ measurements and also provides predictions closer than 10 R where in situ approaches are not available. The results provide unique diagnostics of the ion-scale plasma turbulence amplitude and energy cascade rate spanning over 3 orders of magnitude in solar distance.

A Monte Carlo Simulation on the Scattering Coefficients of Solar Radio Wave Propagation

Radio waves undergo scattering by small-scale density fluctuations during propagation through the solar-terrestrial environment, substantially affecting the observed characteristics of solar radio bursts. This scattering process can be effectively modeled as photon diffusion in phase space. In this study, we present a comprehensive comparison between the quasilinear diffusion coefficients and those calculated by ray-tracing the photon trajectories in numerically generated, broadband, isotropic density fluctuation fields in both 2D and 3D configurations. The comparative analysis demonstrates that for weak scattering, the simulated diffusion coefficients agree well with the quasilinear theoretical predictions. However, when the radio frequency approaches the electron plasma frequency and/or the density fluctuation amplitude becomes significant, photons experience strong scattering. Under such conditions, the quasilinear theory tends to underestimate the scattering strength of photons induced by 2D density fluctuations while overestimating the scattering strength in 3D cases. Furthermore, we implement a group velocity correction to the theoretical diffusion coefficients, based on the effective propagation speed averaged over all test photons. The corrected coefficients provide an accurate quantification of the scattering strength for radio waves propagating through 3D density fluctuations. The physical mechanisms underlying these phenomena are elucidated in the discussion.

Possible contribution of ground motion to modify ionosphere before the 2011 Tohoku-Oki Mw9.1 earthquake

This study examines the complex interrelationships among ionospheric NmF2 variations, solar activity F10.7 index, and crustal movements preceding the 2011 Tohoku-Oki earthquake. NmF2 from 5 ionospheric stations (Yakutsk, Khabarovsk, Wakkanai, Kokubunji, and Jeju) is studied together with vertical movement observed by Hi-net tilt meters in Japan. Results showed that while daytime NmF2 typically correlates with solar activity (F10.7 solar radio flux), significant deviations were observed during specific periods. During these intervals, NmF2 variations did not correlate with space weather parameters as expected but correlate with the vertical ground motion. We explain these phenomena as due to the enhanced dynamo electric field, which is produced originally by the vertical ground motion. Our findings suggest that the ionosphere before the large earthquake is influenced by the complex Lithosphere- Atmosphere-Ionosphere interaction such as crustal movements, acoustic- gravity waves, and dynamo electric field variations.

Effects of X2.8-class solar flare on the ionosphere occurred during the recovery phase of a geomagnetic storm over South American and Antarctic sectors

In this investigation, we present and discuss the effects of an X2.8-class solar flare occurred on 14 December 2023 on the ionospheric F region and on the geomagnetic field over South American and Antarctic sectors. This flare coincides with the recovery phase of a geomagnetic storm. To this end, we rely on vertical total electron content (VTEC) observations from nearly 250 Global Positioning System (GPS) receiver stations over South American and Antarctic sectors, complemented by in- situ electron density observations from Swarm satellites, magnetometer measurements, and ionospheric sounding observations from ionosondes. The magnetic observations show a large increase in the variations of the horizontal component (H) of the geomagnetic field and equatorial electrojet (EEJ) currents at all stations, synchronized with the increase in X-rays flux, indicating solar flare effects or magnetic crochet on the Earth's geomagnetic field. VTEC shows how the impact of the solar flare on the ionosphere is enhanced from east to west of South America in the equatorial and low-latitudes. VTEC from a specific GPS satellite-receiver also shows great effects at mid-latitudes. Results are confirmed and further elaborated through Swarm in-situ observations. In addition, an asymmetry is observed in the equatorial ionization anomaly (EIA), in which the eastern South American sector shows an intensified EIA compared to the western sector. Ionospheric sounding observations by ionosondes show total fade out in the echo traces of the ionograms, characterizing blackouts in the radio signals from equatorial to low-latitudes. Overall, our results show that an X-class solar flare occurring near the limb of the solar disk is capable of producing effects on the Earth's ionosphere with similar or even stronger intensities than flares occurring at the center of the solar disk.

A wide-band high-frequency type-II solar radio burst

Aims. Type-II radio bursts are typically observed below 400 MHz and are characterized by the narrowband, slowly drifting fundamental and harmonic structures. Here we report an unusual high-frequency wide-band type-II burst with a starting frequency as high as 670 MHz and an instantaneous bandwidth as wide as 300 MHz. Methods. We used radio imaging from the Nanay Radio Heliograph, spectroscopic data from ORFEES, extreme-ultraviolet (EUV) observations from Solar Dynamics Observatory, and white-light observations from LASCO to determine the nature and origin of the observed radio burst as well as its propagation in the corona. Results. The estimated average spectral drift is 2.18 MHz s¹, its mean duration at each frequency is 3 min, and the maximum brightness temperature can exceed 10¹¹ to 10¹² K. According to the simultaneous EUV and radio imaging data, the radio sources are distributed over a relatively broad region centered on a dip in the nose front of the shock-like EUV wave structure. The dip is likely caused by the strong interaction of the eruption with the overlying closed dense loops that are enclosed by the large-scale streamer structure, indicating that the type-II burst originates from coronal mass ejection shocks interacting with dense, closed-loop structures. Conclusions. The observations suggest that the unusual wide-band high-frequency type-II radio burst originates from a dense streamer region in the corona; this is further evidenced by an EUV shock-like structure that steepens very close to the solar surface, at 1.23 R, and the fact that the type-II radio source coincides with the shock dip. The wide-band feature is due to the source stemming from a region with significant density variations and not due to the intensity variations across the shock structure.

Study on the Variation of Low-Frequency Time Code Signals During Medium to Large Solar Flare Events

This study is based on the 68.5 kHz signal transmitted by China's low- frequency time code time service system (BPC) and systematically researches the effects of medium to large solar flares (M/X-class) on low-frequency time code signals. By analyzing the field strength and timing deviation data of the BPC signal during 20 typical flare events, the study reveals the variation patterns of low-frequency time code signals under disturbances from medium to large solar flares. Case analyses indicate that, during such flares, the BPC signal intensity exhibits two response patterns: a single-valley structure and a double- valley structure. The BPC signal response is divided into two stagesa rapid change phase and a gradual change phasewhich show a strong linear relationship with the development of the solar flare. Meanwhile, the BPC timing deviation displays a bipolar pulse characteristic, and after the flare, the instability in signal performance is closely associated with the double-valley response in field strength. These phenomena suggest that the changes in the BPC time code signal are closely related to the effects of ionospheric disturbances during solar flares on the superposition characteristics of the BPC ground-wave and sky-wave signals. This first systematic investigation analyzes low-frequency time-code signal variation during medium-to-large solar flares, revealing their response characteristics. It provides significant insights into the low-frequency time-code signal propagation-solar activity association and lays a solid theoretical foundation for improving time-service stability and reliability.

Statistical Study of DH Type II Bursts and Associated CMEs During Solar Cycle 24

Decameter-hectometric (DH) Type II bursts, arising from coronal mass ejection (CME)-driven shock waves, are crucial for understanding solar- terrestrial interactions and space weather forecasting. This study provides a comprehensive statistical analysis of CMEs associated with DH type II solar radio bursts during Solar Cycle 24 (20092019), utilizing data from the Wind/WAVES, Solar TErrestrial RElations Observatory/SWAVES, and Solar and Heliospheric Observatory/LASCO catalogs. Analyzing 180 events, we report key spectral and kinematic properties, including a mean CME speed of (1058 531) km s¹ and a mean width of (288.39 99.3), with 62% classified as halo CMEs. About 12% of the total CMEs are accelerated, 58% of them are decelerated, and 30% of them are constant. Similarly, CMEs having a speed 800 km s¹ are constant, and those with speed 800 km s¹ are decelerated. DH type II bursts displayed a mean starting frequency of (12,169.72 4939) kHz, ending frequency of (2152.69 3022.07) kHz, bandwidth of (10,017 5353) kHz, and an average duration of (345.62 453) minutes. A power-law relationship was established between the drift rate (df/dt) and burst duration (D), characterized by df/dt = 2749.07 D^0.88, highlighting the inverse dependence of drift rate on burst longevity. This suggests a dynamic interplay between shock parameters and the ambient solar corona. The findings underscore the persistent and robust spectral coverage of CME-driven shocks, offering new insights into their evolution and impact on the heliospheric environment.

Radiation efficiency of electromagnetic wave modes from beam-generated solar radio sources

During type III solar radio bursts, electromagnetic waves are radiated at the plasma frequency _p and its harmonics by electrostatic wave turbulence generated by electron beams ejected from the Sun in randomly inhomogeneous solar wind and coronal plasmas. These emissions, detected for decades by spacecraft and radiotelescopes, are split by the plasma magnetic field into three modes, X, O and Z, with different dispersion, polarization and radiation properties. Here, using three independent and converging approachesparticle-in-cell simulations, a theoretical model of waves in a random medium and analytical calculations in the framework of turbulence theorywe demonstrate that only a small fraction of electromagnetic energy radiated at _p (10%) escapes from beam-generated radio sources, mainly as O-mode waves and, depending on plasma conditions, as X-mode waves. Most energy is radiated in the Z -mode and can therefore be observed only close to sources. The results provide strong support for interpretation of observations performed up to close distances from the Sun by spacecraft such as Parker Solar Probe and Solar Orbiter. This work, based on general approaches requiring few assumptions, makes it possible to study the properties of radio emission under realistic solar conditions, and thereby provides a solid basis for the development of theoretical tools for probing space and time variations of beam-plasma systems in the solar wind.

Detecting fast-variation pulsations in solar hard X-ray and radio emissions

Quasi-periodic pulsations (QPPs) at sub-second periods are frequently detected in the time series of X-rays during stellar flares. However, such rapid pulsations are rarely reported in the hard X-ray (HXR) emission of the small solar flare. We explored the QPP patterns with fast-time variations in HXR and radio emissions produced in a small solar flare on 2025 January 19. By applying the fast Fourier transform, the fast-variation pulsations at a quasi-period of about 1 s are identified in the HXR channel of 2080 keV, which were simultaneously measured by the Hard X-ray Imager and the Konus-Wind. The rapid pulsations with a same quasi-period were also detected in the radio emission at a lower frequency range of about 40100 MHz. The restructured HXR images show that the QPP patterns mainly locate in footpoint areas that connect by hot plasma loops, and they appear in the flare impulsive phase. Our observations suggest that the fast-variation pulsations could be associated with non-thermal electrons that are periodically accelerated by the intermittent magnetic reconnection, and the 1-s period may be modulated by the coalescence instability between current-carrying loops and magnetic islands.

An assessment of potentially space weather causing CMEs through analysis of associated interplanetary type II solar radio bursts and solar energetic particle events

This study investigates the space weather implications of coronal mass ejections (CMEs) by analyzing 39 metric type II solar radio bursts with decametrichectometric (DH) counterparts during a segment of Solar Cycle 24. To minimize projection effects, only limb CMEs originating far from the solar disk center (central meridian distances between60 and 90) were considered. The events were categorized into three groups: (i) all metric type II bursts with DH counterparts (m-DH), (ii) those accompanied by solar energetic particle (SEP) events (m-DH-SEP), and (iii) those without SEP events (m-DH-NonSEP). Analysis of CME parameters revealed that m-DH-SEP events are associated with faster (average speed of 1203 km/s) and wider CMEs compared to m-DH-NonSEP events (average speed of 333 km/s). Additionally, the fraction of halo CMEs increased across the groups: m-DH-NonSEP (62.5%), m-DH (74.3%), and m-DH-SEP (93%). A strong positive correlation (Pearson's CC = 0.76;SEcc=0.18) was found between CME speeds and the logarithmic peak intensity of SEP events. Notably, 87% of m-DH-SEP events originated from the western hemisphere in StonyHurst coordinates, consistent with favorable magnetic connectivity to Earth. Further analysis indicated that 62.5% of western hemisphere metric type II bursts with DH counterparts were followed by SEPs at Earth, compared to only 13% without DH counterparts. These findings confirm that fast and wide CMEs associated with DH type II bursts are effective in accelerating energetic particles, underscoring the significance of DH type II bursts as indicators of SEP events and their relevance in space weather forecasting.

demonstration of technical readiness and initial science highlights

Solar radio emissions offer unique diagnostic insights into the solar corona. However, their dynamic and multiscale nature, along with several orders of magnitude variations in intensity, pose significant observational challenges. To date, at gigahertz frequencies, MeerKAT stands out globally with high potential of producing high-fidelity, spectroscopic snapshot images of the Sun, enabled by its dense core, high sensitivity, and broad frequency coverage. Yet, as a telescope originally designed for observing faint galactic and extragalactic sources, observing the Sun at the boresight of the telescope requires customized observing strategies and calibration methods. This work demonstrates the technical readiness of MeerKAT for solar observations at the boresight of the telescope in the UHF (5801015 MHz) and L-band (9001670 MHz) frequency ranges, including optimized modes, a dedicated calibration scheme, and a tailored, entirely automated calibration and imaging pipeline. The quality of solar images is validated through morphological comparisons with the solar images at other wavelengths. Several unique early science results showcase the potential of this new capability of MeerKAT. Once fully commissioned and operational, this will unlock novel solar studies, significantly expand the scientific portfolio of MeerKAT, and lay the groundwork for solar observations with the mid-frequency telescope of the upcoming Square Kilometre Array Observatory, for which MeerKAT serves as a precursor.

The Time Delays in Reaction of the Ionosphere and the Earth's Magnetic Field to the Solar Flares on 8 May and Geomagnetic Superstorm on 10 May 2024

In the paper we consider the pulsed disturbances caused in the ionosphere by an extreme G5-level geomagnetic superstorm on 10 May 2024, and by the X1.0 and M-class solar flares on 8 May 2024, which preceded the storm. Particular attention is paid to the short-term delays and the sequence of disturbance appearance in the ionosphere and geomagnetic field during these extreme events. The results of a continuous Doppler sounding of the ionosphere on an inclined radio path with a sampling frequency of 25 Hz were used, as well as the data of a ground-based mid- latitude fluxgate magnetometer LEMI-008, and an induction magnetometer IMS-008, which operated with a sampling frequency of 66.6 Hz. Ionization of the ionosphere by the intense X-ray and extreme ultraviolet radiation of solar flares was accompanied by the equally sudden and similarly timed disturbances in the Doppler frequency shift (DFS) of the ionospheric signal, which had an amplitude of 2.05.8 Hz. The largest pulsed burst in DFS was registered 68 s after an X1.0 flare on 8 May 2024 at the time when the change of the X-ray flux was at its maximum. Following onto the effect in the ionosphere, a disturbance in the geomagnetic field appeared with a time delay of 35 s. This disturbance is a secondary one that arose as a consequence of the ionosphere response to the solar flare. It was likely driven by the contribution of ionospheric currents and electric fields, which modified the Earth's magnetic field. On 10 May 2024, a G5-level geomagnetic superstorm with a sudden commencement triggered an impulsive reaction in the ionosphere. A response in DFS at the calculated reflection altitude of the sounding radio wave of 267.5 km was detected 58 s after the commencement of the storm. The sudden impulsive changes in Doppler frequencies showed a bipolar character, reflecting complex dynamic transformations in the ionosphere at the geomagnetic storm. Consequently, the DFS amplitude initially rose to 5.5 Hz over 86 s, and then its sharp drop to 3.2 Hz followed. Using the instruments that operated in a mode with a high temporal resolution allowed us to identify for the first time the impulsive nature of the ionospheric reaction, the time delays, and the sequence of disturbance appearances in the ionosphere and geomagnetic field in response to the X1.0 solar flare on 8 May 2024 as well as to the sudden commencement of the extreme G5-level geomagnetic storm on 10 May 2024.

hyperparameter selection

Radio interferometric imaging samples visibility data in the spatial frequency domain and then reconstructs the image. Because of the limited number of antennas, the sampling is usually sparse and noisy. Compressed sensing-based on convex optimization is an effective reconstruction method for sparse sampling conditions. The hyperparameter for the l₁ regularization term is an important parameter that directly affects the quality of the reconstructed image. If its value is too high, the image structure will be missed. If its value is too low, the image will have a low signal-to-noise ratio. The selection of hyperparameters under different levels of image noise is studied in this paper, and solar radio images are used as examples to analyze the optimization results of compressed sensing algorithms under different noise conditions. The simulation results show that when the salt-and- pepper noise density is between 10% and 30%, the compressed sensing algorithm obtains good reconstruction results. Moreover, the optimal hyperparameter value has a linear relationship with the noise density, and the mean squared error of regression is approximately .

Imaging and Radio Signatures of ShockPlasmoid Interaction

Understanding how shocks interact with coronal structures is crucial for understanding the mechanisms of particle acceleration in the solar corona and inner heliosphere. Using simultaneous radio and white-light observations, we investigate the interaction between a coronal mass ejection (CME)-driven shock and a plasmoid. LASCO and STEREO-A COR-2 white-light images are analyzed to track the evolution of the plasmoid, CME, and its associated shock, while the Wind/WAVES and STEREO/WAVES dynamic spectra provide complementary radio signatures of the shockplasmoid interaction at 7 R. An interplanetary type II radio burst was detected as the shock propagated through the plasmoid. The merging of the plasmoid into the CME was accompanied by interplanetary type III radio bursts, suggesting escaping electron beams during the reconnection process. These observations clearly demonstrate that shockplasmoid interactions can enhance the efficiency of particle acceleration associated with CMEs, with implications for electron acceleration in flare and heliospheric current sheets as well.

Measuring the Magnetic Field of a Coronal Mass Ejection from the Low to Middle Corona

A major challenge in understanding the initiation and evolution of coronal mass ejections (CMEs) is measuring the magnetic field of the magnetic flux ropes (MFRs) that drive CMEs. Recent developments in radio imaging spectroscopy have paved the way for diagnosing the CMEs' magnetic field using gyrosynchrotron radiation. We present magnetic field measurements of a CME associated with an X5-class flare by combining radio imaging spectroscopy data in microwaves (118 GHz) and meter waves (2088 MHz), obtained by the Owens Valley Radio Observatory's Expanded Owens Valley Solar Array (EOVSA) and Long Wavelength Array (OVRO-LWA), respectively. EOVSA observations reveal that the microwave source, observed in the low corona during the initiation phase of the eruption, outlines the bottom of the rising MFR- hosting CME bubble seen in extreme ultraviolet and expands as the bubble evolves. As the MFR erupts into the middle corona and appears as a white-light CME, its meter-wave counterpart, observed by OVRO-LWA, displays a similar morphology. For the first time, using gyrosynchrotron spectral diagnostics, we obtain magnetic field measurements of the erupting MFR in both the low and middle corona, corresponding to coronal heights of 0.02 and 1.83 R. The magnetic field strength is found to be around 300 G at 0.02 R during the CME initiation and about 0.6 G near the leading edge of the CME when it propagates to 1.83 R. These results provide critical new insights into the magnetic structure of the CME and its evolution during the early stages of its eruption.

Plasma Instability in Front of Ejected Energetic Electrons and Type III Solar Radio Bursts

Type III radio bursts are signatures of the fluxes of near-relativistic electrons ejected during solar flares. These bursts are frequently observed by spacecraft such as the Parker Solar Probe. It has been traditionally believed that these electron beams generate Langmuir waves through the two-stream instability, which are then converted into electromagnetic waves. In this study, we revise that model, by examining how the electron distribution becomes truncated due to the "time-of- flight" effect, as the beam travels through a randomly inhomogeneous and gently varying solar wind plasma. Rather than the two-stream instability, this truncation destabilizes the distribution and leads to the generation of Langmuir waves via a linear instability; we confine our analysis to this linear regime and do not take into account the backreaction of the generated Langmuir waves on the electron distribution, which is nonlinear. The instability grows until slower electrons arrive and dampen the waves. Our qualitative analysis shows that the resulting wave intensity growth and decay closely match the intensitytime profile of observed type III radio bursts at the fundamental frequency, supporting this modified theory.

Low-Frequency Type II Radio Bursts and Associated Solar Flares, Coronal Holes, and CMEs During 2001 to 2015

We have analyzed 20 low-frequencies (LF) type-II radio bursts associated with solar coronal mass ejections (CMEs), coronal holes (CHs), and solar flares observed during the solar cycle 23 and solar cycle 24, which consist of the period of year 1997 to year 2015. A total number of 505 types-II radio bursts were observed during the above period out of which only 20 types II radio bursts have frequencies 1 MHz. The time duration of 20 LF type II bursts ranges from 5 min to 2020 min. On investigation of 17 type-II bursts associated CMEs, solar flares, and coronal holes we also found that 12 types-II burst related CMEs observed when there were CHs and solar flares within 10 and 5 type-II burst- associated CMEs found when there were CHs and solar flares within 30, respectively. In this paper we have done statistical analysis of low frequency type II radio bursts and related solar phenomena. The LF type- II radio bursts start after the peak time of associated solar flares. In this paper each LF type-II radio burst and other related solar phenomena are seen and analyzed to understand the origin of LF type-II radio bursts from the Sun in the latest scenario of solar heliophysics.

A short-term prediction of ionospheric TEC using the RF-Prophet model for GNSS stations in China

The ionospheric total electron content (TEC) is a crucial parameter for studying ionospheric variability and space weather. Short-term forecasting of the ionosphere is also vital for detecting near-Earth space environment changes. This study proposes a random forest (RF) feature selection method combined with the Prophet model. The best features are selected by random forest training, and the Prophet model is used to forecast ionospheric TEC data for the short term. This paper presents the construction of the RF-Prophet model using TEC data from 16 GNSS stations at CMONOC and five parameters selected through training, namely Lyman alpha, Auroral Electrojet Index (AU index), Polar Cap Index (PC-index), Auroral Lower Index (AL index), and Solar Radio Flux at 10.7 cm (F10.7). The model's performance is evaluated by comparing it with a single Prophet model, using 30 days of historical TEC data as the training set and selecting a one-day sliding forecast experiment for the 2015 high solar activity and 2018 low solar activity years. The experimental results indicate that the RF-Prophet model has a lower one- year average root mean square error (RMSE) of all stations in the 2015 and 2018 test sets compared to the Prophet model. Specifically, in the 2015 test set, the RMSE of the RF-Prophet model improved by 9.40 % compared with the Prophet model; in the 2018 test set, the RMSE of the RF-Prophet model improved by 7.19 % compared with the Prophet model.

Pre-Flare Current Sheet, Build-Up of Eruptive Filament, Flare and Eruption Onset in the Frame of the Tether-Cutting Magnetic Reconnection Scenario

We present multiwavelength analysis of the pre-flare phase, as well as the onset of the powerful X4.9 near-limb eruptive solar flare on February 25, 2014 (SOL2014-02-25T00:39), revealing the tether-cutting geometry. This event provides an excellent opportunity to investigate pre-flare and flare energy release in details utilizing available large volume of observational data in different spectral ranges, suitable limb position of the flare, high power of its energy release, favorable spatial distribution of pre-flare and flare emission sources and well- observed eruption. We aim at determining relationship between the region of pre-flare energy release with the regions where the flare started to develop, and to investigate a detailed chronology of energy release during the pre-flare time interval and the beginning of the impulse phase. Using X-ray, ultraviolet (UV) and radio microwave data we found that the pre-flare energy release site was compact and localized in the vicinity of interaction (tether-cutting type) of larger-scale magnetic structures near the polarity inversion line of the magnetic field. The analysis indicates that a pre-flare current sheet could be in this region. Good correspondence between the location of the pre-flare and flare emission sources visible at the very beginning of the impulsive phase is shown. We found relationship between dynamics of the energy release in the pre-flare current sheet and formation of the future flare eruptive structure. The growth of the magnetic flux rope was associated with activation of plasma emissions, flows and an increase of UV radiation fluxes from the region where the pre-flare current sheet was located. The eruptive flux rope gradually grew due to "feeding" by magnetized plasma ejected from the reconnecting pre-flare current sheet. Finally, it is shown that the most probable trigger of the eruption was a local fast microflare-like magnetic reconnection in the pre-flare current sheet. Some local instability in the pre-flare sheet could lead to a transition from the slow to fast reconnection regime. As a result, an ejection from the sheet was initiated and the eruptive flux rope lost its stability. Then, the eruptive flux rope itself initiated formation of the main reconnecting flare current sheet as in the Standard Flare Model (under the flux rope) during its movement, and intense emissions associated with the impulsive phase were observed.

Bidirectional anisotropic solar energetic particle events observed by Solar Orbiter

Context. Solar energetic particle (SEP) events are critical for understanding particle acceleration and transport in the heliosphere. While most SEP events involve outward streaming particles along open magnetic field lines, bidirectional events characterized by simultaneous sunward and anti-sunward particle flows offer unique insights into magnetic field topology and the interplay of multiple acceleration sources. Aims. We investigate the origin and transport of energetic particles in two rare bidirectional anisotropic SEP events observed by Solar Orbiter, with a particular emphasis on their association with magnetic flux ropes. Methods. Energetic particles, solar wind plasma, magnetic field, and solar radio measurements were analysed. Via the velocity dispersion analysis, we determined release times and path lengths for distinct particle populations. Automated flux rope identification and magnetic helicity diagnostics were used to characterize magnetic flux ropes. Results. Both events showed two clear velocity dispersion signatures with opposite particle anisotropies during their onset phase. The sunward streaming protons, characterized by a delayed release time, a harder spectral index, and higher intensities, may be due to coronal mass ejection-driven shock acceleration, while the promptly released anti-sunward streaming protons are likely linked to flare acceleration. Notably, in both cases, small- scale flux ropes were identified in situ during the time intervals corresponding to the bidirectional particle streaming. Path lengths derived for sunward and anti-sunward injections were substantially greater than nominal values of the Parker field lines, further evidence of the role of the flux rope in shaping particle trajectories. Conclusions. These observations demonstrate that magnetic flux ropes can significantly affect magnetic connectivity to the source region and SEP propagation in the inner heliosphere, and that simultaneous velocity dispersion from two distinct particle sources can be used to place direct constraints on the topology of the flux rope. Our results highlight the value of combining particle anisotropy, release time, source spectra, and magnetic structure diagnostics to unravel SEP transport in complex transient magnetic structures, and also present new challenges for the current SEP transport model.

Modelling gyrosynchrotron emission from coronal energetic electrons in a CME flux rope

Context. Solar flares and coronal mass ejections (CMEs) can accelerate electrons, causing bursts such as type IV emissions in the solar radio continuum. Although radio spectroscopy is a powerful diagnostic tool for the corona, the origin and mechanisms of type IV bursts remain uncertain. In situ measurements can occasionally shed some light on these mechanisms, but they are limited in space and time. Sophisticated numerical modelling offers the best approach to improve our understanding of the physical processes underlying particle dynamics and radio emission. Aims. This research examines type IV radio bursts, exploring the effects of various electron distribution properties and CMEs on their generation and characteristics. To transcend idealised assumptions, we employed realistic anisotropic electron distributions obtained from particle transport simulations within complex magnetohydrodynamic (MHD) environments as input for radio emission models. Methods. We used the three-dimensional coronal MHD model COCONUT to generate coronal background configurations, including a CME modelled as an unstable modified TitovDmoulin magnetic flux rope (MFR). These MHD simulations were used by the PARADISE particle transport code, which injects energetic electrons into the MFR and tracks their evolution. Finally, we fed the electron distributions and solar wind parameters into the Ultimate Fast Gyrosynchrotron Codes to compute radio emission along lines of sight. Results. Electrons injected close to the flux rope's central axis remained largely confined, producing a gyrosynchrotron emission spectrum resembling observed type IV characteristics. Varying observer positions, CME properties, and spectral indices of the electron energy distributions modified the intensities and durations of the observed bursts. The strongest gyrosynchrotron emission was observed as originating from the CME flanks. Conclusions. Our results indicate that gyrosynchrotron emission is the major component in type IV spectra, although additional contributors cannot be ruled out.

Catalogue description and first statistical results

Context. The acceleration of particles at the Sun and their propagation through interplanetary space are key topics in heliophysics. Specifically, solar energetic electrons (SEEs) measured in situ can be linked to solar events such as flares and coronal mass ejections (CMEs) since they are also observed remotely in a broad range of electromagnetic emissions such as in radio and X-rays. Solar Orbiter, equipped with a wide range of remote-sensing and in situ detectors, provides an excellent opportunity to investigate SEEs and their solar origin from the inner heliosphere. Aims. We aim to record all SEE events measured in situ by Solar Orbiter, and to identify and characterise their potential solar counterparts. The results have been compiled in the Comprehensive Solar Energetic Electron event Catalogue (CoSEE-Cat), which will be updated regularly as the mission progresses. The catalogue contains key parameters of the SEEs, as well as the associated flares, CMEs, and radio bursts. In this paper, we describe the catalogue and provide a first statistical analysis. Methods. The Energetic Particle Detector (EPD) was used to identify and characterise SEE events, infer the electron release time at the Sun, and determine the composition of related energetic ions. Basic parameters of associated X-ray flares (timing, intensity, source location) were provided by the Spectrometer/Telescope for Imaging X-rays (STIX). This was complemented by the Extreme Ultraviolet Imager (EUI), which added information on eruptive phenomena. CME observations were contributed by the coronagraph Metis and the Solar Orbiter Heliospheric Imager (SoloHI). Type III radio bursts observed by the Radio and Plasma Waves (RPW) instrument provided a link between the SEEs detected at Solar Orbiter and their potential solar sources. The conditions in interplanetary space were characterised using Solar Wind Analyzer (SWA) and Solar Orbiter Magnetometer (MAG) measurements. Finally, data-driven modelling with the Magnetic Connectivity Tool provided an independent estimate of the solar source position of the SEEs. Results. The first data release of the catalogue contains 303 SEE events observed in the period from November 2020 until the end of December 2022. Based on the timing and magnetic connectivity of their solar counterparts, we find a very clear distinction between events with an impulsive ion composition and ones with a gradual one. These results support the flare-related origin of impulsive events and the association of gradual events with extended structures such as CME- driven shocks or erupting flux ropes. We also show that the commonly observed delays of the solar release times of the SEEs relative to the associated X-ray flares and type III radio burst are at least partially due to propagation effects and not exclusively due to an actual delayed injection. This effect is cumulative with heliocentric distance and is probably related to turbulence and cross-field transport.

The Role of Coronal Shock Waves

We investigate the relationship between the gamma-ray emission measured with the Large Area Telescope on board Fermi (Fermi-LAT9 and radio signatures of coronal shock waves in four behind-the-limb (BTL) solar flares. All events were associated with metric type II radio bursts. Both start and end times of the radio bursts were synchronized with the gamma-ray emission. The type II bursts associated with the BTL gamma-ray flares had higher speeds and lower formation heights than those of an average sample. These findings support the notion that the highly relativistic ions that produce the gamma-rays in BTL flares are accelerated at CME-driven propagating coronal shock waves rather than in large-scale coronal loops.

A new type of fine spectral structure in the solar radio emission

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Solar Activity and Polar Ozone

The paper presents the results of studying the influence of solar activity on polar ozone in the Arctic and Antarctic. The polar ozone satellite measurements in 19792024 presented on the NASA website (USA, http://ozonewatch.gsfc.nasa.gov) and the corresponding characteristics of solar activity, namely, the sunspot number and the magnitude of a solar radio emission flux with a wavelength of 10.7 cm were used. An analysis of the data showed that positive deviations of polar ozone prevail under high solar activity. On average, under high solar activity, polar ozone deviations from long-term means normalized to standard deviations increase with the growth of solar activity. In 19792024, only positive polar ozone anomalies were observed in the Arctic and Antarctic under high solar activity.

Heating Mechanisms and Radio Response from the Solar Chromosphere to Corona

Heating mechanism in the solar atmosphere (from chromosphere to corona) is one of the top-challenges in modern astronomy. The classic mechanisms can be divided into two categories: wave heating (W) and magnetic reconnection heating (X). Both of them still face some problems currently difficult to overcome. Recently, we proposed a new mechanism, called magnetic-gradient pumping heating (MGP, or P) which seems to overcome those difficulties, but still lacks sufficient observational evidence. Which one really explained the physics of hot corona exactly? How can observations be used to identify and verify the heating mechanism? Since different heating mechanisms will generate non-thermal particles from different accelerations and experience different propagations, they will have different responses in the broadband spectral radio observations. Among them, the non-thermal electrons from W mechanisms are closely related to shock-wave acceleration, and their radio response should be a group of spike bursts with random distribution of drifting rates; the non-thermal electrons from X mechanisms are accelerated by reconnecting electric field with bidirectional flow, and their radio response should be type III pairs or spike pairs; P mechanism will produce energetic particle upflows, and their radio response should be unidirectional fiber bursts with moderate negative drifting rates. Therefore, the heating mechanism can be identified and verified from the broadband dynamic spectral radio observations. Additionally, using high-resolution radioheliographs and spectral-imaging observations, the heating mechanisms in different regions can be identified and verified separately, thereby demonstrating the physical essence of the hot corona.

Polarization Ratios of Turbulent Langmuir/Z-mode Waves Generated by Electron Beams in Magnetized Solar Wind Plasmas

The polarization ratios F = E²/E² of beam-generated turbulent Langmuir/Z-mode (LZ) waves and electromagnetic emissions radiated at plasma frequency _p from such sources are studied in weakly magnetized and randomly inhomogeneous plasmas. Large-scale and long-term 2D/3V particle-in-cell simulations with parameters relevant to type III solar radio bursts are performed. Statistical studies using waveforms recorded by virtual satellites are carried out to determine the distributions of polarization ratios as a function of beam and plasma parameters. This efficient method, which mimics waveform recording by spacecraft in the solar wind, leads to results consistent with observations. Moreover, plasma random density fluctuations n turn out to be the key factor responsible for the increase in polarization ratios up to F 1. Indeed, it is demonstrated that linear mode conversion at constant frequency near _p of LZ waves scattering on n is the most efficient and fastest process to produce large polarization ratios in randomly inhomogeneous plasmas. This is due to electromagnetic slow extraordinary Z-mode wave emission by LZ wave turbulence. The results provide guidance to theoretical studies and useful support to estimate the average level of density fluctuations in solar wind plasmas.

A Flare-related Decimetric Type-IV Radio Burst Induced by the X2 Radiation of Electron Cyclotron Maser Emission

The radiation mechanism of decimetric wideband and pulsating radio bursts from the Sun (in terms of decimetric type-IV (t-IVdm) burst) and other flaring stars is a long-standing problem. Early investigations were based on the leading-spot hypothesis for the Sun and yielded contradictory results. Here, we analyzed the flare-associated t-IVdm burst on 20110924 with medium-strong levels of polarization and from sources near a sunspot. We found that the emission is intermittent and the maximum T_B exceeds 10¹¹ K, with well-defined upper and lower frequency cutoffs. The radio sources are left-handed polarized, located above the sunspot with a negative polarity. The sources align well with the sites of the second harmonic of the local electron gyrofrequency. These findings provide essential evidence that the burst is induced by the electron cyclotron maser emission (ECME) in the harmonic X mode. We further modeled the transport of downward- streaming energetic electrons along a coronal loop and found that most electrons get mirrored within the specific altitude range of 20100 Mm. This explains why such bursts tend to have well-defined spectral ranges. We also found the ECME-radiating energetic electrons exhibit a shell- like velocity distribution function instead of the generally presumed loss-cone distribution. The study greatly expands the application of ECME in solar radio astronomy and provides solar samples for similar bursts from other flaring stars.

Observations of Microwave Emission from Solar Jets and Comparison with Magnetohydrodynamic Simulations

We computed the thermal microwave emission from a 3D magnetohydrodynamic (MHD) simulation and compared it with observations of solar jets. The simulation treats the emergence of magnetic flux into the solar atmosphere and its interaction with a low, preexisting ambient magnetic field. This interaction leads to the formation and development of a jet, driven by an eruption. The computed 17 GHz radio emission is compared with a number of observed jets, with respect to their morphology, their flux, and the rise time of the radio flux. We find that the MHD model reproduces the characteristics of lower-intensity jets reasonably well, whereas there are differences with stronger jets. We suggest possible ways to obtain more realistic jets from MHD simulations, so that they better match the real jets.

A Multispacecraft Analysis and Modeling of Type III Radio Burst Exciter Deceleration in Inhomogeneous Heliospheric Plasma

Electron beams accelerated in solar flares and escaping from the Sun along open magnetic field lines can trigger intense radio emissions known as type III solar radio bursts. Utilizing observations by Parker Solar Probe (PSP), STEREO-A, Solar Orbiter, and Wind spacecraft, the speeds and accelerations of type III exciters are derived for simple and isolated type III solar bursts. For the first time, simultaneous four spacecraft observations allow us to determine positions and correct the resulting velocities and accelerations for the location between the spacecraft and the apparent source. We observe velocities and acceleration to change as u(r) r^{0.37 0.14} and a(r) r^{1.71 0.20} with radial distance from the Sun r. To explain the electron beam deceleration, we develop a simple gas-dynamic description of the electron beam moving through plasma with monotonically decreasing density. The model predicts that the beam velocity decreases as u(f) f^1/4(r), so the acceleration changes r^1.58 (and speed as r^0.29) for the plasma density profile n(r) r^2.3. The deceleration is consistent with the average observation values corrected for the type III source locations. Intriguingly, the observations also show differences in velocity and acceleration of the same type III observed by different spacecraft. We suggest the difference could be related to the additional time delay caused by radio-wave scattering between the spacecraft and the source.

Challenging Established Paradigms

Polarimetric radio observations of the Sun can provide rich information about emission mechanisms and the propagation medium. For the past five decades, solar polarimetric studies at low radio frequencies have almost always assumed the absence of linear polarization. This has been based on the expectations from coronal propagation effects. Here we present the first robust evidence of linear polarization from solar emissions at meter wavelengths using simultaneous measurements with two telescopes of very different designs separated by thousands of kilometersthe Murchison Widefield Array and the upgraded Giant Metrewave Radio Telescope. Both data sets show consistent linear polarization fractions, confirming this detection. Rapid changes in morphology, as well as the fractional linear polarization at small time and frequency spans, further rule out any possibilities of an instrumental origin. Assuming the absence of linear polarization in solar radio emissions can result in incorrect interpretation of solar observations as well as those of other flare stars, which are often guided by learnings from solar studies. This discovery highlights the need for relaxing this assumption and is essential for precise estimation of polarization signatures, ultimately leading to a better understanding of the plasma conditions in the Sun and other stars.

Energy Budget in the 2017 September 7 "Cold" Solar Flare

A subclass of early impulsive solar flares, cold flares, was proposed to represent a clean case, where the release of the free magnetic energy (almost) entirely goes to the acceleration of the nonthermal electrons, while the observed thermal response is entirely driven by the nonthermal energy deposition to the ambient plasma. This paper studies one more example of a cold flare, which was observed by a unique combination of instruments. In particular, this is the first cold flare observed with the Expanded Owens Valley Solar Array and, thus, for which the dynamical measurement of the coronal magnetic field and other parameters at the flare site is possible. With these new data, we quantified the coronal magnetic field at the flare site but did not find statistically significant variations of the magnetic field within the measurement uncertainties. We estimated that the uncertainty in the corresponding magnetic energy exceeds the thermal and nonthermal energies by an order of magnitude; thus, there should be sufficient free energy to drive the flare. We discovered a very prominent soft-hard-soft spectral evolution of the microwave-producing nonthermal electrons. We computed energy partitions and concluded that the nonthermal energy deposition is likely sufficient to drive the flare thermal response similarly to other cold flares.

I. The differential rotation profile

Context. Although the differential rotation rate on the solar surface has long been studied using optical and extreme ultraviolet (EUV) observations, associating these measurements with specific atmospheric heights remains challenging due to the temperature-dependent emission of tracers observed in EUV wavelengths. Radio observations, being primarily influenced by coherent plasma processes and/or thermal bremsstrahlung, offer a more height-stable diagnostic and thus provide an independent means to test and validate rotational trends observed at other EUV wavelengths. Aims. We aim to characterise the differential rotation profile of the upper chromosphere using cleaned solar full-disc 17 GHz radio imaging from the Nobeyama Radioheliograph spanning a little over two solar cycles (19922020). Methods. A tracer-independent method based on automated image correlation was employed on daily full-disc 17 GHz radio maps. This method determines the angular velocities in 16 latitudinal bins of 15 each by maximising the 2D cross-correlation of overlapping image segments. Results. The best-fit parameters for the differential rotation profile are A = 14.520 0.006/day, B = 1.443 0.099/day, and C =0.433 0.267/day. These results suggest that the upper chromosphere rotates significantly faster than the photosphere at all latitudes, with a relatively flatter latitudinal profile. We also observed a very weak anti-correlation, _s = 0.383 (94.73%), between the equatorial rotation rate and solar activity. Conclusions. Our findings reaffirm the potential of radio observations to probe the dynamics of the solar chromosphere with reduced height ambiguity. The overlap of the equatorial rotation rate (A) found in this study with that for 304 in the EUV regime lends additional support to the view that the equatorial rotation rates increase with height above the photosphere. Future coordinated studies at wavelengths with better- constrained height formation will be crucial for further understanding the complex dynamics of the solar atmosphere.

The source sizes of type II radio bursts with LOFAR

Context. Solar radio bursts can provide important insights into the underlying physical mechanisms that drive the small and large-scale eruptions on the Sun. Since metric radio observations can give us direct observational access to the inner and middle corona, they are often used as an important tool to monitor and understand the coronal dynamics. Aims. While the sizes of the radio sources that can be observed in the solar corona are essential for understanding the nature of density turbulence within the solar corona and its subsequent influence on the angular broadening observed in radio source measurements, the smallest radio sources associated with solar radio bursts have so far been limited by observational techniques and the radio instrument's baselines. Methods. We selected three type II bursts that were observed with the LOFAR core and remote stations in the Solar Cycle 24. We estimated the sizes and shapes (ellipticity) of the radio sources from 20200 MHz using a two-dimensional (2D) Gaussian approximation. Results. Our analysis shows that the smallest radio source size for type II bursts in the solar corona that can be observed in the solar atmosphere at low frequencies is 1.5'0.5' at 150 MHz. However, even though the observations were taken with remote baselines (with a maximum distance of 85 km), the effective baselines were much shorter (15 km), likely due to snapshot imaging of the Sun. Conclusions. Our results show that the radio source sizes are less affected by scattering than suggested in previous studies. Our measurements indicate smaller source sizes at frequencies below 95 MHz compared to previous reports, though some overlap exists with measurements at higher frequencies when using smaller baselines.

Observation and Modeling of Small Spatial Structures of Solar Radio Noise Storms Using the uGMRT

One of the most commonly observed solar radio sources in the metric and decametric wavelengths is the solar noise storm. These are generally associated with active regions and are believed to be powered by the plasma emission mechanism. Since plasma emission is emitted primarily at the fundamental and harmonic of the local plasma frequency, it is significantly affected by density inhomogeneities in the solar corona. The source can become significantly scatter-broadened due to the multi- path propagation caused by refraction from the density inhomogeneities. Past observational and theoretical estimates suggest some minimum observable source size in the solar corona. The details of this limit, however, depend on the modeling approach and details of the coronal turbulence model chosen. Hence pushing the minimum observable source size to smaller values can help constrain the plasma environment of the observed sources. In this work, we for the first time, use data from the upgraded Giant Metrewave Radio Telescope in the 250 500 MHz band, to determine multiple instances of very small-scale structures in the noise storms. We also find that these structures are stable over timescales of 15 30 minutes. By comparing the past observations of type III radio bursts and noise storms, we hypothesize that the primary reason behind the detection of these small sources in noise storm is due to the local environment of the noise storm. We also build an illustrative model and propose some conditions under which the minimum observable source size predicted by theoretical models, can be lowered significantly.

Multiple Sources of a Type II Radio Burst Within a Coronal Mass Ejection

Solar type II radio bursts are generated through plasma emission caused by energetic electrons that are accelerated by shock waves during solar eruptions. These bursts serve as tracers of shock waves in the corona. However, the complexity of solar eruptions and the lack of radio imaging observations have hampered our understanding of type II bursts. The newly built Daocheng Solar Radio Telescope (DSRT) detected a rare type II burst. Its harmonic shows an initial herringbone (HB), followed by three nearly parallel lanes. These lanes form a framed pattern: a central main lane (termed MAIN) with a higher brightness temperature and wider bandwidth, flanked by two well-defined fringes, F1 and F2. Radio and extreme ultraviolet imaging observations indicate that the sources of the HB are precisely located on the flank of the leading shock wave driven by a coronal mass ejection (CME). In contrast, the MAIN and F2 sources correlate in terms of time, location, electron number density, and propagation velocity with an ascending coronal loop. In contrast, the F1 sources are associated with a nearby but distinct coronal loop. These observations suggest that at least three sources of the type II burst accompany the CME. A scenario involving multiple shock waves within the CME is proposed to explain the presence of the different radio sources.

Theoretical Considerations

The Sun is a powerful source of radio emissions, so much so that, unlike most celestial sources, this emission can dominate the system noise of radio telescopes. We outline the theory of noise in maps formed by Fourier synthesis techniques at radio wavelengths, with a focus on self- noise: that is, noise due to the source itself. As a means of developing intuition we consider noise for the case of a single dish, a two-element interferometer, and an n-element array for simple limiting cases. We then turn to the question of the distribution of noise on a map of an arbitrary source observed at radio wavelengths by an n-element interferometric array. We consider the implications of self-noise for observations of the Sun in a companion paper.

Solar Use Cases

Noise in images of strong celestial sources at radio wavelengths using Fourier synthesis arrays can be dominated by the source itself, so- called self-noise. We outlined the theory of self-noise for strong sources in a companion paper. Here we consider the case of noise in maps of radio emission from the Sun which, as we show, is always dominated by self noise. We consider several classes of science use cases for current and planned arrays designed to observe the Sun in order to understand limitations imposed by self-noise. We focus on instruments operating at decimeter and centimeter wavelengths but the results are applicable to other wavelength regimes.

A novel fine spectral structure of solar radio bursts with periodic beaded stripes observed by CBSm of CMP-II

A novel fine spectral structure in solar radio bursts has been discovered using the Chashan broadband solar radio spectrometer at meter wavelengths (CBSm), an instrument of the Chinese Meridian Project-Phase II (CMP-II). The structure features periodic narrow-band stripes with a typical recurrence time < 1 s (occasionally reaching 8 s), often drifting from high to low frequencies and accompanied by absorptions, with trailing stripes appearing at the end of preceding ones. Some stripes exhibit periodic beaded (or pearl-like) enhancements with a periodicity of 0.1 s. The beaded stripes are reported for the first time ever. Data from the DAocheng Radio Telescope (DART) indicate a radio emission brightness temperature exceeding 10⁹ K, originating above brightening loops in active region AR 13664. We proposed a novel generation mechanism of the periodic stripes on the basis of the double plasma resonance (DPR) instability, and explained the beaded substructure in terms of modulation by low-frequency magnetohydrodynamic (MHD) waves. The study highlights the CBSm's capability to detect high-resolution fine spectral structures and offers novel insights into the emission mechanism and source characteristics of solar radio bursts.

Detecting Ionospheric Disturbances Using High Frequency Coastal Radar Transmissions From the West Coast of the United States

Coastal radar system are located around the world and many happen to transmit at frequencies capable of skywave propagation via the ionosphere. Therefore, they can be detected hundreds to thousands of kilometers away. This paper demonstrates the opportunity to detect 39 Coastal Ocean Dynamics Application Radar transmitters located on the western coast of the United States using three HF radio receivers in Utah and New Mexico. It also illustrates the possibility to use the phase and Doppler measurements of these signals to derive displacements of the refracting ionospheric layer up to meter resolution for the 2023 annular solar eclipse, an M-class solar flare, and a Falcon 9 second stage reentry. This study demonstrates the feasibility and usefulness of coastal radar systems to make ionospheric measurements and conduct research.

Noise Reduction Method for Radio Astronomy Single Station Observation Based on Wavelet Transform and Mathematical Morphology

The 21 cm radiation of neutral hydrogen provides crucial information for studying the early universe and its evolution. To advance this research, countries have made significant investments in constructing large low- frequency radio telescope arrays, such as the Low Frequency Array and the Square Kilometre Array Phase 1 Low Frequency. These instruments are pivotal for radio astronomy research. However, challenges such as ionospheric plasma interference, ambient radio noise, and instrument- related effects have become increasingly prominent, posing major obstacles in cosmology research. To address these issues, this paper proposes an efficient signal processing method that combines wavelet transform and mathematical morphology. The method involves the following steps: Background Subtraction: Background interference in radio observation signals is eliminated. Wavelet Transform: The signal, after removing background noise, undergoes a two-dimensional discrete wavelet transform. Threshold processing is then applied to the wavelet coefficients to effectively remove interference components. Wavelet Inversion: The processed signal is reconstructed using wavelet inversion. Mathematical Morphology: The reconstructed signal is further optimized using mathematical morphology to refine the results. Experimental verification was conducted using solar observation data from the Xinjiang Observatory and the Yunnan Observatory. The results demonstrate that this method successfully removes interference signals while preserving useful signals, thus improving the accuracy of radio astronomy observations and reducing the impact of radio frequency interference.

Confined vs. eruptive M-class flares in solar cycles 23 and 24

This report presents a quantitative comparison between confined, eruptive and all (2177) M-class solar flares (SFs) over the last two solar cycles (SC) and separately in SC 23 and 24. The properties of the SFs, related radio bursts and the parent sunspots (Hale type and total area) are examined. The differences are presented and discussed in the framework of space weather.

A 5055 GHz Millimeter-wave Radiometer Spectrometer for Solar Flare Detection

During solar flare eruptions, millimeter-wave radiation is emitted, which is highly efficient and sensitive to high-energy electrons, allowing for the extraction of unique magnetic field information. Therefore, we have developed a 5055 GHz solar millimeter-wave radiometer system. The system employs a 50 cm diameter Cassegrain antenna to receive circularly polarized solar radiation signals. These signals enter the analog front-end system, where they undergo power division, filtering, and detection operations, resulting in voltage signals. Subsequently, the signals are processed by the digital receiver for analog-to-digital conversion and smoothing and are finally transmitted to the host computer via the RS422 protocol to display the intensity of solar radiation. The system's performance metrics are as follows: a noise figure of <2.5 dB, system linearity 0.9999, a time resolution range of 0.0011 s, and a dynamic range exceeding 30 dB. The system began routine observations in 2024 October and successfully captured the world's first 50 GHz band solar flare data in December. Currently, the system is effectively observing during the 25th solar activity maximum period, which is expected to provide valuable data for solar physics research.

Deep Active LearningBased Classification of Solar Radio Spectrogram Data

The study of solar burst activity can provide early warnings for the environmental protection of the solarterrestrial space environment. With the improvement of solar radio observation techniques, observation devices have generated enormous amounts of observation data. To solve the shortcomings of time-consuming and error-prone manual recognition, researchers have begun to use deep learning to recognize and automatically classify solar radio outbursts. Deep learning will depend on a large number of labeled samples; however, the labeling of samples requires a lot of time and manual labor. This leads to low efficiency. In addition, the labeled samples are not all valuable samples, so it is necessary to improve the effectiveness of the labeled samples and select the high-value samples. The occurrence of active-learning techniques provides an opportunity to solve this problem. In this study, we developed a progressive deep convolutional generative adversarial network model. Then, we combined it with deep active learning to complete the automatic classification of solar radio spectrum data. We used solar radio spectrum data from the Chashan Observatory (CSO) of Shandong University and Learmonth Observatory in Australia. The results show that the method proposed in this paper can achieve high accuracy in the automatic recognition of solar radio spectrum data and solve the time-consuming problem of labeling a huge number of data samples. Finally, we applied the results to the CSO and realized the automatic recognition of solar radio spectral data.

Research on High-precision Fiber Transmission and Compensation for a Solar Radio Heliograph

Solar radio bursts are a major source of space weather hazards. Thus, developing solar observation systems is essential. The heliograph, based on synthetic aperture imaging and a dual interferometer, enables high- resolution solar imaging, offering rich spatial information beyond conventional radio spectrometers. The performance of a synthetic aperture heliograph depends critically on time-frequency synchronization among multiple antennas. On this basis, we have conducted research on fiber time-frequency synchronization. The research allows the long- distance transmission and synchronization of signals generated by the rubidium atomic clock via optical fiber. Analysis and testing demonstrated that the research achieved frequency stability of 7 10¹³/1 s and an accuracy of 0.466 ppm. After compensation, the average time difference achieved 4.6 and 5.4 ps for frequency and time standard signal, respectively. These results not only indicate that our scheme meets the requirements of the synthetic aperture heliograph but also demonstrate good applicability and scalability, providing a solid foundation for the future development of solar observation systems.

Highly Polarized Type III Storm Observed with Parker Solar Probe

The Parker Solar Probe (PSP) spacecraft observed a large coronal mass ejection (CME) on 2022 September 5, shortly before closest approach during the 13th PSP solar encounter. For several days following the CME, PSP detected a storm of Type III radio bursts. Stokes parameter analysis of the radio emission indicates that the Type III storm was highly circularly polarized (with fractional polarization up to 0.4). Left- hand circularly polarized (LHC) emission dominated at the start of the storm, transitioning to right-hand circularly polarized (RHC) emission at the crossing of the heliospheric current sheet on September 6. We analyze the properties of this Type III storm. The drift rate of the Type IIIs indicates a constant beam speed of 0.1c, typical for Type III-producing electron beams. The sense of polarization is consistent with fundamental emission generated primarily in the O-mode. The stable and well organized post-CME magnetic field neatly separates the LHC- and RHC-dominated intervals of the storm, with minimal overlap between the senses of polarization. The proximity of PSP to the source region, both in radial distance and in heliographic longitude, makes this event an ideal case study to connect in situ plasma measurements with remote observations of radio emission.

Small-scale Inhomogeneity Effects on Coherent Solar Radio Emission

The coherent radio emission mechanism of solar radio bursts (SRBs) is one of the most complicated and controversial topics in solar physics. To clarify the mechanism(s) of different types of SRBs, (radio-) wave excitation by energetic electrons in homogeneous plasmas has been widely studied via particle-in-cell (PIC) code numerical simulations. The solar corona is, however, inhomogeneous over almost all spatial scales. Due to the kinetic nature of SRBs, small-scale inhomogeneities in the plasma could influence the excitation and emission properties of SRBs. In this paper, we thus investigate the effects of small-scale inhomogeneity (in the magnetic field as well as plasma density or temperature) of plasmas in the solar corona on radio-wave emission by ring-beam distributed energetic electrons utilizing 2.5-dimensional PIC simulations. The typical length scale of the small-scale inhomogeneity we consider in this study is on the order of the proton gyroradius. Both beam and electron-cyclotron maser instabilities can be triggered with the presence of energetic ring-beam electrons. The resultant spectrum of the excited electromagnetic waves presents a zebra-stripe pattern in the frequency space. The inhomogeneous density or temperature in plasmas influences the frequency bandwidth and location of these excited waves. Our results can hence help to diagnose the plasma properties at the emission sites of SRBs. Applications of our results to SRBs with a zebra-stripe pattern are discussed.

Solar energetic electron events with a spectral bump break

Aims. We present ten solar energetic electron (SEE) events measured by Wind/3DP at 1 to 200 keV with a bump break in the electron peak flux versus energy spectrum. We examined their acceleration sources and/or processes at the Sun. Methods. We assumed that these bump SEE events consist of two electron populations: a primary population (described by the pan-spectrum (PS) function), and a bump population (described by the Gaussian function), which dominate at low and high energies, respectively. We constructed two formulae to fit the SEE energy spectrum by multiplying a PS function with a natural exponential form of the Gaussian function (i.e., the MUL formula) and by adding a PS function with a Gaussian function (i.e., the ADD formula). Results. The fitting results suggest that the MUL fitting can reflect the physical nature in the formation of these bump events. For the primary electron population, the MUL fitting obtains an upward-bending double power-law spectrum for event 10 with a spectral index of 3.58 (1.74) at energies below (above) 4.6 keV, and a single power-law spectrum for the other nine events with a median spectral index of 2.52^+0.29_0.25. For the bump electron population, the fitted center energy has a median value of 59.1_3.2^+18.1 keV. For the events associated with soft X-ray flares (west limb coronal mass ejections), the flare class (angular width of the coronal mass ejection) is positively correlated with the estimated electron number of the power-law population N_pl and of the bump population N_bp (the number ratio N_bp/N_pl at 10400 keV). Conclusions. These results indicate that for these bump SEE events, the power-law electron population can be produced by some flare-related processes that occur high in the corona, while the bump population can be accelerated by some processes related to coronal mass ejections that act on the power-law population. The bump-like spectrum might also be the intermediate spectrum during the evolution from single power-law to downward-bending double power-law.

Comparison of Observed and Predicted Electron Density Profiles From 15 to 26 April 2001

Solar flares significantly affect Mars's ionosphere, yet there are few comparisons between observed and simulated densities in the M1 and M2 ionospheric layers during solar flares. Here we compare observed and simulated electron density profiles for the X14.4 solar flare of 15 April 2001 and the M7.8 solar flare of 26 April 2001. We use observations from Mars Global Surveyor radio occultations and simulations from the Mars Global Ionosphere-Thermosphere Model (M-GITM). Due to poor constraints on the solar spectrum incident upon Mars at this time, simulated M2 electron density values were 50% larger than observed. Yet the relative changes in M2 electron density during these two flares were reproduced to 10% accuracy. When accurate solar irradiance data are available, absolute M2 electron density values are simulated accurately. Due to the omission of electron impact ionization from the M-GITM model, the simulated M1/M2 density ratio was under- predicted by a factor of approximately 3. Yet the relative changes in M1 electron density during these two flares were reproduced to 20% accuracy. The model can accurately predict relative changes in M1 and M2 electron densities during a solar flare. If accurate solar irradiance data are available, it can accurately predict absolute changes in M2 electron densities. If a simple parameterization of electron impact ionization were incorporated into the model, then it would likely predict absolute changes in M1 electron densities accurately as well. The M-GITM model is well-suited to studies of time-varying phenomena in the ionosphere of Mars.

Monitoring Solar Radio Bursts With an Expansive Array of Antennae at High Schools Nationwide

The Sun Radio Interferometer Space Experiment (SunRISE) Ground Radio Lab (GRL) is a Science, Technology, Engineering, Arts, and Mathematics (STEAM) project, sponsored by NASA's SunRISE mission and organized by the University of Michigan College of Engineering. The project aims to engage and train the next generations of scholars. To achieve this, the project deployed antennas to 18 high schools nationwide to observe solar radio bursts (SRB). SRBs are defined as low-frequency radio emissions emanated by accelerated electrons associated with extreme solar activity, including solar flares and coronal mass ejections (CMEs). Type II SRBs were found to predominantly correspond to coronal shocks caused by CMEs, highlighting particle acceleration events in the solar atmosphere and interplanetary space. These bursts can act as early warning signs of upcoming solar disturbances which can lead to geomagnetic storms. The type II bursts were then investigated to estimate the corresponding shock and Alfvn speeds: 277 < v_shock < 1,480 km/s and 194 < v_A < 947 km/s at heliocentric distances of around 12 solar radii, respectively. The Alfvn Mach number was further found to be 1.2 < M_A < 2, while the measured magnetic field strength followed a single power law of B(r) = 0.3 r², where r represents the heliocentric distance. Our results were found to agree with previous studies. Through SunRISE GRL, an ever-expanding catalog of SRBs is being collected by high school students nationwide, curated by a team of solar physics experts, and made publicly available to the scientific community to make progress toward the SunRISE mission's objectives.

Statistics of Coronal Mass Ejections in Solar Flares with Helioseismic Response

This paper presents the results of a statistical analysis of the properties of coronal mass ejections (CMEs) associated with solar flares that exhibit a helioseismic response (''sunquakes'') in comparison with flares that do not show photospheric disturbances. The analysis is based on observations of the solar corona in the ultraviolet range (from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, SDO/AIA) and the visible range (from the Large Angle Spectroscopic Coronagraph on board the Solar and Heliospheric Observatory, SOHO/LASCO). We considered samples of flares with different lower thresholds based on the Geostationary Operational Environmental Satellites (GOES) classification: above M1.0, M5.0, and M7.0. A correlation analysis was also carried out between CME parameters and the total energy of the sunquakes. Additionally, for flares above M7.0-class, information on the presence of radio bursts across a wide range of wavelengths, as well as hard X-ray emission, was analyzed. It was found that CMEs accompanying flares with a helioseismic response tend to have higher velocities in the lower corona (according to AIA data) compared to flares without photospheric disturbances. The distribution of CME masses is approximately the same regardless of the presence or absence of sunquakes during the flares. An analysis of dimming properties showed that they are more impulsive in terms of temporal dynamics in flares with sunquakes. CMEs in flares above M7.0-class that exhibit helioseismic responses are less massive and slower in the outer corona according to LASCO data. The correlation analysis did not reveal strong relationships between acoustic energy and CME parameters based on AIA observations, but for several parameters (kinetic energy, CME mass, and dimming depth), statistically significant correlations were identified according to the -criterion. In contrast to flares with sunquakes, there was an almost complete absence of type III radio bursts and a rare occurrence of type II bursts in the M7.0-class flares without photospheric disturbances. The spectral peak of microwave bursts tends to occur at higher frequencies in flares with sunquakes than in those without. According to our analysis, flares with sunquakes likely possess the ability to efficiently generate fast coronal dimmings and shock waves, even in the presence of poorly developed CMEs in the upper corona (in comparison to flares without photospheric disturbances). These events are also characterized by pronounced signatures of electron acceleration, with particles escaping the acceleration region both toward the solar surface and outward from it. In our view, this indicates that the possibility of an eruptive origin for sunquakes cannot be ruled out. Accelerated electrons may act as both the primary and secondary agents responsible for initiating the photospheric perturbation.

Polarization Analysis of Type III Langmuir/Z-mode Waves with Coherent Magnetic Component Observations by Solar Orbiter

Observations from the Solar Orbiter spacecraft provide unique insights into the interaction between electron beams and the plasma background in the source regions of type III radio emissions. We analyze this interaction by examining the high-frequency electric and magnetic components of in situ wave measurements, focusing on their polarization properties. Using electron data from onboard instruments, we model the electron velocity distribution function and numerically solve the dispersion relation. We compare the predicted polarization of the electric and magnetic components with the observations. Our findings are consistent with propagation in the Langmuir/Z-mode at an oblique wavevector. We explain the magnetic component and transverse polarization by the presence of small density fluctuations, without the need for mode conversion.

First Detection of Low-frequency Striae in Interplanetary Type III Radio Bursts

We report the first detection of type III solar radio burst striae in the 3080 kHz range, observed by the Cluster-4 spacecraft during an exceptionally quiet solar period. These low-frequency fine structures, which drift slowly in frequency and exhibit narrow bandwidths, provide a novel diagnostic of plasma processes in the inner heliosphere. The detected striae, interpreted as fundamental plasma emission, exhibit a frequency drift rate of 0.328 Hz s¹ and a bandwidth of 1.3 kHz. By combining high-resolution radio observations with well- calibrated in situ electron velocity distribution function data from the Wind spacecraft, we characterized the plasma properties of the burst source region near 0.32 au. Our analysis estimates relative density fluctuations, at the effective turbulence scale length, as approximately 3.4% (inferred from striae bandwidths), 0.62% (from intensity fluctuations), and 3.5% (from a heliocentric distance-based empirical model). These findings offer critical insights into small-scale density inhomogeneities and turbulence that affect electron beam propagation. This study underscores the potential of combining well-calibrated in situ electron data with radio burst measurements to probe the physical conditions of the solar wind and to refine our understanding of solar radio bursts across a broad frequency range.

High-resolution Observational Features of Type I Solar Radio Bursts

In this paper, we statistically studied the characteristic features of solar radio Type I bursts observed by Chashan broadband solar radio spectrometer at meter wavelengths from 00:06 to 07:28 UT on 2023 December 13. Based on the image morphology, we develop a method to identify individual Type I bursts on the dynamic spectra from a connected region with a ratio of the duration to bandwidth less than or equal to 3.8 and the area sum (total pixels) greater than 4. In total, 102,073 and 78,773 bursts are detected on right-hand circular polarization and left-hand circular polarization components, respectively. We find Type I bursts with a mean lifetime of 0.7 s, a mean bandwidth of 5.6 MHz, and a relative bandwidth f/f₀ of about 2%3%. Consistent with the previous findings of hard X-ray pulses, microwave pulses, Type III bursts, and decimetric spikes, Type I bursts exhibit their duration, bandwidth, peak intensity, and area sum with a power-law distribution. From the linear fitting in loglog space, we obtain the slope index between 1 and 3.5 for various parameters. Meanwhile, Type I bursts display the mean polarization degree in the range from 22% to 33%, and different bursts have various degrees. We find that Type I bursts tend to appear as a chain, and the short periods of 0.92 and 1.22 s are detected in two burst chains. Our finding would be an observational constraint for the emission mechanism and physical model of solar Type I radio bursts.

Imaging and spectropolarimetric observations of a band-split type II solar radio burst with LOFAR

Context. Type II solar radio bursts are generated by electrons accelerated by coronal shock waves. They appear in dynamic spectra as lanes drifting from higher to lower frequencies at the plasma frequency and its harmonic. These lanes can often be split into two or more sub- bands that have similar drift rates. This phenomenon is called band- splitting, and its physical origins are still under debate. Aims. Our aim is to investigate the origin of band-splitting using novel imaging and spectropolarimetric observations of a type II solar radio burst from the Low Frequency Array (LOFAR). Methods. We used LOFAR imaging at multiple frequencies and time steps to track the locations of the radio sources corresponding to the two components of the band-split emission lane. In addition, we estimated the degree of circular polarisation (dcp) for both components using LOFAR's full Stokes dynamic spectra. Results. From the imaging of the type II burst, we found two close but clearly separated emission regions clustered over several frequencies spanning each split band. One emission region corresponds to the lower frequency band and the other to the higher frequency band of the split lane. Using the full Stokes dynamic spectra, we also found the dcp to be very similar for both bands. Conclusions. The two distinct emission regions suggest that the split bands originate from two separate regions at the shock. The similar values of dcp for both sub-bands correspond to similar values of magnetic field strength in the two regions and indicate little to no change in the emission region plasma. Thus, our findings are in contradiction with previous theories, which have suggested that split bands originate in the same region but upstream and downstream of the shock. Instead, our results suggest that both bands originate in two separate upstream regions since we find a clear separation in locations and no magnetic compression.

Studying the Inner Heliosphere Using Radio Signals Transmitted From Spacecraft Orbiting Mars

In this campaign, we used very long baseline interferometry (VLBI) radio telescopes to track the carrier radio signal transmitted from spacecraft orbiting Mars during the Mars solar conjunction. The campaign extended from September $6\text{th}$ to November $6\text{th}$ 2021 and targeted the Mars Express (MEX) and Tianwen-1 (TIW) spacecraft. The aim was to study the structure of the solar corona within 40 solar radii using the radio sounding technique developed by the Planetary Radio and Doppler Experiment (PRIDE). Previous work by the group has studied the effect of the solar wind and interplanetary scintillation on spacecraft signals at a large range of solar elongations. Here, we apply the same methodology to characterize the inner regions of the solar wind and attempt to capture the formation of transient solar phenomena such as solar flares or coronal mass ejections (CMEs). We found that the phase scintillation measurements fit the expected model of the electron content for the line-of-sight very close to the Sun. Experimental outliers, where the scintillation results deviated significantly from the expected electron content model, were compared with ancillary data from the LASCO C2 and C3 coronagraph instruments and found to correspond directly to solar transient events.

Robust Statistical Techniques for Operational Maintenance of the 10.7 cm Solar Radio Flux

The F10.7 solar radio flux is a critical quantity for operational space weather nowcasting and forecasting, where it is routinely used as a driver for coupled atmospheric models to estimate a variety of important quantities such as the neutral atmospheric density. Although there have been several successful developments in the way of parametric modeling to ensure F10.7 coverage during outages (often using the sunspot number or radio flux observations at neighboring wavelengths), these developments have refrained from employing comprehensive cross- validation schemes to ensure model generalizability, and can benefit from recently-developed techniques for modeling nonlinear phenomena. We present an approach that uses Feature Ordering by Conditional Independence (FOCI) to identify favorable surrogates for the F10.7 index and combines this with modeling of F10.7 with linear models and Generalized Additive Models (GAMs). We find that this approach offers notable improvements in reconstructing F10.7 over gaps of various lengths, with GAMs yielding mean error of ${\sim} $2.8%, compared to polynomial methods that yield mean errors of ${\sim} 3.1$%. We additionally demonstrate the effect of reconstruction error on neutral densities modeled by the NRLMSISE2.0 thermosphere model.

The High-Energy Protons of the Ground Level Enhancement (GLE74) Event on 11 May 2024

High energy solar protons were observed by particle detectors aboard spacecraft in near-Earth orbit on May 11, 2024 and produced the 74^th ground level enhancement (GLE74) event registered by ground-based neutron monitors. This study involves a detailed reconstruction of the neutron monitor response, along with the identification of the solar eruption responsible for the emission of the primary particles, utilizing both in situ and remote-sensing. Observations spanning proton energies from a few MeV to around 1.64 GeV, collected from the Solar and Heliospheric Observatory (SOHO), the Geostationary Operational Environmental Satellite (GOES), the Solar Terrestrial Relations Observatory (STEREO-A), and neutron monitors, were integrated with records of the associated solar soft X-ray flare, coronal mass ejection, and radio bursts, to identify the solar origin of the GLE74. Additionally, a time-shift analysis was conducted to link the detected particles to their solar sources. Finally, a comparison of GLE74 to previous ones is carried out. GLE74 reached a maximum particle rigidity of at least 2.4 GV and was associated with a series of type III, type II, and type IV radio bursts. The release time of the primary solar energetic particles (SEPs) with an energy of 500 MeV was estimated to be around 01:21 UT. A significant SEP flux was observed from the anti-Sun direction with a relatively broad angular distribution, rather than a narrow, beam-like pattern, particularly during the main phase at the particle peak flux. Comparisons with previous GLEs suggest that GLE74 was a typical event in terms of solar eruption dynamics.

Extended Scenarios for Solar Radio Emissions With Downshifted Electron Beam Plasma Excitations

First-principle studies of radiative processes aimed at explaining the origin of type II and type III solar radio bursts raise questions on the implications of downshifted electron beam plasma excitations with frequency (slightly) below the plasma frequency $\left(\omega \lesssim {\omega }_{pe}\right)$ in the generation of radio emissions. Unlike the beam-induced Langmuir waves $\left(\omega rsim {\omega }_{pe}\right)$ in the standard radio emission plasma model, the primary wave excitations of cooler and/or denser beams have predominantly downshifted frequencies. Broadbands of such downshifted excitations are also confirmed by in situ observations in association with terrestrial foreshock and electron beams (in contrast to narrowband Langmuir waves), but their involvement in radiative processes has not been examined so far. We revisit three radiative scenarios specific to downshifted primary excitations, and the results demonstrate their direct or indirect involvement in plasma radio emission. Downshifted excitations of an electron beam primarily play an indirect role, contributing to the relaxation to a plateau-on-tail still able to induce Langmuir beam waves that satisfy conditions for nonlinear wave-wave interactions leading to free radio waves. At longer time scales, the primary excitations can become predominantly downshifted, and then directly couple with the secondary (backscattered) Langmuir waves to generate the second harmonic of radio emissions. Two counter beams are more efficient and lead to faster radiative mechanisms, involving counterpropagating downshifted excitations, which couple to each other and generate intense, broadband and isotropic radio spectra of downshifted second harmonics. Such a long-lasting (second) radio harmonic can thus be invoked to distinguish regimes with downshifted $\left(\omega \lesssim {\omega }_{pe}\right)$ primary excitations.

Ionospheric and Thermospheric Response to the 13 June 2022 M-Class Solar Flare

On 13 June 2022, an M3-class solar flare erupted at 03:00 UT that lasted nearly eight hours, causing increased ionization and shortwave radio blackouts. Here, we combine measurements made by the NASA Ionospheric Connections Explorer (ICON) mission to assess the evolution of the ionosphere-thermosphere system in response to this flare. We find that increased solar extreme ultraviolet (EUV) radiation during the flare did increase O⁺ plasma in the region that was directly exposed to the flare, but this effect was moderated by thermospheric perturbations as evidenced by a decrease in column O/N₂ (O/N₂) that accompanied the sequence of events. Larger increases in O⁺ were seen in the same region the day after the flare as the non-impulsive, long-term solar EUV irradiance continued to increase and the thermospheric O/N₂ recovered. When energetic particles arrived 3 days after the event, the ionospheric impact was delayed by about a day compared to the thermospheric changes. Changes in O/N₂ inferred from far ultraviolet airglow are also strongly correlated with relative changes in atomic oxygen independently determined by simultaneous ICON measurements of EUV airglow. These results demonstrate the magnitude, duration, and complexity of change that even moderate M-class flares can generate in the ionosphere and thermosphere.

Case study of the African low and mid-latitude regions

We simultaneously evaluate the contributions of the mostly used solar activity indices to the modelling of geomagnetic storms using principal component analysis (PCA). The selected indices are the sunspot number (SSN), solar radio flux at a wavelength of 10.7 cm ( F10.7), 12-month running average of SSN (R12), 81-day running average of F 10.7 (< mml:math>F10.781< /mml:mn>), and the modified F10.7 index herein referred to as F10.7p. The assessment of these indices was accomplished by first developing five storm-time empirical models of the ionosphere with ionospheric total electron content (TEC) as dependent variable, and each of the five solar proxies as the independent variable. As the energy from the Sun differs from one latitudinal region to another on Earth, two locations at different latitudes were considered for the analysis. Based on their long data coverage periods, Hartebeesthoek (HRAO, geographic coordinates: 25.89 S, 27.69 E; geomagnetic coordinates: 36.32 S, 94.69 E), South Africa; and Mbarara (MBAR, geographic coordinates: 0.60< mml:mo> S and 30.74 E, geomagnetic coordinates: 10.22 S and 102.36 E), Uganda, were chosen to represent the middle and low latitude ionospheric regions, respectively. Their data coverage periods are 27 September 1996 to 30 March 2024 (HRAO) and 17 July 2001 to 30 March 2024 (MBAR) and only storm-time TEC data within these periods selected based on the criterion Dst50 nT or < mml:mi>Kp4 were considered for the statistical analysis. Through PCA decomposition, TEC data were broken up into a matrix of principal directions of the maximum variances in the dataset (or matrix of eigenvectors of the covariance matrix) and a matrix of principal components (PCs) which represent the projection of data onto the principal directions. For each model, PCs were thereafter modelled in terms of the corresponding solar activity index and the modelled quantities were further combined with the original PC vectors to get the reconstructed TEC for the entire period of the study. With reference to the ionospheric storm-time model implemented using SSN as solar activity representation, a statistical analysis revealed that, overall, the storm-time empirical models developed using either F10< /mml:mn>.7, F10.< mml:msub>781, < mml:mi>R12, or F10.7p, perform about 8%, 15%, 18%, 22%, respectively, better in reconstructing actual TEC than the SSN based model for HRAO, and 11%, 23%, 19%, 24% for MBAR. Validating the models over selected four storms, results showed that running average based indices led to more accurate TEC predictions compared to the usual daily Wolf's SSN and F 10.7.

Solar Radio Burst Detection Based on Deformable DETR

Solar radio burst (SRB) detection is crucial for solar physics research and space weather forecasting. The main challenges faced are noise interference in the spectrum and the diversity of SRBs. However, most research focuses on classifying whether SRBs exist or detecting a single type of SRB. Existing detection models exhibit deficiencies in the accuracy of SRB detection. Moreover, existing detection models cannot effectively handle background noise interference in solar radio spectrograms and the significant scale variations among different burst types. This paper proposes a high-performance detection model for SRBs based on the Deformable DEtection TRansformer (DETR) called DETR4SBRs. First, this study designed a scale-sensitive attention module to better address the scale variations of SRBs. Subsequently, this study introduced collaborative hybrid auxiliary training to mitigate the positivenegative sample imbalance issue in Deformable DETR. The experimental results demonstrate that the proposed model achieves a mAP@50 of 83.5% and a recall rate of 99.4% on the SRBs data set. Additionally, the model exhibits excellent noise-robust performance and can efficiently detect and locate Type II, III, IV, and V SRBs. The model proposed in this study provides robust support for preliminary SRB data processing and has significant implications for space weather forecasting. The source code and data are available on https://github.com/onewangqianqian/SSA-Co-Deformable-DETR.git, and the software is archived on Zenodo.

Impulsive Solar Flares in the Parker Solar Probe Era. I. Low-energy Electron, Proton, and Alpha Beams

Multiple instruments on board the Parker Solar Probe spacecraft have detected signatures of impulsive electron and ion beam events over the course of the first 20 encounters. The energy spectra of these events are characterized by a peak originating in the low-frequency bandwidth of the FIELDS sensor and descending over time in energy until it merges with the bulk of the solar wind as observed by the Solar Wind Electrons, Alphas, and Protons suite. All events are well associated with Type III radio bursts, and some are well correlated with soft and hard X-rays generated by impulsive solar flares. These dispersive energy beam phenomena are essential in understanding particle acceleration, transport, and energy partitioning between electrons and ions as a result of impulsive solar flares. In this work, we present an analysis of said events and leverage multiwavelength observations that are conducted by instruments on multiple platforms. Our results show, for the first time, that solar flares are observed to be the source of low- energy ions in interplanetary space. This discovery has never been previously observed at such low energies by instruments at 1 au.

Investigating an Erupting Metric-decimetric Radio Depression and Its Physical Origin

We present direct metric-decimetric radio imaging observations of a fascinating quiescent filament eruption on 2024 March 17 using data from the DAocheng Radio Telescope, with a combination of the Solar Dynamics Observatory and the Chinese H Solar Explorer. At the radio band, even though the filament is difficult to identify in its early phase, it rapidly became distinct and formed a continuous loop-like dark structure during the eruption, i.e., so-called radio depression. Compared with the fragmentation of the erupting filament observed at the H and EUV bands, the radio depression appeared more coherently. Based on synthetic radio images from a three-dimensional magnetohydrodynamics simulation of a flux-rope-filament eruption, it is suggested that the radio depression originates from the absorption of cold and dense materials within the erupting flux rope to the background emission. The absorption seems to be stronger than that at the H and EUV bands, thus leading to their apparent discrepancies. Moreover, the radio depression is also found to occupy the lower part but not the whole body of the flux rope.

The Efficiency of Harmonic Emissions Excited by Energetic Electrons in Coronal Loops

Magnetic reconnection is a key process that drives the energy release in solar flares. This process can occur at multiple locations along the coronal loop. The reconnection generates energetic electrons capable of exciting wave modes and emissions as they propagate through the loop. In this follow-up study, we investigate the influence of the injection site location of these energetic electrons, either at the looptop (LT) or at the leg of the loop around a footpoint (FP), on the excitation of wave modes, especially the second harmonic emissions (X2) in coronal loops. Our simulations reveal that the injection location significantly impacts the spatial distribution and intensity of excited wave modes. When electrons are injected at the LT, electromagnetic X2, and Z-modes dominate along the loop, with minimal excitation of Langmuir waves. Conversely, the present study reveals that injection close to FP leads to a strong Langmuir wave excitation throughout the loop, particularly as electrons ascend toward the LT. We find that X2 and Z-modes are consistently excited at the injection site with different intensities, regardless of the injection location. However, electron injection near the FP scenario creates favorable conditions for significant Langmuir wave generation, potentially leading to plasma emission under specific circumstances. These findings emphasize the importance of electron injection location in determining the properties of the excited and emitted waves in solar coronal loops.

On the limitations of using metric radio bursts as diagnostic tools for interplanetary coronal mass ejections

Aims. Metric radio bursts are often said to be valuable diagnostic tools for studying the near-sun kinematics and energetics of the interplanetary coronal mass ejections (ICMEs). Radio observations also serve as indirect tools to estimate the coronal magnetic fields. However, how these estimated coronal magnetic fields are related to the magnetic field strength in the ICME at 1 AU has rarely been explored. Our aim was to establish a relation between the coronal magnetic fields obtained from the radio observations very close to the Sun and the magnetic field measured at 1 AU when the ICME arrives at the Earth. Methods. We performed statistical analyses of all metric type II radio bursts in solar cycles 23 and 24 that were found to be associated with ICMEs. We estimated the coronal magnetic field associated with the corresponding CME near the Sun (middle corona) using a split-band radio technique and compared them with the magnetic fields recorded at 1 AU with in situ observations. Results. We found that the estimated magnetic fields near the Sun using radio techniques are not well correlated with the magnetic fields measured at 1 AU using in situ observations. This could be due to the complex evolution of the magnetic field as it propagates through the heliosphere. Conclusions. Our results suggest that while metric radio observations can serve as effective proxies for estimating magnetic fields near the Sun, they may not be as effective close to the Earth. At least, no linear relation could be established using metric radio emissions to estimate the magnetic fields at 1 AU with acceptable error margins.

Non-thermal energy release in the post-impulsive phase of the May 9, 2021 event

Context. In the standard model of solar flares, a magnetic flux rope erupts and gets ejected from the Sun. The current sheets that form in its wake are the seat of magnetic reconnection, which is thought to power energy release throughout the long-lasting decay phase of the thermal X-ray emission. This model has been broadly tested with plasma diagnostics at soft X-ray, EUV, and H wavelengths. Aims. The primary aim of the present investigation is to shed light on the acceleration of non-thermal electrons in the post-impulsive phase through hard X-ray (HXR) radiation and radio spectroscopic imaging at decimeter-to-meter wavelengths. We focus our study on the case of a C4.0 class flare on May 9, 2021. Methods. This event was fully observed by multiple instruments from three different vantage points in space. We analyzed the spectrum and the source configuration of X-ray emission with the Spectrometer- Telescope for Imaging X-rays (STIX) on board the Solar Orbiter spacecraft, complemented by the Gamma-Ray Burst Monitor (GBM) aboard the Fermi mission, and the radio emission with Nanay Radioheliograph (NRH) and the ORFEES spectrograph. The extreme ultraviolet images from both Solar TErrestrial RElations Observatory (STEREO-A) and Solar Dynamics Observatory (SDO) were applied to trace the evolution of thermal plasma and coronal magnetic structures. Results. The radio spectrum at decimeter-to-meter wavelengths shows broadband continuum emission (type IV burst), which is a well-known radio signature of time-extended electron acceleration in eruptive flares. Both moving and stationary radio sources were identified. Energetic electrons were observed in X-rays up to 20 keV, displaying a significant correlation with the time evolution of the stationary type IV radio burst during the long duration decay phase, which lasted over 50 minutes. The X-ray photon spectral index is relatively steep with a value of around 7.5 and the integrated electron flux above 30 keV is on the order of 1.6 10³² electron s¹. Conclusions. This case study provides for the first time evidence that HXR emission accompanies the onset of a stationary type IV radio burst. It ties together several pieces of evidence to support that non-thermal electrons are released into large-scale magnetic flux ropes during the post-impulsive phase of eruptive solar flares. The energies of the non-thermal electrons inferred from the X-ray spectral analysis confirm indirect estimates from radio observations. Electron acceleration processes appear as a significant signature of post-impulsive energy release, with energies in the range from several to tens of kiloelectron volts (keV).

Correlation Between the Delay and Rise Time of VLF/LF Amplitudes During 20 Solar X-Ray Flares Observed in February 2014 at Mid-Latitude

During daylight hours, the concentration of electrons in the ionosphere can be amplified by solar flares, which may subsequently influence the propagation of radio waves. Previous research on Very Low Frequency (VLF) signals focused on X-class and M-class flares. This study expands the scope to include a broader frequency range and C-class flares. During 20-28 February 2014, signals from 15 transmitters (18.3-81.0 kHz) were recorded by a receiver in Bath, UK. 20 solar flares captured during this period are investigated. A new methodology was employed to determine the rise times of the received amplitudes for comparison with the solar X-ray flux recorded by the Geostationary Operational Environmental Satellite geostationary satellite. The time delays between the onset of the X-ray flux and the onset of received amplitude changes are calculated. The general trend shows that shorter delays are linearly correlated to longer rise times of the amplitudes. It is found that the absolute slopes of the linear correlation between the delay and the rise time of M-class flares are larger than those of C-class flares. Two flares showed onset times of received amplitudes preceding the X-ray flux onset. A possible explanation for this is that the received signals might also be influenced by hard X-rays rather than the analyzed soft X-rays. In summary, this study demonstrates the effects of small C-class and M-class flares on the propagation of VLF/LF signals and offers insights for further research on solar flare impacts on radio waves and the lower ionosphere.

Tracing the heliospheric magnetic field via anisotropic radio-wave scattering

Astrophysical radio sources are embedded in turbulent magnetised environments. In the 1 MHz sky, solar radio bursts are the brightest sources, produced by electrons travelling along magnetic field lines from the Sun through the heliosphere. We demonstrate that the magnetic field not only guides the emitting electrons, but also directs radio waves via anisotropic scattering from density irregularities in the magnetised plasma. Using multi-vantage-point type III solar radio burst observations and anisotropic radio wave propagation simulations, we show that the interplanetary field structure is encoded in the observed radio emission directivity, and that large-scale turbulent channelling of radio waves is present over large distances, even for relatively weak anisotropy in the embedded density fluctuations. Tracing the radio emission at many frequencies (distances), the effects of anisotropic scattering can be disentangled from the electron motion along the interplanetary magnetic field, and the emission source locations are unveiled. Our analysis suggests that magnetic field structures within turbulent media could be reconstructed using radio observations and is found consistent with the Parker field, offering a novel method for remotely diagnosing the large-scale field structure in the heliosphere and other astrophysical plasmas.

Exploring the Origin of Multi-Periodic Pulsations During a White-Light Flare

We explored the quasi-periodic pulsations (QPPs) at multiple periods during an X4.0 flare on 2024 May 10 (SOL2024-05-10T06:27), which occurred in the complex active region of NOAA 13664. The flare radiation reveals five prominent periods in multiple wavelengths. A 8-min QPP is simultaneously detected in wavelengths of HXR, radio, UV/EUV, Ly$\alpha $, and white light, which may be associated with nonthermal electrons periodically accelerated by intermittent magnetic reconnection that is modulated by the slow wave. A quasi-period at 14 min is observed in the SXR and high-temperature EUV wavebands, and it may be caused by repeatedly heated plasmas in hot flare loops. A quasi-period at about 18 min is only observed by STIX, with reconstructed SXR images suggesting that the 18-min period pulsations should be considered as different flares. Meanwhile, a 3-min QPP is simultaneously detected in wavelengths of HXR, radio, and UV/EUV, which is directly modulated by the slow magnetoacoustic wave leaking from sunspot umbrae. At last, a 2-min QPP is simultaneously detected in HXR and radio emissions during the pre- flare phase, which is possibly generated by a quasi-periodic regime of magnetic reconnection that is triggered by the kink wave.

Characteristics Near Dawn and Dusk

The characteristics of very low frequency (VLF) radio wave propagation in the Earth-ionosphere waveguide are determined particularly through dawn and dusk using phase and amplitude measurements of man-made signals propagating below the ionospheric D region. For the first time variations of "Wait" height and sharpness parameters, H' and , have been determined for dawn and dusk conditions. These measurements provide observational data to constrain D region modeling efforts, extending the capabilities of VLF propagation monitoring for geophysical phenomena such as lightning, solar flares, and energetic particle precipitation. At mid-latitudes, H' varied from 85 km at night, then, starting from solar zenith angle (SZA) 97.5, rapidly down to 73 km at dawn (SZA = 90), then back up to 78 km at SZA 75 and then down to the appropriate noon value for the latitude (and season). In contrast, from noon through dusk to night, H' varied essentially monotonically from 70 to 75 km through 80 to 85 km. At low latitudes no dawn minimum in H' was observed, due to the reduced effect of galactic cosmic rays (GCR). Sharpness, , varied from its nighttime value of 0.6 km¹ down to a minimum of 0.25 km¹ at SZA 85 near dusk or 75 near dawn, rising again to (SZA-dependent) noon values of 0.350.5 km¹. The results are interpreted through the geophysical effects controlling D region electrons, including the daytime dominant role of solar Lyman- from low to mid-latitudes, and the greater role of GCR at increasingly higher mid-latitudes.

The Effects of Solar Flares and Geomagnetic Storm on the Upper and Lower Ionosphere Across the Malay Archipelago Between 8th and 15th May 2024

The Mother's Day Storm is the strongest solar storm event in Solar Cycle 25 to date and has been the strongest solar storm since the Halloween storm of 2003. This event provides a great opportunity to investigate the effect of the solar storm on the upper and lower ionospheres. In this study, we investigated the response of the ionosphere to solar flares and geomagnetic storms between 8th and 15th May 2024, using very low frequency radio waves (VLF) collected by our newly built UTM-SID VLF receiver and analyzing the Total Electron Contents (TEC), detrended TEC (dTEC), and Rate Of TEC change Index (ROTI) derived from Global Positioning Satellite System (GNSS) signals across the Malay Archipelago Region. UTM-SID successfully detected 38 out of 114 solar flares that occurred during this period, and the detection depended on the intensity and time of the solar flare. 7 dTEC and 5 ROTI responses were noted, along with a rapid enhancement of vertical TEC (vTEC) of up to 0.5 TECU during severe solar flares. Meanwhile, the geomagnetic storm that occurred on 1112 May had induced high dTEC and ROTI variations, with dTEC reaching 5 TECU and ROTI of 0.5 TECU/min, suggesting the occurrence of a Travelling Ionospheric Disturbance (TID). Additionally, Equatorial Plasma Bubbles (EPB) were found to be suppressed on 11 May and a pre- sunrise EPB was noted on 12 May. Background VLF signals are also enhanced during this period.

Simultaneous multi-spacecraft observations with VLBI radio telescopes to study the interplanetary phase scintillation

Ground-based observations of spacecraft signals have been used to study space weather. However, single spacecraft measurements observed from the Earth have limitations in studying the structure and evolution of solar plasma as they are unable to differentiate spatial and temporal variations. To overcome this limitation and improve our understanding of interplanetary scintillation, we simultaneously observed radio signals transmitted by two co-orbiting spacecraft: the ESA Mars Express (MEX) and the Chinese National Space Administration Tianwen-1 (TIW-1). We conducted the observations from April to November 2021 using the University of Tasmania's VLBI radio telescopes at 8.4 GHz. We employed the Planetary Radio Interferometer and Doppler Experiment (PRIDE) technique to determine the topocentric Doppler measurements and residual phase of the carrier signal. These observables were used to quantify the phase fluctuations of the spacecraft signals caused by solar wind and hydrodynamic turbulence in the interplanetary medium. The measured phase fluctuations RMS from both spacecraft show small differences which are caused by factors such as the spacecraft's motion, onboard electronics, and variations in the uplink signal path through Earth's ionosphere. These fluctuations decrease with solar elongation and correlate with solar radio flux at 10.7 cm (2800 MHz), indicating solar activity. The estimated total electron contents along MEX and TIW-1's radio lines of sight are similar, with higher values at lower solar elongations. Simultaneous multi-spacecraft observations also enable RFI characterization, frequent spacecraft performance comparisons, and investigation of solar activity effects on spacecraft performance and scientific outcomes.

X-class Flare on 2023 December 31 Observed by the Solar Ultraviolet Imaging Telescope on Board Aditya-L1

We present the multiwavelength study of the ejection of a plasma blob from the limb flare SOL2023-12-31T21:36:00 from NOAA 13536 observed by the Solar Ultraviolet Imaging Telescope (SUIT) on board Aditya-L1. We use SUIT observations along with those from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory and Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter to infer the kinematics and thermal nature of the ejected blob and its connection to the associated flare. The observations show that the flare was comprised of two eruptions. The blob was ejected during the first eruption and later accelerated to velocities over 1500 km s¹ measured at a maximum projected height of 178 Mm from the Sun's surface. The acceleration of the ejected plasma blob is cotemporal with the bursty appearance of the hard X-ray light curve recorded by STIX. Radio spectrogram observations from STEREO-A/WAVES and RSTN reveal type III bursts at the same time, indicative of magnetic reconnection. DEM analysis using AIA observations suggests the plasma blob is comprised of cooler and denser plasma in comparison to the ambient corona. To the best of our knowledge, this is the first observation of such a plasma blob in the near-ultraviolet providing crucial measurements for eruption thermodynamics.

Two Phases of Particle Acceleration of a Solar Flare Associated with In Situ Energetic Particles

How impulsive solar energetic particle (SEP) events are produced by magnetic-reconnection-driven processes during solar flares remains an outstanding question. Here we report a short-duration SEP event associated with an X-class eruptive flare on 2021 July 3, using a combination of remote sensing observations and in situ measurements. The in situ SEPs were recorded by multiple spacecraft including the Parker Solar Probe. The hard X-ray (HXR) light curve exhibits two impulsive periods. The first period is characterized by a single peak with a rapid rise and decay, while the second period features a more gradual HXR light curve with a harder spectrum. Such observation is consistent with in situ measurements: the energetic electrons were first released during the early impulsive phase when the eruption was initiated. The more energetic in situ electrons were released several minutes later during the second period of the impulsive phase when the eruption was well underway. This second period of energetic electron acceleration also coincides with the release of in situ energetic protons and the onset of an interplanetary type III radio burst. We conclude that these multimessenger observations favor a two-phase particle acceleration scenario: the first, less energetic electron population was produced during the initial reconnection that triggers the flare eruption, and the second, more energetic electron population was accelerated in the region above the loop-top below a well-developed, large-scale reconnection current sheet induced by the eruption.

A robust preprocessing pipeline for RATAN-600 solar radio observations data

The advancement of observational technologies and software for processing and visualizing spectro-polarimetric microwave data obtained with the RATAN-600 radio telescope opens new opportunities for studying the physical characteristics of solar plasma at the levels of the chromosphere and corona. These levels remain some difficult to detect in the ultraviolet and X-ray ranges. The development of such methods allows for more precise investigation of the fine structure and dynamics of the solar atmosphere, thereby deepening our understanding of the processes occurring in these layers. The obtained data also can be utilized for diagnosing solar plasma and forecasting solar activity. However, using RATAN-600 data requires extensive data processing and familiarity with the RATAN-600. This paper introduces RatanSunPy, an open-source Python package developed for accessing, visualizing, and analyzing multi-band radio observations of the Sun from the RATAN-600 solar complex. The package offers comprehensive data processing functionalities, including direct access to raw data, essential processing steps such as calibration and quiet Sun normalization, and tools for analyzing solar activity. This includes automatic detection of local sources, identifying them with NOAA (National Oceanic and Atmospheric Administration) active regions, and further determining parameters for local sources and active regions. By streamlining data processing workflows, RatanSunPy enables researchers to investigate the fine structure and dynamics of the solar atmosphere more efficiently, contributing to advancements in solar physics and space weather forecasting.

Dynamic spectra of solar radio emissions from weak-turbulence simulation

Context. In recent decades, serious efforts have been made in the analytical and numerical modeling of solar radio bursts generated by the electron beam interacting with the background plasma, including the dynamic spectra with decreasing frequency over time/space. These are type II and type III radio bursts, with the fundamental components at the local plasma frequency (_p = 2f_p) and the harmonics (n_p = 2nf_p). Synthetic spectra built for a number of radio events were able to reproduce the decreasing frequency profiles reasonably well, despite the limitations of the approximate analytical theory. Aims. We propose new modeling of dynamic radio emission spectra using weak-turbulence (WT) theory. This novel approach also aims at a self-consistent and quantitative evaluation of radio emissions, based on first-principles modeling of electron beam plasma instabilities and nonlinear wave interaction. Methods. We performed the WT simulation, which has the ability to quantitatively describe the standard plasma emission involving the nonlinear interaction of Langmuir (L), ion-sound (S), and transverse electromagnetic (T) waves. The composite dynamic spectra are constructed for type II- and type III-like events, against the background electron density model that behaves as an inverse square of the distance from the solar source. Results. The new dynamic spectra are obtained distinctly, with a rapid frequency shift for type III emissions (generated by fast electron beams from coronal sources), as well as a less steep frequency drop for type II spectra (whose sources move away from the Sun along with interplanetary shocks). Upon making a qualitative comparison with typical solar radio emission events, we find that our first-principle- based synthetic dynamic spectra are in good agreement. Conclusions. The findings of the present study demonstrate that the theoretical approach taken in this paper can be further applied to obtain (i) quantitatively relevant predictions and replications of the observed dynamic spectra of radio bursts, and (ii) more realistic large-scale models of the solar radio source, for example the type II and type III source models computed from the large-scale magnetohydrodynamics (MHD) simulations or even from direct spacecraft observations.

Electron beam propagation and radio-wave scattering in the inner heliosphere using five spacecraft

Context. Solar energetic particles such as electrons can be accelerated to mildly relativistic velocities in the solar corona. These electrons travel through the turbulent corona, generating radio waves, which are then severely affected by scattering. Aims. The physical interpretation of the discrepancies between the actual and observed radio sources is still subject to debate. We used radio emission observed by an unprecedented total of five spacecraft to track the path of radio sources from the low corona to the inner heliosphere (1575 R or 0.070.35 au generated during a solar event on 4 December 2021. Methods. We used the Bayesian multilateration technique known as BELLA to track the apparent path of radio sources observed by Parker Solar Probe, STEREO A, Wind, Solar Orbiter, and Mars Express. To validate the accuracy of the tracked path, we used Nanay Radioheliograph interferometric imaging at 150 MHz, which was found to agree with the estimated footpoints predicted by BELLA. We further validated our results using ACE in situ measurements. Results. We find that the apparent radio sources followed the path of an Archimedean Parker spiral, with an associated solar wind velocity of approximately 493 km s¹ (consistent with the corresponding speed observed at 1 au at the relevant longitude), and connected to the solar surface at 75 longitude east. Finally, we made quantitative estimates of the scattering of radio waves, which we found to be in good agreement with contemporary models of scattering in which the radio waves primarily propagate along the local Parker spiral. Conclusions. This work shows conclusive evidence that the cause of the widely observed 'higher-than- expected' electron densities at interplanetary distances is due to radio-wave scattering, and provides a more detailed understanding of the propagation of radio waves emitted near the local plasma frequency in turbulent astrophysical plasmas.

First Global Machine Learning Model to Predict the Rate of TEC Index (ROTI) Response to X-Class Solar Flares

Solar flares are bursts of electromagnetic radiation originating in the Sun's atmosphere. Solar flares cause a rapid increase in ionization in the ionosphere, resulting in radio signal interference. This paper aims to predict the ionospheric response to the solar flare of various characteristics in all latitudes around the dayside ionosphere. X-ray flux measured by the Geostationary Operational Environmental Satellite (GOES) satellite associated with 84 solar flare events between 2000 and 2017 are obtained. Global total electron content (TEC) data from more than 5,000 ground Global Navigation Satellite System receivers are used. The rate of the TEC Index (Rate of TEC Index (ROTI)) is calculated to examine the time evolution of ionospheric response. Three selected events are studied in detail by eliminating the ROTI associated with stations on the nightside. A nonlinear response of the ionosphere associated with solar flare characteristics including rise/fall time and maximum amplitude is discussed. The first global machine learning (ML) model to predict solar flare impact on Earth's ionosphere through ROTI parameter is developed. Solar flare parameters measured by the GOES satellite along with solar radiation angle and ROTI data from 5-degree latitude ranges are selected as an input to the ML model. Thek-nearest neighbors and random forest algorithms are used. Quantitative and qualitative results show that the random forest provides better accuracy in predicting the time evolution of ionospheric response to X-class solar flare. The coefficient of determination (${\mathrm{R}}^{2}$) and the Pearson Correlation Coefficient (r) are used to provide a quantitative comparison of the model prediction with the actual data.

II. Features of the Flare and Its Atypical Microwave Emission

As known, large near-Earth proton enhancements usually occur after major eruptive solar flares accompanied by strong microwave bursts. Typically, the spectral-maximum frequency of such a burst exceeds 10 GHz, and the flux exceeds 104 sfu. Ground-level cosmic-ray enhancements (GLEs) are the most energetic subset of large proton events, and it seems that microwave bursts in GLE-associated flares should follow this pattern. This is true in most cases, but in individual events that have produced GLEs, only moderate microwave bursts have been observed. In particular, in the SOL2012-05-17 event responsible for GLE71, the spectral-maximum frequency of the microwave burst did not exceed 10 GHz, and the flux did not reach 103 sfu. We found that the temporal profile of the microwave burst followed the smoothed magnetic-reconnection rate, lagging behind it by about 50 s and that the burst properties were determined by the following circumstances: i) the magnetic configuration was asymmetric, and ii) the sources of the gyrosynchrotron emission were the entire flare arcade and a compact region above the sunspot umbra. Observations directly demonstrated these features, which were previously inferred for the SOL2001-12-26 event responsible for GLE63. A long-known discrepancy was observed between the estimates of the electron spectrum obtained from hard X-rays and microwaves. However, the hardening of the spectrum of trapped electrons that has been invoked to explain this discrepancy was not found in this event. Indications of a relationship between flare processes and proton acceleration are discussed.

Two-Part Interplanetary Type II Solar Radio Bursts

Two similar-looking, two-part interplanetary type II burst events from 2003 and 2012 are reported and analysed. The 2012 event was observed from three different viewing angles, enabling comparisons between the spacecraft data. In these two events, a diffuse wide-band type II radio burst was followed by a type II burst, which showed emission at the fundamental and harmonic (F-H) plasma frequencies, and these emission bands were also slightly curved in their frequency-time evolution. Both events were associated with high-speed, halo-type coronal mass ejections (CMEs). In both events, the diffuse type II burst was most probably created by a bow shock at the leading front of the CME. However, for the later appearing F-H type II burst, there are at least two possible explanations. In the 2003 event, there is evidence of CME interaction with a streamer, with a possible shift from a bow shock to a CME flank shock. In the 2012 event, a separate white-light shock front was observed at lower heights, and it could have acted as the driver of the F-H type II burst. There is also some speculation on the existence of two separate CMEs, launched from the same active region, close in time. The reason for the diffuse type II burst being visible only from one viewing direction (STEREO-A) and the ending of the diffuse emission before the F-H type II burst appears still need explanations.

Features of correlation curves of the Siberian Radioheliograph

Correlation curves of the multi-frequency Siberian Radioheliograph (SRH) provide a sensitive indication and demonstrative representation of monitoring the microwave life of the active Sun. We derive approximate relationships and briefly discuss the contribution of the quiet Sun, active regions, radio bursts, satellites, and atmospheric absorption to the radioheliograph's correlation response. The estimates are obtained under the assumption that the activity centers and the quiet Sun are homogeneous disks of different sizes and brightness. The sensitivity of the correlation curves to weak sources of small angular sizes is due to their wide spatial spectrum. The wide spectrum means that each pair of antennas produces a noticeable interferometric response, so the total response is significant. The correlation curves allow us to estimate spatial sizes of the radio burst source at different frequencies, but do not allow us to calculate the shape of its radio spectrum. Variability in the atmospheric water content over time creates fluctuations of the received solar radio flux. The correlation response is much less susceptible to this factor.

Fully convolutional neural networks for processing observational data from small remote solar telescopes

Heliophysics phenomena on the Sun, such as radio bursts, can strongly affect satellites and ground-based electronic systems. Therefore, an insight into the actual image of the Sun with good spatial and temporal resolution is crucial. In this paper, we explore the possibility of using fully convolutional networks (FCNs) to improve the images acquired from remotely operated small solar telescopes whose resolution is limited by the size of the lens aperture and by atmospheric turbulence. For this purpose, we use chromosphere data from the 50 mm small H Telescope of the Silesian University of Technology acquired over many months under various atmospheric conditions. We compare the obtained results with the results of raw data processing by a state-of-the-art deterministic algorithm, multi-frame blind deconvolution (MFBD). In our research, we investigate the impact of the amount of data and the complexity of FCNs on the quality of the results and their processing time. We show that the use of FCNs is a very attractive alternative to MFBD because they are more energy efficient and allow for the obtaining of comparable results in orders of magnitude shorter time.

Forecasting solar energetic particles using multi-source data from solar flares, CMEs, and radio bursts with machine learning approaches

This study presents a consistent method to the inherently imbalanced problem of predicting solar energetic particle (SEP) events, using a variety of datasets that include solar flares, coronal mass ejections (CMEs), and radio bursts. We applied several machine learning (ML) methods, including Random Forests (RF), Decision Trees (dtree), and Support Vector Machines (SVM) with both linear (linSVM) and nonlinear (svm) kernels. To assess model performance, we used standard metrics such as Probability of Detection (POD), False Alarm Rate (FAR), True Skill Statistic (TSS), and Heidke Skill Score (HSS). Our results show that the RF model consistently outperforms the other algorithms across datasets containing flares, CMEs, and radio bursts. For the sweep frequency dataset, RF achieved a POD of , a FAR of , a TSS of ,and a HSS of ). For the fixed-frequency dataset, RF produced a POD of , a FAR of , a TSS of ,and a HSS of ). Key features for SEP prediction include CME linear speed and angular width across both datasets. For sweep frequency, flare intensity and integral soft X-ray (SXR) flux are crucial, while for fixed frequency, the rise time and duration of radio bursts at 1415 MHz are significant.

Effects on the attenuation and phase velocities of the waveguide modes

Solar flares are sudden bursts of X-rays and UV rays emitted from coronal magnetic loops in active regions near sunspots on the sun's surface. Soft X-rays below 1 nm penetrate the D-region ionosphere, causing excess ionization and altering its conductivity profile. This study examines solar flares (C-class and M-class) from the 25th solar cycle using two Very Low Frequency (VLF, 330 kHz) radio receivers in the low-latitude Indian region, located in Cooch Behar (CHB) and Kolkata (CUB). The work focuses on the Indian VLF transmitter VTX at a frequency of 18.2 kHz to study the effects of solar flares on the D-region ionosphere and VLF signal propagation characteristics in the earth- ionosphere waveguide. The solar zenith angle at the CUB station has a significant impact on the magnitude of VLF amplitude disruption caused by solar flares, showing a positive correlation (r = +0.82, r = +0.61) with flare power during low and high solar activity, respectively. In contrast, CHB exhibits both positive and negative amplitude perturbations, with a negative correlation (r = 0.83, r = 0.78) between flare power and VLF amplitude under similar conditions. The Long Wave Propagation Capability (LWPC) code has been used to explain the differences in the observed amplitude perturbations due to solar flares in both receivers. Solar flares weaker than C2.0 at CHB reduce attenuation and phase velocity of the propagating waveguide modes in the earth-ionosphere waveguide, causing positive amplitude perturbations, while stronger flares increase these parameters, resulting in negative perturbations. In contrast, solar flares of all classes cause an increase in phase velocities and a decrease in attenuation coefficients of the propagating waveguide modes along the VTX-CUB propagation path, resulting in positive VLF amplitude responses. This study highlights distinct responses of VLF signals to solar flares in different propagation paths, emphasizing the complex interactions between solar activity and earth-ionosphere waveguide properties.

Trends of Solar, Interplanetary, and Geomagnetic Parameters during the Recent Five Solar Cycles

This study examined the variations of solar, interplanetary, and geomagnetic (SIG) parameters from 1974 to 2024 to assess the changes in the solar cycle. Eleven SIG parameters were analyzed, including the sunspot number (SSN), solar magnetic field, 10.7 cm solar radio flux, total solar irradiance, and Ap index. This study also aimed to predict Solar Cycle 25 using the seasonal autoregressive integrated moving average (SARIMA) statistical forecasting model. The results showed that consistent with previous studies, all SIG parameters exhibited a strong correlation with the SSN. The change in SSN strongly influences the variations in all SIG parameters, even though some exhibit time-lagged responses. The cross-correlation analysis revealed a high correlation coefficient of 0.9678 between the SSN and the 10.7 cm solar radio flux without delay. Most SIG parameters showed a general weakening trend toward Solar Cycles 2224. This suggests that solar activity is waning over time. In particular, the solar polar magnetic field (SPMF) showed a large decrease in the solar minimum 23/24, and specifically, the SPMF at the south pole weakened more rapidly than at the north pole. Hence, the SPMF is changing asymmetrically between the north and south poles. This weakening of the solar magnetic field suggests an increase in galactic cosmic rays within the heliosphere, exposing the Earth to higher levels of cosmic rays. Finally, forecasts for Solar Cycle 25 using the SARIMA model predict that the SSN will continue to decline after the solar maximum in 2024, with the predicted minimum SSN of 9.42 in October 2028, and will likely enter a solar minimum period around 2030.

The first analysis of the outward H fluxes measured by IBEX-Lo in 2050 R_E geocentric distances

In this study, we analyze the energetic neutral atom (ENA) observations measured in the lowest energy channel (1021 eV) of the IBEX-Lo instrument on Interstellar Boundary Explorer (IBEX) during two spring seasons, day of year (DOY) 101146, 2009, and DOY 88178, 2013, confirming the existence of outward hydrogen (H) fluxes at 15 eV. The outward H flux decreases slightly with distance, showing an intensity of approximately 10⁶ cm² s¹ sr¹ keV¹. Results also suggest that the outward H fluxes are not influenced by solar radio flux. We compute the expected H ENA fluxes at 15 eV using ion flux measurements from the Helium, Oxygen, Proton, and Electron (HOPE) mass spectrometer aboard the Radiation Belt Storm Probes (RBSP) during the corresponding period of the 2013 spring season, combined with a simple exospheric density model (n H = n H 0 r 0 / r 3, where r 0 = 10 R_E). The expected ENA fluxes similarly show a decrease in the intensity with increasing geocentric distance, which is on the order of 10⁵10⁶ cm^-2 s¹ sr¹ keV¹. These consistent features suggest that the outward H fluxes observed by IBEX-Lo are closely related to escaping H ENAs produced within the inner exosphere (<4 R_E).

Decay of Turbulent Upper-hybrid Waves in Weakly Magnetized Solar Wind Plasmas

Large-scale and long-term two-dimensional particle-in-cell simulations of high resolution are performed for the first time to study the dynamics of electrostatic decay of upper-hybrid wave turbulence generated by electron beams into Langmuir/-mode () waves in weakly to moderately magnetized plasmas, in conditions relevant to type III solar radio bursts. Simulations use parameters characteristic of beamplasma interactions between 0.1 and 1 au. The impact of plasma magnetic field on decay is shown, and magnetic properties of waves are determined. During their energy transport through k wavevector scales, waves undergo several decay cascades, acquiring increasing magnetic energy until they reach electromagnetic -mode dispersion below the plasma frequency. Whereas the impact of magnetic field on decaying waves of large k = k is weak, important differences with respect to the unmagnetized plasma case manifest at small k-scales, where a boundary layer delimiting a spectral domain free of energy is revealed. It prevents decayed waves from reaching the -mode cutoff frequency and a high level of left-handed polarization, and it modifies the conditions for the appearance of modulational instabilities and strong turbulence phenomena at k 0. Ordinary -mode waves are generated jointly with -mode waves at comparable energy levels, via electromagnetic decay, whereas -mode emissions are much weaker in most cases. These results provide support for the interpretation of observations by satellites such as Parker Solar Probe and Solar Orbiter, and they supply a solid basis for tackling the more complex problem of dynamics of upper-hybrid wave turbulence in magnetized plasmas where random density fluctuations cannot be neglected.

Multi-category solar radio burst detection based on task-aligned one-stage object detection model

Accurate identification of solar radio bursts (SRBs) is essential for advancing research in solar physics and predicting space weather. However, the majority of current studies mainly concentrate on detecting whether SRBs are present or absent, often focusing on only one particular type of burst. Moreover, the neural network models used for SRB detection are typically complex, involving a large number of parameters, which results in slower processing speeds. This study establishes a dataset encompassing Type II, Type III, Type IIIs, Type IV, and Type V SRBs collected from e-CALLISTO, including 8,752 SRB spectrum images and achieving annotations for 10,822 SRBs. We propose a multi-category SRB detection model based on task-aligned one-stage object detection (TOOD). TOOD can solve the problem of inconsistent predictions in classification and localization tasks, and it improves the detection recall rate. This model aligns classification and localization tasks and optimizes the neck network by incorporating a channel attention mechanism. This model achieves higher recall and accuracy with fewer parameters. This model can accurately detect five types of SBRs. The experimental results show that the model achieved an accuracy of 79.9% (AP50) and a recall rate of 95.1% on the SBRs dataset. A higher recall rate than other models means fewer SRBs are missed in automatic detection. The model we propose has the potential to make a substantial impact on solar physics research and space weather studies. Additionally, the findings in this paper could provide valuable insights for processing other small-sample astronomical, datasets. The source code and data is available at https://github.com/onewangqianqian/MobileNetVitv2-TOOD.git.

Particle Acceleration in Solar Flares From Radio and Hard X-Ray Spectra

For a deeper understanding of the physical processes at play in solar flares, it is necessary to analyze the flare emissions at multiple wavelengths. This multifrequency approach enables the characterization of energetic electrons accelerated from hundreds of keV and up to several tens of MeV. This study reports on the observation of 10 solar flares, in which the spectral parameters were determined for the cm/mm and x-ray bands. The radio spectrum was fitted using gyrosynchrotron emission whereas the hard x-rays fit considered a model of thermal plus nonthermal emission of accelerated electrons. The results show that the spectral indices of the energy distribution of nonthermal electrons emitting in millimeter and hard x-rays do not agree, with the millimeter spectral index being approximately 2 units harder than that of hard x-rays. These findings are consistent with previous research and suggest the existence of a break in the energy spectrum of accelerated electrons. Moreover, for the only flare where photons exceeding 1 MeV were detected, the hard x-ray spectra exhibited a broken power-law where the index of the electron distribution above ~500 keV agreed with the inferred radio spectral index.

Effects of the September 2014 coronal mass ejection chain in the inner Solar System and the response of the Martian ionosphere

Context. During September 2014, intense solar activity led to a number of coronal mass ejections (CMEs) propagating in the heliosphere. The strong perturbation in the interplanetary magnetic field and the remarkable enhancements in the energetic particle fluxes accelerated by the shock waves associated with the CMEs affected the environments of the inner planets of the Solar System. Aims. Taking advantage of a relatively favorable position in terms of angular distance among Mercury, Earth, and Mars, our purpose is to observe the evolution and impact of strong solar events, providing an overview of the impact of the same solar phenomena on different planetary environments, with special interest in the response of Mars' ionosphere as this may have implications for future exploration of the red planet. Methods. We used observations from a fleet of spacecraft distributed in the inner Solar System, such as STEREO B, MESSENGER, Mars Express, and SOHO, to perform a characterization of the interaction with the planets, investigating some of the main effects of the CMEs on the different planetary environments. Besides, we applied a numerical simulation to reconstruct the magnetic connection from Mercury, Earth, and Mars to the solar corona on the dates on which the CME events occurred. Results. We find that the CMEs events analyzed here induced remarkable effects that affected all the environments of the inner planets of the Solar System. Enhancements in the solar energetic particle fluxes were observed at Mercury, Earth, and Mars, with different characteristics. In addition, a solar radio burst was observed both at Earth and Mars, together with strong disturbances in the geomagnetic field, and diffuse echoes and radio black outs in the Martian ionosphere. Conclusions. The proposed multi-spacecraft and multiparameter analysis, along with the numerical simulations for reconstructing the magnetic footpoints of the Parker spiral on the Sun's surface, offer a detailed cause-and-effect framework for studying space weather events in the Solar System.

Bursty acceleration and 3D trajectories of electrons in a solar flare

Context. During a solar flare, electrons are accelerated to non-thermal energies as a result of magnetic reconnection. These electrons then propagate upwards and downwards from the energy release site along magnetic field lines and produce radio and X-ray emission. Aims. On 11 November 2022, an M5.1 solar flare was observed by the Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter together with various ground- and space-based radio instruments. The flare was associated with several fine hard X-ray (HXR) structures and a complex set of metric radio bursts (type III, J, and narrowband). By studying the evolution of X-ray, extreme ultraviolet, and radio sources, we aim to study the trajectories of the flare-accelerated electrons in the lower solar atmosphere and low corona. Methods. We used observations from the STIX on board Solar Orbiter to study the evolution of X-ray sources. Using radio imaging from the Nanay Radio heliograph (NRH) and the Newkirk density model, we constructed 3D trajectories of 14 radio bursts. Results. Imaging of the HXR fine structures shows several sources at different times. The STIX and NRH imaging shows correlated changes in the location of the HXR and radio source at the highest frequency during the most intense impulsive period. Imaging and 3D trajectories of all the bursts show that electrons are getting accelerated at different locations and along several distinct field lines. Some of the trajectories from the same origin show expansion on the order of 4 over a height of 110 Mm. The longitude and latitude extent of the trajectories are 30 and 152. Conclusions. We find that the electrons producing HXR and radio emission have similar acceleration origins. Importantly, our study supports the scenario that the flare acceleration process is temporally and spatially fragmentary, and during each of these small-scale processes, the electron beams are injected into a very fibrous environment and produce complex HXR and radio emission.

The reason for the widespread energetic storm particle event of 13 March 2023

Context. On 13 March 2023, when the Parker Solar Probe spacecraft (S/C) was situated on the far side of the Sun as seen from Earth, a large solar eruption took place, which created a strong solar energetic particle (SEP) event observed by multiple S/C all around the Sun. The energetic event was observed at six well-separated locations in the heliosphere, provided by the Parker Solar Probe, Solar Orbiter, BepiColombo, STEREO A, near-Earth S/C, and MAVEN at Mars. Clear signatures of an in situ shock crossing and a related energetic storm particle (ESP) event were observed at all inner-heliospheric S/C, suggesting that the interplanetary coronal mass ejection (CME)-driven shock extended all around the Sun. However, the solar event was accompanied by a series of pre-event CMEs. Aims. We aim to characterize this extreme widespread SEP event and to provide an explanation for the unusual observation of a circumsolar interplanetary shock and a corresponding circumsolar ESP event. Methods. We analyzed data from seven space missions, namely Parker Solar Probe, Solar Orbiter, BepiColombo, STEREO A, SOHO, Wind, and MAVEN, to characterize the solar eruption at the Sun, the energetic particle event, and the interplanetary context at each observer location as well as the magnetic connectivity of each observer to the Sun. We then employed magnetohydrodynamic simulations of the solar wind in which we injected various CMEs that were launched before as well as contemporaneously with the solar eruption under study. In particular, we tested two different scenarios that could have produced the observed global ESP event: (1) a single circumsolar blast-wave-like shock launched by the associated solar eruption, and (2) the combination of multiple CMEs driving shocks into different directions. Results. By comparing the simulations of the two scenarios with observations, we find that both settings are able to explain the observations. However, the blast-wave scenario performs slightly better in terms of the predicted shock arrival times at the various observers. Conclusions. Our work demonstrates that a circumsolar ESP event, driven by a single solar eruption into the inner heliosphere, is a realistic scenario.

Resolving spatial and temporal shock structures using LOFAR observations of type II radio bursts

Context. Collisionless shocks are one of the most powerful particle accelerators in the Universe. In the heliosphere, type II solar radio bursts are signatures of electrons accelerated by collisionless shocks launched at the Sun. Spectral observations of these bursts show a variety of fine structures often composing multiple type II lanes. The origin of these lanes and structures is not well understood and has been attributed to the inhomogeneous environment around the propagating shock. Aims. Here, we aim to determine the large-scale local structures near a coronal shock wave using high-resolution radio imaging observations of a complex type II radio burst observed on 3 October 2023. Methods. By using inteferometric imaging from the Low Frequency Array (LOFAR), combined with extreme ultraviolet observations, we investigate the origin of multiple type II lanes at low frequencies (3080 MHz) relative to the propagating shock wave. Results. We identify at least three radio sources at metric wavelengths corresponding to a multi-lane type II burst. The type II burst sources propagate outwards with a shock driven by a coronal mass ejection. We find a double radio source that exhibits increasing separation over time, consistent with the expansion rate of the global coronal shock. This suggests that the overall shock expansion is nearly self-similar, with acceleration hotspots forming at various times and splitting at a rate proportional to the shock's expansion. Conclusions. Our results show the importance of increased spatial resolution in determining either the small-scale spatial properties in coronal shocks or the structuring of the ambient medium. Possible shock corrugations or structuring of the upstream plasma at the scale of 10⁵ km can act as hotspots for the acceleration of suprathermal electrons. This can be observed as radiation that exhibits double sources with increasing separation at the same expansion rate as the global shock wave.

Radio dimming associated with filament eruptions in the meter and decimeter wavebands

Filament eruptions are considered to be a common phenomenon on the Sun and other stars, yet they are rarely directly imaged in the meter and decimeter wavebands. Using imaging data from the DAocheng solar Radio Telescope (DART) in the 150450 MHz frequency range, we present two eruptive filaments that manifest as radio dimmings (i.e., emission depressions). Simultaneously, portions of these eruptive filaments are discernible as dark features in the chromospheric images. The sun-as-a- star flux curves of brightness temperature, derived from the DART images, exhibit obvious radio dimmings. The dimming depths range from 1.5% to 8% of the background level and show a negative correlation with radio frequencies and a positive correlation with filament areas. Our investigation suggests that radio dimming is caused by free-free absorption during filament eruptions obscuring the solar corona. This may provide a new method for detecting stellar filament eruptions.

On a Possible Scenario of Solar Coherent Bursts

The first burst of solar microwave coherent emission observed simultaneously with two multifrequency two-dimensional radio telescopes is reported, making it possible to unambiguously interpret the mechanism of the radiation and to propose a scenario that explains all the observed features of the burst. Recently, many studies have appeared that explain coherent bursts of radio emission from the Earth's magnetosphere and solar corona by an electron cyclotron maser (ECM) driven by horseshoe distribution. The result of this study is that the observed coherent burst near the frequency 4.8 GHz is caused by a hollow beam distribution formed by the oblique injection of electrons into a magnetic loop. If the pitch-angle is large enough, then the absence of HXR and the relatively large (about 1 s) pulse duration can be explained. The measured size of the ECM source, 2.2, corresponds to a brightness temperature of 5.81010 K. The displacement of the spike sources with respect to the gyroresonance source is consistent to the second-harmonic ECM emission, whereas the gyroresonance source is consistent to the third gyrolayer.

Performance of the segment anything model in various RFI/events detection in radio astronomy

The emerging era of big data in radio astronomy demands more efficient and higher-quality processing of observational data. While deep learning methods have been applied to tasks such as automatic radio frequency interference (RFI) detection, these methods often face limitations, including dependence on training data and poor generalisation, which are also common issues in other deep learning applications within astronomy. In this study, we investigate the use of the open-source image recognition and segmentation model, Segment Anything Model (SAM), and its optimised version, HQ-SAM, due to their impressive generalisation capabilities. We evaluate these models across various tasks, including RFI detection and solar radio burst (SRB) identification. For RFI detection, HQ-SAM (SAM) shows performance that is comparable to or even superior to the SumThreshold method, especially with large-area broadband RFI data. In the search for SRBs, HQ-SAM demonstrates strong recognition abilities for Type II and Type III bursts. Overall, with its impressive generalisation capability, SAM (HQ-SAM) can be a promising candidate for further optimisation and application in RFI and event detection tasks in radio astronomy.

Enhancing Deep Learning Ionospheric Modeling With Solar Radiation and Flare Classes

The ionosphere is pivotal for satellite navigation, radio communication, and the modeling of space weather. However, the accurate three- dimensional modeling of ionospheric features remains a challenge. Since solar activity introduces changes in space weather, we collected COSMIC radio occultation observations of 20102020 with a suite of indices related to solar and geomagnetic activities, especially including solar EUV and X-ray radiation fluxes, to develop a deep learning model for the global ionospheric electron density. This model, which is called the Solar Flare and Radiation Neural Network (SFRNN) and is based on Embedding, Long Short-Term Memory and fully connected layers, presented excellent performance in reconstructing ionospheric profiles. In this study, 28-min was found to be the best input solar radiation interval for SFRNN with annual RMSEs of 6.24 10⁴ to 1.56 10⁵ el/cm³. Significantly, during solar flare events, SFRNN had a lower reconstruction error than the former artificial neural network (ANN) model that only uses space weather indices. The most substantial improvement was observed under X-class flares, where SFRNN exhibited a 18.3% lower Root Mean Squared Error than ANN. To further validate the modeling accuracy, electron density profiles derived from Jicamarca incoherent scatter radar (ISR) were used. SFRNN successfully provided profiles with high consistency with the ISR observation in the ionospheric layers. Our modeling results demonstrate that refined solar activity parameters can effectively improve reconstruction performance.

On Thermospheric Molecular Oxygen and Its Relationship to Solar Activity

Observations from the Global-scale Observations of Limb and Disk (GOLD) mission have provided a new remote-sensing data source of molecular oxygen profiles in Earth's lower-to-middle thermosphere (120200 km). GOLD O₂ observations indicate increasing densities of molecular oxygen at 170 km with rising solar activity between solar radio flux F10.7 values of 60 and 120 solar flux units (sfu). This is also seen in comparisons with solar extreme ultraviolet irradiance Q_EUV between 1 and 2.25 erg cm² s¹. However, the empirical Mass Spectrometer Incoherent Scatter radar 2.0 (MSIS 2.0) model overestimates O₂ densities at 170 km at these low levels of solar activity and predicts a decreasing density with increasing solar flux below 120 sfu. Additional data sets validate GOLD observations of O₂ and their relationship with solar activity. Accurately determining and forecasting O₂ is critical for accurately modeling plasma densities in the ionosphere and thermospheric density in the lower-to-middle thermosphere.

data and modeling

Solar flares, which are powerful explosions on the Sun's surface, are well recognized driving forces that have a significant impact on the near-Earth environment, causing extra ionization within the sunlit Earth's atmospheric layers. Based on how they affect the lower ionosphere and its electron density profile, X-ray solar flares can be categorized. In order to forecast the effects of potential solar occurrences during the waning phase of Solar Cycle 25, this study focuses on the disturbances caused by X-ray solar flares. In this paper we examined Solar Cycle progression i.e. solar activity of highest intensity (strongest 50 solar flares) during the ascending phase of Solar Cycle 25 by conducting numerical ionospheric modeling based on the Geostationary Operational Environmental Satellite (GOES) database on solar X-ray radiation.

Radiosolariz Solar Radio Telescope Short Monopole Antenna

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Universal Constants in Self-organized Criticality Systems

The occurrence frequency distributions of fluxes (F) and fluences or energies (E) observed in the majority (in 18 out of 23 cases) of astrophysical phenomena are found to be consistent with the predictions of the fractal-diffusive self-organized criticality (FD-SOC) model, which predicts power-law slopes with universal constants of _F = (9/5) = 1.80 for the flux and _E = (5/3) 1.67 for the fluence. The theoretical FD-SOC model is based on the fractal dimension, the flux-volume proportionality, and classical diffusion. The universal scaling laws predict the size distributions of numerous astrophysical phenomena, such as solar flares, stellar flares, coronal mass ejections, auroras, blazars, active galactic nuclei, black hole systems, galactic fast radio bursts, gamma-ray bursts, and soft gamma-ray repeaters. In contrast, we identify five outliers of astrophysical phenomena, i.e., coherent solar radio bursts, random solar radio bursts, solar energetic particles, cosmic rays, and pulsar glitches, which are not consistent with the standard FD-SOC model, and thus require different physical mechanisms.

Flares in TESS Data and Optical Spectra

We studied the flaring activities of M4.5 dwarf AD Leo to understand its stellar atmospheres and magnetic activities. We present new observational results on this highly active star, using nonsimultaneous measurement of Transiting Exoplanet Survey Satellite (TESS) data, time- series optical spectra, and Giant Meter Radio Telescope 325 MHz radio data. We revisited the rotation period of 2.23 0.04 days from TESS, which matches well with previously measured literature values. We estimated an extremely rare high-energy superflare of 4.9 10³⁵ erg and 400 minute duration with a high magnetic field strength of 1.2 kG. Interestingly, we correlated the duration of a flare event with its energy, , and noticed a discrepancy between stellar and solar flares suggesting a difference in coronal magnetic field strength. From time-series spectra, we observed H spectral flares in the range of 10³⁰10³¹ erg. A 12 minute delay in a spectral flare event was observed between the emission of the H and Ca II H and K lines, possibly due to their origination at different spatial locations in the chromosphere. We noticed the deviation in flare rate distribution and orbital phases indicates the presence of highly active regions. Furthermore, an occasional radio detection with a flux density of 9.46 1.63 mJy at a frequency of 325 MHz might be coherent emission in the presence of the magnetic field, giving a hint of starplanet interaction.

X-Ray/Radio Quasiperiodic Pulsations Associated with Plasmoids in Solar Flare Current Sheets

Plasmoids (or magnetic islands) are believed to play an important role in the onset of fast magnetic reconnection and particle acceleration during solar flares and eruptions. Direct imaging of flare current sheets and the formation/ejection of multiple plasmoids in extreme- ultraviolet images, along with simultaneous X-ray and radio observations, offers significant insights into the mechanisms driving particle acceleration in solar flares. Here, we present direct imaging of the formation and ejection of multiple plasmoids in flare plasma/current sheets and the associated quasiperiodic pulsations (QPPs) observed at X-ray and radio wavelengths, using observations from the Solar Dynamics Observatory/Atmospheric Imaging Assembly, RHESSI, and the Fermi Gamma-ray Burst Monitor. These plasmoids propagate bidirectionally upward and downward along the flare current sheet beneath the erupting flux rope during two successive flares associated with confined/failed eruptions. The flux rope exhibits evidence of helical kink instability, with the formation and ejection of multiple plasmoids in the flare current sheet, as predicted in an MHD simulation of a kink-unstable flux rope. RHESSI X-ray images show double coronal sources ("looptop" and higher coronal sources) located at both ends of the flare current/plasma sheet. Moreover, we detect an additional transient faint X-ray source (612 keV) located between the double coronal sources, which is cospatial with multiple plasmoids in the flare current sheet. X-ray (soft and hard) and radio (decimetric) observations unveil QPPs (periods 10 s and 100 s) associated with the ejection and coalescence of plasmoids. These observations suggest that energetic electrons are accelerated during the ejection and coalescence of multiple plasmoids in the flare current sheet.

Delay of Near-relativistic Electrons with Respect to Type III Radio Bursts throughout the Inner Heliosphere

Energetic electrons accelerated by solar eruptive events are frequently observed to have inferred injection times that appear significantly delayed with respect to electromagnetic emission including type III radio bursts. This is noteworthy because type III radio emission is produced by streaming suprathermal electrons, and thus this observed delay implies either a delayed injection/release of higher-energy electrons, compared with the suprathermal population, and/or a delay of the electrons observed in situ in transit through the interplanetary medium. A number of studies have investigated these delays with spacecraft located at 1 au. In this study, we examine energetic electron onsets and type III radio bursts observed by the Integrated Science Investigation of the Sun (ISIS) and the FIELDS Radio Frequency Spectrometer instrument on Parker Solar Probe at a variety of heliocentric distances. With these observations, we can uniquely decouple the effects of acceleration and transport and shed light on the source of these delays. We present a survey of electron events observed by ISIS within the first 6 yr of the mission, including their delays with respect to type III emission between 0.1 and 0.8 au. These results suggest that energetic electron delays with respect to type III radio bursts are not purely produced by a delayed injection/release as has been suggested, implying that transport processes play a role.

ALMA Observations of Solar Spicules in a Polar Coronal Hole

We report observations of solar spicules at millimeter wavelengths (mm-) using the Atacama Large Millimeter/submillimeter Array (ALMA). These are supplemented by observations in optical, ultraviolet (UV), and extreme ultraviolet (EUV) wavelengths. The observations were made on 2018 December 25 of the northern polar coronal hole. ALMA obtained time- resolved imaging observations at wavelengths of 3 mm (100 GHz; 2 s cadence) and 1.25 mm (239 GHz; 2 minutes cadence) with an angular resolution of 22 13 and 15 07, respectively. Spicules observed at mm- are easily seen low in the chromosphere whereas spicules in UV bands are seen to extend higher. The spicules observed at mm- are seen in absorption against coronal EUV emission, allowing us to estimate the column depth of neutral hydrogen. Spicular emission at mm-, due to thermal freefree radiation, allows us to estimate the electron number density as a function of height. We find that spicule densities, inferred from the mm- data are uniquely insensitive to assumptions regarding the temperature of plasma in spicules. We suggest that the upward mass flux carried by spicules is unlikely to play a significant role in the mass budget of the solar corona and solar wind, and the transport of hot material into the corona by spicules may not play a significant role in coronal heating. However, the possibility that electric currents, fast kink and torsional waves, or other wave modes carried by spicules may play a role in transporting energy into the solar corona cannot be excluded.

Constraining Solar Emission Radius at 42 MHz During the 2024 Total Solar Eclipse Using a Student-commissioned Radio Telescope

Low-frequency solar radio emission is sourced in the solar corona, with sub-100 MHz radio emission largely originating from the 10⁵ K plasma around 2 optical radii. However, the region of emission has yet to be constrained at 3545 MHz due to both instrumentation limitations and the rarity of astronomical events, such as total solar eclipses, which allow for direct observational approaches. In this work, we present the results from a student-led project to commission a low- frequency radio telescope array situated in the path of totality of the 2024 total solar eclipse in an effort to probe the middle corona. The Deployable Low-Band Ionosphere and Transient Experiment (DLITE) is a low-frequency radio array comprised of four dipole antennas, optimized to observe at 3545 MHz, and capable of resolving the brightest radio sources in the sky. We constructed a DLITE station in Observatory Park, a dark-sky park in Montville, Ohio. Results of observations during the total solar eclipse demonstrate that DLITE stations can be quickly deployed for observations and provide constraints on the radius of solar emission at our center observing frequency of 42 MHz. In this work, we outline the construction of DLITE Ohio and the solar observation results from the total solar eclipse that transversed North America in 2024 April.

Maximum Energy of Particles in Plasmas

Particles are accelerated to very high, nonthermal energies in space, solar, and astrophysical plasma environments. In cosmic-ray physics, the Hillas limit is often used as a rough estimate (or the necessary condition) of the maximum energy of particles. This limit is based on the concepts of one-shot direct acceleration by a system-wide motional electric field, as well as stochastic and diffusive acceleration in strongly turbulent environments. However, it remains unclear how well this limit explains the actual observed maximum energies of particles. Here, we show, based on a systematic review, that the observed maximum energy of particlesthose in space, solar, astrophysical, and laboratory environmentsoften reach the energy predicted by the Hillas limit. We also found several exceptions, such as electrons in solar flares and jet-terminal lobes of radio galaxies, as well as protons in planetary radiation belts, where deviations from this limit occur. We discuss possible causes of such deviations, and we argue in particular that there is a good chance of detecting ultra-high-energy (100 GeV) solar flare electrons that have not yet been detected. We anticipate that this study will facilitate further interdisciplinary discussions on the maximum energy of particles and the underlying mechanisms of particle acceleration in diverse plasma environments.

Evaluating F10.7 And F30 Radio Fluxes As Long-Term Solar Proxies Of Energy Deposition In The Thermosphere

We use model simulations and observations to examine how well the F10.7 and F30 solar radio fluxes have represented solar forcing in the thermosphere during the last 60 years of weakening solar activity. We found that increased saturation of radio fluxes during the last two extended solar minima leads to an overestimation of solar energy deposition, which manifests as a change in the linear relation between thermospheric parameters and F10.7. On the other hand, the linear relation between thermospheric parameters and F30 remains nearly the same throughout the whole studied period because of a recently found relative increase of F30 with respect to F10.7. This explains the earlier finding that F30 correlates better with several ionospheric and thermospheric parameters than F10.7 during recent decades. We note that continued evaluation is needed to see how well F10.7 and F30 will serve as solar proxies in the future when solar activity may start increasing toward the next grand maximum.

Fine structures of a solar type III radio bursts observed with LOFAR

We present spectral and imaging LOFAR (LOw-Frequency ARray) observations in the 20 - 40 MHz range of solar radio bursts fine structures, such as flag-like, sail-like, and dot-like that appeared on 8 April 2019. These structures were associated with type III solar radio bursts that occurred in the 40 - 80 MHz band. The mean duration and spectral widths of the fine structures range from 1.0 to 3.4 s and from 0.3 to 0.9 MHz, respectively. Additionally, we investigated the radio images of eight fine structures - two flags, two sails and four dots. This allowed us to determine their emission source sizes, which ranged from 240 to 392 arcmin2, and their frequencies from 25.58 to 39.25 MHz as well as their location. They occurred on the east side of the Sun and were most likely associated with an emerging active region NOAA AR 12738, where a weak B1.7 flare was observed.

Based on X-ray, EUV (DEM analysis), and type-III radio burst observations

Context. In solar flares, non-potential magnetic field energy is transferred to particle acceleration, heating, and radiation. Multi- wavelength observations of the solar corona in extreme ultraviolet (EUV) and X-ray show the consequences of magnetic reconfiguration in lower heights, while radio observations contain information about the access of flare-accelerated electrons to greater heights in the solar atmosphere. Signs of downward and upward particle propagation do not always appear symmetrically, they depend on the acceleration process, the sensitivity of the instruments, and the magnetic connectivity. The magnetic connectivity in various flare phases is therefore a key element to study in order to gain a better understanding of combined flare observations. Aims. We aim to draw conclusions about the magnetic connectivity of specific coronal loops in the active region, the acceleration region, and the higher corona with respect to different phases of the flare process. Methods. We investigate the evolution of particle acceleration, loops heated by the energy release, and the trajectories of flare-accelerated electrons observed up to one solar radius (1 R_S) above the active region in a B-class flare on 6 June 2020. We studied the downward particle acceleration and thermal evolution with observations by the Spectrometer/Telescope for Imaging X-rays and with a reconstruction code based on EUV observations. Traces of flare-accelerated electrons, namely, type-III radio bursts, were investigated with a spectroscopic solar dynamic radio imager (LOw Frequency ARray). The radio source positions in heights of 0.4 R_S1.0 R_S were compared with the thermal evolution of coronal loops and with a solar magnetic field model. Results. In this flare event, similar magnetic reconnection processes and accompanied heating processes are triggered several times. For loops and periods with access to the higher corona, type-III bursts from similar reconnection process are emitted along similar propagation trajectories. There are no radio bursts associated with the heating process of the main flaring loop. Conclusions. In this event, the large-scale magnetic field is rather stable and seems not to be affected by the flare. The access to loops reaching heights of half a solar radius or more is suppressed during the main flare phase for flare-accelerated electrons. This may lead to more effective heating and absent type-III radio bursts between 20 MHz and 85 MHz.

Connecting energetic electrons at the Sun and in the heliosphere through X-ray and radio diagnostics

Context. Solar flares release huge amounts of energy, a considerable part of which is channeled into particle acceleration in the lower corona. Hard X-ray (HXR) emissions are used to diagnose the accelerated electrons that bombard the chromosphere, while type III radio bursts result from energetic electron beams propagating through the corona and into interplanetary space. The Solar Orbiter mission, launched in 2020, aims to link solar flare remote observations with heliospheric events, thus producing useful observations for our understanding of particle acceleration and propagation from the Sun to the heliosphere. Aims. While both hard X-Ray and radio emissions result from flare-accelerated electrons, their relationship is not straightforward. By comparing the evolution of the X-ray emitting sites and the timing of type III bursts, our aim is to determine the conditions for associations between X-ray flares and interplanetary (IP) type III bursts. Methods. We analyzed 15 interplanetary type III bursts that are associated with HXR bursts in the first available period for simultaneous X-ray/radio observations of type III bursts from Solar Orbiter (using the RPW and STIX instruments). X-ray imaging was performed around the onset of the type III bursts, complemented by EUI 174 images to assess the magnetic configuration of the corona. Results. All 15 X-ray flares originated from the same active region on the west limb as observed by Solar Orbiter. In each of the events, a change in X-ray source morphology occurred shortly (< 6 minutes) before the onset of type III radio bursts, indicating a change in the electron acceleration region preceding the radio emission. Considering the delays observed between the two emissions, these findings describe complex scenarios with multiple reconnection episodes, some of which may allow accelerated electrons to escape into IP space when open magnetic field lines are involved (interchange reconnection). In some cases, X-ray source elongations toward open field lines in the UV were observed, reinforcing this idea.

The widespread solar energetic particle event on 2022 January 20

Context. On 2022 January 20, the Energetic Particle Detector (EPD) on board Solar Orbiter measured a solar energetic particle (SEP) event showing unusual first arriving particles from the anti-Sun direction. Near-Earth spacecraft separated by 17 in longitude to the west of Solar Orbiter measured classic anti-sunward-directed fluxes. STEREO-A and MAVEN, separated by 18 to the east and by 143 to the west of Solar Orbiter, respectively, also observed the event, suggesting that particles spread over at least 160 in the heliosphere. Aims. The aim of the present study is to investigate how SEPs are accelerated and transported towards Solar Orbiter and near-Earth spacecraft, as well as to examine the influence of a magnetic cloud (MC) present in the heliosphere at the time of the event onset on the propagation of energetic particles. Methods. We analysed remote-sensing data, including flare, coronal mass ejection (CME), and radio emission to identify the parent solar source of the event. We investigated energetic particles, solar wind plasma, and magnetic field data from multiple spacecraft. Results. Solar Orbiter was embedded in a MC erupting on 16 January from the same active region as that related to the SEP event on 20 January. The SEP event is related to a M5.5 flare and a fast CME-driven shock of 1433 km s¹, which accelerated and injected particles within and outside the MC. Taken together, the hard SEP spectra, the presence of a Type II radio burst, and the co-temporal Type III radio burst being observed from 80 MHz that appears to emanate from the Type II burst, suggest that the shock is likely the main accelerator of the particles. Conclusions. Our detailed analysis of the SEP event strongly suggests that the energetic particles are mainly accelerated by a CME-driven shock and are injected into and outside of a previous MC present in the heliosphere at the time of the particle onset. The sunward-propagating SEPs measured by Solar Orbiter are produced by the injection of particles along the longer (western) leg of the MC still connected to the Sun at the time of the release of the particles. The determined electron propagation path length inside the MC is around 30% longer than the estimated length of the loop leg of the MC itself (based on the graduated cylindrical shell model), which is consistent with the low number of field line rotations.

Correlation between active regions' spectra at high radio frequencies and solar flare occurrences

High radio frequencies observations with the Italian network of large single-dish radio telescopes resulted in 450 solar images between 2018 and 2023 in K-band frequency range (18-26 GHz). Solar radio mapping at these frequencies allows the probing of the Active Regions (ARs) chromospheric magnetic field close to the Transition Region, where strong flares and coronal mass ejection events occur. Enhanced magnetic fields up to 1500-2000 G determine anomalous spectra in the ARs brightness compared to pure free-free emission, due to the addition of a steeper gyro- resonance component also associated with circular polarisation up to 40%. When a significant AR spectral flattening is detected, the probability of a strong flare occurrence within 30 hours is high (89% in terms of statistical precision). Despite an approximate weekly cadence of our observations, only 12% of strong flares are missed/unpredicted within this time interval. Through a correlation analysis, we assess the trade-off on the sensitivity and the robustness of this physics-based flare forecast method.

Long-term trends of ionospheric electron density related to global warming

Long-term trends of ionospheric electron density have been studied using vertical sounding measurements at 10 ionosonde stations from European, Asian, and American longitude sectors. The analysis focuses on studying the relationship of ionospheric F2 layer noontime peak electron density (NmF2) data covering a long time period of up to 71 years with the 30 cm solar radio flux index F30. The long-term behavior of 11-year sliding averages of noontime NmF2 data shows a substantial decrease from a stable reference level that is specific for each ionosonde station. The reference level is defined by a linear model of the noontime F2 layer 11-year sliding peak electron density NmF2* as a function of the associated 11-year sliding F30 solar activity index F30*. Whereas NmF2* is proportional to F30* within a small variability range of 1.5% over nearly two solar cycles until 1982, NmF2* decouples from this linear relationship with F30* afterwards. The deviation (reduction) may reach up to 20.6% in 2022 or up to about 5% per decade in the Northern hemisphere and up to 18.2% in 2022 or about 4% per decade in the Southern hemisphere. It is expected that such strong changes should have serious consequences for the accuracy of empirical ionosphere models utilizing a database that was established before the 1980s. For the first time, it has been demonstrated that there is a significant correlation between the observed long-term decrease in ionospheric electron density and the temperature anomaly (TA) measured at the Earth's surface. This finding highlights a close connection between atmospheric changes at lower altitudes, as indicated by surface temperature records, and variations in the ionospheric electron density observed over extended periods. Similar to the temperature anomaly (TA), the concept of "electron density anomaly" (EDA) has been introduced to characterize deviations in ionospheric electron density from expected values. Analysis indicates that the EDA exhibits a more pronounced effect in the Northern Hemisphere compared to the Southern Hemisphere. A similar pattern occurs with the TA, which supports the idea that a shared physical mechanism may explain both the EDA and TA phenomena. This long-term reduction of the electron density reflects ongoing modifications in the structure and behavior of the Earth's magnetosphere-ionosphere-thermosphere (MIT) system. The findings suggest that these changes are closely linked to the increasing concentrations of greenhouse gases accumulating in the thermosphere. As greenhouse gas levels rise, their effects extend beyond the lower atmosphere, impacting the upper atmospheric regions and contributing to observable trends in ionospheric electron density.

Theoretical algorithm and its validation

The Soil Moisture and Ocean Salinity (SMOS) satellite, launched in November 2009, is equipped with the Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) payload, which collects high temporal resolution polarimetric data at 1.413 GHz (L-band), corresponding to a wavelength of 21.2 cm, with a time resolution of 2.4 s. The Sun is present in the SMOS images as an alias and is removed from the images to improve the accuracy of soil moisture and ocean salinity maps. This paper illustrates the algorithm's theoretical basis for the retrieval of the solar flux from the SMOS dataset at the time scale of 50 min, and details the validation process and results. Validation and long-term stability are evaluated at daily and monthly scales, using the radio- telescopes and other solar activity proxies as references. Comparison with the F10.7 index and the flux at 1 GHz from the Nobeyama Radio Polarimeters (NoRP) shows good agreement with the SMOS solar flux dataset, with no significant seasonal or temporal drift, unlike data collected by the Radio Solar Telescope Network (RSTN) at 1.415 GHz. The SMOS solar flux spectrum reveals both the solar rotation and solar cycle frequencies, resembling the F10.7 index. The uncertainties in the 50 min time resolution dataset are assessed, and the precision of the daily data is further evaluated through auto-regressive modelling, comparing favourably to results obtained by the radio telescopes, with errors consistently below 3%. The correlation between SMOS solar flux and both the Wolf sunspot number and the Mg II proxy shows high agreement, and suggests that the correlation between the SMOS solar flux and the Mg II is mostly linear, and stronger than the one between the latter and the F10.7. Lastly, a prototype for solar radio burst detection is introduced, and a few solar radio burst events are discussed. This work demonstrates the added value of the SMOS solar flux in solar physics studies, thanks to its 15-year-long time series dataset and its proven stability. The full polarimetric measurements acquired in the L-band at high temporal resolution represent an innovative point of view for solar flux observations, alleging SMOS solar flux as a valid resource in the space weather field, complementing established solar indicators and contributing to Space Weather applications and studies. The SMOS solar flux at 50 min time resolution dataset is publicly accessible from Zenodo as daily files.

Estimation of the impact of solar flare spectra on the Earth's ionosphere using the GAIA model

The rapid increase in X-ray and extreme ultraviolet (EUV) emissions owing to solar flares enhances ionization in the ionosphere, increasing radio wave attenuation. Among these phenomena, the shortwave communication disturbance caused by the increased electron density in the ionospheric D region is known as the shortwave fadeout (SWF). We investigated the relationship between SWF's magnitude and solar flare emission, and evaluated the electron density variation in the ionospheric D region associated with flare. We defined the minimum frequency (fmin) observed in Japan's ionograms as the SWF's magnitude. We analyzed ionosonde data for 38 SWF events observed during daytime in Japan between May 2010 and May 2014. To investigate the relationship between flares and SWF, we compared the observed X-ray and EUV emissions during flares with the dfmin (background subtracted fmin). X-ray (0.10.8 nm) and EUV (1114 nm) emissions correlate with dfmin. Then, using the GAIA model, a numerical model that treats the entire Earth's atmosphere, we investigated the effect of the X-ray and EUV solar flare emissions on the ionosphere, which affects the SWF. The results showed that the main ionization source in the ionospheric D region was X-ray emission, and shortwaves were attenuated by 90%. In contrast, in the ionospheric E and F regions, the primary ionization source was EUV emission, with only 10% shortwave attenuation. Finally, we estimated the fmin values and blackout (total fadeout of the ionospheric echo observed in ionograms) and compared the simulated and observed fmin values. The hit rate of blackouts was 35% when we only used the GAIA calculations. Therefore, we estimated fmin using the electron density variation in the ionospheric D region corresponding to X-ray solar emission. As a result, the hit rate of the blackout was 68%, and the linear correlation coefficient between the simulated and observed fmin values was 0.85. The estimated magnitude of the SWF was improved by incorporating the effects of X-ray emissions into the ionospheric D region of GAIA. We are the first to implement a method for evaluating the electron density in the ionospheric D region using the fmin value.

Investigating the drivers of long-term trends in the upper atmosphere over Rome across four decades

The nature of the long-term changes in the upper atmosphere morphology at mid-latitude remains a subject of debate, particularly regarding whether these changes are purely driven by geomagnetic and solar activities or whether forcing from the lower atmosphere, such as CO₂ variations, may play a role. To contribute to this debate, we investigate the nature of the long-term trends of the ionospheric and thermospheric parameters by leveraging on ionosonde data digitally recorded at the Rome Observatory since 1976. The following parameters have been investigated under sunlit conditions (12:00 Local Time): critical frequency of the F1 layer (foF1); critical frequency of the F2 layer (foF2), atomic oxygen concentration at 300 km ([O]); ratio between atomic oxygen and molecular nitrogen concentrations at 300 km altitude ([O]/[N₂]); exospheric temperature (T_ex); thermospheric density at 300 km (). The ionospheric parameters are manually scaled from digital ionograms, whereas thermospheric parameters are retrieved using the THERmospheric parameters from IONosonde observations (THERION) method, which utilises ionosonde observations and a physical model of the ionospheric F region. To investigate the influence of the solar and geomagnetic activity on long term variations, we consider the solar radio flux at 10.7 cm (F10.7) and the geomagnetic disturbance index Ap. To identify the various frequency/period components of the time series under consideration and identify the trends, we leverage the high scale/time resolution offered by the Fast Iterative Filtering (FIF) algorithm. A regression analysis of thermosphere/ionosphere parameters against geomagnetic/solar activity indices has then been conducted to investigate the drivers of long-term variability. Our findings reveal that the identified trends are predominantly controlled by external drivers, particularly long-term solar and geomagnetic activity variations. The adopted methodology, based on regression modelling, demonstrates that variability in F10.7 and Ap accounts for nearly all of the observed changes, with the exception of atomic oxygen ([O]), which displays a slightly higher unexplained variability (~7%). The inclusion of CO₂ concentration as an additional driver improves the regression model for [O]. However, the effect remains statistically limited, indicating that the impact of CO₂ on thermospheric cooling might be of little significance. Further studies with extended time series are necessary to better quantify this relationship and evaluate its importance. These results highlight the predominant influence of solar and geomagnetic activity in determining upper atmosphere long-term trends at mid- latitudes.

Exploring daily fluctuations of cosmic ray muon components at a low latitude site and their associations with space weather variables

In this study, cosmic rays (CR) data from the King Abdulaziz University muon detector in Jeddah (Rc = 14.8 GV), Saudi Arabia, were utilized to investigate the amplitude and phase components of diurnal variations in CR muons. The data covered the period from 2007 to 2012 and were fitted using a single cosine function with a 24-h period and two cosine functions with periods of 24 h and 12 h, respectively. The distributions of the phases and amplitudes resulting from these fits were analyzed across different time spans. The findings of this analysis provided valuable insights into the diurnal characteristics of CR muons. The mean amplitude and phase obtained from the single fit were reported as 0.11 0.51% and 11:00 4.30 UT, respectively. Furthermore, employing the two-cosine fit revealed that the first phase had a mean occurrence time of 06:00 6.90 UT, accompanied by an amplitude of 0.10 0.62%. The second phase occurred at 13:00 3.51 UT, with an amplitude of 0.11 0.25%. The study observed diverse distributions and trends in amplitude and phase values across different time scales, including months, seasons, and years. Additionally, the study investigated the influences of five solar activity parameters on the diurnal CR components using Pearson linear, non-parametric Spearman, and Kendall correlations. These parameters included the interplanetary magnetic field, solar wind speed, Kp index, Dst index, and solar radio flux at 10.7 cm. The results revealed that the relationships between solar activity variables and the diurnal CR parameters were not uniform. There were varying degrees of correlation, with differences in strength and magnitude depending on the specific variable and correlation coefficient being examined.

A Solar-Powered Mobile Phased Array for Radio Astronomy Observations at Low Frequencies

In this paper, we present initial results from test observations conducted with a compact, broadband (meter-decameter range) mobile antenna array designed for autonomous operation in field conditions. Powered exclusively by solar energy and independent of stationary infrastructure, the system is rapidly deployable and well-suited for remote low-frequency (<100 MHz) radio astronomy. Solar radio bursts were used as illustrative examples to demonstrate system functionality and sensitivity under natural conditions. In addition to solar activity, the antenna array is capable of detecting other astrophysical sources such as Jupiter and Cas A, confirming its broader applicability. Key technical characteristics of the antenna and its integrated solar power station are also described.

Correlation between rate of TEC index and positioning error during solar flares and geomagnetic storms using navigation with Indian constellation receiver measurements

The real-time position accuracy of the Global Navigation Satellite System (GNSS)/Navigation with Indian Constellation (NavIC) receiver is limited by the dynamic behavior of the ionosphere, particularly in adverse conditions like solar flares and geomagnetic storms. The NavIC satellites broadcast dual coherent radio beacon signals on L5 (1,164.5 MHz) and S (2,472.5 MHz) bands for providing position, velocity, and timing services in all weather conditions. The Total Electron Content (TEC) and Rate of TEC Index (ROTI) are the potential indicators for characterizing the ionosphere and its irregularities. In this research work, the TEC and ROTI are computed from the code and carrier phase observations of the NavIC receiver located at Kurnool low latitude station (15.79 N, 78.07 E) with geomagnetic coordinates (7.30 N, 151.65 E). This paper presents a statistical study of TEC, ROTI, and the correlation between ROTI and NavIC positioning error during highly intense solar flares (X9.3 and X2.2) and geomagnetic storm conditions. Compared to quiet days mean TEC, the enhancement is 3 TECU due to X9.3 flares, and the maximum peak of TEC on storm day (September 8, 2017) is 80.92 TECU. Moreover, the correlation coefficient between ROTI and position error is 0.76 on a quiet day (September 4, 2017), 0.54 on an intense solar flares day (September 6, 2017), and 0.24 on a storm day (September 8, 2017), this indicates positional accuracy degradation on a geomagnetic storm day. The outcome of this research work would be helpful for investigating characteristics of the northern low latitude ionospheric irregularities and, in turn, useful for developing suitable ionospheric nowcasting/prediction models for GNSS applications.

Radio absorption in the dayside martian ionosphere enhanced by solar flares

Solar flares can cause radio absorption in the D region of the Earth's ionosphere and consequently interrupt high-frequency radio communication systems, also known as short-wave fadeout or the Dellinger effect. We present an analogous radio absorption event observed at Mars during a solar flare. In this event, the Mars Express Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument fortuitously operated at low altitudes on the dayside around the terminator in a favorable configuration for surface echo measurements at the flare peak. The surface echo power during the flare is abnormally weak compared to nominal echo powers at corresponding solar zenith angles, suggesting flare-induced radio absorption in the dayside lower ionosphere of Mars. Additionally, long-term MARSIS data statistically demonstrate the radio absorption dependence on solar soft X-ray fluxes. Our results point to the need for Martian space weather prediction including ionospheric effects on radio waves.

Joint Optimization for Cooperative Service-Caching, Computation-Offloading, and Resource-Allocations Over EH/MEC 6G Ultra-Dense Mobile Networks

Service-caching, computation-offloading, and mobile edge-computing (MEC) have been widely recognized as three key 6G mobile wireless neworking techniques which can efficiently support implementing the ultra-dense networks (UDNs) with massive small-cell base stations (SBSs). But, these impose the new challenges for the UDNs to solely rely on grid power for energy supplying and to jointly optimize service-caching, computation- offloading, and resource-allocations. To overcome the above described difficulties, integrating energy-harvesting (EH) techniques with MEC- enabled 6G UDNs, we propose to develop the joint optimization schemes for cooperative service-caching, computation-offloading, and resource- allocations. In our considered UDNs, there exist a large number of EH- based stationary users (SUs) or mobile users (MUs), and a mixture of on- grid SBSs powered by electric grid and off-grid SBSs power-supplied by solar, radio frequency (RF) energy, etc. Specifically, first we formulate an energy minimization problem under a non-linear RF-energy EH model to minimize the sum of weighted energy consumption of users and off-grid SBSs. Second, for scenarios with SUs, we develop a two- timescale based joint cooperative service-caching, computation- offloading, and resource-allocations scheme using the hierarchical multi-agent deep reinforcement learning. We derive cooperative service- caching in each time frame, and then derive computation-offloading and resource-allocations in each time slot. Third, we extend our work to scenarios with MUs, where MUs can move with certain trajectories at low speeds. Finally, we validate and evaluate the performances of our proposed schemes through the extensive simulations.

Development of an FX Digital Correlator for Millimeter-Wave Solar Radio Interferometry and Calibration of AmplitudePhase Consistency

Interferometry technology stands out as a significant trend in radio observation systems, where correlators act as pivotal components responsible for real-time data processing. Responding to the operational needs of Shandong University Chashan Solar Radio Observatory's 39.540-GHz solar radio binary interferometer, a 1.25-Gs/s FX-type digital correlator was developed. Following a modular framework, the digital correlator comprises three core modules: data acquisition, correlation processing, and data transmission. Additionally, it incorporates adjustable delay compensation and amplitudephase consistency calibration functionalities to ensure precise signal alignment and system coherence. The coherent noise injection method is employed to assess inherent amplitudephase discrepancies in the analog front end (AFE), while real-time compensation calibration factors are implemented in FPGA to rectify the amplitudephase consistency of the AFE. Experimental design and validation were executed, showcasing that the correlator, featuring dual channels, a 1.25-Gs/s sampling rate, and 14-bit resolution, achieves an adjustable delay compensation accuracy of 0.8 ns. Post-amplitudephase consistency calibration of the AFE links reveals an amplitude error of approximately 0.2 dB and a phase error of approximately 7, indicating notable calibration efficacy. Subsequently, the RTL design of the digital correlator presented in this article was verified, confirming the logical coherence of the RTL design and validating the accuracy of the correlation output results through experimental design. The development of this correlator not only finds application in other interferometric measurement system scenarios but also establishes a research groundwork for future digital correlator advancements in the comprehensive aperture solar imager slated for development by the research group.

Enhancing Receiver Linearity Through Behavioral Modeling of Measured Data and Digital Postcompensation Techniques

The challenge of receiver nonlinearity in radio observation systems results in diminished system linearity, dynamic range, and sensitivity, particularly in low-amplitude and linear regions. To address these issues, this article proposes a digital postcompensation calibration method based on a nonlinear model of radio receivers. By using nonlinear digital postcompensation technology, we analyze the nonlinear effects exhibited by radio receivers. A nonlinear model of radio receivers is constructed using empirical data. The compensation-linearized output is generated by compensating for the observed input of the radio observation system's output response through the inverse model of the nonlinear model. Subsequently, the receiver response compensates for the difference between the actual radio receiver response and the response corresponding to linear gain, thereby eliminating nonlinear distortion and achieving real-time digital postcompensation linearization of these effects. We introduce a method for modeling the nonlinear behavior of radio receivers based on empirical data, along with a nonlinear postcompensation linearization approach rooted in this model. Experimental validation via radio observation systems reveals a mean square error of approximately 0.8226% between the model output and the system response. Furthermore, the nonlinear postcompensation linearization method, which is based on the nonlinear model of radio receivers, significantly enhances system linearity, particularly in the low-amplitude region and nonlinear region. The amplitude response difference of the system, concerning the quiet sun and cold space, shifts from approximately 5 dB to approximately 11 dB, thereby amplifying the sensitivity of the solar radio observation system.

A Modern VLF Radio Receiver Designed for the Array for VLF Imaging of the D-Region (AVID)

Very-low-frequency (VLF) radio waves are commonly used to remotely sense the D-region ionosphere. In the past few decades, a few prominent designs for VLF receivers have come to the forefront of the community, such as the AWESOME receiver developed at Stanford in the early 2000s. In the last decade, advancements in off-the-shelf electrical components and the obsoletion of others motivated a redesign of this receiver system. The result of this receiver design work is presented in this article with the measured characteristics of over ten complete systems. The newly operational array for VLF imaging of the D-region (AVID), incorporating the updated VLF receiver, is also presented. One early result from AVID is the characterization of the drift rates of the phase of two VLF transmitters. The NLK transmitter at 24.8 kHz was found to drift by 0.07886/s /s, while the NML transmitter at 25.2 kHz was found to drift by 0.07201/s /s. We also show that the transmitted power of these transmitters can change over time, by up to ~7% in the case of NLK. Finally, two early case studies are presented, showing AVID's ability to observe and track: 1) the effects of an M7.2 solar flare on the D-region ionosphere and 2) the onset of a 333 nT Dst geomagnetic storm and the D-region's response to it. These case studies demonstrate the tools necessary for future work incorporating spatial estimation algorithms to better characterize disturbances to the D-region ionosphere.

The Outlook of Lunar Observation by 1.3-m Wavelength EISCAT_3D Radar With Large Telescope Antennas of Chinese Meridian Project

Earth-based radar (EBR) is an important type of remote sensing instrument for planetary observation. EBR takes advantages in large- scale imaging swath, high repeatability, great flexibility, and so on. The upcoming 233-MHz-frequency European Incoherent Scatter Scientific Association (EISCAT) 3-D radar system will provide important features to lunar observation as introduced in this study. EISCAT_3D (E3D) radar is a powerful multistatic radar system. The 1.3-m wavelength wave of E3D can penetrate deeper, about 30 m below the lunar average surface, which can reach the second layer, i.e., layer of ejecta. E3D radar supports dual/quadrature polarimetry, which gives it good flexibility and lower ambiguity in the inference of scatter's properties. Also, there is less ambiguity in scattering regimes between icy and nonicy scatters for 1.3-m wavelength than for shorter wavelengths as given from the simulation results. Besides, the high topographic resolution (which requires forming interferometric baselines with distant telescope antennas) of E3D radar along with its penetration depth makes it possible for detection of sublunarean cavities by signatures of depression. As a whole, the 1.3-m wavelength 3-D polarimetric imaging of the Moon by E3D radar, on a spatial resolution of about 200 m, will be valuable for obtaining new information about the geology and subsurface structure of the Moon and can be used in search of buried water ice, sublunarean cavity, and so on. Furthermore, we envision the collaboration of E3D radar with large telescope antennas in China, including Daocheng Solar Radio Telescope (DSRT) and Mingantu spectral radioheliograph (MUSER) for better imaging ability and detectivity.

Frequency-Scanning Waveguide Antenna for Solar Radio Bursts Detection in the UHF Band

This paper proposes a frequency-scanning antenna designed to operate in the 400 MHz to 800 MHz band for solar radio astronomy applications. It is constructed with perforated metallic walls and cylinders, that form a rectangular leaky waveguide. By adjusting the metallic cylinders in the appropriate subwavelength holes, the scanning angle of the directive beams and directivity can be effectively controlled while assuring high radiation efficiency and gain. Unlike previous leaky waveguides, the proposed design allows manual tuning of both the leaky-mode phase and the leakage factors. A 2-meter-long antenna prototype has been fabricated, and the measurement results show an angular scanning range from 70 to 30 above the horizon, with a peak gain of 14 dBi and radiation efficiency exceeding 70% over the entire scanning band. The application of this antenna to detect solar radio bursts without the need for Sun tracking is demonstrated.

Passive Radar Sounding for Firn Aquifer Monitoring

Firn aquifers may play an important role in the mass balance of the Greenland ice sheet, but changes in their water storage are challenging to measure due to their spatiotemporal variability. Here, we explore the feasibility of passive radar sounding using the Sun as a radio source to monitor daily to seasonal changes in the firn aquifer water table level. Our simulations account for time-varying environmental factors (e.g., changes in firn temperature, structure, and saturation, as well as snow accumulation) that impact passive radar measurements of height fluctuations in the firn aquifer water table. Our results suggest that passive sounding could be a novel observational tool for quasi- continuous, seasonal monitoring of firn aquifer properties in polar regions of interest, such as the Helheim firn aquifer on the Greenland ice sheet and firn aquifers on Antarctic ice shelves. We conclude by discussing the opportunities and challenges of near-surface passive radar sounding, with the goal of enabling low-resource, low-cost glaciological sensors that complement existing observational tools.

Classification of Equatorial Ionospheric Irregularities Using Unsupervised Machine Learning Based on Spatiotemporal ROTI Keograms

Equatorial ionospheric irregularities, particularly those associated with equatorial plasma bubbles (EPB), can significantly disrupt satellite navigation and communication systems. As the demand for reliable Global Navigation Satellite System (GNSS) and communication services grows, the prediction of ionospheric irregularities becomes critical. A key step in the prediction process is to identify distinct spatiotemporal patterns of irregularities, including day-to-day, longitudinal, and seasonal variations. However, with large datasets, manually classification or identification of these irregularities is a complex and challenging task. In this work, we propose unsupervised machine learning techniques to recognize and group irregularity patterns in large, unlabeled Rate of Total Electron Content (TEC) Index (ROTI) keograms. Specifically, two machine learning models: Gaussian Mixture Model and k-means clustering are employed. The ROTI keograms are constructed using GNSS data from two low-latitude receiver stations in Thailand. To reduce redundancy in the keogram images, three feature extraction techniques are applied before the clustering process. A comparative analysis is performed to determine the optimal number of clusters using these models. Based on the results, the optimal combination of feature extraction and clustering technique is determined for the proposed clustering model. The resulting k-means model with contour extractor classifies five distinct patterns of ionospheric irregularity patterns, providing valuable insights for enhancing EPB prediction models and deepening our understanding of ionospheric dynamics. Furthermore, these five irregularity patterns are analyzed in relation to space weather parameters such as the solar radio flux index (F10.7), and the geomagnetic index (Kp). The findings contribute to the development of robust prediction models, improving the reliability of satellite-based communication and navigation systems.

Analysis of the Variability of Diurnal Profiles of Critical foF2 Frequencies in the Light of Solar Radiation

This present article was written to analyze the variability of the diurnal profiles of the critical foF2 frequencies in the light of solar radiation based on in situ measurements of the ionosonde stations of Dakar (latitude 14.8 N, longitude 342.6 E) and Ouagadougou (latitude 12.5 N, longitude 358.5 E), respectively, for Sunspot Cycles 21 and 22. The objective being to deduce interpretations in terms of propensity to occur on the five profiles B, M, R, D, and P referenced to occurrence between the latitudes of 20 N and 20 S. Thus, on the analyses of conjunctions in phases or in phase shift observed in the variation of the sunspot cycle (VSC) and monthly foF2 frequencies (MFVs), we will note the following: (1) during periods of geomagnetic calm, (a) a propensity for the formation of the R profile during the growth phases of the solar cycle and during the periods from the solstices to the equinoxes and this is due to the conjunction in the growthgrowth phase between VSC and MFV; (b) a propensity for the formation of the M profile during the waning phases of the solar cycle and during the periods from the equinoxes to the solstices and this is due to the conjunction in the waningdecaying phase between VSC and MFV; (c) a propensity for the formation of profiles B, D, and P on the one hand during the growth phases of the solar cycle and during the periods from the equinoxes to the solstices and on the other hand during the waning phases of the solar cycle and during the periods from the solstices to the equinoxes and all these are due to the nonconjunction in the phase between VSC and MFV. It therefore appears that out of the 16 possible propensities for profile formations, the R and M profiles are each counted for 12.5%. Profiles B, P, and D each counted 25%. (2) During periods of geomagnetic disturbances, notably by SFEs, CMEs, or ionization losses, a profile can transform into one or other of the other four profiles. Finally, as an importance or application of this study, we note that (a) foF2 contributes more to the good knowledge of the ionosphere; (b) using the diurnal profiles of foF2, we could a priori identify periods of lulls in radio or satellite transmission; (c) by induction, we could count the solar flares during the day; and (d) the proposed mathematical model is a basic tool for analyzing foF2 variability.

Modeling the transport and anisotropy of energetic electrons in solar flares

Transport of energetic electrons in the flare loop is important to understanding nonthermal emissions in solar flares. In this work, we model the propagation of electrons by numerically solving the particle transport equation which includes the physics of magnetic mirroring and turbulent pitch-angle diffusion. We find that both the fractions of electrons trapped in the looptop and precipitating into the solar surface display a non-monotonic behavior with increasing scattering rate. In the moderate diffusion regime, the precipitation fraction is highest and we expect intense nonthermal HXR and microwave emissions at the footpoints. With no or weak pitch-angle scattering, the velocity space distribution can be highly anisotropic both in the looptop and loopleg regions. Different patterns of stripes with positive gradients in the perpendicular direction can drive the electron cyclotron maser instability with higher efficiency than the classical loss-cone distribution, facilitating the excitation of coherent solar radio bursts. Our simulation results highlight the effects of turbulent pitch- angle scattering on electron trap/precipitation and anisotropic distribution in solar flares, which may help us understand the precipitation of magnetospheric electrons accounting for the aurora as well.

Analysis of 42 Years of Cosmic Ray Measurements by the Neutron Monitor at Lomnick tt Observatory

The correlation and physical interconnection between space weather indices and cosmic ray flux has been well-established with extensive literature on the topic. Our investigation is centered on the relationships among the solar radio flux, geomagnetic field activity, and cosmic ray flux, as observed by the Neutron Monitor at the Lomnick tt Observatory in Slovakia. We processed the raw neutron monitor data, generating the first publicly accessible data set spanning 42 years. The curated continuous data are available in.csv format in hourly resolution from December 1981 to July 2023 and in minute resolution from January 2001 to July 2023 (Institute of Experimental Physics SAS, 2024, https://doi.org/10.5281/zenodo.10790915). Validation of this processed data was accomplished by identifying distinctive events within the data set. As part of the selection of events for case studies, we report the discovery of TGE-s visible in the data. Applying the Pearson method for statistical analysis, we quantified the linear correlation of the data sets. Additionally, a prediction power score was computed to reveal potential non-linear relationships. Our findings demonstrate a significant anti-correlation between cosmic ray and solar radio flux with a correlation coefficient of 0.74, coupled with a positive correlation concerning geomagnetic field strength. We also found that the neutron monitor measurements correlate better with a delay of 721 hr applied to the geomagnetic field strength data. The correlation between these data sets is further improved when inspecting periods of extreme solar events only. Lastly, the computed prediction power score of 0.22 for neutron flux in the context of geomagnetic field strength presents exciting possibilities for developing real-time geomagnetic storm prediction models based on cosmic ray measurements.

Machine Learning Solar Full Disk Flare Operational Forecasting

Solar flare forecasting is an essential component in space environment forecasting. Most of the deep learning flare forecasting models constructed are based on the magnetograms of active regions. Affected by the projection effect, these models can only forecast the active region in the center of the Sun. It is difficult to meet the need of operational flare forecasting of the solar full disk. Based on the traditional solar activity parameters, in this study, the relationships between the magnetic type of the active region, area of the active region, the history of the flare outburst, the 10 cm radio flux and flares from January 1996 to December 2022 were statistically analyzed. By using the fully connected neural network, an operational flare forecasting model for solar full disk active regions was constructed. This model can forecast the eruption of the M-class or above flares of the full solar disk active regions in the next 48 h. The F1 score of the model is 0.4304, the TSS is 0.3689, and the HSS is 0.3906. The model is compared with the deep learning flare forecasting model constructed in the previous work, and the results show that the operational forecasting model constructed in this paper has a better forecasting performance. Meanwhile, in order to explore the influence of the projection effect, the solar full disk active regions flare forecasting model constructed was tested for test data within 30 degrees from the center of the solar disk, within the interval from 30 degrees to 60 degrees, and over 60 degrees, respectively. The results show that the projection effect has little influence on the flare forecast model constructed in this study. The model can be used to forecast flare in the active region of the full solar disk, and provide an effective tool for operational solar flare forecasting.

Simulation on the Mechanism of Harmonic Maser Emission Through the Wave Coalescence Process

Electron Cyclotron Maser Emission (ECME) driven by electrons with loss- cone distribution is the main mechanism for explaining solar radio spikes. However, in strongly magnetized plasmas, the losscone-driven ECME mainly generates fundamental X mode emissions, which can be efficiently absorbed when escaping through the second-harmonic layer in the solar corona. To solve the "escaping difficulty, recent studies suggested a new mechanism of harmonic maser emission (X2) involving nonlinear wave coupling process of Z-mode and fundamental X-mode (X1) waves (Z+ZX2, Z+X1X2). It is necessary to verify the nonlinear wave coalescence process with theoretical analyses and numerical simulations. Here, the possibility of a nonlinear wave coupling process is examined via solving the matching conditions for three-wave resonant interaction based on the dispersion relation of cold plasma in magneto-ionic theory.The matching conditions for the Z and/or X1 waves were found to be satisfied over a wide range of parameters, leading to the production of X2 emissions that propagate perpendicularly and obliquely relative to the direction of the background magnetic field. Based on the solutions obtained in the matching condition analysis, we selected four sets of solutions of Z+Z and Z+X1 to perform particle-in-cell simulations using wave pumping method, to verify the nonlinear process of wave coalescence generating second harmonic X-mode emissions. With X1 and/or Z modes correctly pumped in the simulation domain, efficient generation of X2 emissions was observed, with saturation achieved within 400 _ce^-1. The conversion rate of energies of X2 emissions to Z mode waves varies from 2% to 8%. The study presents strong evidence to support the new mechanism of harmonic maser emission, which can be widely applied to explain the solar and space radio emissions.

Research on F10.7 Index Prediction Based on Factor Decomposition and Feature Enhancement

The F10.7 index is crucial for assessing solar activity, significantly impacting communication, navigation, and satellite operations. The intrinsic complexity and variability of solar activity often result in sudden perturbations in the F10.7 index, compromising the accuracy and stability of forecasts. To address this challenge, we propose a novel prediction strategy that separately forecasts fundamental trends driven by the medium-to-long-term evolution of the solar cycle and the 27 day rotational modulation, along with transient disturbances caused by solar flares and the rapid evolution of active regions. These forecasts are then integrated to enhance overall prediction accuracy. We incorporate additional features such as the soft X-ray flare index (FI_SXR), magnetic type of the active region (new_Mag), and X-ray background flux (XBF) to enhance the understanding of the underlying physical processes of solar activity. Our experiments, conducted using advanced forecasting models on the SG-F10.7-All data set, validate the efficacy of our proposed strategy. Notably, the iTransformer model demonstrates superior performance in both short-term and medium-term forecasting scenarios. The inclusion of FI_SXR, new_Mag, and XBF significantly improves forecasting accuracy, highlighting their importance in improving the F10.7 index predictions. Our method outperforms international models from the Space Weather Prediction Center, British Geological Survey, and Collecte Localisation Satellites, exhibiting greater accuracy and adaptability across various solar activity phases. This finding provides a novel approach for precise forecasting of the F10.7 index.

A Study of Real-time Detection Methods for Solar Radio Burst Identification

Solar flares, coronal mass ejections, and other solar radio burst phenomena release substantial amounts of solar radiation energy, resulting in adverse space weather conditions and posing significant hazards in space. Spectrum analysis conducted manually or with traditional image processing algorithms is limited by low efficiency and accuracy. This paper investigates solar radio burst detection methods and their applications. Five solar radio burst detection methodsContinuous-3, Sum Flux-3, Continuous Slope, Sum Flux Slope, and Sum Flux Continuous-3are developed and validated using data from the Japanese NoRP and the Australian Learmonth Solar Radio Observatory. The results show that all five methods can detect solar radio bursts to some degree. Considering the combined metrics of success rate, false detection rate, and real-time performance, the Sum Flux Continuous-3 method is deemed the optimal method among the five. Additionally, the Sum Flux Slope method, which is not reliant on historical data, demonstrates superior universality. Finally, we implement the Sum Flux Slope method on a 39.540 GHz two-element interferometer, achieving real-time solar radio burst detection in the upper computer software. The method also includes functionalities for email alerts, burst information recording, and control parameter adjustment, confirming its effectiveness and practicality. Test results demonstrate the method's effectiveness in real-time solar radio burst detection.

Type IV-like Solar Radio Burst Consisting of a Series of Short-time Bursts Observed by PSP

Solar and interplanetary radio bursts can reflect the existence and motion of energetic electrons and are therefore a kind of vital phenomenon in solar activities. The present study reported a solar radio storm observed by the Parker Solar Probe (PSP) in its eighth orbital encounter phase, and it lasted about 20 hr in a frequency range of 0.515 MHz, called the type IV-like burst. It consists of a series of numerous short-time (ST) bursts with the central frequency drifting slowly from ~5 to ~1 MHz, and each individual ST burst appears at a much faster frequency drifting rate and has a typical frequency range of a few MHz and a short duration of about 14 minutes. Based on the empirical models of the solar atmosphere adopted commonly, combining the in situ measurement by PSP, we analyzed and compared some possible mechanisms for the generation of these small-scale ST bursts and proposed that they were generated probably by a group of solitary kinetic Alfvn waves (SKAWs) in a magnetic loop accompanying coronal mass ejection and slowly moving outward, in which the frequency drifting of individual ST burst is caused by the SKAW's propagation and the central frequency drifting may be attributed to the motion of the magnetic loop.

Emission Characteristics of Energetic Electrons with Crescent-shaped Velocity Distributions

Solar flares release magnetic energy through reconnection, accelerating electrons into nonthermal velocity distributions, including crescent- shaped electron populations. These energetic electron distributions are crucial in driving instabilities that can lead to distinct electromagnetic emissions. This study investigates the emission properties of crescent-shaped electron velocity distribution functions under different frequency ratios (_pe/_ce), critical for understanding plasma conditions in various astrophysical environments, by comparing the emissions and intensities of waves among different cases. Here, we study and analyze three distinct frequency ratio conditions (2.2, 10, and 1, designated as cases A, B, and C, respectively). We find that the beam-Langmuir and upper-hybrid modes can be efficiently excited, leading to further plasma emissions in different cases. Our study reveals that the fundamental (O/F) emission can reach a maximum value of 10⁴E_k0, while the harmonics (H) can extend to 1.5 10⁵E_k0, depending on the frequency ratio of the environment. The intensity of the fundamental mode exceeds previous findings for pure-ring, pure-beam, and ringbeam distributions, highlighting the impact of crescent-shaped electron velocity distributions on wave excitation and emission processes. This effect is notably influenced by different frequency ratios, offering new insights into the way that nonthermal electron distributions affect the plasma emission process.

Magnetic Field Geometry and Anisotropic Scattering Effects on Solar Radio Burst Observations

The fine structures of solar radio bursts reveal complex dynamics in the corona, yet the observed characteristics of these subsecond bursts are additionally complicated by radio wave scattering in the turbulent solar corona. We examine the impact of anisotropic turbulence in radio wave propagation simulations with nonradial magnetic field structures in shaping the morphology, time characteristics, and source positions of fine structures. The apparent sources are found to move along the direction of the magnetic field lines and not along the density gradient, whereas the major axis of the scattered source is perpendicular to the local magnetic field (the scattering anisotropy axis). Using a dipolar magnetic field structure of an active region, we reproduce observed radio fine-structure source motion parallel to the solar limb associated with a coronal loop and provide a natural explanation for puzzling observations of solar radio burst position motions with the Low Frequency Array. Furthermore, the anisotropy aligned with a dipolar magnetic field causes the apparent-source images to bifurcate into two distinct components, with characteristic sizes smaller than in unmagnetized media. The temporal broadening induced by scattering reduces the observed frequency drift rate of fine structures, depending on the contribution of scattering to the time profile. The findings underscore the role of magnetic field geometry and anisotropic scattering for the interpretation of solar radio bursts and highlight that anisotropic scattering produces more than a single source.

Revisiting Sunspot Number As An Extreme Ultraviolet (Euv) Proxy For Ionospheric F2 Critical Frequency

This study reconsiders sunspot number (Sn) as a solar extreme ultraviolet (EUV) proxy for modeling the ionospheric F2 layer's critical frequency (foF2) over the period 1960-2023. We compare the performance of Sn with F10.7 and F30 solar radio fluxes, focusing on their ability to model the Ionospheric Global index (IG). Our results reveal that while F30 has shown a better correlation in recent solar cycles, Sn is the most stable and reliable over the entire dataset, obtaining the highest correlation. In addition, if we remove the saturation effects from considering a maximum value of Sn, the correlation increases, outperforming all other proxies and correctly predicting the long-term trend estimated by general circulation models.

Investigation Of The October Effect In Very Low-Frequency (Vlf) Signals

Subionospheric very low-frequency (VLF) radio signals are reflected by free electrons in the ionospheric D-region at about 60-90 km altitude and can propagate over long distances, which makes them useful for monitoring the state of the D-region or perturbations due to solar flares. At the D-region height, the ionosphere is mainly ionized by solar Lyman- radiation. The reflection characteristics of VLF signals depend on the state and dynamics of the D-region, which is highly influenced by Lyman- radiation. Although the amplitude of the received terrestrial VLF signal changes as a function of solar zenith angle over the course of the year, the VLF amplitude shows a distinctive sharp decrease around October, which is hence called the "October effect". This study investigates the occurrence of the October effect and its dependencies on latitude and longitude. We developed a method to detect the occurrence of the October effect in the long-term VLF data and derive key parameters characterizing (start and end date, intensity) the sudden decrease in the signal amplitude. This investigation using a network of VLF stations distributed over low-, middle-, and high- latitude regions shows that the occurrence of the October effect has a clear latitudinal dependency, occurring earlier in high-latitude regions than at midlatitudes. No low-latitude signature is found.

Large solar energetic particles and solar radio emissions during Cycle 25. A comparative analysis of trends and characteristics with cycles 23 and 24

Solar energetic particles (SEPs) affect space weather in both the heliosphere and on Earth. The present study investigates the occurrences of large SEP events focusing on their influence on the Earth, as well as their correlation with solar radio bursts (SRBs). The velocity dispersion analysis (VDA) is used to calculate the release times of large SEP events from their launch locations, as well as their apparent path length that connects them to interplanetary magnetic fields lines. According to the study, 122 large SEP events impacted the planet Earth from 1997 to 2024. The comparison of occurrence rates with previous solar cycles (SCs) suggests that SC 25 will peak with greater solar activity than the cycle 24 in terms of large SEP occurrences since large SEP events correlate well with the pattern of the sunspot cycle progression. In general, a few (35 out of 122) large SEP events are typically released from the Sun at times that coincide with the peak of associated solar flares and the onsets of corresponding SRBs indicating no delay while the rest have delayed in the release. The projected apparent lengths (L) range from 1.0 to 3.0 AU, with L exceeding 1.5 AU due to particle scattering and launch site pitch angles. The majority (115/122) of SEP occurrences are accelerated by shock waves from solar flares, CMEs, and fast plasma flow in the magnetic reconnection regions. Relevant SRBs for space weather study as they precede large SEP events diagnose the properties of particle populations propelled by solar flares and CMEs. This study finds that 90% of the large SEP events are preceded by solar radio emissions of type II, III and IV; and WAVES/STEREO revealed 76% of SRBs have continuation in IP medium indicating the dynamics of associated shocks and electron beams traveling along open and quasi-open magnetic field lines. Thus, SRB monitoring continues to be a valuable tool for studying space weather and understanding physical phenomena in the solar corona and IP medium, such as particle populations that cause large SEP occurrences.

The Impact of Space Weather on Ultraviolet Radiation at Mid Latitudes

Solar ultraviolet (UV) radiation plays a crucial role in various environmental, atmospheric, and industrial applications, despite covering only a small portion of the electromagnetic solar radiation. The distribution of UV radiation reaching the Earth's surface is influenced by multiple environmental, atmospheric, and astronomical factors, some of which require further investigation. The aim of this study is to explore the potential impact of cosmic rays and space weather parameters on solar UV radiation. To achieve this, the study utilized solar UV data collected from the King Abdulaziz City for Science and Technology (KACST) radiometric station, situated in central Saudi Arabia (latitude 24 43, longitude 46 40, altitude 613 m) for the period 20152021, along with space weather parameters. These parameters include solar radio flux at 10.7 cm (F10.7 cm) and sunspot number (SSN), as well as cosmic ray data from the Oulu neutron monitor for the same period as the UV data. The study conducted correlation analyses between the time series UV data and the considered parameters, revealing mutual relationships between these variables. UV radiation demonstrated a positive correlation with the SSN and F10.7 cm radio flux while showing a significant anticorrelation with the CR data. Furthermore, the mean daily time series of the variables underwent power spectral analyses to identify potential common periodicities. The spectral analyses showed several common periodicities in all the variables considered, such as ~410, 172, 165, and 66 days. Based on the correlations and spectral results, the study suggests that solar signals could have a direct or indirect impact on the observed solar UV radiation at a specific site. This finding has significant implications for our understanding of the factors that affect UV radiation.

Analysis of Seismic Ionospheric Effects and Prediction of TEC During Earthquakes Occurred in Indonesia Based on GPS Data

Total electron content (TEC), which quantifies the quantity of free electrons in the Earth's ionosphere, is a crucial parameter that experiences discrepancies during seismic events. This study investigates the potential of utilizing TEC prediction at the BAKO position in Indonesia during earthquakes. TEC data and solar parameters were collected for six preselected earthquakes, encompassing the earthquake event periods. Three prediction models, namely, ARMA, OKSM 1, and OKSM 2, were employed to predict TEC for a period spanning 8 days. The input parameters required for TEC prediction were obtained from the IONOLAB and OMNIWeb database. The OKSM 1 model is constructed with the input parameters like solar radio flux at 10.7 cm (F10.7), disturbance storm time index (Dst), solar wind (Sw), sunspot number (SSN), and TEC values, while the OKSM 2 model is developed with the parameters like geomagnetic indices (Kp and Ap) and solar indices SSN and F10.7 along with TEC data. The ARMA model is constructed with TEC data. The primary objective of this research is to assess the utility of TEC prediction based on the influence on input parameters for the kriging models and to identify the most effective model for predicting TEC variations associated with seismic events. Four evaluation metrics were systematically utilized to gauge the performance of each model. This rigorous evaluation aims to deliver perceptions into the predictive accuracy, reliability, and potential practical implications of TEC predicting during earthquakes. Upon comparison, the OKSM 2 model demonstrated superior predictive accuracy, exhibiting a notable agreement with the true TEC. The results suggest that OKSM 2 holds promise as a reliable model for earthquakerelated TEC prediction. The average RMSE values range from 4.06 to 8.06, indicating the models' ability to predict seismic events with a reasonable magnitude of error. Similarly, the average MAE values, ranging from 3.32 to 6.71, underscore the models' overall accuracy in predicting the absolute differences between actual and predicted TEC. The CC values, averaging between 0.97 and 0.99, highlight a strong relationship between predicted and actual TEC values. Additionally, the average sMAPE values, ranging from 0.11 to 0.21, demonstrate the models' effectiveness in minimizing percentagebased errors. While variations exist across different earthquakes, these average metrics collectively suggest promising predicting capabilities.

Horizontal Short Monopole Antenna for Radiosolariz Solar Radio Telescope

This paper elaborates on the possible implementation of a horizontally shortened monopole antenna in broadband radio reception of radio emissions originating from the Sun. The spectrum of interest is in the low portion of the very high frequency (VHF) radio band. The frequency range extends down to the top end of the high frequency (HF) spectrum near 20 MHz and up to 65 MHz. The ionosphere is mostly radio-transparent to these radio waves, permitting observations of solar activity in the mentioned radio band. The proposed monopole antenna is an un-tuned wideband antenna suitable for reception only. This aerial is inadequate for the transmission of radio waves due to its low efficiency. Another prerequisite for implementing this antenna is the presence of high background noise the background noise level commonly encountered in the low part of the VHF band, even in rural regions, is of the expected magnitude. The suggested antenna has superior qualities than most other antennas suitable for the task. It shows benefits such as small dimensions, being hard to break, simplicity of construction, and consisting of a single element both electrically and mechanically. Other advantages of the proposed antenna include a very low cost of building and maintenance, straightforward operation, small weight, portability, an omnidirectional gain pattern in the plane perpendicular to the antenna element, and a uniformly and monotonically increasing smooth gain curve as a function of frequency. This aerial has been built for the aims of the radioSolariz project (www.radiosolariz.space), which is a Bulgarian radio telescope project initiated in 2019 at the Space Research and Technology Institute Bulgarian Academy of Sciences. The project is backed by a number of designs and innovations protected by patents and utility models registered at the Patent Office of the Republic of Bulgaria.

Solar flare observations with the Radio Neutrino Observatory Greenland (RNO-G)

The Radio Neutrino Observatory Greenland (RNO-G) seeks discovery of ultra-high energy neutrinos from the cosmos through their interactions in ice. The science program extends beyond particle astrophysics to include radioglaciology and, as we show herein, solar observations, as well. Currently seven of 35 planned radio-receiver stations (24 antennas/station) are operational. These stations are sensitive to impulsive radio signals with frequencies between 80 and 700 MHz and feature a neutrino trigger threshold for recording data close to the thermal floor. RNO-G can also trigger on elevated signals from the Sun, resulting in nanosecond resolution time-domain flare data; such temporal resolution is significantly shorter than from most dedicated solar observatories. In addition to possible RNO-G solar flare polarization measurements, the Sun also represents an extremely useful above-surface calibration source. Using RNO-G data recorded during the summers of 2022 and 2023, we find signal excesses during solar flares reported by the solar-observing Callisto network and also in coincidence with 2/3 of the brightest excesses recorded by the SWAVES satellite. These observed flares are characterized by significant time-domain impulsivity. Using the known position of the Sun, the flare sample is used to calibrate the RNO-G absolute pointing on the radio signal arrival direction to sub-degree resolution. We thus establish the Sun as a regularly observed astronomical calibration source to provide the accurate absolute pointing required for neutrino astronomy.

Determining the acceleration regions of in situ electrons using remote radio and X-ray observations

Context. Solar energetic particles in the heliosphere are produced by flaring processes on the Sun or by shocks driven by coronal mass ejections. These particles are regularly detected remotely as electromagnetic radiation (X-rays or radio emission), which they generate through various processes, or in situ by spacecraft monitoring the Sun and the heliosphere. Aims. Our aim is to combine remote-sensing and in situ observations of energetic electrons to determine the origin and acceleration mechanism of these particles. Methods. Here we investigate the acceleration location, escape, and propagation directions of electron beams producing radio bursts observed with the Low Frequency Array (LOFAR), hard X-ray (HXR) emission, and in situ electrons observed at Solar Orbiter on 3 October 2023. These observations are combined with a three-dimensional (3D) representation of the electron acceleration locations and results from a magnetohydrodynamic (MHD) model of the solar corona in order to investigate the origin and connectivity of electrons observed remotely at the Sun to in situ electrons. Results. We observed a type II radio burst with good connectivity to Solar Orbiter, where a significant electron event was detected. However, type III radio bursts and hard X-rays were also observed co-temporally with the electron event, but likely connected to Solar Orbiter by different far-side field lines. The injection times of the Solar Orbiter electrons are simultaneous with both the onset of the type II radio burst, the group of type III bursts, and the presence of a second HXR peak; however, the most direct connection to Solar Orbiter is that of the type II burst location. The in situ electron spectra point to shock acceleration of electrons with a short-term connection to the source region. Conclusions. We propose that there are two contributions to the Solar Orbiter electron fluxes based on the results and magnetic connectivity determined from remote-sensing data: a smaller flare contribution from the far-side of the Sun and a main shock contribution from the region close to the eastern limb as viewed from Earth. We note that these two electron acceleration regions are distinct and separated by a large distance and are connected via two separate field lines to Solar Orbiter.

Simulation of weak electron beam injection into plasma with open boundary conditions

Context. Different high-energy events lead to the generation of electron beams in the solar atmosphere as well as in planetary magnetospheres. The propagation of these beams through space plasma becomes a main source of non-thermal emission, primarily on the harmonics of the fundamental plasma frequency. Due to the high level of non-linearity and the complexity of such systems, theoretical studies of them are largely based on numerical simulations. However, it is still common practice to use a simplified model in which periodic boundary conditions for fields and particles are used to simulate an infinite plasma. Aims. In this work, the first attempt at high-resolution studies of the dynamics of a weak beam in space plasma using a model with open boundary conditions is reported. The general results of the simulations are compared with those obtained previously using the approximation of infinite plasma. Methods. The continuous injection of an electron beam with an average velocity of v_b = 0.25c (c speed of light) and a relative density of n_b/n₀ = 5 10⁴ (n₀ plasma density) into an unmagnetised plasma was simulated in a quasi-1D approximation using a collisionless electromagnetic particle-in-cell code. The background plasma was initially homogeneous and consisted of electrons and protons with the real mass ratio. The total simulation time was 10 000 _p0¹, where _p0 is the Langmuir frequency for the given n₀. Results. The present simulations demonstrate the formation of a spatially localised Langmuir turbulence in the close vicinity of the beam injection site. The continuous injection of fresh beam particles increases the amplitude of the plasma waves to values larger than those possible when simulating the same parameters in a simplified model. Plasma waves in this region turn out to be unstable against the modulation instability, so the formation of density wells followed by plasma wave trapping is observed. Some of the beam particles are significantly accelerated by previously arisen plasma waves. On average, only 10% of the beam energy gets lost in the system, but the distribution function is transformed into a flat- top form with a supra-thermal tail. Conclusions. The obtained results demonstrate several significant differences from the results of simulations using the approximation of infinite plasma. This fact emphasises the importance of using of a more realistic model for simulations of beam-plasma systems. In addition, using the model with open boundaries, in contrast to the simplified model, will allow us to correctly investigate the influence of not only random gradients of the plasma parameters, but also regular ones.

V. Magnetohydrodynamic waves in a fibrillar structure

Magnetohydrodynamic (MHD) waves, playing a crucial role in transporting energy through the solar atmosphere, manifest in various chromospheric structures. Here, we investigated MHD waves in a long-lasting dark fibril using high-temporal-resolution (2 s cadence) Atacama Large Millimeter/submillimeter Array (ALMA) observations in Band 6 (centered at 1.25 mm). We detected oscillations in brightness temperature, horizontal displacement, and width at multiple locations along the fibril, with median periods and standard deviations of 240 114 s, 225 102 s, and 272 118 s, respectively. Wavelet analysis revealed a combination of standing and propagating waves, suggesting the presence of both MHD kink and sausage modes. Less dominant than standing waves, oppositely propagating waves exhibit phase speeds (median and standard deviation of distributions) of 74 204 km/s, 52 197 km/s, and 28 254 km/s for the three observables, respectively. This work demonstrates ALMA's capability to effectively sample dynamic fibrillar structures, despite previous doubts. This provides valuable insights into wave dynamics in the upper chromosphere.

Possible Mechanisms and Impacts

One of the most intense geomagnetic storms of recent times occurred on 10-11 May 2024. With a peak negative excursion of Sym-H below -500 nT, this storm is the second largest of the space era. Solar wind energy transferred through radiation and mass coupling affected the entire Geospace. Our study revealed that the dayside magnetopause was compressed below the geostationary orbit (6.6 RE) for continuously 6 hr due to strong Solar Wind Dynamic Pressure (SWDP). Tremendous compression pushed the bow-shock also to below the geostationary orbit for a few minutes. Magnetohydrodynamic models suggest that the magnetopause location could be as low as 3.3RE. We show that a unique combination of high SWDP (15 nPa) with an intense eastward interplanetary electric field (IEF_Y 2.5 mV/m) within a super-dense Interplanetary Coronal Mass Ejection lasted for 409 min-is the key factor that led to the strong ring current at much closer to the Earth causing such an intense storm. Severe electrodynamic disturbances led to a strong positive ionospheric storm with more than 100% increase in dayside ionospheric Total Electron Content (TEC), affecting GPS positioning/navigation. Further, an HF radio blackout was found to occur in the 2-12 MHz frequency band due to strong D- and E-region ionization resulting from a solar flare prior to this storm.

Improving Solar Proton Event Forecasting by Means of Automated Recognition of Type-III Radio Bursts

This work reports on an attempt toward improving the Relativistic Electron Alert System for Exploration (REleASE): the occurrence of a type-III radio burst as a precondition for a REleASE forecast. REleASE forecasts are based on the detection of early arrival of near- relativistic electrons ahead of more hazardous protons from Solar Energetic Particle (SEP) events. The goal is to allow astronauts on a Lunar or Mars mission sufficient advance warning to reach a radiation shelter to minimize radiation dose exposure. We test a new system that sets a condition of the occurrence of a type-III radio burst, thus adding independent evidence of particle escape from the Sun, with the aim of reducing known sources of false-alarms of the existing REleASE system. The High Energy Solar Particle Events foRecastIng and Analysis (HESPERIA) REleASE+ system, which takes advantage of availability of real-time solar radio observations during the passage of STEREO-A by Earth in 2023, has now been incorporated in the HESPERIA framework. We discuss the techniques used for automatic detection of type-III radio bursts preparing for its real-time implementation, the determination of selection criteria for type-III bursts that are candidates for solar proton events in the Earth-moon system, and first results of the combined system.

Spectral Analysis of Two-Year Ionospheric Data Series

This study demonstrates a rich complexity of the time-frequency ionospheric signal spectrum, dependent on the measurement type and platform. Different phenomena contributing to satellite-derived and ground-derived geophysical data that only selected signal bands can be potentially sensitive to seismicity over time, and they are applicable in lithosphere-atmosphere-ionosphere coupling (LAIC) studies. In this study, satellite-derived and ground-derived ionospheric observations are filtered by a Fourier-based band-pass filter, and an experimental selection of potentially sensitive frequency bands has been carried out. This work focuses on band-pass filtered ionospheric observations and seismic activity in the region of the Aegean Sea over a two-year time period (2020-2021), with particular focus on the entire system of tectonic plate junctions, which are suspected to be a potential source of ionospheric disturbances distributed over hundreds of kilometers. The temporal evolution of seismicity power in the Aegean region is represented by the record of earthquakes characterized by M 4.5, used for the estimation of cumulative seismic energy. The ionospheric response to LAIC is explored in three data types: short inspections of in situ electron density (Ne) over a tectonic plate boundary by Swarm satellites, stationary determination of three Ne density profile parameters by the Athens Digisonde station AT138 (maximum frequency of the F2 layer: foF2; maximum frequency of the sporadic E layer: foEs; and frequency spread: ff), and stationary measure of vertical total electron content (VTEC) interpolated from a UPC-IonSAT Quarter-of-an-hour time resolution Rapid Global ionospheric map (UQRG) near Athens. The spectrograms are made with the use of short-term Fourier transform (STFT). These frequency bands in the spectrograms, which show a notable coincidence with seismicity, are filtered out and compared to cumulative seismic energy in the Aegean Sea, to the geomagnetic Dst index, to sunspot number (SN), and to the solar radio flux (F10.7). In the case of Swarm, STFT allows for precise removal of long-wavelength Ne signals related to specific latitudes. The application of STFT to time series of ionospheric parameters from the Digisonde station and GIM VTEC is crucial in the removal of seasonal signals and strong diurnal and semi- diurnal signal components. The time series formed from experimentally selected wavebands of different ionospheric observations reveal a moderate but notable correlation with the seismic activity, higher than with any solar radiation parameter in 8 out of 12 cases. The correlation coefficient must be treated relatively and with caution here, as we have not determined the shift between seismic and ionospheric events, as this process requires more data. However, it can be observed from the spectrograms that some weak signals from selected frequencies are candidates to be related to seismic processes.

Regional features of ionospheric disturbances during the intense geospace storm of November 4-5, 2023

Relevance. The ionosphere is the main channel that ensures the functioning of radio communication, radionavigation, radar, remote radio sensing, and radio astronomy systems. The parameters of this channel significantly affect the quality and functionality of both terrestrial and space technological systems. The channel parameters are shaped by various factors within the Earthatmosphereionospheremagnetosphere (EAIM) system. The most influential factor is solar storms, which are accompanied by solar flares, solar cosmic radiation, and coronal mass ejections. These events greatly disturb the atmosphereionospheremagnetosphere radio channels, leading to disruptions in the operation of radio systems for various purposes. Therefore, comprehensive research into radio channel disturbances is a relevant task. The aim of this work is to describe the results of research on the regional peculiarities of ionospheric disturbances through the analysis of derivatives from global ionospheric maps, specifically maps of percentage increases in total electron content (TEC) values. Methods and Methodology. The primary data used for this study are global ionospheric maps compiled by the Center for Orbit Determination in Europe, which are freely available on the website of The Crustal Dynamic Data Information System. Results. For the first time, using GNSS technologies, maps of percentage increases in TEC values in the ionosphere were constructed and studied, which can be interpreted as an ionospheric disturbance index. The response of TEC to the powerful geospace storm of November 4-5, 2023, was investigated. It was found that during most of the main phase of the magnetic storm, the largest ionospheric disturbances were observed at latitudes significantly lower than the Arctic and Antarctic Circles, indicating a reconfiguration of the ionospheremagnetosphere current system, the emergence of significant currents, and a change in ionospheric weather conditions.

Heliospheric Effect on Solar Activity Parameters during Maximum Phase of Solar Cycle 24 (20122015)

The time series of daily data on solar activity proxies, namely the sunspot number (SSN), sunspot area (SSA), solar radio flux (F10.7), modified coronal index (MCI), solar flare index (FI), and cosmic ray intensity (CRI), were analyzed to understand the solar activity modulations and short-term periodicities therein. Rieger-type and other short-term periods include the solar rotational period that covers the maximum activity phase period (maximum phase of solar cycle 24). The wavelet power spectra and Periodogram of SSN, SSA, F10.7, MCI, FI, and CRI exhibited a significant short-term period. The heliospheric effects exist for a particular period (27 days) and they are related to the solar activity phenomena. The cross-correlation coefficients and time lags between the CRI and solar activity parameters were estimated to be 200, 46, 281, 39, and 47 days for SSN, SSA, F10.7, MCI, and FI respectively during the time series 20122015 (maximum phase of solar cycle 24).

Stellar flares

Magnetic storms on stars manifest as remarkable, randomly occurring changes of the luminosity over durations that are tiny in comparison to the normal evolution of stars. These stellar flares are bursts of electromagnetic radiation from X-ray to radio wavelengths, and they occur on most stars with outer convection zones. They are analogous to the events on the Sun known as solar flares, which impact our everyday life and modern technological society. Stellar flares, however, can attain much greater energies than those on the Sun. Despite this, we think that these phenomena are rather similar in origin to solar flares, which result from a catastrophic conversion of latent magnetic field energy into atmospheric heating within a region that is relatively small in comparison to normal stellar sizes. We review the last several decades of stellar flare research. We summarize multi-wavelength observational results and the associated thermal and nonthermal processes in flaring stellar atmospheres. Static and hydrodynamic models are reviewed with an emphasis on recent progress in radiation- hydrodynamics and the physical diagnostics in flare spectra. Thanks to their effects on the space weather of exoplanetary systems (and thus in our search for life elsewhere in the universe) and their preponderance in Kepler mission data, white-light stellar flares have re-emerged in the last decade as a widely-impactful area of study within astrophysics. Yet, there is still much we do not understand, both empirically and theoretically, about the spectrum of flare radiation, its origin, and its time evolution. We conclude with several big-picture questions that are fundamental in our pursuit toward a greater understanding of these enigmatic stellar phenomena and, by extension, those on the Sun.

Effects of a Solar Flare on Global Propagation of Extremely Low Frequency Waves

Solar flares have profound impacts on the lower ionosphere and long- distance radio propagation. Extremely low frequency (ELF: 33,000 Hz) waves are challenging to observe and experience unique interactions with the lower ionosphere. The primary natural sources of ELF waves are thunderstorm lightnings across the globe. Using a newly developed azimuth determination technique and improved observation hardware we show that ELF attenuation in the Earth-Ionosphere spherical cavity decreases and propagation velocity increases under the influence of an M-class solar flare. Using a two-parameter model of the lower ionosphere, the observations are shown to be consistent with increased electron density and sharper gradients in the D-region resulting from X-ray radiation. The sharper electron density gradient is primarily responsible for the propagation velocity increase, suggesting a unique capability that ELF observations can bring to global remote sensing of the lower ionosphere under space weather perturbations.

Differential responses of total ozone content to solar activity parameters at two Saudi Arabian locations

This study examines the correlations between Total Ozone Content (TOC) at two locations in Saudi ArabiaAbha and Jeddahand various solar activity indicators (sunspot numbers, solar radio flux) and cosmic rays, using data spanning from 1979 to 2023. The research employs correlation analyses and spectral techniques, such as Fast Fourier Transform and wavelet analysis, to explore these relationships. The results reveal significant non-zero correlations between changes in TOC at both Saudi sites and the studied solar activity indicators and cosmic rays, with these correlations varying in strength and significance across different solar cycles and seasons. Spectral analysis suggests the presence of several periodicities in the TOC data from both sites, including cycles of 3.9 years, 2.63 years, 1.65 years, 1.11.2 years, 325 days (0.88 years), 285293 days (0.780.80 years), 273 days (0.75 years), 249-232 days (0.68 years), and 202-188 days (0.52 years). Notable shared periodicities between TOC and solar activity and cosmic rays data include 2.6 years, 3.83.9 years, 1.56 years, 325 days, 273 days, and 166 days. The findings from both correlation and spectral analyses suggest a potential connection between variations in TOC and solar activity at the specific locations studied. This aligns with previous research indicating that increased UV radiation during periods of high solar activity enhances ozone production, particularly at lower latitudes, and that increased magnetic activity reduces the influx of cosmic rays into the heliosphere, impacting atmospheric ionization.

On Possible Additional Sources of Solar Protons in the Events of September 410, 2017

The article considers the period from September 4 to September 10, 2017, inclusive, during which the last proton events of solar cycle 24 occurred. In order to detect possible additional proton sources and verify the sources already listed in various catalogs, we apply an empirical method for predicting proton events to all solar flares detected during this period. It is based on the threshold criteria of the parent flares. In addition, we apply an algorithm for automatic search of proton flares obtained by machine learning. Two variants of the automatic search algorithm are used: the first one (method 319) does not take into account the duration of the radio emission, while the second one (method 189) imposes a condition on its duration (>2 min). The empirical method shows that, except for the source flares found by the time of the first arrival of solar protons on Earth, other flares of this period do not fulfill all the criteria of "protonicity." An additional test of the automatic method is the detection of proton flares that we selected by the protonicity criteria but that did not make it to the training sample. Method 319 considers proton flares X9.3 on September 6, 2017, M1.4 and X1.3 on September 7, 2017, and C8.3 on September 8, 2017, as proton flares. Method 189 does not consider the flares of September 7 and 8, 2017, as credible proton sources, which is consistent with expert empirical estimates of the protonicity criteria.

Prediction of Range Error in GPS Signals during X-Class Solar Flares Occurred between JanuaryApril 2023 Using OKSM and RNN

Positioning, navigation and time are the cornerstones of satellite navigation. These aspects are frequently affected by ionospheric variations caused by solar flares (SF). In this study, we have attempted to predict the range error (RE) caused by ionospheric delay in Global Positioning System (GPS) signals during six different X-class SF that occurred in the 25th solar cycle using two different approaches, namely, a recurrent neural network (RNN) and the ordinary Kriging-based surrogate model (OKSM). The total electron content (TEC) collected from Hyderabad station along with other input parameter includes the Planetary A and K index (Ap and Kp), solar sunspot number (SSN), disturbance storm time index (Dst), and radio flux measured at 10.7 cm (F10.7) were used for prediction. The OKSM uses the previous six days of datasets to predict the RE on the seventh day, whereas the RNN model uses the previous 45 days of datasets to predict the RE on the 46th day. The performance of both models is evaluated using statistical parameters such as root mean square error (RMSE), normalized root mean square error (NRMSE), Pearson's correlation coefficient (CC), and symmetric mean absolute percentage error (sMAPE). The results indicate that the OKSM performs well in adverse space weather conditions when compared to RNN.

Mechanisms of Zebra Pattern Generation in Solar Radio Emission on the Background of Complex Dynamic Spectra

The discussion about the origin of the zebra pattern has been going on for more than 50 years. In many papers it is usually postulated that the double plasma resonance mechanism always works in the presence of fast particles in the magnetic trap. Due to a number of difficulties encountered by this mechanism, works on its improvement began to appear, mainly in a dozen papers by Karlick and Yasnov, where the whole discussion is based on variability of the ratio of the magnetic field and density height scales and the assumption of some plasma turbulence in the source. Here we show possibilities of an alternative model of the interaction between plasma waves and whistlers. Several phenomena were selected in which it is clear that the ratio of height scales does not change in the magnetic loop as the source of the zebra pattern. It is shown that all the main details of the sporadic zebra pattern in the phenomenon of August 1, 2010 (and in many other phenomena), can be explained within the framework of a unified model of zebra patterns and radio fibers (fiber bursts) in the interaction of plasma waves with whistlers. The main changes in the zebra pattern stripes are caused by scattering of fast particles by whistlers leading to switching of the whistler instability from the normal Doppler effect to the anomalous one. In the end, possibilities of laboratory experiments are considered and the solar zebra pattern is compared with similar stripes in the decameter radio emission of Jupiter.

Geomagnetic field variations due to solar tides at the Indian Observatories

This study investigates the response of solar (S) tidal signatures on the horizontal component of the geomagnetic field at two observatories in India during 19802002 over solar cycles (SC) 2123: Hyderabad (HYB), located in the low-latitude region, and Ettaiyapuram (ETT), situated at the magnetic equator. HYB represents the characteristics of solar quiet (Sq), while ETT is under the equatorial electrojet (EEJ) effect. Our results show the additional information about ter (S3), and quarta- diurnal (S4) tidal signatures of Sq and EEJ, along with diurnal (S1) and semi-diurnal (S2) at both observatories. In Sq solar tide, the average amplitude of S1 tide is consistently higher than that of EEJ tide by ~ 10%. During the same period, the S2, S3, and S4 tidal signatures of Sq are weaker than EEJ by ~ 2%, 5%, and 2.5%, respectively. During solar cycle maxima, the amplitude of the Sq tide is higher in SC-21 than in SC-22 and SC-23 by ~ 13% and 16%, while SC-22 has higher EEJ tidal amplitudes than other SCs by ~ 9%. We observe that the tidal signatures of Sq and EEJ closely follow the trend of solar radio flux (F10.7), except for S4 of Sq. The Pearson correlation coefficients (P) between F10.7 and Sq/EEJ tidal amplitudes exhibit negative to positive correlation coefficients during different phases of SCs. The solar tidal amplitudes of Sq/EEJ (S1-S4) with F10.7 during D, E, and J seasons have varying correlation coefficients, indicating that each tide has a distinct response on the geomagnetic field.

Electron Cyclotron Maser with Moderately Relativistic Electrons

Electron cyclotron maser (ECM) emission is an important coherent emission mechanism for the direct amplification of electromagnetic waves by nonthermal electrons in a magnetized plasma. This paper will report on our recent study on ECM emission by fast electron beams with moderately relativistic energy. The results show that, similar to the spontaneous emission by the magnetic cyclotron motion of energetic electrons in a magnetic field, the coherent emission also exhibits the characteristic of a gradual transition from harmonic emission to continuous emission as the energy of the energetic electrons increases from subrelativistic to relativistic. The effects of the characteristic beam electrons energy (E_c) and the plasma-to-cyclotron frequency ratio (_pe/_ce) on the growth rate, the peak-value frequency, and the spectral width are discussed further. These results are helpful for us to understand phenomena associated with astrophysical radio bursts.

On the Possible Mechanisms of the SEP Event and Electron Enhancement over the SEP Decay Phase on 2023 August 5

We carry out this study on the solar energetic particle (SEP) event that occurred on 2023 August 5 over the ascending phase of the current solar cycle 25. It is found that the SEP event might have been initiated by the M1.6 flare, while the SEP peak was caused by the coronal shock manifested in DH-type II radio burst over the propagation phase of a halo coronal mass ejection (CME; 1000 km s¹), thus creating a mixed SEP event. There were two enhancements of the electron fluxes lying over the SEP rise and decay phase. It is surprising that, despite a stronger flare (X1.6) and a faster halo CME (1647 km s¹), there was no SEP enhancement during the second enhancement of the electron fluxes. In order to investigate this, we make an additional effort to analyze the X1.6 flare based on the availability of the temporal, spectral, and spatial evolution of the electromagnetic radiation components. It is observed that the CME shock was aligned with the flare eruption direction and was close to the western limb (W77), and thus the radially moving CME shock missed the Earth. In another development, it is observed that the electron impulsive phase lies over the type III radio bursts, indicating that the electrons might have escaped directly during the eruption. The radio flux and radio dynamic spectra of a higher frequency lie over the rise phase of the soft X-ray derivative, indicating that a large number of electrons travelled through magnetic fields.

Advection-nonlinear-diffusion Model of Flare Accelerated Electron Transport in Type III Solar Radio Bursts

Electrons accelerated by solar flares and observed as type III solar radio bursts are not only a crucial diagnostic tool for understanding electron transport in the inner heliosphere but also a possible early indication of potentially hazardous space weather events. The electron beams traveling in the solar corona and heliosphere along magnetic field lines generate Langmuir waves and quasilinearly relax toward a plateau in velocity space. The relaxation of the electron beam over the short distance in contrast to large beam-travel distances observed is often referred to as Sturrok's dilemma. Here, we develop a new electron transport model with quasilinear distance/time self-consistently changing in space and time. This model results in a nonlinear advection- diffusion equation for the electron beam density with a nonlinear diffusion term that is inversely proportional to the beam density. The solution predicts slow super-diffusive (ballistic) spatial expansion of a fast-propagating electron beam. This model also provides the evolution of the spectral energy density of Langmuir waves, which determines brightness temperature of plasma radiation in solar bursts. The model solution is consistent with the results of numerical simulation using kinetic equations and can explain some characteristics of type III solar radio bursts.

math> index and its application study on the Chinese Langfang dataset

The 10.7 cm solar radio flux (F10.7) is a key indicator of solar activity. Accurately forecasting of F10.7 is crucial for reducing the impact of solar activity on fields such as radio communication, navigation, and satellite communication. In this work, we present a novel channel-independent patch time series Transformer (PatchTST) for F10.7 forecasting. This is the first time that the PatchTST model is applied to F1 0.7 forecasting. We construct the F10.7< /mml:msub> dataset, which is measured by the Dominion Radio Astrophysical Observatory (DRAO) in Canada. We compare the performance of PatchTST, N-Beats, BiGRU, and CNN-BiGRU on DRAO data. The root mean squared error (RMSE), mean absolute percentage error (MAPE), and correlation coefficient (R) of our PatchTST model are 4.731, 2.351%, and 0.986, respectively, which outperforms those of the other models when the prediction length is 1 day. Especially in mid-term forecasting, the PatchTST model performs much better than those of the other models. We make uncertainty analyses on these models, and the PatchTST model exhibits superior adaptability to model uncertainty compared to the N-Beats, BiGRU, and CNN-BiGRU. The PatchTST model shows a 62.9% improvement in mean error (ME) and a 40.5% improvement in standard mean error (STDE) compared to the benchmark data provided by Space Environment Technologies (SET). This work also shows that our PatchTST model generalizes well by applying it to other F< mml:mrow>10.7 observational data originating from Long and Short-band Solar Precision Flux Radiotelescope (L&S) in China.

Automated classification of solar radio burst type II, III and IV for CALLISTO spectra using physical properties during maximum of solar cycle 24

Continuous observation of solar radio bursts (SRBs) throughout the year using the CALLISTO spectrometer generates a huge volume of spectral data. This study introduces a burst-classifier algorithm, which is an automated algorithm, to classify the SRB spectrum into three solar radio bursts, namely Type II (SRBT II), Type III (SRBT III) and Type IV (SRBT IV). The proposed algorithm was designed using four characteristic parameters derived from a collection of training dataset files. The characteristic parameters were derived from the intensity bursts observed on frequency channels and timesteps of the spectrum. This dataset consisted of 50 spectra of SRBT II and SRBT III, along with 40 spectra for SRBT IV, collected during the solar maximum of 2014 (Solar Cycle 24). After observations and analysis of the training dataset, each burst type was set up with a threshold. A training dataset of 80 data spectra from 2013 to 2016 was used to test the algorithm. Accuracy of the proposed algorithm was calculated using the percentage of true positives (TP) and false positives (FP). Findings demonstrate an accuracy of 74 % with 57 out of 80 spectra classified as TP and 23 spectra as FP.

Key Characteristics and Trends

This paper presents a statistical analysis of doublet type II radio bursts, specifically the first type II (type II₁) and the second type II (type II₂), during solar cycles 23 and 24. The study focuses on radio-rich CMEs that are accompanied by type II solar radio bursts, covering a period of 15 years from November 1997 to December 2015. The characteristics of these doublet type II radio bursts were analyzed using data from the Culgoora radio spectrograph. Key parameters such as lifetime, start frequency, end frequency, bandwidth, drift rate, and estimated shock speed (ESS) of both the types of radio burst were examined. During solar cycles 23 and 24, the start frequencies of first type II bursts were generally higher than those of the second type II bursts, suggesting different driving mechanisms for the two types. A good correlation was found between start frequency and drift rate throughout the study period, with correlation coefficients of for solar cycle 23 and for solar cycle 24. This indicates that doublet type II radio bursts consistently drift from a high start frequency to a low end frequency, reflecting the rapid propagation of the shock away from the sun. However, the correlation between other solar parameters, such as lifetime, bandwidth, drift rate, and ESS was weak, implying that various drivers or mechanisms might influence the occurrence of these radio bursts. Additionally, the ESS of the first type II bursts was observed to be greater than that of the second type II bursts suggesting that doublet type II radio bursts may originate from eruptions of different structures.

Radio Spectrum Observations and Studies of the Solar Broadband Radio Dynamic Spectrometer (SBRS)

Solar radio spectral observation is one of the essential approaches for solar physics research, which helps us study the plasma dynamics in the solar atmosphere. The Solar Broadband Radio Dynamic Spectrometer (SBRS) started observing the Sun at Huairou Solar Observing Station in Beijing, China, in 1999. It has obtained a large amount of high-quality observation data of solar radio dynamic spectra in the centimeterdecimeter wavelengths (1.107.60 GHz). In particular, the observations with high-temporal resolution of millisecond and high- frequency resolution of MHz display plenty of superfine structures in the dynamic spectrum, which provide crucial information on the radiation process of various radio bursts. We review the past history of solar radio spectral observation and scientific results of SBRS. It is meaningful and will undoubtedly help us inspire new ideas for future research. The understanding of the basic plasma processes in solar plasma could also promote the development of solar physics, astrophysics, and space weather. To broaden the observation frequency range, we propose a new spectrometer at millimeter wavelengths (20100 GHz) with ultra-wideband and high timefrequency resolution to study the physical processes in the solar transition region. This will open a new window for solar physics research and will provide crucial observational evidence for exploring a series of major issues in solar physics, including coronal heating, solar eruptions, and the origin of solar winds.

Solar Radio Emissions

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Studying the Properties of Spacetime with an Improved Dynamical Model of the Inner Solar System

Physical properties of the Sun (orientation of rotation axis, oblateness coefficient J2, and change rate of the gravitational parameter ) are determined using a dynamical model describing the motion of the Sun, planets, the Moon, asteroids, and Trans-Neptunian objects (TNOs). Among the many kinds of observations used to determine the orbits and physical properties of the bodies, the most important for our study are precise interplanetary ranging data: EarthMercury ranges from MESSENGER spacecraft and EarthMars ranges from Odyssey and MRO. The findings allow us to improve the model of the Sun in modern planetary ephemerides. First, the dynamically determined direction of the Sun's pole is 2 off the visible axis of rotation of the Sun's surface, which is corroborated by present knowledge of the Sun's interior. Second, the change rate of the Sun's gravitational parameter is found to be smaller (in absolute value) than the nominal value derived from the estimate of mass loss through radiation and solar wind. Possible interpretations are discussed.

Aurora Observations and Low Latitude Space Weather Effects in Mexico

On 10 May 2024, a severe geomagnetic storm coinciding with Mother's Day in Mexico lasted over 40 hr and produced polar auroras observable at low latitudes. This storm, the most intense since 2003, resulted from a series of solar flares and coronal mass ejections from active region 3664. The event was significant for space weather studies in Mexico, marking a milestone by enabling comprehensive measurements of its effects. The Mexico Space Weather Service (SCIESMEX) and the National Space Weather Laboratory (LANCE) had prepared for such an event since their inception. LANCE's instrument networks recorded solar chromospheric images, solar radio bursts, geomagnetic variations, Schumann resonances, ionospheric disturbances, and energetic particle flows. They also monitored Geomagnetically Induced Currents (GICs) in three strategic substations of the national electrical system. This provided unprecedented insights into the dynamics of severe space weather events at the North-American low-latitude environment. Citizen science efforts documented auroras and regional responses, capturing variations in geomagnetic indices, ionospheric disturbances, cosmic ray fluxes, GICs, and technological impacts. SCIESMEX worked with the National Civil Protection System (SINAPROC) to issue warnings, ensuring public awareness and preparedness. This coordination underscores the importance of effective communication and collaboration in mitigating impacts. The May 2024 geomagnetic storm demonstrated the critical role of preparedness, research, and public education in reducing the effects of future space weather events in Mexico.

Time-varying trends from Arctic ozonesonde time series in the years 19942022

Although evidence of recovery in Antarctic stratospheric ozone has been found, evidence of recovery in Arctic ozone is still elusive, even though 25 years have passed since the peak in ozone depleting substances. Here we have used a Dynamic Linear Model to derive time- varying trends over 20-year periods in the Arctic ozone time series, measured in-situ by ozonesondes from 6 stations, from 1994 to 2022. The model accounts for seasonality, external forcing and 1st-order correlation in the residuals. As proxies for the external forcing, we have used tropopause pressure (replaced with Arctic Oscillation in the troposphere), eddy heat flux, the volume of polar stratospheric clouds multiplied by effective equivalent stratospheric chlorine, and solar radio flux at 10.7 cm for the 11-year solar cycle. Our results indicate that the ozone recovery in the lower Arctic stratosphere is not detectable. Though significant positive trends have been detected prior to 2017 at some stations, there are no statistically significant positive trends after 2017. Moreover, at a number of stations the trends after 2019 are rather negative and significant, varying between 0.30 0.25 and 1.00 0.85% per decade. Furthermore, the Arctic troposphere exhibited only statistically significant negative trends over 20-year periods ending in 2017 or later, varying between 0.31 0.27 and 1.76 0.41% per decade. These results highlight the importance of continued monitoring of the Arctic ozone.

An Artificial-Intelligence-Assisted Optimization of Imperceptible Multimode Rectenna

In this letter, we propose and experimentally demonstrate compact, low- profile, and optically transparent antennas for multiband and multirange wireless power transfer (WPT) applications. Specifically, we put forward new types of transparent multiband antennas that can perform the near- field reactive WPT (13.56 MHz), as well as the far-field radiative WPT (980 MHz and 2.45 GHz) within a single device. Furthermore, such an antenna is integrated with compact frequency-scalable rectifying circuits to form an unseeable multimode WPT device. We show that a hybrid inductive (13.56 MHz) and radiative (980 MHz and 2.4 GHz) WPT device can be realized with a modified inverted-F antenna structure connected to spiral coil virtual ground. To meet the stringent design requirements of this unobtrusive multiband antenna, a state-of-the-art machine-learning-assisted global optimization method (parallel- surrogate-model-assisted hybrid differential evolution for antenna optimization) is exploited for global optimization. We envision that the proposed transparent and flexible WPT and energy harvesting devices can be beneficial for many applications, including ubiquitous wireless charging based on smart windows and glasses, solar radio frequency integrated power supply, wearable or textile electronics, and Internet of Things.

Investigating the Behavior and Spatiotemporal Variations of Green-line Emission in the Solar Corona

Understanding coronal structure and dynamics can be facilitated by analyzing green-line emission, which enables the investigation of diverse coronal structures such as coronal loops, streamers, coronal holes, and various eruptions in the solar atmosphere. In this study, we investigated the spatiotemporal behaviors of green-line emissions in both low and high latitudes across nine solar cycles, ranging from Solar Cycle 17 to the current Solar Cycle 25, using the modified homogeneous data set. We employed methodologies such as cross correlation, power spectral density, and wavelet transform techniques for this analysis. We found distinct behaviors in green-line energy across various latitudinal distributions in the solar atmosphere. The trends observed at higher latitudes differ from those at lower latitudes. The emission behaviors show a close association with other solar phenomena like solar flares, sunspots, and coronal mass ejections throughout the solar cycles. The observed variations exhibit harmonic periods. The emission activity is significantly higher in the low latitudes, accounting for over 70% of the emissions, while the higher latitudes contribute less than 30%. The emissions exhibit asymmetric behavior between the northern and southern hemispheres, leading to a 44 yr cycle of solar hemispheric dominance shifts. Various factors, such as Alfvn waves, solar magnetic fields, sunspots, differential rotation, and reconnection events, influence the observed differences in behavior between lower and higher latitudes, suggesting the existence of potential underlying phenomena contributing to deviations in properties, intensity, temporal dynamics, and spatiotemporal lifetime.

Direct Measurements of Synchrotron-emitting Electrons at Near-Sun Shocks

In this study, we present the first-ever direct measurements of synchrotron-emitting heliospheric traveling shocks, intercepted by the Parker Solar Probe (PSP) during its close encounters. Given that much of our understanding of powerful astrophysical shocks is derived from synchrotron radiation, these observations by PSP provide an unprecedented opportunity to explore how shocks accelerate relativistic electrons and the conditions under which they emit radiation. The probe's unparalleled capabilities to measure both electromagnetic fields and energetic particles with high precision in the near-Sun environment has allowed us to directly correlate the distribution of relativistic electrons with the resulting photon emissions. Our findings reveal that strong quasi-parallel shocks emit radiation at significantly higher intensities than quasi-perpendicular shocks due to the efficient acceleration of ultrarelativistic electrons. These experimental results are consistent with theory and recent observations of supernova remnant shocks and advance our understanding of shock physics across diverse space environments.

Estimating the Total Energy Content in Escaping Accelerated Solar Electron Beams

Quantifying the energy content of accelerated electron beams during solar eruptive events is a key outstanding objective that must be constrained to refine particle acceleration models and understand the electron component of space weather. Previous estimations have used in situ measurements near the Earth, and consequently suffer from electron- beam propagation effects. In this study, we deduce properties of a rapid sequence of escaping electron beams that were accelerated during a solar flare on 2013 May 22 and produced type III radio bursts, including the first estimate of energy density from remote-sensing observations. We use extreme-ultraviolet observations to infer the magnetic structure of the source active region NOAA 11745, and Nanay Radioheliograph imaging spectroscopy to estimate the speed and origin of the escaping electron beams. Using the observationally deduced electron-beam properties from the type III bursts and cotemporal hard X-rays, we simulate electron- beam properties to estimate the electron number density and energy in the acceleration region. We find an electron density (above 30 keV) in the acceleration region of 10^2.5 cm³ and an energy density of 2 10⁵ erg cm³. Radio observations suggest the particles travelled a very short distance before they began to produce radio emission, implying a radially narrow acceleration region. A short but plausibly wide slab-like acceleration volume of 10²⁶10²⁸ cm³ atop the flaring loop arcade could contain a total energy of 10²³10²⁵ erg (100 beams), which is comparable to energy estimates from previous studies.

Study on the Temporal Evolution of the Radial Differential Rotation of Solar Corona Using Radio Emissions

The daily measurements of the disk-integrated solar radio flux, observed by the Radio Solar Telescope Network, at 245, 410, 610, 1415, 2695, 4995, and 8800 MHz during the time interval of 1989 January 1 to 2019 December 17, are used to investigate the temporal evolution of radial differential rotation of the solar corona using the methods of ensemble empirical mode decomposition (EEMD) and wavelet analysis. Overall, the results reveal that over the 30 yr period, the rotation rates for the observed solar radio flux within the frequency range of 2458800 MHz show an increase with frequency. This verifies the existence of the radial differential rotation of the solar corona over long timescales of nearly three solar cycles. Based on the radio emission mechanism, to some extent, the results can also serve as an indicator of how the rotation of the solar upper atmosphere varies with altitude within a specific range. From the temporal variation of rotation cycle lengths of radio flux, the coronal rotation at different altitudes from the low corona to approximately 1.3 R exhibits complex temporal variations with the progression of the solar cycle. However, in this altitude range, over the past 30 yr from 1989 to 2019, the coronal rotation consistently becomes gradually slower as the altitude increases. Finally, the EEMD method can extract rotation cycle signals from these highly randomized radio emissions, and so it can be used to investigate the rotation periods for the radio emissions at higher or lower frequencies.

Spectral Characteristics of FundamentalHarmonic Pairs of Interplanetary Type III Radio Bursts Observed by PSP

Based on the observations by the Parker Solar Probe (PSP) during its encounter phases of approaching the Sun, I. C. Jebaraj et al. found that fundamentalharmonic (F-H) pairs constitute a majority of interplanetary (IP) type III radio bursts. In the present Letter, spectral characteristics of the IP F-H pairs are identified and analyzed further. The observations were made with the Radio Frequency Spectrometer (RFS) experiment on the PSP spacecraft in its encounter phase from the first to the ninth orbit as it traveled from 0.17 to 0.074 au from the Sun. The result shows that the occurrence rate of F-H pairs rises significantly with the rise in the number of IP type III radio bursts detected by the PSP or the enhancement in the time resolution of the RFS instrument. In particular, we compare the relationship between F and H spectral characteristics, such as the frequency-drift rate, emission intensity, relative bandwidth, duration, and fine structure. The results will be helpful for us to understand the physics underlying the generation and evolution of the IP F-H pairs as well as other IP type III radio bursts.

A statistical study

In this paper, we present for the first time a comprehensive statistical study between type II radio bursts from the metric (m) to the dekamerichectometric (DH) domain and their associated solar and space weather (SW) phenomena, namely, solar flares (SFs), sunspot (SN) configurations, filament eruptions, coronal mass ejections (CMEs), their interplanetary (IP) counterparts (ICMEs) and shocks, in situ detected particles and geomagnetic storms (GSs). The m-only and m + DH radio signatures are identified from dynamic spectra provided by the ground- based RSTN stations distributed over the globe together with Wind/WAVES satellite data. The DH-only type IIs are adopted from a ready catalog based on Wind/WAVES spacecraft data. We perform the temporal and spatial association between the radio emission and the listed above activity events during solar cycle (SC) 24, separately for the three sub- categories, m-only, m + DH and DH-only type IIs. A quantitative assessment on the occurrence rates is presented as a function of the strength of the specific SW phenomena: highest rates are obtained with CMEs, SFs, filament eruptions, and SN configurations, whereas a much weaker relationship is found with ICMEs, IP shocks, energetic particles, and GSs. The potential of the obtained rates to be used in empirical or physics-based models for SW forecasting is discussed.

Novel scaling laws to derive spatially resolved flare and CME parameters from sun-as-a-star observables

Coronal mass ejections (CMEs) are often associated with X-ray (SXR) flares powered by magnetic reconnection in the low corona, while the CME shocks in the upper corona and interplanetary (IP) space accelerate electrons often producing the type II radio bursts. The CME and the reconnection event are part of the same energy release process as highlighted by the correlation between reconnection flux (_rec) that quantifies the strength of the released magnetic free energy during the SXR flare, and the CME kinetic energy that drives the IP shocks leading to type II bursts. Unlike the Sun, these physical parameters cannot be directly inferred in stellar observations. Hence, scaling laws between unresolved sun-as-a-star observables, namely SXR luminosity (L_X) and type II luminosity (L_R), and the physical properties of the associated dynamical events are crucial. Such scaling laws also provide insights into the interconnections between the particle acceleration processes across low-corona to IP space during solar-stellar "flare-CME-type II" events. Using long-term solar data in the SXR to radio waveband, we derived a scaling law between two novel power metrics for the flare and CME-associated processes. The metrics of "flare power" (P_flare = (L_X_rec)) and "CME power" (P_CME = (L_RV_CME²)), where V_CME is the CME speed, scale as P_flare P_CME^{0.76
0.04}. In addition, L_X and _rec show power- law trends with P_CME with indices of 1.12 0.05 and 0.61 0.05, respectively. These power laws help infer the spatially resolved physical parameters, V_CME and _rec, from disk- averaged observables, L_X and L_R during solar- stellar flare-CME-type II events.

A type II radio burst associated with solar filamentfilament interaction

Aims. Solar radio type II bursts are often associated with coronal shocks driven by solar eruptions. In this study, we report a type II burst associated with filamentfilament interaction. Methods. Combining the high-quality multiwavelength observations from CHASE, SDO, STEREO, and CALLISTO, we conducted a detailed study of the type II burst associated with filamentfilament interaction. Results. On 2023 September 11, an erupting filament (F1) likely disturbed a nearby long filament (F2), causing F2 to subsequently erupt. As a result of possible magnetic reconnection between ejective materials from the two filaments, loop-like structures formed perpendicular to them. Subsequently, the expansion of these loop-like structures triggered a strong coronal mass ejection (CME). Interestingly, a type II burst appeared on the solar spectrum around the time when the loop-like structures formed and the CME appeared above the occulting disk of STEREO/COR1. By converting the frequency of the type II burst to the coronal height using polarization brightness data recorded by the COR1 coronagraph and the spherically symmetric polynomial approximation technique, we determined the formation height of the type II burst to be around 1.45 R, with a speed of approximately 440 km s¹. This is comparable to the observed height of the CME (1.43 R), although slightly lower in speed (540 km s¹). Conclusions. All these results indicate that the type II burst was closely associated with filamentfilament interactions and was possibly excited by the accompanying CME at the flank. We suggest that the filamentfilament interactions played an important role in producing the type II burst by acting as a piston to trigger a strong CME.

Extreme Space Weather Impacts on GNSS Timing Signals for Electricity Grid Management

Extreme space weather events can have serious impacts on critical infrastructure, including Global Navigation Satellite Systems (GNSS). The use of GNSS, particularly as sources of accurate timing signals, is becoming more widespread, with one example being the measurement of electricity grid frequency and phase information to aid grid management and stability. Understanding the likelihood of extreme space weather impacts on GNSS timing signals is therefore becoming vital to maintain national electricity grid resilience. This study determines critical intensity thresholds above which the complete failure of a GNSS based timing system may occur. Solar radio bursts are identified as a simple example to investigate in more detail. The probability of occurrence of an extreme space weather event with an intensity equal to or greater than the critical intensity is estimated. Both a power law and extreme value theory were used to evaluate recurrence probabilities based on historical event frequencies. The probability was estimated to be between 3%-12% per decade to cause the complete failure of any GNSS- based timing system.

Nonlinear Evolution of the Electron Two-Stream Instability

Solar type V radio bursts are associated with type III bursts. Several processes have been proposed to interpret the association, electron distribution, and emission. We present the observation of a unique type V event observed by e-CALLISTO on 7 May 2021. The type V radio emission follows a group of U bursts. Unlike the unpolarized U bursts, the type V burst is circularly polarized, leaving room for a different emission process. Its starting edge drifts to higher frequency four times slower than the descending branch of the associated U burst. The type V processes seem to be ruled by electrons of lower energy. The observations conform to a coherent scenario where a dense electron beam drives the two-stream instability (causing type III emission) and, in the nonlinear stage, becomes unstable to another instability, previously known as the electron firehose instability (EFI). The secondary instability scatters some beam electrons into velocities perpendicular to the magnetic field and produces, after particle loss, a trapped distribution prone to electron cyclotron masering (ECM). A reduction in beaming and the formation of an isotropic halo are predicted for electron beams continuing to interplanetary space, possibly observable by Parker Solar Probe and Solar Orbiter.

Galileo and BeiDou AltBOC Signals and Their Perspectives for Ionospheric TEC Studies

For decades, GNSS code measurements were much noisier than phase ones, limiting their applicability to ionospheric total electron content (TEC) studies. Ultra-wideband AltBOC signals changed the situation. This study revisits the Galileo E5 and BeiDou B2 AltBOC signals and their potential applications in TEC estimation. We found that TEC noises are comparable for the single-frequency AltBOC phase-code combination and those of the dual-frequency legacy BPSK/QPSK phase combination, while single- frequency BPSK/QPSK TEC noises are much higher. A two-week high-rate measurement campaign at the ACRG receiver revealed a mean 100 sec TEC RMS (used as the noise proxy) of 0.26 TECU, 0.15 TECU, and 0.09 TECU for the BeiDou B2(a+b) AltBOC signal and satellite elevations 0-30, 30-60, and 60-90, correspondingly, and 0.22 TECU, 0.14 TECU, and 0.09 TECU for the legacy B1/B3 dual-frequency phase combination. The Galileo E5(a+b) AltBOC signal corresponding values were 0.25 TECU, 0.14 TECU, and 0.09 TECU; for the legacy signals' phase combination, the values were 0.19 TECU, 0.13 TECU, and 0.08 TECU. The AltBOC (for both BeiDou and Galileo) SNR exceeds those of BPSK/QPSK by 7.5 dB-Hz in undisturbed conditions. Radio frequency interference (the 28 August 2022 and 9 May 2024 Solar Radio Burst events in our study) decreased the AltBOC SNR 5 dB-Hz more against QPSK SNR, but, due to the higher initial SNR, the threshold for the loss of the lock was never broken. Today, we have enough BeiDou and Galileo satellites that transmit AltBOC signals for a reliable single- frequency vTEC estimation. This study provides new insights and evidence for using Galileo and BeiDou AltBOC signals in high-precision ionospheric monitoring.

Ionospheric Absorption Variation Based on Ionosonde and Riometer Data and the NOAA D-RAP Model over Europe During Intense Solar Flares in September 2017

A novel method was developed based on the amplitude data of the EM waves measured by Digisondes to calculate and investigate the relative ionospheric absorption changes. The effect of 13 solar flares (>C8) that occurred from 4 to 10 September 2017 were studied at three European Digisonde stations (Juliusruh (54.63N, 13.37E), Prhonice (49.98N, 14.55E) and San Vito (40.6N, 17.8E)). The present study compares the results of the amplitude method with the absorption changes measured by the Finnish Riometer Network and determined by the NOAA D-RAP model during the same events. The X-class flares caused 1.5-2.5 dB of attenuation at 30-32.5 MHz based on the riometer data, while the absorption changes were between 10 and 15 dB in the 2.5-4.5 MHz frequency range according to the amplitude data. The impact caused by energetic particles after the solar flares are clearly seen in the riometer data, while among the Digisonde stations it can be observed only at Juliusruh in some certain cases. Comparing the results of the amplitude method with the D-RAP model it seems evident that the observed absorption values almost always exceed the values given by the model both at 2.5 MHz and at 4 MHz during the investigated period. According to the comparison between the riometer data with the D-RAP, generally, the model underestimates the absorption values obtained from the riometers during solar flares except at the highest latitude stations, while D-RAP overestimates the impact during the particle events.

Trends in Ionospheric Solar Activity Indices

The article presents the first results of identifying trends in annual average ionospheric indices IG₁₂ and T₁₂, which are obtained after excluding from IG₁₂ and T₁₂ the dependence of these indices on solar activity indices. In this case, solar activity indices are F10 and F30solar radio emission fluxes at 10.7 and 30 cm. It was found that for the interval of 19572023, all analyzed linear trends are negative, i.e., quantities IG₁₂ and T₁₂ decrease over time, and these trends are significant. In absolute value, they are maximum for IG₁₂, taking into account the IG₁₂ dependence on F10₁₂, and minimum for T₁₂, taking into account the T₁₂ dependence on F30₁₂. Account for the nonlinearity of trends shows that, e.g., after 2010, they intensified. Relations are presented that make it possible, based on data from trends of the ionospheric indices (IG₁₂ or T₁₂), to judge the nature of the foF2 trend over a specific point. For this, using the IRI model for foF2, a coefficient was obtained that gives the relationship between the trends of the ionospheric index and foF2 over this point. Comparison with experimental data at mid-latitudes revealed that trends of the ionospheric indices make it possible to correctly determine the sign of the foF2 trend and the general tendency for this trend change, but the calculated value of the trend over a specific point may differ markedly from the experimental data.

Temporally Resolved Type III Solar Radio Bursts in the Frequency Range 313 MHz

Radio observations from space allow to characterize solar radio bursts below the ionospheric cutoff, which are otherwise inaccessible, but suffer from low, insufficient temporal resolution. In this Letter we present novel, high-temporal resolution observations of type III solar radio bursts in the range 313 MHz. A dedicated configuration of the Radio and Plasma Waves (RPW) High Frequency Receiver (HFR) on the Solar Orbiter mission, allowing for a temporal resolution as high as 0.07 s (up to 2 orders of magnitude better than any other spacecraft measurements), provides for the very first time resolved measurements of the typical decay time values in this frequency range. The comparison of data with different time resolutions and acquired at different radial distances indicates that discrepancies with decay time values provided in previous studies are only due to the insufficient time resolution not allowing to accurately characterize decay times in this frequency range. The statistical analysis on a large sample of 500 type III radio bursts shows a power low decay time trend with a spectral index of 0.75 0.03 when the median values for each frequency are considered. When these results are combined with previous observations, referring to frequencies outside the considered range, a spectral index of 1.00 0.01 is found in the range 0.05300 MHz, compatible with the presence of radio-wave scattering between 1 and 100 R .

Implications for Turbulence and Electron Acceleration

The excess broadening of high-temperature spectral lines, long observed near the tops of flare arcades, is widely considered to result from magnetohydrodynamic turbulence. According to different theories, plasma turbulence is also believed to be a candidate mechanism for particle acceleration during solar flares. However, the degree to which this broadening is connected to the acceleration of nonthermal electrons remains largely unexplored outside of recent work, and many observations have been limited by limited spatial resolution and cadence. Using the Interface Region Imaging Spectrometer, we present spatially resolved observations of loop-top (LT) broadenings using hot (11 MK) Fe XXI 1354.1 line emission at 9 s cadence during the 2022 March 30 X1.3 flare. We find nonthermal velocities upward of 65 km s¹ that decay linearly with time, indicating the presence and subsequent dissipation of plasma turbulence. Moreover, the initial Fe XXI signal was found to be cospatial and cotemporal with microwave emission measured by the Expanded Owens Valley Solar Array, placing a population of nonthermal electrons in the same region as the LT turbulence. Evidence of electron acceleration at this time is further supported by hard X-ray measurements from the Spectrometer/Telescope for Imaging X-rays on board Solar Orbiter. Using the decay of nonthermal broadenings as a proxy for turbulent dissipation, we found the rate of energy dissipation to be consistent with the power of nonthermal electrons deposited into the chromosphere, suggesting a possible connection between turbulence and electron acceleration.

Real-time automated detection of multi-category solar radio bursts

Accurate real-time solar radio burst (SRB) detection is crucial for solar physics research and space weather forecasting. Currently, most studies on solar radio burst detection focus on single-category identification and simple discrimination of bursts. There are limited existing studies on multi-category detection. This paper proposes a real-time multi-category solar radio burst detection method to meet the requirements of real-time detection, detection accuracy, and classification accuracy in solar radio bursts. First, solar radio burst spectrums were collected from e-CALLISTO. The spectrums are labeled using LabelImg, and a dataset containing solar radio bursts of Type II, Type III, Type IIIs, Type IV, and Type V was established. Second, a full-dimensional dynamic convolution was introduced in the backbone module of the YOLOv8n model, enhancing the model's feature extraction capability. Third, a multi-scale feature fusion network based on ConvNeXt was created to prevent feature information loss and optimize the loss function. The experimental results show that the proposed method achieves an average detection accuracy of 82.4% on the established solar radio burst dataset. Compared with the original YOLOv8n model, the accuracy increased by 3.5%. Additionally, the model operates at 140.9 frames per second, with each frame representing a spectrum of 15 minutes duration. Thus, the improved YOLOv8n model enhances the detection accuracy and speed of solar radio bursts, enabling automatic detection and localization of solar radio bursts of Type II, Type III, Type IIIs, Type IV, and Type V.

C6.2 class flare parameters inferred with a 3D geometry of flare database

We analysed a GOES C6.2 class solar flare that occurred on 27 October 2003 at the heliographic position S20E29. Utilising a database developed from a simplified 3D magnetic loop model, we inferred a set of geometric and physical parameters to characterise the flare. This model, based on general properties of known solar flares and adjusted for instrument resolution, was used to replicate observations from the Nobeyama Radio Polarimeter (NoRP) spectra and the Nobeyama Radioheliograph (NoRH) brightness maps. We identified a possible range of parameters by comparing observed spectra and brightness distribution maps with modelled counterparts. The analysis involved computing a weighted mean of the one hundred best-fit model parameters, ranked by increasing 2 values.Across six discrete time intervals representing the gradual, impulsive, and decay phases of the flare, we observed slight variations in values for ten analysed parameters, with notable exceptions at the burst maxima, including an energy spectral index of approximately 2.34, a photospheric magnetic field strength of around 2308 G, and a non-thermal electron density of about 107.11cm-3. In general, our results replicated the soft-hard-soft behaviour in the gradual-peak- decay phases of a solar flare. Therefore, we infer that the physical parameters (magnetic field strength, energy spectral index, and non- thermal electron density) were accurately recovered. However, fitting geometric parameters such as inclination, asymmetry, azimuth, loop radius, loop height, and footpoint separation proved challenging due to the limited resolution of the instruments, which affects the precise determination of loop-like flare geometries.

Origin of the type III radiation observed near the Sun

Aims. We investigate processes associated with the generation of type III radiation using Parker Solar Probe measurements. Methods. We measured the amplitudes and phase velocities of electric and magnetic fields and their associated plasma density fluctuations. Results. 1. There are slow electrostatic waves near the Langmuir frequency and at as many as six harmonics, the number of which increases with the amplitude of the Langmuir wave. Their electrostatic nature is shown by measurements of the plasma density fluctuations. From these density fluctuations and the electric field magnitude, the k-value of the Langmuir wave is estimated to be 0.14 and k_d = 0.4. Even with the large uncertainty in this quantity (more than a factor of two), the phase velocity of the Langmuir wave was < 10 000 km/s. 2. The electromagnetic wave near the Langmuir frequency has a phase velocity lower than 50 000 km/s. 3. We cannot determine whether there are electromagnetic waves at the harmonics of the Langmuir frequency. If they existed, their magnetic field components would be below the noise level of the measurement. 4. The rapid (less than one millisecond) amplitude variations typical of the Langmuir wave and its harmonics are artifacts resulting from the addition of two waves, one of which has small frequency variations that arise because the wave travels through density irregularities. None of these results are expected in or consistent with the conventional model of the three-wave interaction of two counter-streaming Langmuir waves that coalesce to produce the type III wave. They are consistent with a new model in which electrostatic antenna waves are produced at the harmonics by radiation of the Langmuir wave, after which at least the first harmonic wave evolved through density irregularities such that its wave number decreased and it became the type III radiation.

Magnetic structure, confinement, and escape into the heliosphere

Context. Filament eruptions and coronal mass ejections (CMEs) reveal large-scale instabilities of magnetic structures in the solar corona. Some of them are accompanied by radio emission, which at decimetric and longer wavelengths is a signature of electron acceleration that may be different from the acceleration in impulsive flares. The radio emission is part of the broadband continua at decimetre and metre wavelengths called type IV bursts. Aims. In this article we investigate a particularly well-observed combination of a filament eruption seen in H and at extreme ultraviolet (EUV) wavelengths and a moving type IV burst on 2021 August 24. The aim is to shed light on the relationship between the large-scale erupting magnetic structure and the acceleration and transport of non-thermal electrons. Methods. We used imaging observations of a moving radio source and associated burst groups with the refurbished Nanay Radioheliograph and whole-Sun radio spectrography from different ground-based and space-borne instruments, in combination with X-ray, radio, and in situ electron observations at tens of keV from Solar Orbiter and EUV imaging by SDO/AIA. The radio sources are located with respect to the erupting magnetic structure traced by the filament (EUV 30.4 nm), and the timing of the electrons detected in situ is compared with the timing of the different radio emissions. Results. We find that the moving radio source is located at the top of the erupting magnetic structure outlined by the filament, which we interpret as a magnetic flux rope. The flux rope erupts in a strongly non-radial direction, guided by the overlying magnetic field of a coronal hole. The electrons detected at Solar Orbiter are found to be released mainly in two episodes, 1040 minutes after the impulsive phase. The releases coincide with two groups of radio bursts, which originate respectively on the flank and near the top of the erupting flux rope. Conclusions. The observation allows an unusually clear association between a moving type IV radio burst, an erupting magnetic flux rope as core structure of a CME, and particle releases into the heliosphere. Non-thermal electrons are confined in the flux rope. Electrons escape to the heliosphere mainly in two distinct episodes, which we relate to magnetic reconnection between the flux rope and ambient open field lines.

Slowly positively drifting bursts generated by large-scale magnetic reconnection

Context. The slowly positively drifting bursts (SPDBs) are rarely observed in radio emission of solar flares. Aims. To understand how the SPDBs are generated, we studied the radio observations at 6005000 MHz together with the imaging observations made in ultraviolet (UV) and extreme ultraviolet (EUV) during the SPDB-rich C8.7 flare of 2014 May 10 (SOL2014-05-10T0702). Methods. Because the SPDBs propagate towards locations of higher plasma density, we studied their associations with individual flare kernels, located either within the flare core itself, or distributed at longer distances, but connected to the flaring region by large-scale hot loops. For each kernel we constructed light curves using 1600 and 304 observations and compared these light curves with the temporal evolution of radio flux at 1190 MHz, representing all observed groups of SPDBs. We also analysed the UV/EUV observations to understand the evolution of magnetic connectivity during the flare. Results. The flare starts with a growing hot sigmoid observed in 131 . As the sigmoid evolves, it extends to and interacts with a half dome present within the active region. The evolving sigmoid reconnects at the respective hyperbolic flux tube, producing large-scale magnetic connections and an EUV swirl. Three groups of SPDBs are observed during this large-scale magnetic reconnection, along with a group of narrow- band type III bursts. The light curves of a kernel corresponding to the footpoint of spine line analogue show good agreement with the radio flux at 1190 MHz, indicating that the SPDBs are produced by the large-scale magnetic reconnection at the half dome. In addition, one of the kernels appeared in the neighbouring active region and also showed a similar evolution to the radio flux, implying that beams of accelerated particles can synchronize radio and UV/EUV light curves across relatively large distances.

Modelling non-radially propagating coronal mass ejections and forecasting the time of their arrival at Earth

We present the study of two solar eruptive events observed on December 7 2020 and October 28 2021. Both events were associated with full halo coronal mass ejections (CMEs) and flares. These events were chosen because they show a strong non-radial direction of propagation in the low corona and their main propagation direction observed in the inner heliosphere is not fully aligned with the Sun-Earth line. This characteristic makes them suitable for our study, which aims to inspect how the non-radial direction of propagation in the low corona affects the time of CMEs' arrival at Earth. We reconstructed the CMEs using SOHO/LASCO and STEREO/COR observations and modelled them with the 3D MHD model EUHFORIA and the cone model for CMEs. In order to compare the accuracy of forecasting the CME and the CME-driven shock arrival time at Earth obtained from different methods, we also used so-called type II bursts, radio signatures of associated shocks, to find the velocities of the CME-driven shocks and forecast the time of their arrival at Earth. Additionally, we estimated the CME arrival time using the 2D CME velocity obtained from the white light images. Our results show that the lowest accuracy of estimated CME Earth arrival times is found when the 2D CME velocity is used (time difference between observed and modelled arrival time, t 29 h and 39 h, for the two studied events, respectively). The velocity of the type II radio bursts provides somewhat better but still not very accurate results (t +21 h and 29 h, for the two studied events, respectively). Employing, as an input to EUHFORIA, the CME parameters obtained from the graduated cylindrical shell (GCS) fittings at consequently increasing heights, results in a strongly improved accuracy of the modelled CME and shock arrival time; t changes from 20 h to 10 min in the case of the first event, and from 12 h to 30 min in the case of the second one. This improvement shows that when we increased the heights of the GCS reconstruction we accounted for the change in the propagation direction of the studied CMEs, which allowed us to accurately model the CME flank encounter at Earth. Our results show the great importance of the change in the direction of propagation of the CME in the low corona when modelling CMEs and estimating the time of their arrival at Earth.

Effect of temperature anisotropy formed by fast electron beams moving in the flare loop on its excited electron-cyclotron maser instability

Context. The electron-cyclotron maser instability (ECMI) is a significant coherent radio emission mechanism widely utilized in various astrophysical radio phenomena. It is well known that the velocity anisotropic distribution of energetic electrons, which leads to an inverted perpendicular population in the vertical direction with f_b/v > 0, can provide the free energy necessary for the ECMI. Aims. The initial velocity distribution of energetic electrons leaving the acceleration region is typically isotropic or beam-like. However, as these energetic electrons travel along the magnetic field as fast electron beams (FEBs) in magnetic plasma, various velocity anisotropic distributions can emerge. In this paper, we examine the impact of temperature anisotropy formed by beam electrons traveling along a flare loop on the ECMI. Methods. By neglecting the energy loss of energetic electrons as they traverse the corona and invoking the conservation of energy and magnetic moments, we established the relationship between momentum dispersion and the magnetic field. Utilizing the magnetic field model of the flare loop, we calculated the evolution of momentum dispersion and the growth rates of the ECMI as FEBs precipitate along the flare loop. Results. The results demonstrate that the temperature anisotropy arising as FEBs descend along the flare loop significantly impacts the ECMI. The maximum growth rates of the excited modes exhibit a gradual increase initially and then decline rapidly after reaching a critical height for ₀ = 0.2c and 0.15c. The results also show that the growth rates of the O2 mode are one order of magnitude smaller than those of the O1 and X2 modes. This indicates that the harmonic radiation is X-mode polarized. Notably, the temperature anisotropy of FEBs as they precipitate along the flare loop with different magnetic field models or at different heights has similar effects on the ECMI.

Long-period energy releases during a C2.8 flare

Context. The study of quasi-periodic pulsations (QPPs) is a key diagnostic of intermittent or periodic energy releases during solar flares. Aims. We investigated the intermittent energy-releasing processes by analyzing the long-period pulsations during a C2.8 flare on 2023 June 3. Methods. The solar flare was simultaneously observed by the solar X-ray detector on board the Macau Science Satellite-1B, the Geostationary Operational Environmental Satellite, the Chinese H Solar Explorer, the Expanded Owens Valley Solar Array, the Atmospheric Imaging Assembly, and the Extreme Ultraviolet Variability Experiment for the Solar Dynamics Observatory. Results. The C2.8 flare shows three successive and repetitive pulsations in soft X-ray (SXR) and high- temperature extreme ultraviolet (EUV) emissions, which may imply three episodes of energy releases during the solar flare. The QPP period is estimated to be as long as 7.5 minutes. EUV imaging observations suggest that these three pulsations come from the same flare area dominated by the hot loop system. Conversely, the flare radiation in wavelengths of radio/microwave, low-temperature EUV, ultraviolet (UV), and H only reveals the first pulsation, which may be associated with nonthermal electrons accelerated by magnetic reconnection. The other two pulsations in wavelengths of SXR and high-temperature EUV might be caused by the loop-loop interaction. Conclusions. Our observations indicate that the three episodes of energy releases during the C2.8 flare are triggered by different mechanisms, namely the accelerated electron via magnetic reconnection, and the loop-loop interaction in a complicated magnetic configuration.

MEMPSEP-III. A Machine Learning-Oriented Multivariate Data Set for Forecasting the Occurrence and Properties of Solar Energetic Particle Events Using a Multivariate Ensemble Approach

We introduce a new multivariate data set that utilizes multiple spacecraft collecting in-situ and remote sensing heliospheric measurements shown to be linked to physical processes responsible for generating solar energetic particles (SEPs). Using the Geostationary Operational Environmental Satellites (GOES) flare event list from Solar Cycle (SC) 23 and part of SC 24 (1998-2013), we identify 252 solar events (>C-class flares) that produce SEPs and 17,542 events that do not. For each identified event, we acquire the local plasma properties at 1 au, such as energetic proton and electron data, upstream solar wind conditions, and the interplanetary magnetic field vector quantities using various instruments onboard GOES and the Advanced Composition Explorer spacecraft. We also collect remote sensing data from instruments onboard the Solar Dynamic Observatory, Solar and Heliospheric Observatory, and the Wind solar radio instrument WAVES. The data set is designed to allow for variations of the inputs and feature sets for machine learning (ML) in heliophysics and has a specific purpose for forecasting the occurrence of SEP events and their subsequent properties. This paper describes a data set created from multiple publicly available observation sources that is validated, cleaned, and carefully curated for our ML pipeline. The data set has been used to drive the newly-developed Multivariate Ensemble of Models for Probabilistic Forecast of SEPs (MEMPSEP; see MEMPSEP-I (Chatterjee et al., 2024, https://doi .org/10.1029/2023SW003568) and MEMPSEP-II (Dayeh et al., 2024, https://doi.org/10.1029/2023SW 003697) for accompanying papers).

Comparative Study of Solar Rotation of Transition Region and Corona using Solar Irradiance and Radio Flux

We study the temporal variation of solar rotation profiles based on solar irradiance at 93.5 nm and solar radio flux at 10.7 cm originating from the transition region and lower corona, respectively. The autocorrelation technique is used to calculate the period in periodic time series data. The sidereal rotation periods for normalized and detrended data are studied for 2011 2021. The sidereal rotation periods for solar irradiance and radio flux for 2011 2021 vary from 22.75 to 26.17 days and 19.42 to 28.14 days, respectively. The mean of the sidereal rotation periods for solar irradiance and radio flux are 24.76 and 23.76 days, respectively. The mean sidereal rotation period for solar irradiance is higher than the mean sidereal rotation period for solar radio flux. The sidereal rotation period for solar irradiance is greater than or equal to the sidereal rotation period for solar radio flux for almost all the years between 2011 and 2021. It is found that the lower corona rotates faster than the transition region during 2011 2021, i.e., the lower corona is found to be moving 4% faster than the transition region during 2011 2021. We found a linear relationship between the normalized daily irradiance and radio flux with a correlation coefficient of 0.986. Using cross-correlation analysis, we investigated a phase relationship between solar irradiance and radio flux and found no time lag between solar irradiance and radio flux.

A Discussion of First Proton Arrival Times in Wide-Spread Solar Energetic Particle Events

This work analyzes the appearance of wide-spread deka-MeV solar energetic proton (SEP) events, in particular the arrival of the first protons within 4.5 45 MeV measured at EarthSun L1, and their relationship with their relative solar source longitude. The definition of "wide-spread SEP event" for this study refers to events that are observed as a 25 MeV proton intensity increase at near 1 AU locations that are separated by at least 130 in solar longitude. Many of these events are seen at all three of the spacecraft, STEREO (Solar- Terrestrial Relations Observatory) A, STEREO B, and SOHO (Solar and Heliospheric Observatory), and may therefore extend far beyond 130 in longitude around the Sun. A large subset of these events have already been part of a study by Richardson et al. (Solar Phys., 289, 3059, 2014). The event source region identifications draw from this study; more recent events have also been added. Our focus is on answering two specific questions: (1) What is the maximum longitude over which SEP protons show energy dispersion, i.e., a clear sign of arrival of higher-energy protons before those of lower energy? (2) What implications can be drawn from the ensemble of events observed regarding either direct magnetic connectivity to shocks and/or cross-field transport from the site of the eruption in the onset phase of the event?

Heating manifestations at the onset of the 29 June 2012 flare

Analysis of GOES data for the SOL2012-06-29T04:09 flare, class C4.6, shows a thermal character of the energy release for several minutes before the impulsive stage. Plasma heating to temperatures above 10 MK leads to the appearance of plasma jets along open field lines and in large loops. This work examines the relationship between the heated plasma and the flare structure and its dynamics, using observations in the X-ray, extreme ultraviolet (EUV), and radio-wave ranges. Particular attention is drawn to the detection of narrow-band fine temporal structures of radio emission before and after the impulsive stage of the flare in dynamic spectra. In the initial stage, broadband pulses in the decimeter range are observed which can be associated with the formation of thermal fronts in the jets. A series of super-bright drifting bursts in the centimeter range occurs after the end of the impulsive energy release in the flare kernel. Using data from the Siberian Solar Radio Telescope (5.7 GHz), we managed to localize the position of the source of the fine structure of drifting bursts at the remote footpoint of the large-scale flare loop.

A Novel Two-dimensional Low-redundancy Array Design for Solar Radio Imaging

The radioheliograph is an extensive array of antennas operating on the principle of aperture synthesis to produce images of the Sun. The image acquired by the telescope results from convoluting the Sun's true brightness distribution with the antenna array's directional pattern. The imaging quality of the radioheliograph is affected by a multitude of factors, with the performance of the "dirty beam" being simply one component. Other factors such as imaging methods, calibration techniques, clean algorithms, and more also play a significant influence on the resulting image quality. As the layout of the antenna array directly affects the performance of the dirty beam, the design of an appropriate antenna configuration is critical to improving the imaging quality of the radioheliograph. Based on the actual needs of observing the Sun, this work optimized the antenna array design and proposed a two-dimensional low-redundancy array. The proposed array was compared with common T-shaped arrays, Y-shaped arrays, uniformly spaced circular arrays, and three-arm spiral arrays. Through simulations and experiments, their performance in terms of sampling point numbers, UV coverage area, beam-half width, sidelobe level, and performance in the absence of antennas are compared and analyzed. It was found that each of these arrays has its advantages, but the two-dimensional low-redundancy array proposed in this paper performs best in overall evaluation. It has the shortest imaging calculation time among the array types and is highly robust when antennas are missing, making it the most suitable choice.

System for control of the power of an on-board signal generator of an unmanned aerial vehicle

We consider the issues of calibration of the transmission coefficient of an antenna system of the ground station used for monitoring of the energy characteristics of signals from global navigation satellite systems. We present the most commonly used method intended for calibration according to the solar radio-emission of the transmission coefficient of large-aperture reflector antenna systems used as basic in the construction of ground control stations. We describe the advantages and disadvantages of the method of calibration according to the radio- emission. To eliminate the disadvantages of calibration based on solar radio-emission (the limited range of working elevation angles and the necessity of using third-party data on the spectral intensity of solar radio-emission), it is proposed to consider an alternative approach, namely, the calibration based on the use of an artificial source of radio-emission placed on board of an unmanned aerial vehicle. The specific features of this approach, namely, the necessity of operation in the near-field zone of the investigated antenna system and the motion of an artificial source of radio-emission in a plane parallel to the aperture plane of the antenna system, are indicated. We develop a block diagram of the payload of an unmanned aerial vehicle for measuring in the near-field zone of the analyzed antenna system. We present the results of investigations of the characteristics of a payload signal generator of an unmanned aerial vehicle. We propose a solution, which eliminates the error in specifying the output power of the signal generator (the difference between the nominal and actual signal power) according to the results of measurement. The necessity of using a control signal power meter and a bandpass filter as components of the payload is demonstrated. A system for monitoring of the output power of signal generator is designed. The components of the payload satisfying the restrictions imposed on the weight and size characteristics are selected. The possibility of remote connection of an operator to the payload equipment of an unmanned aerial vehicle is realized. We also describe the prospects of subsequent development of the payload model.

Relationship between microwave and metre ranges during an impulsive solar flare

An analysis of solar flares with simple temporal structures can help us to understand the features and mechanisms of energy and particle propagation. A weak impulsive solar flare that occurred on 2021 June 3 provided such opportunity. For the purposes of the study, we used microwave observations with spatial resolution from the Siberian Radioheliograph (3-6 GHz) combined with various spectral radio and X-ray data. Flare topology analysis revealed a configuration consisting of a small loop or dome-like structure associated with a compact bright source, and a long high loop system associated with the diffuse source. This indicates a compact and relatively low-lying site of acceleration and initial energy release. The radio metre and the microwave emission demonstrated a peak-to-peak correlation in three of the four bursts. The delays obtained from comparing microwave and metre radio ranges are in good agreement with the delays from the metre dynamic spectrum analysis, but the different radio bursts had different delays. We found that the electron energies derived from metre dynamic spectrum analysis are lower than those shown by hard X-ray emissions. According to the results of theoretical simulations, this can be explained by the expansion of magnetic loops with altitude. The difference in drift velocities for various radio bursts can be the result of the different size of the loop where the electron beams are propagated. This can be a feature related to the configuration type of the studied flare.

Unraveling the Origins of an Extreme Solar Eruptive Event with Hard X-Ray Imaging Spectroscopy

Hard X-ray (HXR) observations are crucial for understanding the initiation and evolution of solar eruptive events, as they provide a key signature of flare-accelerated electrons and heated plasma. The potential of high-cadence HXR imaging for deciphering the erupting structure, however, has not received adequate attention in an era of extreme ultraviolet (EUV) imaging abundance. An extreme solar eruptive event on 2022 September 5 observed on the solar far side by both Parker Solar Probe and Solar Orbiter provides the opportunity to showcase the power of HXR imaging in the absence of high-cadence EUV imaging. We investigate the evolution of flare energy release through HXR timing, imaging, and spectral analyses using data from the Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter. STIX provides the highest cadence imaging of the energy release sites for this far-side event and offers crucial insight into the nature of energy release, timing of flare particle acceleration, and evolution of the acceleration efficiency. We find that this is a two-phase eruptive event, rather than two distinct eruptions, as has been previously suggested. The eruption begins with an initial peak in flare emission on one side of the active region (AR), marking the rise/destabilization of a loop system followed by notable episodes of energy release across the AR and an eruptive phase associated with a very fast coronal mass ejection, type III radio bursts, and solar energetic particles. We demonstrate that high-cadence HXR imaging spectroscopy is indispensable for understanding the formation of powerful, space-weather relevant eruptions.

Time Profile Study of Type III Solar Radio Bursts Using Parker Solar Probe

Solar type III radio bursts are crucial indicators of energetic electron activity in the solar corona and interplanetary space. Our assessment of 43 interplanetary type III bursts, recorded by the FIELDS instrument on board the Parker Solar Probe during Encounters 05 to 11, has led to significant and complex findings. We have analyzed time profile features across a frequency range of 190.5 MHz, revealing dependencies on frequency and providing insights into duration, burst speeds, bandwidths, and drift rates. This novel analysis has unveiled a spectral index of 0.63 0.04 for rise, 0.69 0.03 for decay time, and 0.68 0.03 for the total duration. We have determined the average electron beam velocities for front, middle, and back as 0.15c, 0.13c, and 0.08c, respectively. Our findings show that faster electron beams generate emissions with shorter duration. The average ratio of the front-to-back velocity is 1.87, and the ratio of front-to-middle velocity is 1.23. We have also discovered a strong relationship between burst duration with rise, peak, and decay times, particularly pronounced with decay time (correlation coefficient = 0.95). This indicates that the entire temporal profile, including rise, peak, and decay phases, collectively contributes to event duration and is not solely influenced by external factors like plasma conditions or electron beam dynamics but also by internal burst processes. These complex findings shed light on the physical mechanisms governing burst dynamics, revealing intricate interactions between electron beam characteristics and observed temporal and spectral traits of type III solar radio bursts.

Observations and interpretation

Context. Shock waves in the solar corona are associated with solar flares and coronal mass ejections (CMEs). Type II solar bursts are radio signatures of shock waves in the solar corona. They are driven by solar flares or CMEs. Despite extensive studies, the intricate spectral patterns observed in type II solar bursts occasionally pose new challenges for the theory of electron acceleration in shocks. Aims. We study a newly identified feature in type II solar bursts called spectral cleaving. This feature is characterized by the actual branching of a type II radio emission lane in radio spectral data. Methods. We analyzed the type II burst exhibiting spectral cleaving in high-fidelity dynamic spectra obtained using the URAN-2 radio telescope (8.2533 MHz; Poltava region, Ukraine) on 2011 February 14. The high-resolution spectrograms were examined to ascertain its spectral morphology. Results. Our research represents the first recognition of spectral cleaving as a peculiarity of type II bursts that is yet to be classified. This effect occurs due to the shift (or migration) of radio source(s) along a shock front, which in turn is caused by changes in the magnetic field orientation ahead of the propagating shock front. Conclusions. The spectral cleaving observed in solar type II bursts reveals a distinct phenomenon that indicates complex interactions between shock waves and magnetic fields in the solar corona. This discovery enhances our understanding of the mechanisms behind solar radio emissions and emphasizes the need for further observational studies to verify these findings.

Periods and Frequency Drifts of Groups of the Decimetric Spikes in Two Solar Flares

We studied the radio emission occurring as narrowband decimetric spikes observed during the 10 May 2022 and 26 August 2022 flares. In the radio spectra, these spikes were distributed in groups that occurred quasi- periodically with the periods 5.1 s in the 10 May 2022 flare and 9.1 s in the 26 August 2022 flare. In some parts of these groups, even subgroups of spikes distributed with the quasi-periods of 0.19 s (10 May 2022 flare), and 0.17 s and 0.21 s (26 August 2022 flare) were found. Some of these subgroups even drifted to higher or lower frequencies, which was observed for the first time. At the time of the dm-spikes observation, a pair of reconnecting loops are identified in the SDO/AIA EUV observations of the 10 May 2022 flare, one of which is interpreted as belonging to a small erupting filament. We propose that these loops reconnect in the dynamic quasi-periodic regime (the period 0.19 s) and this reconnection is modulated by an oscillation of one of the interacting loops (the period 5.1 s). Accelerated electrons from this process are trapped in reconnecting plasma outflows, and thus the drifting groups of spikes are generated. The 26 August 2022 flare is a complex event with several systems of bright loops; nevertheless, it also shows a disintegrating erupting filament similar to the 10 May 2022 flare, meaning that the dm-spikes are likely generated by similar reconnection processes.

The Multifaceted M1.7 GOES-class Flare Event of 21 April 2023 in AR13283

On 21 April 2023, a significant M1.7 solar flare erupted from Active Region 13283, accompanied by a filament eruption and a full-halo Coronal Mass Ejection, which reached Earth on 23 April, triggering a severe geomagnetic storm, with Kp reaching 8 (G4) and Dst plummeting to 212 nT together with a sharply distinguished long-lasting negative double-dip behavior of the z-component of the interplanetary magnetic field. This event led to remarkable auroral displays, even at mid-latitudes in Europe. The flare-induced filament eruption caused distinct intensity dimming in the solar corona, observed in specific EUV wavelengths. We observed the dimming region growing at its fastest rate before the flare reached its peak of intensity. Notably, the proximity of the flare to a large southern coronal hole influenced the expansion and propagation of the coronal mass ejection toward Earth, probably impacting the solar wind speed and density. Additionally, we observed a sudden expansion of the coronal hole during the flare, leading us to speculating that the adjacent flare may have further stimulated the flow of solar-wind particles along the open magnetic-field lines. In accordance with the severe Dst-index disturbance, we also report changes in the potential of the pipeline of an Italian energy infrastructure company with respect to the surrounding soil as well as double-dip variation in the H-component of the terrestial magnetic field observed locally (reminiscent to what reported in Dst-index and IMF B_z) temporal profiles, confirming the effects of the geomagnetic storm at Italy mid-latitudes. Several solar radio events have been observed too. Therefore this study provides insights into the dynamic solar phenomena and their potential geomagnetic implications.

Verification of the Empirical Model of Ionization of the Lower Ionosphere during Solar Flares of Different Classes

The results of measuring VLF signal parameters propagating in the Earth- D-region of the ionosphere waveguide to assess changes in the state of the lower ionosphere as a result of the impact of X-ray radiation of solar flares make it possible to obtain qualitative data on the nature and magnitude of the impact. Obtaining accurate data on the relationship between changes in electron concentration and flare parameters and reliable prediction of the conditions of LF radio signal propagation during strong geophysical disturbances is complicated by the lack of complete information on the frequency spectrum of X-ray radiation for a particular flare and data on the ionization rate of the ionosphere for flares of different classes. The technique of determining the X-ray spectrum in a wide range of wavelengths and calculating the ionization coefficients of the lower ionosphere as a function of the ionizing radiation parameters of flares, presented by Ryakhovsky et al. (2023), makes it possible to improve the accuracy in estimating variations in the parameters of the lower ionosphere. The present paper is devoted to verifying the performance of the developed empirical model of lower ionization of the lower ionosphere at the solar flare front and comparing the results with experimental data on the variation of VLF radio parameters.

lista completa de eventos observados en TNR

In the present work, a database of low frequency radio events is described, analyzing all dynamic spectra from the instrument Thermal Noise Receiver, belonging to the Radio and Plasma Wave Investigation (WAVES) suite aboard the NASA Wind spacecraft. This database expands the one built in a previous work, and encompasses the years 1994-2021, covering more than two full solar cycles. The radio events are also linked to the detection of other interplanetary structures such as shock waves and interplanetary coronal mass ejections. We found a total of 320 events, out of which 136 had not been previously catalogued. Moreover, 121 shock waves that arrived to Earth's vicinity could be associated with these radiofrequency events.

Efecto Neupert Anlisis para fulguraciones del Ciclo Solar 24

The Neupert effect empirically states that, for many solar flares, the soft X-ray (SXR) time derivative nearly fits the hard X-ray (HXR) or microwave time profiles. This simple relationship supports flare models in which the HXR emission is non-thermal bremsstrahlung by accelerated electrons as they gradually lose their energy in the lower corona and chromosphere, and the SXR emission is thermal bremsstrahlung from plasma heated by the same electrons. We analize this effect for the most energetic flares (classified as M or X) during the Solar Cycle 24, using SXR observations and microwave data (MW), taking into account that the energetic electrons responsible for HXR emission at chromospheric levels are the same that, at low corona levels, produce MW radiation by gyrosynchrotron emission. We observe for the first time that, at higher MW frequencies, the Neupert effect works better.

Giant Postflare Loops in Active Regions with an Extremely Strong Coronal Magnetic Field

We report for the first time the detection of thermal freefree emission from post-flare loops at 34 GHz in images from the Nobeyama Radioheliograph. We studied eight loops, seven of which were from regions with an extremely strong coronal magnetic field reported by Fedenev et al. Loop emission was observed in a wide range of wavelength bands, up to soft X-rays, confirming their multitemperature structure and was associated with noise storm emission in metric . The comparison of the 17 GHz emission with that at 34 GHz, after a calibration correction of the latter, showed that the emission was optically thin at both frequencies. We describe the structure and evolution of the loops and we computed their density, obtaining values for the top of the loops between 1 and 6 10¹⁰ cm³, noticeably varying from one loop to another and in the course of the evolution of the same loop system; these values have only a weak dependence on the assumed temperature, 2 10⁶ K in our case, as we are in the optically thin regime. Our density values are above those reported from EUV observations, which go up to about 10¹⁰ cm³. This difference could be due to the fact that different emitting regions are sampled in the two domains and/or due to the more accurate diagnostics in the radio range, which do not suffer from inherent uncertainties arising from abundances and non-LTE excitation/ionization equilibria. We also estimated the magnetic field in the loop tops to be in the range of 1030 G.

A Comprehensive Catalog and Statistical Results

Decameter hectometric (DH; 114 MHz) type IV radio bursts are produced by flare-accelerated electrons trapped in postflare loops or the moving magnetic structures associated with the coronal mass ejections (CMEs). From a space weather perspective, it is important to systematically compile these bursts, explore their spectrotemporal characteristics, and study the associated CMEs. We present a comprehensive catalog of DH type IV bursts observed by the Radio and Plasma Wave Investigation instruments on board the Wind and Solar TErrestrial RElations Observatory spacecraft covering the period of white-light CME observations by the Large Angle and Spectrometric Coronagraph on board the Solar and Heliospheric Observatory mission between 1996 November and 2023 May. The catalog has 139 bursts, of which 73% are associated with a fast (>900 km s¹) and wide (>60) CME, with a mean CME speed of 1301 km s¹. All DH type IV bursts are white-light CME- associated, with 78% of the events associated with halo CMEs. The CME source latitudes are within 45. Seventy-seven events had multiple- vantage-point observations from different spacecraft, letting us explore the impact of the line of sight on the dynamic spectra. For 48 of the 77 events, there were good data from at least two spacecraft. We find that, unless occulted by nearby plasma structures, a type IV burst is best viewed when observed within a 60 line of sight. Also, bursts with a duration above 120 minutes have source longitudes within 60. Our inferences confirm the inherent directivity in the type IV emission. Additionally, the catalog forms a Sun-as-a-star DH type IV burst database.

Energetic Electrons Accelerated and Trapped in a Magnetic Bottle above a Solar Flare Arcade

Where and how flares efficiently accelerate charged particles remains an unresolved question. Recent studies revealed that a "magnetic bottle" structure, which forms near the bottom of a large-scale reconnection current sheet above the flare arcade, is an excellent candidate for confining and accelerating charged particles. However, further understanding its role requires linking the various observational signatures to the underlying coupled plasma and particle processes. Here we present the first study combining multiwavelength observations with data-informed macroscopic magnetohydrodynamics and particle modeling in a realistic eruptive flare geometry. The presence of an above-the-loop- top magnetic bottle structure is strongly supported by the observations, which feature not only a local minimum of magnetic field strength but also abruptly slowing plasma downflows. It also coincides with a compact above-the-loop-top hard X-ray source and an extended microwave source that bestrides the flare arcade. Spatially resolved spectral analysis suggests that nonthermal electrons are highly concentrated in this region. Our model returns synthetic emission signatures that are well matched to the observations. The results suggest that the energetic electrons are strongly trapped in the magnetic bottle region due to turbulence, with only a small fraction managing to escape. The electrons are primarily accelerated by plasma compression and facilitated by a fast-mode termination shock via the Fermi mechanism. Our results provide concrete support for the magnetic bottle as the primary electron acceleration site in eruptive solar flares. They also offer new insights into understanding the previously reported small population of flare- accelerated electrons entering interplanetary space.

Radio Signature of the Strong Compression between a Streamer and a Coronal Hole Boundary

We present evidence of the first detection of the radio signature at metric wavelengths of the strong compression between a helmet streamer (HS) and the boundary of a coronal hole (CH) using radio observations from the Callisto MEXICO-LANCE and ALASKA-HAARP systems and white-light observations obtained by the STEREO-A/COR1-COR2 coronagraphs. The event occurred very close to the Sun (3.4 solar radii) and produced an intense and unusually broad drifting radio feature at metric wavelengths after a downward-drifting band of emission related to a metric Type II radio burst. The compression is caused by the interaction between an expanding structure (coronal mass ejection/shock) and the HS against the CH boundary. Observations in white light show a sharp compressive feature that propagates radially outward, while STEREO-A/EUVI images show loop oscillations at the same position angle, indicating that the interaction occurs across a range of heights. The loop oscillations cease when the compressive front loses its sharp boundary. This transition indicates a reduction of the density compression at the front and the cessation of the radio emission.

A detection of the 22P3/222S1/2 fine-structure transition of hydrogen in the radio spectrum of the Sun?

The hyperfine transition of atomic hydrogen at a wavelength of about 21 cm is an essential tool for studies of interstellar gas. It has been argued that also fine-structure transitions of hydrogen could be detected in astronomical sources. Our aim is to detect the fine- structure transition 22P3/222S1/2 of hydrogen at 10 GHz in the radio spectrum of the Sun, with a spectral resolution which enables a detailed study of the line profile. The Sun was observed with the 13.7 m radio telescope at the Metshovi Radio Observatory, in Finland. We detect emission from two of the three hyperfine components of the transition. The width of the components is 15 MHz, much less than the expected natural line width of 100 MHz (broadened solely by the uncertainty principle). At red-shifted Doppler velocities, the lines show enhanced emission and possibly self-absorption. If the absorption happens at the chromosphere, our observations challenge the traditional view that chromospheric temperature increases gradually towards higher altitudes. Our unconventional results have to be confirmed by further observations. This transition would be the only known spectral line in the Sun at radio frequencies.

A comparative study of solar activity parameters during the period 20092012 and 20202023 (ascending phase of solar cycles 24 and 25)

In this paper, we performed solar synodic period (27 days) and heliospheric effect for selected solar activity parameters: sunspot number (SSN), sunspot area (SSA), modified coronal index (MCI), solar radio flux (F10.7), chromospheric composite Mg II index and Galactic cosmic rays (GCRs), during the ascending phase of solar cycles 24 and 25 (20092012 and 2020 to 2023). The Wavelet analyses of daily data of SSN, SSA, MCI, Mg II, and F10.7, reveal a solar rotational period of 27 days. The R Robper method is used to validate the observed periods; near one solar rotational period during the ascending phase of solar cycles 24 and 25. We observed cross-correlation and time lag for solar activity parameters (SSN, SSA, MCI, Mg II, and F10.7) with GCRs during the ascending phase of solar cycles 24 and 25 (20092012 and 2020 to 2023). We found the highest time lag for SSA is 300 days, and for SSN is 270 days during the ascending phase of solar cycle 25. We also found the highest cross-correlation values are 0.998 and 0.996 for chromospheric composite Mg II index with Galactic cosmic rays (GCRs) during the ascending phase of solar cycle 24 and 25 respectively. We found the chromospheric composite Mg II index is a good indicator of solar activity indices and it is strongly correlated to SSN.

Algorithms and Resources for the Monitoring of Very-Low-Frequency Signal Deviations Due to Solar Activity Using a Web-Based Software-Defined Radio-Distributed Network

This work presents the development and testing of an experimental web- based SDR (software-defined radio) monitoring system for indirect solar activity detection, which has the ability to estimate and potentially predict various events in space and on earth, including solar flares, coronal mass ejections, and geomagnetic storms. The proposed system can be used to investigate the effect of solar activity on the propagation of very-low-frequency (VLF) signals. The advantages and benefits of the given approach are as follows: increasing measurement accuracy and eventual solar activity identification by combining measurements from multiple spatially distributed SDRs. The verification process involves carrying out several experiments comparing data from the GOES satellite system and the Dunksin SuperSID system with information received by the SDR monitoring system. Then, utilizing Pearson correlation coefficients, the measured data from the SDRs, along with those from the GOES satellite system and the Dunsing monitoring station, are investigated. At the time of a solar flare, the correlation value is above 90% for most of the stations used. Combining the signal-to-noise ratio via summation also shows an improvement in the results, with a correlation above 98%.

GUI for the e-CALLISTO data

Solar radio bursts are sudden peaks in the low-frequency radio emissions originating from the sun. These emissions, while revealing important insights into underlying physical mechanisms in solar physics, can also help predict space weather events that could have adverse effects on satellite communications and the global energy grid. A thorough understanding of this phenomena demands the collection and analysis of solar emission data over vast geographical and time scales. In this regard, the e-CALLISTO network plays a major role through having already archived more than 20 years worth of solar radio burst data. Leveraging on the advances in data analysis techniques, this data can be used to review the statistical significance of burst properties of type II and type III solar radio bursts and hence more importantly the magnetic field measurements of the active regions. In order to process the e-CALLISTO data, a software containing several data reduction processes is introduced to optimize the data analysis via a graphical user interface (GUI). The program is capable of reading out data from any CALLISTO receiving station, while offering visualization capabilities such as the color-corrected spectrum view, the plot of frequencies of the highest intensity, the individual frequency spectrum, the solar burst isolation portal, the fitting model for the radio burst, and the drift rate curve of the burst. These are achieved through using the raw "fits" files of spectra to perform background RFI reduction, identify and isolate solar radio burst regions, model the peak frequency variation using curve fitting, and thereby determine the frequency drift rates. The method can be directly applied to Type II and III solar bursts while providing space for tailoring and modification. In this work, the slow drift type II radio bursts were fitted by exponential decay and the fast drift type III radio bursts were approximated as linear decay. Hence, the frequency drift rates were computed for type II and type III radio bursts. The application is used to analyze several Type II and Type III solar radio bursts and depending on the bust type shock speed and electron velocity were determined. The GUI interface eliminates the time-consuming subjective manual analysis of e-CALLISTO data thereby making the analysis of solar radio bursts a routine and rapid process.

Quasi-periodic oscillations in rotating and deformed space-times

Quasi-periodic oscillation (QPOs) analysis is important for understanding the dynamical behaviour of many astrophysical objects during transient events such as gamma-ray bursts, solar flares, magnetar flares, and fast radio bursts. In this paper, we analyse QPO data in low-mass X-ray binary (LMXB) systems, using the Lense-Thirring, Kerr, and approximate Zipoy-Voorhees metrics. We demonstrate that the inclusion of spin and quadrupole parameters modifies the well- established results for the fundamental frequencies in the Schwarzschild space-time. We interpret the QPO data within the framework of the standard relativistic precession model, allowing us to infer the values of the mass, spin, and quadrupole parameters of neutron stars in LMXBs. We explore recent QPO data sets from eight distinct LMXBs, assess their optimal parameters, and compare our findings with results in the existing literature. Finally, we discuss the astrophysical implications of our findings.

Development of a VLF receiver based on Red Pitaya for space weather studies

A new VLF (Very low Frequency) receiver has been developed by the Peruvian Space Agency (CONIDA) for space weather studies. The receiver has been designed based on a Red Pitaya board which performs an SDR (Software Defined Radio) to digitize, process and store the signal. The receiver is composed of a vertical antenna, a preamplifier to filter and amplify the incoming VLF signals from several transmitters located around the world. The receiver is able to cover a bandwidth from 1 up to 50 kHz and it has been developed in such a way as to be cost-effective, autonomous and solar-powered, making it suitable for installation in multiple locations with different geographic conditions. We show the performance of the receiver, the typical daily pattern of the lower ionosphere for the NAA VLF signal, as observed in Peru, and the first solar flares observed. The VLF amplitude curves recorded are validated by comparing them with data from SAVNET (The South American VLF Network) receiver installed in Peru. In a first effort to investigate the impact of solar flares on the lower ionosphere, we conducted a statistical analysis between VLF amplitude perturbations and 18 solar X-rays flux provided by GOES satellites, resulting in a linear relationship.

Ionospheric TEC modeling using COSMIC-2 GNSS radio occultation and artificial neural networks over Egypt

The ionospheric delay significantly impacts GNSS positioning accuracy. To address this, an Artificial Neural Network (ANN) was developed using the high-quality COSMIC-2 ionospheric profile dataset to predict the Total Electron Content (TEC). ANNs are adept at addressing both linear and nonlinear challenges. For this research, eight distinct ANNs were cultivated. These ANNs were designed with the following inputs Year, Month, Day, Hour, Latitude, and Longitude. Along with solar and geomagnetic parameters such as the F10.7 solar radio flux index, the Sunspot Number (SSN), the Kp index, and the ap index. The goal was to discern the most influential parameters on ionosphere prediction. After pinpointing these key parameters, an enhanced model utilizing a pioneering technique of a secondary ANN was employed with the main ANN to predict TEC values for events in 2023. The study's findings indicate that solar parameters markedly enhance the model's accuracy. Notably, the augmented model featuring a prelude secondary network achieved a stellar correlation coefficient of 0.99. Distributionally, 41 % of predictions aligned within the (-1 TEC 1) TECU spectrum, 28 % nestled within the (1< TEC 2) and (-2 TEC <-1) TECU ambit, while a substantial 30 % spanned the broader (2< TEC 5) and (-5 TEC <-2) TECU range. In essence, this research underscores the potential of incorporating solar parameters and advanced neural network techniques to refine ionospheric delay predictions, thus boosting GNSS positioning precision.

Probing turbulence in solar flares from SDO/AIA emission lines

Multiple pieces of evidence have revealed the important role of turbulence in physical processes in solar eruptions, from particle acceleration to the suppression of conductive cooling. Radio observations of density variation have established a Kolmogorov-like spectrum for solar wind density disturbance. Close to the Sun, measurements from extreme ultraviolet (EUV) bands have been used to examine turbulence in the solar atmosphere. The Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory (SDO/AIA) has been frequently used for diagnosing plasma properties due to its complex coverage of temperature response. We compute structure functions (SFs) using SDO/AIA emission measurements for two example of plasma sheets. With the relationship of v b n and I(n0+n)2n (v, b, n, and I are turbulent velocity, magnetic field, number density, and intensity, respectively, and n0 is the background density), SFs of I can be regarded as a proxy for those of the turbulent v and b fields in the plasma sheet. We show that by properly accounting for the radial dependence of the emission line intensity, an SF method is capable of probing the presence of turbulence from SDO/AIA emission lines. Compared to in situ observations, performing SFs on EUV emissions is advantageous in studying turbulence behavior in the wave-vector space, and it opens a new window for investigating turbulence from massive SDO/AIA observations.

Progress of GECAM Observation Research

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a constellation with four instruments (launch date): GECAM-A/B (10 December 2020), GECAM-C (27 July 2022) and GECAM-D (13 March 2024), which are dedicated to monitoring gamma-ray transients in all-sky. The primary science objectives of GECAM include Gamma-Ray Bursts (GRBs), Soft Gamma-ray Repeaters (SGRs), high energy counterparts of Gravitation Wave (GW) and Fast Radio Burst (FRB), Solar Flares (SFLs), as well as Terrestrial Gamma-ray Flashes (TGFs) and Terrestrial Electron Beams (TEBs). A series of observations and research have been made since the launch of GECAM-A/B. GECAM observations provide new insights into these high-energy transients, demonstrating the unique role of GECAM in the "multi-wavelength, multi-messenger" era.

Unveiling the Interplanetary Solar Radio Bursts of the 2024 Mother's Day Solar Storm

We report on a comprehensive study of interplanetary type III radio bursts linked to X-class solar flares from NOAA active region 13664, which instigated the intense 2024 Mother's Day solar storm, marked by a geomagnetic storm of 412 nT, the strongest in over two decades. Utilizing novel localization techniques with direction-finding data from STEREO-A, we identify an average eastward drift of 13.42 11.63 in radio source locations relative to GOES observations. Our analysis reveals a significant correlation between solar flare intensity and longitude (Kendall's tau = 0.535) and a strong correlation between radio flux at 1 MHz and GOES 18 soft X-ray flux (Kendall's tau = 0.648). The timing analysis shows that peak soft X-ray fluxes typically follow electron beam liftoff by 3.24 4.42 minutes. These insights into solar radio burst propagation and localization enhance our understanding of solarterrestrial interactions and improve space weather forecasting capabilities.

Localizing Quasiperiodic Pulsations in Hard X-Ray, Microwave, and Ly Emissions of an X6.4 Flare

We report the simultaneous observations of quasiperiodic pulsations (QPPs) in wavelengths of hard X-ray (HXR), microwave, Ly, and ultraviolet (UV) emissions during the impulsive phase of an X6.4 flare on 2024 February 22 (SOL2024-02-22T22:08). The X6.4 flare shows three repetitive and successive pulsations in HXR and microwave wavebands, and they have an extremely large modulation depth. The onset of flare QPPs is almost simultaneous with the start of magnetic cancellation between positive and negative fields. The wavelet power spectra suggest the presence of double periods, which are centered at 200 and 95 s, respectively. The long-period QPP can also be detected in Ly and UV wavebands at the flare area, and it could be observed in the adjacent sunspot. Our observations indicate that the flare QPPs are most likely triggered by accelerated electrons that are associated with periodic magnetic reconnections. The long period at 200 s is probably modulated by the slow magnetoacoustic wave originating from the neighboring sunspot, while the short period at 95 s could be regarded as its second harmonic mode.

Study of Particle Acceleration Using Fine Structures and Oscillations in Microwaves from the Electron Cyclotron Maser

The accelerated electrons during solar flares produce radio bursts and nonthermal X-ray signatures. The quasi-periodic pulsations (QPPs) and fine structures in spatialspectraltemporal space in radio bursts depend on the emission mechanism and the local conditions, such as magnetic fields, electron density, and pitch-angle distribution. Radio burst observations with high-frequency time resolution imaging provide excellent diagnostics. In converging magnetic field geometries, the radio bursts can be produced via the electron cyclotron maser (ECM). Recently, using observations made by the Karl G. Jansky Very Large Array (VLA) at 12 GHz, Yu et al. reported a discovery of long-lasting auroral-like radio bursts persistent over a sunspot and interpreted them as ECM-generated emission. Here we investigate the detailed second and subsecond temporal variability of this continuous ECM source. We study the association of 5 s period QPPs with a concurrent GOES C1.5-class flare, utilizing VLA's imaging spectroscopy capability with an extremely high temporal resolution (50 ms). We use the density and magnetic field extrapolation model to constrain the ECM emission to the second harmonic O-mode. Using the delay of QPPs from X-ray emission times, combined with X-ray spectroscopy and magnetic extrapolation, we constrain the energies and pitch angles of the ECM-emitting electrons to 48 keV and >26. Our analysis shows that the loss-cone diffusion continuously fuels the ECM via Coulomb collisions and magnetic turbulence between a length scale of 5 and 100 Mm. We conclude that the QPP occurs via the LotkaVolterra system, where the electrons from solar flares saturate the continuously operating ECM and cause temporary oscillations.

Recurrent Nova V2487 Oph Had Superflares in 1941 and 1942 with Radiant Energies of 10^42.5+1.6 erg

V2487 Ophiuchi (V2487 Oph) is a recurrent nova with classical nova eruptions in 1900 and 1998, and it is also the most extreme known superflare star. These superflares are roughly hour-long flares with amplitudes and optical energies reaching up to 1.10 mag and 10^39.21 erg, respectively, with the superflares recurring once a day. The V2487 Oph superflares are certainly operating with the same mechanism as all the other types of superflare stars, where magnetic loops are twisted and stretched until reconnection occurs, whereupon ambient electrons are accelerated to relativistic energies and then emit bremsstrahlung radiation from X-ray to radio. V2487 Oph is unique among known superflare stars in that one of the loop footprints is in an accretion disk. This exact mechanism was theoretically predicted by M. R. Hayashi and colleagues in 1996. Now, I have found two superflares recorded on Harvard archival photographs from 1941 and 1942. These two superflares have B-magnitude amplitudes of >1.83 and >2.00 mag and total radiated energies of 10^42.4 and 10^42.5 erg, respectively, with bolometric corrections. Each has emitted energies of 30 billion Carringtons, in units of the most energetic solar flare. Further, I find superflares in Zwicky Transient Factory light curves, so V2487 Oph has been superflaring from 1941 to 2023. For the observed number distribution of dN/dE = 4E ² superflares per year, for E in units of 10⁴¹ erg, the emitted energy in superflare light is 10^42.1 erg in each year, or 10^44.1 erg from 1941 to 2023.

Electron Cyclotron Maser Emission and the Brightest Solar Radio Bursts

This paper investigates the incidence of coherent emission in solar radio bursts, using a revised catalog of 3800 solar radio bursts observed by the Nobeyama Radio Polarimeters from 1988 to 2023. We focus on the 1.0 and 2.0 GHz data, where radio fluxes of order 10¹⁰ Jy have been observed. Previous work has suggested that these bursts are due to electron cyclotron maser (ECM) emission. In at least one well- studied case, the bright emission at 1 GHz consists of narrowband spikes of millisecond duration. Coherent emission at 1 GHz can be distinguished from traditional incoherent gyrosynchrotron flare emission based on the radio spectrum: Gyrosynchrotron emission at 1 GHz usually has a spectrum rising with frequency, so bursts in which 1 GHz is stronger than higher- frequency measurements are unlikely to be incoherent gyrosynchrotron. Based on this criterion, it is found that for bursts exceeding 100 sfu, three-quarters of all bursts at 1 GHz and half of all 2 GHz bursts have a dominant coherent emission component, assumed to be ECM. The majority of the very bright bursts at 1 GHz are highly circularly polarized, consistent with a coherent emission mechanism, but not always 100% polarized. The frequency range from 1 to 2 GHz is heavily utilized for terrestrial applications, and these results are relevant for understanding the extreme flux levels that may impact such applications. Further, they provide a reference for comparison with the study of ECM emission from other stars and potentially exoplanets.

type II radio emission on 27th of September 2001

This study focus on atypical Type II radio bursts observed in conjunction with three simultaneous coronal mass ejections (CMEs) on September 27, 2001. These CMEs originated from a single active region (AR) and were linked to relatively weak solar flares. Analysis of the CME sequences revealed distinct periods of interplanetary (IP) Type II radio emissions, characterized by pronounced increases in intensity. The first radio enhancement, lasting 20 minutes, exhibited very low density and frequency (1.651.5 MHz) at a height range of (7.88.2) solar radii (). Subsequently, the second radio signature persisted for 40 minutes with a frequency range of (900700) kHz and a height range of (10.912.6) . The third radio signature spanned 1 hour and 20 minutes, featuring a frequency range of (660390) kHz and a height range of (13.218.6) . The fourth enhancement extended over 3 hours, ranging from (550250) kHz in frequency and (14.625.0) in height. We concluded that the initial low-density radio signature resulted from a shock wave generated by reconnection of magnetic field lines, without an intense flare or extreme ultraviolet imaging telescope (EIT) wave. This shock wave then accelerated subsequent CMEs. Alternatively, the radio burst could have formed in the wake of the initial slow CME, creating a low- density environment. The second radio enhancement coincided with the accelerated propagation of CME1's core and was attributed to enhanced radio emission resulting from the Type II shock encountering filament material. The third radio enhancement aligned with the concept of a CME bow shock, indicating that the shock was positioned at the leading front of the CME. This enhancement occurred when the shock met remnant material from earlier CMEs, yet the shock continued propagating at a constant speed. The fourth enhancement progressed to higher frequencies due to the merging of CME1's core with CME2, propagating along CME3's path. This comprehensive analysis provides valuable insights into the complex dynamics and interactions associated with these unique Type II radio bursts and their correlation with coronal mass ejections.

Low-Frequency Solar Radio Type II Bursts And Their Association With Space Weather Events During The Ascending Phase Of Solar Cycle 25

Type II solar radio bursts are signatures of the coronal shocks and, therefore, particle acceleration events in the solar atmosphere and interplanetary space. Type II bursts can serve as a proxy to provide early warnings of incoming solar storm disturbances, such as geomagnetic storms and radiation storms, which may further lead to ionospheric effects. In this article, we report the first observation of 32 type II bursts by measuring various plasma parameters that occurred between May 2021 and December 2022 in solar cycle 25. We further evaluated their accompanying space weather events in terms of ionospheric total electron content (TEC) enhancement using the rate of TEC index (ROTI). In this study, we find that at heliocentric distance 12 R, the shock and the Alfvn speeds are in the range 5041282 and 368826 km1, respectively. The Alfvn Mach number is of the order of 1.2MA1.8 at the above- mentioned heliocentric distance. In addition, the measured magnetic field strength is consistent with the earlier reports and follows a single power law B(r)=6.07r-3.96G. Based on the current analysis, it is found that 19 out of 32 type II bursts are associated with immediate space weather events in terms of radio blackouts and polar cap absorption events, making them strong indications of space weather disruption. The ROTI enhancements, which indicate ionospheric irregularities, strongly correlate with GOES X-ray flares, which are associated with the type II radio bursts recorded. The diurnal variability in ROTI is proportional to the strength of the associated flare class, and the corresponding longitudinal variation is attributed to the difference in longitude. This article demonstrates that since type II bursts are connected to space weather hazards, understanding various physical parameters of type II bursts helps to predict and forecast the space weather.

First determination of the angular dependence of rise and decay times of solar radio bursts using multi-spacecraft observations

A large arsenal of space-based and ground-based instruments is dedicated to the observation of radio emissions, whether they originate within our solar system or not. Radio photons interact with anisotropic density fluctuations in the heliosphere which can alter their trajectory and influence the properties that are deduced from observations. This is particularly evident in solar radio observations, where anisotropic scattering leads to highly directional radio emissions. Consequently, observers at varying locations will measure different properties, including different source sizes, source positions, and intensities. However, it is not known whether the measurements of the decay time of solar radio bursts are also affected by the observer's position. Decay times are dominated by scattering effects, and so are frequently used as proxies of the level of density fluctuations in the heliosphere, making the identification of any location-related dependence crucial. We combine multi-vantage observations of interplanetary Type III bursts from four non-collinear, angularly separated spacecraft with simulations to investigate the dependence of the decay- and rise-time measurements on the separation of the observer from the source. We propose a function to characterise the entire time profile of radio signals, allowing for the simultaneous estimation of the peak flux, decay time, and rise time, while demonstrating that the rise phase of radio bursts is non- exponential, having a non-constant growth rate. We determine that the decay and rise times are independent of the observer's position, identifying them as the only properties that remain unaffected and thus do not require corrections for the observer's location. Moreover, we examine the ratio between the rise and decay times and find that it does not depend on the frequency. Therefore, we provide the first evidence that the rise phase is also significantly impacted by scattering effects, adding to our understanding of the plasma emission process.

The solar cycle 25 multi-spacecraft solar energetic particle event catalog of the SERPENTINE project

Context. The solar energetic particle analysis platform for the inner heliosphere (SERPENTINE) project, funded through the H2020-SPACE-2020 call of the European Union's Horizon 2020 framework program, employs measurements of the new inner heliospheric spacecraft fleet to address several outstanding questions on the origin of solar energetic particle (SEP) events. The data products of SERPENTINE include event catalogs, which are provided to the scientific community.
Aims: In this paper, we present SERPENTINE's new multi-spacecraft SEP event catalog for events observed in solar cycle 25. Observations from five different viewpoints are utilized, provided by Solar Orbiter, Parker Solar Probe, STEREO A, BepiColombo, and the near-Earth spacecraft Wind and SOHO. The catalog contains key SEP parameters for 25-40 MeV protons, ~1 MeV electrons, and ~100 keV electrons. Furthermore, basic parameters of associated flares and type II radio bursts are listed, as are the coordinates of the observer and solar source locations.
Methods: An event is included in the catalog if at least two spacecraft detect a significant proton event with energies of 25-40 MeV. The SEP onset times were determined using the Poisson-CUSUM method. The SEP peak times and intensities refer to the global intensity maximum. If different viewing directions are available, we used the one with the earliest onset for the onset determination and the one with the highest peak intensity for the peak identification. We furthermore aimed to use a high time resolution to provide the most accurate event times. Therefore, we opted to use a 1-min time resolution, and more time averaging of the SEP intensity data was only applied if necessary to determine clean event onsets and peaks. Associated flares were identified using observations from near Earth and Solar Orbiter. Associated type II radio bursts were determined from ground-based observations in the metric frequency range and from spacecraft observations in the decametric range.
Results: The current version of the catalog contains 45 multi-spacecraft events observed in the period from November 2020 until May 2023, of which 13 events were found to be widespread (observed at longitudes separated by at least 80 from the associated flare location) and four could be classified as narrow-spread events (not observed at longitudes separated by at least 80 from the associated flare location). Using X-ray observations by GOES/XRS and Solar Orbiter/STIX, we were able to identify the associated flare in all but four events. Using ground-based and space-borne radio observations, we found an associated type II radio burst for 40 events. In total, the catalog contains 142 single event observations, of which 20 (45) have been observed at radial distances below 0.6 AU (0.8 AU). It is anticipated that the catalog will be extended in the future.

Available at https://doi.org/10.5281/zenodo.10732268

A Catalog of Metric Type II Radio Bursts Detected by RSTN During Solar Cycle 24

In this study, we compile a catalog of metric type II radio bursts using the Radio Solar Telescope Network (RSTN) to study the occurrence, associations, and properties of the emission and their parent solar activity phenomena. According to the intensity and clarity of the radio emission features, we have divided the m-type II radio bursts into two qualitative categories, namely certain and uncertain. We analyzed RSTN data in Solar Cycle 24 (2009 2019), which is freely available from four worldwide stations: Learmonth, Sanvito, Sagamore Hills, and Palehua. Through careful visual inspection, we have collected all metric type II bursts detected in the range of 25 180 MHz. The relationships between these bursts and solar eruptive events, such as solar flares and coronal mass ejections (CMEs), are studied, and the results are presented and discussed. The outcomes could be used to reveal the occurrence of solar and space-weather activities based on the ground- based radio perspective. The newly assembled catalog of metric type II and associated solar events will be made freely available to the solar scientific community.

Radio emission from SN 1181 hosting a white dwarf merger product

The remnant of the historical supernova 1181 is claimed to be associated with a white dwarf merger remnant J005311. The supernova remnant (SNR) shock, and a termination shock expected to be formed by the intense wind of J005311, are potential sites for radio emission via synchrotron emission from shock-accelerated electrons. In this paper, we estimate the radio emission from these two shocks, and find the peak radio flux to be 0.1-10 mJy (at 0.01-1 GHz) in the outer SNR shock and 0.01-0.1 mJy (at 1-10 GHz) in the inner termination shock. We also search for radio emission from this source in the archival data of the Karl G. Jansky Very Large Array (VLA) Sky Survey at 3 GHz, the NRAO VLA Sky Survey at 1.4 GHz and the Canadian Galactic Plane Survey at 408 MHz, finding no significant detection. While targeted observations with higher sensitivity are desired, we particularly encourage those at higher frequency and angular resolution to probe the inner termination shock and its evolution.

Semantic Segmentation of Solar Radio Spikes at Low Frequencies

Solar radio spikes are short lived, narrow bandwidth features in low frequency solar radio observations. The timing of their occurrence and the number of spikes in a given observation is often unpredictable. The high temporal and frequency of resolution of modern radio telescopes such as NenuFAR mean that manually identifying radio spikes is an arduous task. Machine learning approaches to data exploration in solar radio data is on the rise. Here we describe a convolutional neural network to identify the per pixel location of radio spikes as well as determine some simple characteristics of duration, spectral width and drift rate. The model, which we call SpikeNet, was trained using an Nvidia Tesla T4 16GB GPU with ~100000 sample spikes in a total time of 2.2 hours. The segmentation performs well with an intersection over union in the test set of ~0.85. The root mean squared error for predicted spike properties is of the order of 23%. Applying the algorithm to unlabelled data successfully generates segmentation masks although the accuracy of the predicted properties is less reliable, particularly when more than one spike is present in the same 64 X 64 pixel time-frequency range. We have successfully demonstrated that our convolutional neural network can locate and characterise solar radio spikes in a number of seconds compared to the weeks it would take for manual identification.

Scaling and universality in the temporal occurrence of repeating FRBs

Fast Radio Bursts (FRBs) are energetic phenomena that have significant implications for understanding fundamental physics and the Universe. Recent observations of FRB 121102, FRB 20220912A, and FRB 20201124A by the Five-hundred-meter Aperture Spherical Telescope showed high-burst rates and distinctive energy distribution and temporal properties. In this study, we examine these observations to investigate the scale invariance of the waiting times between bursts for intervals longer than approximately 1 s. Our analysis revealed a unified scaling law for these longer intervals, which is similar to the behaviour of solar flares. This discovery inspires us to suggest a dual analogy of the FRB scenario across the entire time intervals: with earthquake dynamics at subsecond scales and with solar flare dynamics beyond the one-second threshold. This threshold potentially aligns with the dynamic time-scale of neutron star crusts, offering insight of the occurrence of FRBs into the internal processes of neutron stars.

on alert for high energy transients

We present Daksha, a proposed high energy transients mission for the study of electromagnetic counterparts of gravitational wave sources, and gamma ray bursts. Daksha will comprise of two satellites in low earth equatorial orbits, on opposite sides of the Earth. Each satellite will carry three types of detectors to cover the entire sky in an energy range from 1 keV to >1 MeV. Any transients detected on-board will be announced publicly within minutes of discovery. All photon data will be downloaded in ground station passes to obtain source positions, spectra, and light curves. In addition, Daksha will address a wide range of science cases including monitoring X-ray pulsars, studies of magnetars, solar flares, searches for fast radio burst counterparts, routine monitoring of bright persistent high energy sources, terrestrial gamma- ray flashes, and probing primordial black hole abundances through lensing. In this paper, we discuss the technical capabilities of Daksha, while the detailed science case is discussed in a separate paper.

Features of the Spectra of Microwave Sources above Sunspots Inferred from Observations with RATAN-600

The flux density spectra of cyclotron sources above the sunspots were obtained by observations with RATAN-600 in the range of 1.710 cm with high spectral () resolution and their spectral indices were estimated. The spectral index was then utilized to determine the magnitude of the magnetic field at the base of the transition region on the Sun. By analyzing the relationship between the emisson flux density of the sources and the magnetic field it was found that the observed characteristics of sunspot-associated sources did not align with predictions from a simplified radiation model commonly used for data interpretation. Specifically, there was an excess flux density in the spectrum of ordinary mode radiation compared to what was expected.

Plasma Motions and Compressive Wave Energetics in the Solar Corona and Solar Wind from Radio Wave Scattering Observations

Radio signals propagating via the solar corona and solar wind are significantly affected by compressive waves, impacting the properties of solar bursts as well as sources viewed through the turbulent solar atmosphere. While static fluctuations scatter radio waves elastically, moving, turbulent, or oscillating density irregularities act to broaden the frequency of the scattered waves. Using a new anisotropic density fluctuation model from the kinetic scattering theory for solar radio bursts, we deduce the plasma velocities required to explain observations of spacecraft signal frequency broadening. The inferred velocities are consistent with motions that are dominated by the solar wind at distances 10 R , but the levels of frequency broadening for 10 R require additional radial speeds (100300) km s¹ and/or transverse speeds (2070) km s¹. The inferred radial velocities also appear consistent with the sound or proton thermal speeds, while the speeds perpendicular to the radial direction are consistent with nonthermal motions measured via coronal Doppler-line broadening, interpreted as Alfvnic fluctuations. Landau damping of parallel propagating ion-sound (slow MHD) waves allows an estimate of the proton heating rate. The energy deposition rates due to ion-sound wave damping peak at a heliocentric distance of (13) R are comparable to the rates available from a turbulent cascade of Alfvnic waves at large scales, suggesting a coherent picture of energy transfer, via the cascade or/and parametric decay of Alfvn waves to the small scales where heating takes place.

A Possible Indication of the Insufficiency of Homogeneous Models

The geo-effectiveness of coronal mass ejections (CMEs) is determined primarily by their magnetic fields. Modeling of gyrosynchrotron (GS) emission is a promising remote sensing technique to measure the CME magnetic field at coronal heights. However, faint GS emission from CME flux ropes is hard to detect in the presence of bright solar emission from the solar corona. With high dynamic-range spectropolarimetric meter wavelength solar images provided by the Murchison Widefield Array, we have detected faint GS emission from a CME out to 8.3 R , the largest heliocentric distance reported to date. High-fidelity polarimetric calibration also allowed us to robustly detect circularly polarized emission from GS emission. For the first time in the literature, Stokes V detection has jointly been used with Stokes I spectra to constrain GS models. One expects that the inclusion of polarimetric measurement will provide tighter constraints on the GS model parameters. Instead, we found that homogeneous GS models, which have been used in all prior works, are unable to model both the total intensity and circular polarized emission simultaneously. This strongly suggests the need for using inhomogeneous GS models to robustly estimate the CME magnetic field and plasma parameters.

A Multipeak Solar Flare with a High Turnover Frequency of the Gyrosynchrotron Spectra from the Loop-top Source

The origin of multiple peaks in light curves of various wavelengths remains illusive during flares. Here we discuss the flare of SOL2023-05-09T03:54M6.5 with six flux peaks as recorded by a tandem of new microwave and hard X-ray (HXR) instruments. According to its microwave spectra, the flare represents a high-turnover-frequency (>15 GHz) event. The rather-complete microwave and HXR spectral coverage provides a rare opportunity to uncover the origin of such an event together with simultaneous EUV images. We concluded that (1) the microwave sources originates around the top section of the flaring loops with a trend of source spatial dispersion with frequency; (2) the visible movement of the microwave source from peak to peak originates from the process of new flaring loops appearing sequentially along the magnetic neutral line; (3) the optically thin microwave spectra are hard with the indices ( _tn) varying from 1.2 to 0.4, and the turnover frequency always exceeds 15 GHz; (4) higher turnover/peak frequency corresponds to stronger peak intensity and harder optically thin spectra. Using the FokkerPlanck and GX Simulator codes we obtained a good fit to the observed microwave spectra and spatial distribution of the sources at all peaks, if assuming the radiating energetic electrons have the same spatial distribution and single-power-law spectra but with the number density varying in a range of 30%. We conclude that the particle acceleration in this flare happens in a compact region nearing the loop-top. These results provide new constraints on the acceleration of energetic electrons and the underlying flare intermittent reconnection process.

Mechanisms of Fundamental Electromagnetic Wave Radiation in the Solar Wind

Large-scale and long-term two-dimensional particle-in-cell simulations performed for parameters relevant to type III solar radio bursts have provided new results on the generation mechanisms of fundamental electromagnetic waves radiated at the plasma frequency _p. The paper first considers the nonlinear wave interaction process of electromagnetic decay (EMD) in a homogeneous solar wind plasma with an electron-to-ion temperature ratio T _e/T _i > 1. The dynamics of ion-acoustic waves (dispersion, spectra, growth/damping) is studied, and signatures confirming the three-wave interactions (cross-bicoherence, correlations between waves' phases and between waves' growths, resonance conditions) are provided. The decisive role played in EMD by the backscattered Langmuir waves coming from the electrostatic decay (ESD) is demonstrated. EMD can be triggered by ion acoustic waves coming from the two cascades of the faster and more intense ESD. The same study is then performed in a solar wind plasma with random density fluctuations. In this case, EMD is not suppressed but develops only within plasma regions of reduced or quasi-uniform density. It coexists with linear mode conversion (LMC) of Langmuir waves into electromagnetic radiation, which is the fastest and most prominent process, as well as with ESD. LMC can lead to enhanced occurrence of EMD in the early stage. Moreover, the impact of T _e/T _i on electromagnetic energy growth and saturation is shown to be rather weak. Ion-acoustic waves are heavily damped at T _e T _i, so that EMD is overcome by nonlinear induced scattering on thermal ions. In actual solar wind plasmas, EMD should be more easily observed in plasma regions weakly perturbed by the background density turbulence and where ion temperature is decreased.

Deep learning-based prediction of CME-driven shock standoff distances in metric type II radio emissions

Type II radio emissions are events mostly found to be associated with coronal mass ejections (CMEs) and accelerated by the CME-driven shock in the heliosphere. This study reports on the estimation of the CME-shock standoff distance at the commencement of metric type II radio emissions by combining the CME-deprojected speed and spectral features of radio bursts using a robust TensorFlow Deep-Learning Sequential (TFDLS) technique. The dataset of 96 CMEs at the commencement of type II radio bursts was used between Solar cycle 24 and the ascending phase of Solar Cycle 25. The measured root mean squared error (RMSE) was 0.145 (Rs), with an average height difference of 0.096 Rs between the observed and predicted CME-shock heights. Five (5) CMEs/radio bursts energetic events associated with solar flares were selected from the test data, and the CME shock stand-off heights were forecasted using the TFDLS and flare- onset (FL) methods. The data were used to compare the leading-edge (LE) and dynamic spectra (DS) methods. The RMSE measured between the FL and LE was 0.35 Rs, and the RMSE estimated between the TFDLS and LE approaches was 0.04 Rs. The RMSE between FL and DS was 0.34. Rs, and the RMSE between the TFDLS and the DS was 0.04 Rs. We also used the findings gained from the five selected events and compared them to the 3D shock- fitting (3D-SF) approach. The RMSE found between the TFDLS and the 3D-SF was 0.18 Rs, while the RMSE estimated between the FL and the 3D-SF was 0.23 Rs. This shows that the TFDLS has satisfactory performance and can be used as an alternative technique.

An Assessment of Solar Cycle 25 progress through observation of SRBs and associated Geomagnetic Storms

Geomagnetic storms are severe aspects of Space Weather. They originate due to solar transient emissions such as coronal mass ejections (CMEs), whose energetic materials propagate in the Interplanetary medium and are coupled with the magnetosphere system. CME driven Geomagnetic storms are often associated with solar radio bursts (SRBs), particularly type II and type IV bursts. In this study, we present the preliminary results of solar radio observations and their associated geomagnetic activity during solar cycle 25 (SC 25) from January 2020 to June 2023, focusing on the cycle's first four intense geomagnetic storms. The study used various radio telescopes, mainly the compound astronomical low-frequency low-cost instrument for spectroscopy and transportable observatory (CALLISTO), as well as OMNI data and the World Data Center for Geomagnetism. During the study period, it was found that 23 solar radio bursts diagnosed the geomagnetic storms with Dst <-50nT from 35 reported, including three severe storms of the SC 25. The time delay between the solar radio bursts and the arrival of CMEs and/or HSS near the Earth's magnetosphere is estimated with an average value of 79 h within the [48120 h] range for 23 geomagnetic storms associated with solar radio bursts. Among 35 geomagnetic storms recorded, five are recurring geomagnetic storms associated with coronal high-speed streams (HSS), while CMEs cause the rest with average speeds of 750 km/s. The current SC 25 recognizes four major storms within the scope of the study. On 21 April 2023, a type II radio burst followed by a type IV burst diagnosed the first severe geomagnetic storm on 24 April 2023. The second severe storm was unusual and detected in the absence of the precursor as a solar radio burst. The SRBs of type II burst and type IV burst extending in IP medium on 1 November 2021 tracked the third major storm of the cycle while the group of type III radio bursts, type II and type IV bursts on 24 February 2023, predicted the major storm on 27 February 2023. These major geomagnetic storms are linked to CMEs that show expanding flux ropes, which are signatures of type II and moving type IV radio bursts identified. Furthermore, the detected SRBs and related major geomagnetic storms are proof of high solar and magnetic activity of the ascending phase of SC 25. The SC 25 has been characterized overall, and its current progress is being tracked using observations of SRBs and magnetic activity during its rising phase.

Generation of relativistic electrons at the termination shock in the solar flare region

Context. Solar flares are accompanied by an enhanced emission of electromagnetic waves from the radio up to the -ray range. The associated hard X-ray and microwave radiation is generated by energetic electrons. These electrons play an important role, since they carry a substantial part of the energy released during a flare. The flare is generally understood as a manifestation of magnetic reconnection in the corona. The so-called standard CSHKP model is one of the most widely accepted models for eruptive flares. The solar flare event on September 10, 2017 offers us a unique opportunity to study this model. The observations from the Expanded Owens Valley Solar Array (EOVSA) show that 1.6 10⁴ electrons with energies > 300 keV are generated in the flare region.
Aims: There are signatures in solar radio and extreme ultraviolet (EUV) observations as well as numerical simulations that a "termination shock" (TS) appears in the magnetic reconnection outflow region. Electrons accelerated at the TS can be considered to generate the loop-top hard X-ray sources. In contrast to previous studies, we investigate whether the heating of the plasma at the TS provides enough relativistic electrons needed for the hard X-ray and microwave emission observed during the solar X8.2 flare on September 10, 2017.
Methods: We studied the heating of the plasma at the TS by evaluating the jump in the temperature across the shock by means of the Rankine-Hugoniot relationships under coronal circumstances measured during the event on September 10, 2017. The part of relativistic electrons was calculated in the heated downstream region.
Results: In the magnetic reconnection outflow region, the plasma is strongly heated at the TS. Thus, there are enough energetic electrons in the tail of the electron distribution function (EDF) needed for the microwave and hard X-ray emission observed during the event on September 10, 2017.
Conclusions: The generation of relativistic electrons at the TS is a possible mechanism of explaining the enhanced microwave and hard X-ray radiation emitted during flares.

Estimating the lateral speed of a fast shock driven by a coronal mass ejection at the location of solar radio emissions

Context. Fast coronal mass ejections (CMEs) can drive shock waves capable of accelerating electrons to high energies. These shock- accelerated electrons act as sources of electromagnetic radiation, often in the form of solar radio bursts. Recent findings suggest that radio imaging of solar radio bursts can provide a means to estimate the lateral expansion of CMEs and associated shocks in the low corona.
Aims: Our aim is to estimate the expansion speed of a CME-driven shock at the locations of radio emission using 3D reconstructions of the shock wave from multiple viewpoints.
Methods: In this study, we estimated the 3D location of radio emission using radio imaging from the Nanay Radioheliograph and the 3D location of a CME-driven shock. The 3D shock was reconstructed using white-light and extreme ultraviolet images of the CME from the Solar Terrestrial Relations Observatory, Solar Dynamics Observatory, and the Solar and Heliospheric Observatory. The lateral expansion speed of the CME-driven shock at the electron acceleration locations was then estimated using the approximate 3D locations of the radio emission on the surface of the shock.
Results: The radio bursts associated with the CME were found to reside at the flank of the expanding CME-driven shock. We identified two prominent radio sources at two different locations and found that the lateral speed of the shock was between 800 and 1000 km s¹ at these locations. Such a high speed during the early stages of the eruption already indicates the presence of a fast shock in the low corona. We also found a larger ratio between the radial and lateral expansion speed compared to values obtained higher up in the corona.
Conclusions: We estimated for the first time the 3D expansion speed of a CME-driven shock at the location of the accompanying radio emission. The high shock speed obtained is indicative of a fast acceleration during the initial stage of the eruption. This acceleration leading to lateral speeds in the range of 8001000 km s¹ is most likely one of the key parameters contributing to the presence of metric radio emissions, such as type II radio bursts.

Properties of Type-II Radio Bursts in Relation to Magnetic Complexity of the Solar Active Regions

Type-II radio bursts are believed to occur as a result of the shock driven by flares or coronal mass ejections (CMEs). While the shock waves are important for the acceleration of electrons necessary for the generation of the radio emission, the exact nature of the shock and coronal conditions necessary to produce type-II radio emission is still under debate. In this investigation, we probe the relationship of kinematic characteristics of the type-II radio bursts with the magnetic- field complexity (M_j) of the active regions visible on the photosphere. Our investigation of 64 type-II solar radio bursts, which are associated with flares and CMEs, reveals that M_j is linearly correlated in the logarithmic scale with the starting frequency (f_s) and drift-rate (f/t) of type-II radio burst. Further, M_j exhibits a linear correlation with the shock height (r) and electron density (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$n_{\rm e}$\end{document}) in logarithmic scale. This indicates that high frequency (f_s 100\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}${\rm MH_{z}}$\end{document}) bursts, which occur at the reconnection site near the solar surface, are produced from a strong magnetically complex region. Further, strong and complex magnetic-field regions produce shocks of higher speeds. Based on the derived plasma parameters of the radio bursts and their relationship with f_s as well as with M_j, we propose that the high-frequency type- II bursts were generated in a special situation when the shock is produced due to magnetic reconnection occurring in the low-lying coronal loops. We conclude that type-II radio bursts can occur even in the inner corona as well as in the outer corona; however, it depends on the magnetic complexity of the active region in which the event occurs.

Coronal Signatures of Flare Generated Fast-Mode Wave at EUV and Radio Wavelengths

This paper presents a detailed study of the type II solar radio burst that occurred on 06 March 2014 using combined data analysis. It is a classical radio event consisting of type III radio burst and a following type II radio burst in the dynamic spectrum. The type II radio burst is observed between 235 130 MHz (120 60 MHz) in harmonic (fundamental) bands with the life time of 5 minutes between 09:26 09:31 UT. The estimated speed of type II burst by applying two-fold Saito model is 650 km s¹. An extreme ultraviolet (EUV) wave is observed with Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). The very close temporal onset association of the EUV wave and flare energy release indicates that the EUV wave is likely produced by a flare pressure pulse. The eruption is also accompanied by a weak coronal mass ejection (CME) observed with the coronagraphs onboard the Solar and Heliospheric Observatory (SOHO) and the twin Solar Terrestrial Relations Observatory (STEREO). The plane of sky speed of the CME was 252 km s¹ in the SOHO/LASCO-C2 and 280 km s¹ in the STEREO-B/SECCHI-COR1 images. The EUV wave has two wave fronts, one expanding radially outward and the other one moving along the flare loop arcade. The source position of the type II burst imaged by the Nanay Radio Heliograph (NRH) shows that it was associated with the outward moving EUV wave. The CME is independent of the shock wave as confirmed by the location of NRH radio sources below the CME's leading edge. Therefore the type II radio burst is probably ignited by the flare. This study shows the possibility of EUV wave and coronal shock triggered by flare pressure pulse, generating the observed type II radio burst.

A Statistical Investigation of the Neupert Effect in Solar Flares Observed with ASO-S/HXI

The Neupert effect refers to the strong correlation between the soft X-ray (SXR) light curve and the time-integrated hard X-ray (HXR) or microwave flux, which is frequently observed in solar flares. In this article, we therefore utilized the newly launched Hard X-ray Imager (HXI) on board the Advanced Space-based Solar Observatory to investigate the Neupert effect during solar flares. By checking the HXR light curves at 20 50 keV, a sample of 149 events that cover the flare impulsive phase were selected. Then, we performed a crosscorrelation analysis between the HXR fluence (i.e., the time integral of the HXR flux) and the SXR 1 8 flux measured by the Geostationary Operational Environmental Satellite. All the selected flares show high correlation coefficients (>0.90), which seem to be independent of the flare location and class. The HXR fluences tend to increase linearly with the SXR peak fluxes. Our observations indicate that all the selected flares obey the Neupert effect.

Radio-astronomical monitoring of active regions in the microwave range in the service of forecasting solar flares

One of the key factors of space weather is solar flare activity, the monitoring and prediction of which is an important task of specialized dedicated groups of space experts and solar astronomers. Solar flare forecasts are based on identifying and detecting the so-called precursors, specific processes in solar activity events that occur before flares. Collecting data for space weather analysis and prediction comes down to several types of measurements performed by more than a dozen spacecraft. Ground-based observations and monitoring nowadays are becoming more or less complimentary. One of the reasons for this is the limitation of observation time with ground-based telescopes due to adverse Earth weather conditions. However, solar radio astronomy is immune to almost any weather activity, and the main question here is what new quality it can bring. Observational data accumulated in the 20th century show that solar radio bursts can be associated with flare activity. In addition, the existing network of solar radio telescopes is already well established. As an example, in this paper, we describe the possibilities of a fully steerable 32-meter radio telescope of Ventspils International Radio Astronomy Centre (VIRAC), Latvia, which can be useful for searching for new precursors of solar flares.

The Characteristic Properties of Solar Activity in Solar Cycle 24

Solar cycle 24 began in December 2008 and ended in December 2019. Maximum of solar cycle 24 occurred in April 2014. Magnetic field intensity has been reported via data from the Wilcox Solar Observatory. Sunspot numbers are reported via the data from WDC-SILSO, Royal Observatory of Belgium. Sunspot area distribution was determined using the data from the Max Planck Institute. Flare Index intensity is indicated, and the data recorded by the Kandilli Observatory at Bogazici University is presented. Hemisphere asymmetries in terms of sunspots and solar flare index are calculated. The number of solar flares that occur at the highest intensity (X-class) during this cycle are presented, the data for which from the NOAA/SWPC. The characteristics of Coronal Mass Ejections are given, as determined using the LASCO coronagraph operating on the SOHO mission. Solar radio flux distribution and comparison with previous cycles was studied using data from Space Weather Canada.

Periodicities observed in the solar and geomagnetic indexes and in SABER thermospheric infrared power measurements

We analyze periodic features in the F10.7 cm solar radio flux and the Ap geomagnetic index to examine solar-terrestrial coupling in Earth's atmosphere above 100 km. The coupling is indicated by the same periodic features observed in the global infrared power radiated by Nitric Oxide (NO) and Carbon Dioxide (CO₂). The infrared power data have been measured by the SABER instrument on the NASA TIMED satellite since January 2002. A strong dominant feature for the length of the solar cycle (11 years) can be observed in all the datasets. The length of this periodicity can vary from 9.95 years to 12.25 years for different solar cycles. Periodic features larger than the length of the solar cycle are also observed in the F10.7 and Ap index (22 years and 44 years). A strong 180-day periodic feature can be observed in the Ap index which corresponds to the well-known Russell-McPherron effect. Additionally, the NO power and the Ap index exhibit statistically significant periodic features for shorter periods (e.g., 27, 13.5 and 9 days). This similarity indicates a strong link between the thermosphere's infrared energy budget and the geomagnetic conditions of the upper atmosphere.

Electron cyclotron maser instability by evolving fast electron beams in the flare loops

The electron cyclotron maser instability (ECMI) stands as a pivotal coherent radio emission mechanism widely implicated in various astrophysical phenomena. In the context of solar activity, ECMI is primarily instigated by energetic electrons generated during solar eruptions, notably flares. These electrons, upon leaving the acceleration region, traverse the solar atmosphere, forming fast electron beams (FEBs) along magnetic field lines. It is widely accepted that as these FEBs interact with the ambient plasma and magnetic fields, they give rise to radio and hard X-ray emission. Throughout their journey in the plasma, FEBs undergo modifications in their energy spectrum and velocity spatial distribution due to diverse energy loss mechanisms and changes in ambient plasma parameters. In this study, we delve into the impact of the evolving energy spectrum and velocity anisotropic distribution of FEBs on ECMI during their propagation in flare loops. Our findings indicate that if we solely consider the progressively flattened lower energy cutoff behavior as FEBs descend along flare loops, the growth rates of ECMI decrease accordingly. However, when accounting for the evolution of ambient magnetic plasma parameters, the growth rates of ECMI increase as FEBs delve into denser atmosphere. This underscores the significant influence of the energy spectrum and velocity anisotropy distribution evolution of FEBs on ECMI. Our study sheds light on a more comprehensive understanding of the dynamic spectra of solar radio emissions.

Development of a 90600 MHz Meter-wave Solar Radio Spectrometer

Radio observation is important for understanding coronal mass ejections (CMEs), coronal shock waves, and high-energy electron acceleration. Here, we developed a new Chashan broadband solar radio spectrometer at a meter wavelength for observing the (super)fine structure of the solar radio burst spectrum. In the signal-receiving unit, we adopt an antenna system consisting of a 12 m large-aperture parabolic reflector and dual- line polarized logarithmic periodic feed source, as well as a high- precision Sun-tracking turntable system, all of which ensure the high- precision acquisition of solar radiation signals. For the digital receiver, we use a high-speed analog-to-digital converter with a sampling rate of 1.25 GSPS to directly sample the signal amplified and filtered by the analog receiver, simplifying the structure of the analog receiver, and design a 16k-point fast Fourier transform algorithm in the field programmable gate array to perform timefrequency transformation on the sampled signals. The default frequency and temporal resolution of the system are 76.294 kHz and 0.839 ms (up to 0.21 ms), respectively. The noise coefficient of the system is less than 1 dB, the dynamic range is more than 60 dB, and the sensitivity is as high as 1 sfu. We have observed a large number of radio bursts, including type I radio storms, hundreds of type III, 20 type II, and 15 type IV bursts in the past year. These high-quality data are useful in the further study of CMEs and associated particle acceleration and the origins of solar radio bursts.

A Joint Microwave and Hard X-Ray Study toward Understanding the Transport of Accelerated Electrons During an Eruptive Solar Flare

The standard flare model, despite its success, is limited in comprehensively explaining the various processes involving nonthermal particles. One such missing ingredient is a detailed understanding of the various processes involved during the transport of accelerated electrons from their site of acceleration to different parts of the flare region. Here we use simultaneous radio and X-ray observations from the Expanded Owens Valley Solar Array and the Spectrometer/Telescope for Imaging X-rays on board the Solar Orbiter, respectively, from two distinct viewing perspectives, to study the electron transport processes. Through detailed spectral modeling of the coronal source using radio data and footpoint sources using X-ray spectra, we compare the nonthermal electron distribution at the coronal and footpoint sources. We find that the flux of the nonthermal electrons precipitated at the footpoint is an order of magnitude smaller than that trapped in the looptop, consistent with earlier works that primarily used X-ray for their studies. In addition, we find that the electron spectral indices obtained from X-ray footpoints are significantly softer than the spectral hardness of the nonthermal electron distribution in the corona. We interpret these differences based on transport effects and the difference in sensitivity of microwave and X-ray observations to different regimes of electron energies. Such an understanding is crucial for leveraging different diagnostic methods of nonthermal electrons simultaneously to achieve a more comprehensive understanding of the electron acceleration and transport processes of solar flares.

Intensity Calibration for the Mingantu Spectral Radioheliograph Images

The Mingantu Spectral Radioheliograph (MUSER), a new generation of solar dedicated radio imaging spectroscopic telescope, has realized high-time, high-angular, and high-frequency resolution imaging of the Sun in the 0.4--15 GHz ultra-broadband frequency range. The radio brightness temperature is an important parameter to describe the solar physical process. It plays a very important role in the study of different radio radiation mechanisms, solar magnetic field and the acceleration of non- thermal particles in the solar burst process. Therefore, the image of radioheliograph must be calibrated for brightness temperature. This paper introduces a method suitable for radioheliograph image intensity calibration. The solar radio image contains the structural information of the solar disk. The radio radius and intensity of the quiet solar disk in the image can be obtained by fitting the first kind of Bessel function with the visibilities of short baselines of the radioheliograph. Then, the Rayleigh-Jeans law and the daily solar radio flux can be used to calculate the calibration factor $G_{\rm c}$ of the daily image, so as to realize the calibration of the MUSER image intensity. Applying this method to the actual observation data of MUSER, including different situations such as the quiet sun and solar radio bursts, the error of the daily calibration factor $G_{\rm c}$ is within 10\% of its value, and the obtained bright temperature of the quiet sun has a high correlation with the result obtained by other people. These indicate the feasibility and effectiveness of this method.

Batch Measurement and Calibration Method of DSRT Three-axis Low Frequency Antenna Pointing Error

Daocheng Solar Radio Telescope (DSRT) is an important part of the Solar Interplanetary exploration subsystem of the second phase of The Chinese Meridian Project. It operates in the 150\;MHz to 450\;MHz frequency band and provides high-resolution spatial and temporal images of solar eruption brightness and temperature. Aiming at the high precision pointing measurement of the DSRT antenna and the requirement of batch calibration and correction of pointing errors, this paper establishes the 3-parameter, antenna encoder zero point offset pointing error model by quaternion rotation transformation method according to the unique three-axis mount system of DSRT. In this paper, the drift scanning method based on radio source is proposed to obtain the radiation pattern of 16 antennas and determine the boresight according to the two- dimensional power pattern to accurately measure the pointing error of the DSRT antenna. Finally, the least square method is used to get the model parameters, and the antenna control software is used to adjust the encoder zero point of each axis, and then the adjustment results are verified. The results show that the pointing calibration method is reliable and effective. The pointing accuracy of 16 antennas after correction is within 0.5$^\circ$, which is significantly better than the pointing error of 3.5$^\circ$ before calibration, and the error is less than one-tenth of HBPW (half power beam width) under the maximum working frequency of the DSRT antenna.

Changes in the solar atmosphere during the decay of the Modern Maximum

Context. The Sun experienced a period of unprecedented activity during the 20th century, now called the Modern Maximum (MM). The decay of the MM after its maximum in cycle 19 has changed the Sun, the heliosphere, and the planetary environments in many ways. However, studies disagree on whether this decay has proceeded synchronously in different solar parameters or not.
Aims: One of the related key issues is if the relation between two long parameters of solar activity, the sunspot number and the solar 10.7 cm radio flux, has remained the same during this decay. A recent study argues that there is an inhomogeneity in the 10.7 cm radio flux in 1980, which leads to a step-like jump ("1980 jump") in this relation. If true, this result would reduce the versatility of possible long-term studies of the Sun during the MM. Here we aim to show that the relation between sunspot number and 10.7 cm radio flux does indeed vary in time, not due to an inhomogeneous radio flux but due to physical changes in the solar atmosphere.
Methods: We used radio flux measurements made in Japan at four different wavelengths, and studied their long-term relation with the sunspot number and the 10.7 cm radio flux during the decay of MM. We also used two other solar parameters, the MgII index and the number of solar active regions, in order to study the nature of the observed long-term changes in more detail.
Results: We find that the 1980 jump is only the first of a series of 1-2-year "humps" that mainly occur during solar maxima. All five radio fluxes depict an increasing trend with respect to the sunspot number from the 1970s to 2010s. These results exclude the interpretation of the 1980 jump as an inhomogeneity in the 10.7 cm flux, and reestablish the 10.7 cm flux as a homogeneous measure of solar activity. The fluxes of the longer radio waves are found to increase with respect to the shorter waves, which suggests a long-term change in the solar radio spectrum. We also find that the MgII index of solar UV irradiance and the number of active regions also increased with respect to the sunspot number, further verifying the difference in the long-term evolution in chromospheric and photospheric parameters.
Conclusions: Our results provide evidence for important structural changes in solar magnetic fields and the solar atmosphere during the decay of the MM, which have not been reliably documented so far. We also emphasize that the changing relation between the different (e.g., photospheric and chromospheric) solar parameters should be taken into account when using the sunspot number or any single parameter in long- term studies of solar activity.

Solar Radio Burst Prediction Based on a Multimodal Model

Solar radio bursts are intense radio radiation sources that occur during the energy-release process and represent a hot topic in solar-physics and space-weather research. In this paper, we present a multimode prediction model for daily solar radio bursts. The model uses deep learning and machine learning to obtain data information from different dimensions and to establish the relationship between the characteristics of the solar active region on the solar surface and solar radio bursts. For this model, we use data from the Solar and Heliospheric Observatory (SOHO)/Michelson Doppler Imager (MDI) total solar magnetic map, the Royal Observatory of Belgium World Data Centre in Brussels, and NOAA sunspot parameters (including number, area, and type of sunspots) as inputs. The output results are then compared with the list of solar radio bursts recorded by the Radio Solar Telescope Network (RSTN) to determine whether solar radio bursts are present and to determine the key parameters for determining radio bursts. Based on 5449 days of observational data, we find that the prediction accuracy of the model is 0.898 0.011, and that the number of sunspots is a key parameter in determining the occurrence of solar radio bursts. Specifically, when the number of sunspots is greater than 15, the probability of occurrence of solar radio bursts is greater than 90%. We have identified the key parameters and thresholds for determining solar radio bursts and highlighted the key parameters for space-weather prediction. In addition, the prediction model can also be used for predicting in other fields.

Forecasting Medium-Term F10.7 Using the Deep-Learning Informer Model

The daily 10.7-cm solar radio flux (F10.7) is one of the most important solar activity indices and has been widely applied in various space environment modeling as a crucial parameter. In this study, we adopt a deep-learning Informer model, based on the transformer architecture to predict the medium-term F10.7 index, which uses 48 historical daily F10.7 indices as input to directly forecast the following 1 27 days' F10.7 index. The model is demonstrated to be effective and to have superior performance compared with other widely-used forecasting techniques: two statistical methods provided by British Geological Survey (BGS), Space Weather Prediction Center (SWPC), and a multiflux neural network method provided by Collecte Localisation Satellites (CLS). In comparison, the Informer model significantly improves the forecast accuracy for the prediction horizon larger than 6 days, especially during the solar activity descending phase and at the solar activity minimum. For its effectiveness, accurate prediction capability and the advantage in F10.7 forecasting with longer horizon, the Informer could be potentially used as a candidate model for space weather operational forecasting.

The arrival time and energy of FRBs traverse the time-energy bivariate space like a Brownian motion

The origin of fast radio bursts (FRBs), the brightest cosmic explosion in radio bands, remains unknown. We introduce here a novel method for a comprehensive analysis of active FRBs' behaviors in the time-energy domain. Using "Pincus Index" and "Maximum Lyapunov Exponent", we were able to quantify the randomness and chaoticity, respectively, of the bursting events and put FRBs in the context of common transient physical phenomena, such as pulsar, earthquakes, and solar flares. In the bivariate time-energy domain, repeated FRB bursts' behaviors deviate significantly (more random, less chaotic) from pulsars, earthquakes, and solar flares. The waiting times between FRB bursts and the corresponding energy changes exhibit no correlation and remain unpredictable, suggesting that the emission of FRBs does not exhibit the time and energy clustering observed in seismic events. The pronounced stochasticity may arise from a singular source with high entropy or the combination of diverse emission mechanisms/sites. Consequently, our methodology serves as a pragmatic tool for illustrating the congruities and distinctions among diverse physical processes.

Solar radio spectrogram segmentation algorithm based on improved fuzzy C-means clustering and adaptive cross filtering

Solar radio spectrograms contain essential information, such as the duration type; therefore, recognizing and detecting solar radio spectrograms are significant for the further study of solar radio. With the upgrading of solar radio observation, considering the equipment that has already generated amounts of data, researchers have begun to use machine learning methods to recognize and detect solar radio spectrograms to resolve the weaknesses of manual identification, such as time consumption. However, the spectrograms are characterized by noise or insignificant outburst features, which affect the recognition and detection of solar radio spectrograms. In contrast, extracting the burst region separately and the more distinctive spectrogram features will help identify and detect it. Therefore, to remove the burst domain of the radio spectrogram better, this paper combines the idea of image segmentation and proposes a solar radio spectrogram segmentation algorithm based on improved fuzzy C-means (FCM) clustering and adaptive cross filtering for the extraction of the burst domain of solar radio spectrograms. This algorithm has multiple processing steps. The first step is solar radio spectrogram segmentation with the improved FCM based on the kernel-induced distance by incorporating spatial constraints combined with random walk and adaptive affiliation linking (RWAKFCM_S). The second step is adaptive cross filtering, eliminating the noise clustered in bursts. The results show the following. (1) The RWAKFCM_S proposed in this paper has better anti-noise and segmentation performance than other methods in the synthetic, natural, and solar radio spectrogram segmentation experiments; it can also overcome the problems of noise sensitivity when segmenting spectrograms by traditional FCM. (2) The RWAKFCM_S can satisfy the high accuracy and rate of solar radio spectrogram segmentation demands. (3) The adaptive cross filtering proposed in this paper can eliminate noise clustered in the eruption domain. (4) The proposed method enables burst region extraction.

Spatial structure of resonance cavities in sunspots

We present a study of wave processes in sunspots from active regions NOAA 11131 on 2010 December 10 and NOAA 12565 on 2016 July 14 observed by SDO/AIA in the 1600, 304, and 171 temperature channels. To study the spatial structure of the resonance cavities previously found by Jess et al., we applied spectral data processing techniques such as pixelized wavelet filtering and mode decomposition. For the first time, we found stable regions as waveguides of the oscillations in the sunspot umbra, occupying specific frequency ranges without spatial overlap. The sizes of these regions depend on the frequency oscillations, and the maximum frequency coincides with the values of the harmonics of the main oscillation mode. Frequency drifts were observed in the band occupied by these regions, with different spectral slopes depending on the location of the sources in the sunspot umbra. We suggest that the observed distribution of wave sources in the umbra is a set of resonant cavities where successive amplification of oscillations at selected multiple harmonics is observed. The distribution of sources at low frequencies indicates the influence of the atmospheric cut-off due to the inclinations of the magnetic field lines.

Sources of Solar Protons in the Events of February 24-25 and July 16-17, 2023

From the beginning of January 2021 to the end of August 2023, the radiation monitor of the Spektr-RG spacecraft registered three enhancements in the count rate, which exceed the background variations during the solar activity cycle and have a comparable maximum value. These enhancements are associated with solar proton events (SPEs) from the flares X1.0 on October 28, 2021; M6.3 on February 25, 2023; and M5.7 on July 17, 2023. Using the example of these events, as well as smaller SPEs from the flares M3.7 on February 24, 2023, and M4.0 on July 16, 2023, threshold criteria for "proton" flares are discussed. In powerful SPEs, the contribution of solar protons to the radiation dose can exceed the total contribution of galactic cosmic rays (GCR) over a sufficiently long period of time. Therefore, such SPEs are sources of increased radiation hazard and require prediction based on real-time observations. It was shown that, in these five flares, thresholds were overcome according to three criteria: plasma temperature >12 MK (soft X-ray source), duration (>5 min) of microwave or hard X-ray (HXR) radiation (electron acceleration >100 keV), and height of flare development process >60 Mm (radio emission at plasma frequencies <610 MHz). The arrival of the first solar protons >100 MeV to the Earth's orbit was expected no earlier than 10 min relative to the beginning of HXR or microwave radiation, i.e., could have been predicted in advance. To study the relationship between solar flares and SPEs, we used data from the anticoincidence shield of the spectrometer on INTEGRAL (ACS SPI), which is an effective but uncalibrated detector of HXR >100 keV and protons >100 MeV, as well as patrol observations of radio emission at fixed frequencies (Radio Solar Telescope Network). It is noted that the X2.2 (N25E64) flare on February 17, 2023 satisfied all three "protonity" criteria and could become the source of a powerful SPE near the Earth in a case of favorable location on the Sun. In the M8.6 (N27W29) flare on February 28, 2023, the third criterion was not met, and it did not lead to an SPE as expected (it developed in a plasma with a density >2.5 10¹⁰ cm^-3 and plasma frequency >1415 MHz).

Comparison of Solar Multifrequency Microwave Data with Other Solar Indices for Understanding Solar and Stellar Microwave Data

Thermal microwave emissions detected from stellar atmospheres contain information on stellar activity. However, even for the Sun, the relationship between multifrequency microwave data and other activity indices remains unclear. We investigated the relationships among the thermal microwave fluxes with 1, 2, 3.75, and 9.4 GHz, their circular polarizations, and several activity indices recorded during recent solar cycles and observed that these relationships can be categorized into two groups. In the first group, the relationship between the microwave fluxes and solar indices, which are strongly related to the active regions, can be well-fitted by using a linear function. In the second group, the fitting function is dependent on frequency. Specifically, the microwave fluxes at 1 and 2 GHz can be well-fitted to the total unsigned magnetic and extreme ultraviolet fluxes by employing a power-law function. The trend changes around 3.75 GHz, and the trend for the 9.4 GHz fluxes can be fitted by using a linear function. For the first time, we present the relationship between circular polarization and solar indices. Moreover, we extrapolated these relationships of the solar microwave fluxes to higher values and compared them with the solar-type stars. We found that Eri, whose microwave emission originates from thermal plasma, follows the extrapolated relationship. However, to date, only one star's emission at 110 GHz has been confirmed as thermal emission. More solar-type stars should be observed with future radio interferometers to confirm that relationships based on solar data can be applied to stellar microwave data.

The Slipping Magnetic Reconnection and Damped Quasiperiodic Pulsations in a Circular Ribbon Flare

The study of circular ribbon (CR) flares is important to understand the three-dimensional magnetic reconnection in the solar atmosphere. We investigate the slipping brightenings and damped quasiperiodic pulsations in a CR flare by multiwavelength observations. During the flaring process, two extreme ultraviolet brightenings (SP1 and SP2) slip synchronously along the ribbon in a counterclockwise direction. The ribbon and fans between them show synchronous enhancement with the microwave and hard X-ray (HXR) CR source. In the magnetohydrostatic extrapolation results and observations, the dome and outer spine display an evident counterclockwise twisting feature. We propose the slipping reconnection occurs between the fan and outer spine in the null point, which covers the region from SP1 to SP2. The fan of SP1 shows the strongest twist and produces the most efficient reconnection. The ribbon after SP1 becomes weak due to the destruction of the fan configuration. The fan of SP2 is in the front of the slipping motion, which initiates new reconnection and brightens the local ribbon. The twisting of the dome continuously promotes new reconnection in the null point, which brightens the ribbon in sequence to display a counterclockwise slipping feature. Thus, the twist of the dome may trigger and dominate the slipping reconnection, and the rotation of the central positive pole could be one possible cause of the twist. After the peak, the microwave and HXR emission shows damped oscillations at a period of 15 s. The collapse of the fanspine structure may lead to the standing kink oscillations of the fan to modulate the reconnection and particle acceleration process.

Imaging a Large Coronal Loop Using Type U Solar Radio Burst Interferometry

Solar radio U-bursts are generated by electron beams traveling along closed magnetic loops in the solar corona. Low-frequency (<100 MHz) U-bursts serve as powerful diagnostic tools for studying large-sized coronal loops that extend into the middle corona. However, the positive frequency drift component (descending leg) of U-bursts has received less attention in previous studies, as the descending radio flux is weak. In this study, we utilized LOFAR interferometric solar imaging data from a U-burst that has a significant descending leg component, observed between 10 and 90 MHz on 2020 June 5th. By analyzing the radio source centroid positions, we determined the beam velocities and physical parameters of a large coronal magnetic loop that reached just about 1.3 R in altitude. At this altitude, we found the plasma temperature to be around 1.1 MK, the plasma pressure around 0.20 mdyn, cm², and the minimum magnetic field strength around 0.07 G. The similarity in physical properties determined from the image suggests a symmetric loop. The average electron beam velocity on the ascending leg was found to be 0.21c, while it was 0.14c on the descending leg. This apparent deceleration is attributed to a decrease in the range of electron energies that resonate with Langmuir waves, likely due to the positive background plasma density gradient along the downward loop leg.

Electrostatic Wave Decay in the Randomly Inhomogeneous Solar Wind

Despite a few space observations where Langmuir and ion acoustic waves are expected to participate in the mechanism of electrostatic decay, this is to date believed to be the main and fastest nonlinear wave process in the solar wind. However, in such a plasma where random density fluctuations are ubiquitous, the question of whether nonlinear wave processes play a significant role in Langmuir wave turbulence generated by electron beams associated with type III solar radio bursts remains still open. This paper provides several answers by studying, owing to two-dimensional challenging particle-in-cell simulations, the dynamics and the properties of the ion acoustic waves excited by such Langmuir wave turbulence and the role they play in the electrostatic decay. The impact on this process of plasma background density fluctuations and electron-to-ion temperature ratio is studied. Moreover, it is shown that, for a typical solar wind plasma with an average level of density fluctuations of a few percent of the ambient density and a temperature ratio of the order of 1, nonlinear induced scattering off ions occurs, with small intensity low-frequency quasi-modes and only in localized plasma regions where density is depleted or weakly perturbed by low-frequency turbulence.

Episodic Energy Release during the Main and Post-impulsive Phases of a Solar Flare

When and where the magnetic field energy is released and converted in eruptive solar flares remains an outstanding topic in solar physics. To shed light on this question, here we report multiwavelength observations of a C9.4-class eruptive limb flare that occurred on 2017 August 20. The flare, accompanied by a magnetic flux rope eruption and a white light coronal mass ejection, features three post-impulsive X-ray and microwave bursts immediately following its main impulsive phase. For each burst, both microwave and X-ray imaging suggest that the nonthermal electrons are located in the above-the-loop-top region. Interestingly, contrary to many other flares, the peak flux of the three post-impulsive microwave and X-ray bursts shows an increase for later bursts. Spectral analysis reveals that the sources have a hardening spectral index, suggesting a more efficient electron acceleration into the later post-impulsive bursts. We observe a positive correlation between the acceleration of the magnetic flux rope and the nonthermal energy release during the post-impulsive bursts in the same event. Intriguingly, different from some other eruptive events, this correlation does not hold for the main impulse phase of this event, which we interpret as energy release due to the tether-cutting reconnection before the primary flux rope acceleration occurs. In addition, using footpoint brightenings at conjugate flare ribbons, a weakening reconnection guide field is inferred, which may also contribute to the hardening of the nonthermal electrons during the post-impulsive phase.

Solar Type J Radio Bursts and the Associated Coronal Loop

The solar type J radio burst is a variant of type III bursts, which are a probe for understanding solar energetic electrons and local electron density. This study investigates a type J burst event on 2017 September 9. We have combined the data from the extreme-ultraviolet (EUV) imaging and the EUV Imaging Spectrometer (EIS) to analyze the event. Within 4 minutes several type J bursts with similar morphology occur. Two of them, with clear fundamental and second harmonic bands, are studied in detail. We find a delay of 2 0.5 s between their different harmonic bands. During type J bursts, only one coronal loop brightens significantly at its northern footpoint, in correlation with the continuous injection of erupting jets into the loop. The EUV intensity of the brightening footpoint is correlated with the radio flux at 245 and 410 MHz, with correlation coefficients of 0.2 and 0.4, respectively. These observations suggest that the type J bursts should originate from this coronal loop. By analyzing the electron number density distribution along the coronal loop diagnosed from the EIS data and the time evolution of the plasma frequency calculated from the type J burst, we determine that the velocities of the energetic electrons exciting the two type Js are 0.10 0.02c and 0.12 0.02c. Our results confirm previous studies on type J bursts.

Long-term variations in the mesopause region derived from OH*(3,1) rotational temperature observations at Wuppertal, Germany, from 1988 - 2022

We analysed the time series of OH*(3,1) rotational temperatures observed from Wuppertal in the time interval 1988 - 2022. The long-term evolution of the time series is characterised by two components. Firstly, we derived a significant correlation of the temperatures and the F10.7 cm solar radio flux. The sensitivity to the 11-year cycle of solar activity is 4.8 0.7 K (100 SFU)^-1. Then we show that the second major component in addition to this solar influence is a significant long-period oscillation. The oscillation has a period of P = 22.2 1.5 years with an amplitude of A = 1.8 0.4 K. A significant linear trend cannot be derived for the time series of OH*(3,1) rotational temperatures. Due to the long-period oscillation the derivation of linear trends heavily depends on the analysed time interval. This explains the different linear trends that have been derived in past studies for this time series. All of the previous results for this quantity are in good agreement with the long-period oscillation determined here, because different parts of the time series and, therefore, different parts of the oscillation (e.g mainly downswing or a complete cycle) have been analysed which led to largely different linear trends.

Imaging spectroscopy of a spectral bump in a type II radio burst

Context. Observations of solar, type II radio bursts provide a unique opportunity to analyze the nonthermal electrons accelerated by coronal shocks and diagnose the plasma density distribution in the corona. However, there are very few high-frequency resolution interferometric observations of type II radio bursts that are capable of tracking these electrons.
Aims: Recently, more spatially resolved high-resolution observations of type II radio bursts have been recorded with the Low- Frequency Array (LOFAR). Using these observations, we aim to track the location of a type II radio burst that experienced a sudden spectral bump.
Methods: We present the first radio imaging observations of a type II burst with a spectral bump. We measured the variation in source location and frequency drift of the burst and deducted the density distribution along its propagation direction.
Results: We have identified a type II burst that experiences a sudden spectral bump in its frequency-time profile. The overall frequency drift rate is 0.06 MHz s¹, and this corresponds to an estimated speed of 295 km s¹. The projected velocity of the radio source obtained from imaging is 380 km s¹ toward the east. At the spectral bump, a deviation in the source locations of the type II split bands is observed. The band separation increases significantly in the north-south direction.
Conclusions: The spectral bump shows an 8 MHz deviation at 60 MHz, which corresponds to a 25% decrease in the plasma density. The estimated crossing distance during the spectrum bump was 29 mm, suggesting that this density variation occurs in a confined area. This indicates that the shock most likely encountered the upper extent of a coronal hole.

Movie associated to Fig. 1 is available at https://www.aanda. org.

Tracking the motion of a shock along a channel in the low solar corona

Context. Shock waves are excited by coronal mass ejections (CMEs) and large-scale extreme-ultraviolet (EUV) wave fronts and can result in low- frequency radio emission under certain coronal conditions.
Aims: In this work, we investigate a moving source of low-frequency radio emission as a CME and an associated EUV wave front move along a channel of a lower density, magnetic field, and Alfvn speed in the solar corona.
Methods: Observations from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, the Nanay Radio Heliograph (NRH), and the Irish Low Frequency Array (I-LOFAR) were analysed. Differential emission measure maps were generated to determine densities and Alfvn maps, and the kinematics of the EUV wave front was tracked using CorPITA. The radio sources' positions and velocity were calculated from NRH images and I-LOFAR dynamic spectra.
Results: The EUV wave expanded radially with a uniform velocity of 500 km s¹. However, the radio source was observed to be deflected and appeared to move along a channel of a lower Alfvn speed, abruptly slowing from 1700 km s¹ to 250 km s¹ as it entered a quiet-Sun region. A shock wave with an apparent radial velocity of > 420 km s¹ was determined from the drift rate of the associated Type II radio burst.
Conclusions: The apparent motion of the radio source may have resulted from a wave front moving along a coronal wave guide or by different points along the wave front emitting at locations with favourable conditions for shock formation.

Localising pulsations in the hard X-ray and microwave emission of an X-class flare

Aims: The aim of this work is to identify the mechanism driving pulsations in hard X-ray (HXR) and microwave emission during solar flares. Using combined HXR and microwave observations from Solar Orbiter/STIX and EOVSA, we investigate an X1.3 GOES class flare, 2022-03-30T17:21:00, which displays pulsations on timescales evolving from 7 s in the impulsive phase to 35 s later in the flare.
Methods: We analysed the temporal, spatial, and spectral evolution of the HXR and microwave pulsations during the impulsive phase of the flare. We reconstructed images for individual peaks in the impulsive phase and performed spectral fitting at high cadence throughout the first phase of pulsations.
Results: Our imaging analysis demonstrates that the HXR and microwave emission originates from multiple sites along the flare ribbons. The brightest sources and the location of the emission change in time. Through HXR spectral analysis, the electron spectral index is found to be anti-correlated with the HXR flux, showing a "soft-hard-soft" spectral index evolution for each pulsation. The timing of the associated filament eruption coincides with the early impulsive phase.
Conclusions: Our results indicate that periodic acceleration and/or injection of electrons from multiple sites along the flare arcade is responsible for the pulsations observed in HXR and microwave emission. The evolution of pulsation timescales is likely a result of changes in the 3D magnetic field configuration over time related to the associated filament eruption.

Movie is available at https:// www.aanda.org.

Tracking solar radio bursts using Bayesian multilateration

Context. Solar radio bursts (SRBs), such as Type IIs and IIIs, are emitted by electrons propagating through the corona and interplanetary space. Tracking such bursts is key to understanding the properties of accelerated electrons and radio wave propagation as well as the local plasma environment that they propagate through.
Aims: In this work, we present a novel multilateration algorithm called BayEsian LocaLisation Algorithm (BELLA) and validate the algorithm using simulated and observed SRBs. In addition, apparent SRB positions from BELLA are compared with comparable localisation methods and the predictions of solar wind models.
Methods: BELLA uses Bayesian inference to create probabilistic distributions of source positions and their uncertainties. This facilitates the estimation of algorithmic, instrumental, and physical uncertainties in a quantitative manner.
Results: We validated BELLA using simulations and a Type III SRB observed by STEREO A and STEREO B at 116 from the Sun-Earth line and by Wind at L1. BELLA tracked the Type III source from 10-150 R (2-0.15 MHz) along a spiral trajectory. This allowed for an estimate of an apparent solar wind speed of v_sw 400 km s¹ and a source longitude of ₀ 30. We compared these results with well-established methods of positioning: Goniopolarimetric (GP), analytical time-difference-of-arrival (TDOA), and Solar radio burst Electron Motion Tracker (SEMP). We found them to be in agreement with the results obtained by BELLA. Additionally, the results aligned with solar wind properties assimilated by the Heliospheric Upwind Extrapolation with time dependence (HUXt) model.
Conclusions: We have validated BELLA and used it to identify apparent source positions as well as velocities and densities of the solar wind. Furthermore, we identified higher than expected electron densities, suggesting that the true emission sources were at lower altitudes than those identified by BELLA, an effect that may be due to appreciable scattering of electromagnetic waves by electrons in interplanetary space.

Study of solar brightness profiles in the 18-26 GHz frequency range with INAF radio telescopes. II. Evidence of coronal emission

Context. One of the most important objectives of solar physics is to gain a physical understanding of the solar atmosphere, whose structure can also be described in terms of the density (N) and temperature (T) distributions of the atmospheric matter. Several multi-frequency analyses have shown that the characteristics of these distributions are still under debate, especially for outer coronal emission.
Aims: We aim to constrain the T and N distributions of the solar atmosphere through observations in the centimetric radio domain. We employed single-dish observations from two of the INAF radio telescopes at the K-band frequencies (18-26 GHz). We investigated the origin of the significant brightness temperature (T_B) detected up to the upper corona (at an altitude of 800 Mm with respect to the photospheric solar surface).
Methods: To probe the physical origin of the atmospheric emission and to constrain instrumental biases, we reproduced the solar signal by convolving specific 2D antenna beam models. We performed an analysis of the solar atmosphere by adopting a physical model that assumes the thermal bremsstrahlung as the emission mechanism, with specific T and N distributions. We compared the modelled T_B profiles with those observed by averaging solar maps obtained at 18.3 and 25.8 GHz during the minimum of solar activity (2018-2020).
Results: We probed any possible discrepancies between the T and N distributions assumed from the model and those derived from our measurements. The T and N distributions are compatible (within a 25% of uncertainty) with the model up to 60 Mm and 100 Mm in altitude, respectively.
Conclusions: Our analysis of the role of the antenna beam pattern on our solar maps proves the physical nature of the atmospheric emission in our images up to the coronal tails seen in our T_B profiles. Our results suggest that the modelled T and N distributions are in good agreement (within 25% of uncertainty) with our solar maps up to altitudes of 100 Mm. A subsequent, more challenging analysis of the coronal radio emission at higher altitudes, together with the data from satellite instruments, will require further multi-frequency measurements.

Study of solar brightness profiles in the 18-26 GHz frequency range with INAF radio telescopes. I. Solar radius

Context. The Sun is an extraordinary workbench, on which several fundamental astronomical parameters can be measured with high precision. Among these parameters, the solar radius R plays an important role in several aspects, for instance, in evolutionary models. Moreover, it conveys information about the structure of the different layers that compose the solar interior and its atmosphere. Despite the efforts to obtain accurate measurements of R, the subject is still debated, and measurements are puzzling and/or lacking in many frequency ranges.
Aims: We determine the mean, equatorial, and polar radii of the Sun (R_c, R_eq, and R_pol) in the frequency range 18.1 26.1 GHz. We employed single-dish observations from the newly appointed Medicina Gavril Grueff Radio Telescope and the Sardinia Radio Telescope (SRT) in five years, from 2018 to mid-2023, in the framework of the SunDish project for solar monitoring.
Methods: Two methods for calculating the radius at radio frequencies were employed and compared: the half-power, and the inflection point. To assess the quality of our radius determinations, we also analysed the possible degrading effects of the antenna beam pattern on our solar maps using two 2D models (ECB and 2GECB). We carried out a correlation analysis with the evolution of the solar cycle by calculating Pearson's correlation coefficient in the 13-month running means.
Results: We obtained several values for the solar radius, ranging between 959 and 994 arcsec, and , with typical errors of a few arcseconds. These values constrain the correlation between the solar radius and solar activity, and they allow us to estimate the level of solar prolatness in the centimeter frequency range.
Conclusions: Our R measurements are consistent with the values reported in the literature, and they provide refined estimates in the centimeter range. The results suggest a weak prolateness of the solar limb (R_eq > R_pol), although R_eq and R_pol are statistically compatible within 3 errors. The correlation analysis using the solar images from the Grueff Radio Telescope shows (1) a positive correlation between solar activity and the temporal variation in R_c (and R_eq) at all observing frequencies, and (2) a weak anti-correlation between the temporal variation of R_pol and solar activity at 25.8 GHz.

Ionospheric TEC Prediction Based on Ensemble Learning Models

In this paper, we propose the usage of an ensemble learning approach for predicting total electron content (TEC). The training data set spans from 2007 to 2016, while the testing data set is set to the year 2017. The model inputs in our study included Solar radio flux (F107), Solar Wind plasma speed, By, Bz, Dst, Ap, AE, day of year, universal time, 30-day and 90-day TEC averages. Specifically, eXtreme Gradient Boosting (XGBoost), Gradient Boosting Decision Tree, and Decision Tree were utilized for 1-hr TEC prediction at high- (80W, 80N), mid- (80W, 40N), and low- latitudes (80W, 10N). Results indicate that all three models performed well in predicting TEC, with a mean error of only approximately 0.6 TECU at high- and mid- latitudes and 1.13 TECU at low latitudes. At the same time, we compared the model with 1-day Beijing University of Aeronautics and Astronautics model during the period of magnetic storm from 25 August 2018 to 27 August 2018 and a quiet period from 13 December 2018 to 15 December 2018. In the magnetic storm period, Our model showed an average reduction of 1.83 TECU compared to BUAA model. During the quiet period, XGBoost exhibit an average error that is 1.14 TECU lower than that of BUAA model. Moreover, TEC prediction over the region between the 20N-45N and 70E120E during geomagnetic storm has an error of 2.74 TECU, showing the stability and superiority of XGBoost. Overall, the ensemble learning approach exhibits its advantage in predicting TEC.

Multiwavelength Observations of Quasiperiodic Pulsations in the Impulsive Phase of an Eruptive Flare with the Hard X-Ray Imager On Board ASO-S and Other Instruments

We investigated the quasiperiodic pulsations (QPPs) of the X1.2 solar flare (SOL2023-01-06T00:57) based on multi-instrument observations, especially the Hard X-ray Imager (HXI) on board the Advanced Space-based Solar Observatory (ASO-S). The quasiperiod of 27 s was identified in hard X-rays (HXR) and microwaves using the Markov Chain Monte Carlo method, while no corresponding oscillation was found in soft X-rays. The HXI imaging demonstrates that HXR pulsations arise from the double- footpoint HXR sources. Moreover, the fluctuation of 27 s period was also present in the nonthermal electron power-law index derived from the X-ray spectra, which is the signature of periodic electron acceleration/precipitation during the impulsive phase. The electron spectral indices not only exhibited the well-known "softhardsoft" evolution, but also showed the negative correlation with the HXR pulsations. These results suggest that QPPs directly originate from quasiperiodic injections of accelerated electrons into flare-loop footpoints. We also discuss the possible generation mechanisms for QPPs.

Circular-ribbon flares and the related activities

Solar flares are an impulsive increase of emissions as a result of impetuous release of magnetic free energy. This paper will present the recent progress on circular-ribbon flares (CRFs) and their related activities, including coronal jets, filaments, coronal mass ejections (CMEs), radio bursts, coronal dimmings, and coronal loop oscillations. Owing to the prevalence of three-dimensional (3D) magnetic null points and the corresponding fan-spine topology in the solar atmosphere, CRFs are regularly observed in ultraviolet (UV), extreme-ultraviolet (EUV), and H passbands. Spine reconnection and fan reconnection around the null points are predominantly responsible for the energy release and subsequent particle acceleration. Slipping reconnection at quasi- separatrix layers (QSLs) may explain the sequential brightening or rapid degradation of the circular ribbons. Periodic or quasi-periodic acceleration and precipitation of non-thermal particles in the chromosphere produce observed quasi-periodic pulsations (QPPs) of CRFs in multiple altitudes as well as wavelengths. Like two-ribbon flares, the injected high-energy particles result in explosive evaporation in circular and inner ribbons, which is characterized by simultaneous blueshifts in the coronal emission lines and redshifts in the chromospheric emission lines. Homologous CRFs residing in the same active region present similar morphology, evolution, and energy partition. The peculiar topology of CRFs with closed outer spines facilitates remote brightenings and EUV late phases, which are uncommon in two-ribbon flares. Besides, CRFs are often accompanied by coronal jets, type III radio bursts, CMEs, shock waves, coronal dimmings, and kink oscillations in coronal loops and filaments. Magnetohydrodynamics numerical simulations are very helpful to understand the key problems that are still unclear up to now. Multiwavelength and multipoint observations with state-of-the-art instruments are enormously desired to make a breakthrough. The findings in CRFs are important for a comprehensive understanding of solar flares and have implication for stellar flares.

a first-principles precursor for astrophysical radio bursts

Electromagnetic fundamental and harmonic emission is ubiquitously observed throughout the heliosphere, and in particular it is commonly associated with the occurrence of type II and III solar radio bursts. Classical analytic calculations for the plasma-emission process, though useful, are limited to idealized situations; a conclusive numerical verification of this theory is still lacking, with earlier studies often providing contradicting results on e.g. the precise parameter space in which fundamental and harmonic emission can be produced. To accurately capture the chain of mechanisms underlying plasma emission - from precursor plasma processes to the generation of electromagnetic waves over long times - we perform large scale, first-principles simulations of beam-plasma instabilities. By employing a very large number of computational particles we achieve very low numerical noise, and explore (with an array of simulations) a wide parameter space determined by the beam-plasma density ratio and the ion-to-electron temperature ratio. In particular, we observe direct evidence of both fundamental and harmonic plasma emission when the beam-to-background density ratio 0.005 (with beam-to-background energy ratio ~0.5), tightly constraining this threshold. We observe that, asymptotically, in this regime $\sim 0.1~{{\ \rm per\ cent}}$ of the initial beam energy is converted into harmonic emission, and $\sim 0.001~{{\ \rm per\ cent}}$ into fundamental emission. In contrast with previous studies, we also find that this emission is independent of the ion-to-electron temperature ratio. In addition, we report the direct detection of third-harmonic emission in all of our simulations, at power levels compatible with observations. Our findings have important consequences for understanding the viable conditions leading to plasma emission in space systems, and for the interpretation of observed electromagnetic signals throughout the heliosphere.

Absence of High Frequency Echoes From Ionosondes During the 23-25 April 2023 Geomagnetic Storm; What Happened?

We report an unusual event on absence of high frequency (HF) echoes in ionosonde observations from the ionospheric F2 region during the geomagnetic storm of 23-25 April 2023. This event was observed in both southern and northern hemispheres over two stations, Grahamstown (33.3S, 26.5E), South Africa and Pruhonice (50.0N, 14.6E), Czech Republic. Significant O/N₂ depletion over the stations was observed by TIMED/GUVI, indicating a strong negative ionospheric storm. This is unique since absence of echoes in ionosonde measurements is usually due to strong radio absorption in the ionosphere associated with solar flares. However, there was no flare activity during the periods of "absent" F2 HF echoes. On the other hand, the ionosonde detected echoes from E-layer. TIEGCM simulation reproduced TIMED/GUVI O/N₂ depletion and showed that NmE was larger than NmF2 on dayside over Pruhonice. TIMED/GUVI O/N₂ also showed a clear spatial gradient in the O/N₂ depleted regions, suggesting F-region ionosphere was tilted. By estimating the critical frequency of the F2 layer using GNSS observations, we have shown that it wasn't possible for the ionospheric electron density to reach depletion levels prohibiting reflection of HF echoes from ionosondes. We suggest that this phenomena may have been caused by either (a) maximum electron density of E layer exceeding that of F2 layer and/or (b) ionospheric tilting which made the signals to be reflected far away from the ionosonde locations.

Element abundance and the physics of solar energetic particles

The acceleration and transport of solar energetic particles (SEPs) cause their abundance, measured at a constant velocity, to be enhanced or suppressed as a function of the magnetic rigidity of each ion, and hence, of its atomic mass-to-charge ratio of A/Q. Ion charges, in turn, depend upon the source electron temperature. In small "impulsive" SEP events, arising from solar jets, acceleration during magnetic reconnection causes steep power-law abundance enhancements. These impulsive SEP events can have 1,000-fold enhancements of heavy elements from sources at 2.5 MK and similar enhancements of 3He/4He and of streaming electrons that drive type-III radio bursts. Gamma-ray lines show that solar flares also accelerate 3He-rich ions, but their electrons and ions remain trapped in magnetic loops, so they dissipate their energy as X-rays, -rays, heat, and light. "Gradual" SEPs accelerated at shock waves, driven by fast coronal mass ejections (CMEs), can show power-law abundance enhancements or depressions, even with seed ions from the ambient solar corona. In addition, shocks can reaccelerate seed particles from residual impulsive SEPs with their pre- existing signature heavy-ion enhancements. Different patterns of abundance often show that heavy elements are dominated by a source different from that of H and He. Nevertheless, the SEP abundance, averaged over many large events, defines the abundance of the corona itself, which differs from the solar photosphere as a function of the first ionization potential (FIP) since ions, with FIP <10 eV, are driven upward by forces of electromagnetic waves, which neutral atoms, with FIP >10 eV, cannot feel. Thus, SEPs provide a measurement of element abundance in the solar corona, distinct from solar wind, and may even better define the photosphere for some elements.

Toward a General Empirical Model for the Study of Planetary Aeronomy

Accurate estimation of the solar vacuum ultraviolet irradiance between 0.1 and 200 nm is critical for the study of planetary aeronomy. Previous empirical models have relied on a limited number of reference spectra, or on multiple data sets with various degrees of uncertainty, and on an empirical selection of solar proxies. Here we propose a novel method for the development of empirical models based on Fourier transform and least-squares fitting of the long-term measurements from the Solar EUV Experiment on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics mission. A Fourier transform analysis is performed to examine a large number of solar proxies, which reveals that the solar radio flux at 10.7 cm and the solar Ly flux at 121.6 nm are better proxies for solar irradiance below and above 120 nm, respectively. Using these two proxies, a nonlinear empirical model is developed through Fourier transform and least-squares fitting of solar irradiance measurements, which can reproduce the solar irradiance with uncertainties of only 1%2% above 120 nm, 2%4% within 45120 nm, and 4%8% below 45 nm. Comparison with measurements from the Extreme Ultraviolet Monitor on the Mars Atmosphere and Volatile Evolution mission indicates that the solar irradiance at Mars can be predicted with uncertainties of less than 8% by geometric extrapolation of the solar irradiance measured from Earth, provided that the measurements from Earth can be calibrated accurately. Our study provides a general method for the development of empirical models using long-term observations in planetary aeronomy.

Second Harmonic Electromagnetic Wave Emissions from a Turbulent Plasma with Random Density Fluctuations

In the solar wind, electromagnetic waves at the harmonic plasma frequency 2 _p can be generated as a result of coalescence between forward- and backward-propagating Langmuir waves. A new approach to calculate their radiation efficiency in plasmas with external background density fluctuations is developed. The evolution of Langmuir wave turbulence is studied by solving numerically the Zakharov equations in a two-dimensional randomly inhomogeneous plasma. Then, the dynamics of the nonlinear electric currents modulated at frequencies close to 2 _p are calculated, as well as their radiation into harmonic electromagnetic waves. In the frame of this non-self-consistent approach where all transformations of Langmuir waves on density inhomogeneities are taken into account, the electromagnetic wave radiation rate (emissivity) is determined numerically as well as analytically, providing in both cases similar results. Moreover, scaling laws of the harmonic wave emissivity as a function of the ratio of the light velocity to the electron plasma thermal velocity are found. It is also shown how the emissivity depends on the average level of density fluctuations and on the isotropic/anisotropic character of the Langmuir waves' and density fluctuations' spectra.

Evidence for flare-accelerated particles in large scale loops in the behind-the-limb gamma-ray solar flare of September 29, 2022

We report on the detection of the gamma-ray emission above 100 MeV from the solar flare of September 29, 2022, by Fermi LAT with simultaneous coverage in HXR by Solar Orbiter STIX. The Solar Orbiter-Earth separation was 178 at the time of the flare as seen from Earth, with Solar Orbiter observing the east limb. Based on STIX imaging, the flare was located 16 behind the eastern limb as seen from Earth. The STIX and GBM non-thermal emission and the LAT emission above 100 MeV all show similarly shaped time profiles, and the Fermi profiles peaked only 20 s after the STIX signal from the main flare site, setting this flare apart from all the other occulted flares observed by Fermi LAT. The radio spectral imaging based on the Nanay Radioheliograph and ORFEES spectrograph reveal geometries consistent with a magnetic structure that connects the parent active region behind the limb to the visible disk. We studied the basic characteristics of the gamma-ray time profile, in particular, the rise and decay times and the time delay between the gamma-ray and HXR peak fluxes. We compared the characteristics of this event with those of four Fermi LAT behind-the-limb flares and with an on-disk event and found that this event is strikingly similar to the impulsive on-disk flare. Based on multiwavelength observations, we find that the gamma-ray emission above 100 MeV originated from ions accelerated in the parent active region behind the limb and was transported to the visible disk via a large magnetic structure connected to the parent active region behind the limb. Our results strongly suggest that the source of the emission above 100 MeV from the September 29, 2022 flare cannot be the CME-driven shock.

Movie is available at https ://www.aanda.org.

Spatially resolved radio signatures of electron beams in a coronal shock

Context. Type II radio bursts are a type of solar radio bursts associated with coronal shocks. Type II bursts usually exhibit fine structures in dynamic spectra that represent signatures of accelerated electron beams. So far, the sources of individual fine structures in type II bursts have not been spatially resolved in high-resolution low- frequency radio imaging.
Aims: The objective of this study is to resolve the radio sources of the herringbone bursts found in type II solar radio bursts and investigate the properties of the acceleration regions in coronal shocks.
Methods: We used low-frequency interferometric imaging observations from the Low Frequency Array to provide a spatially resolved analysis for three herringbone groups (A, B, and C) in a type II radio burst that occurred on 16 October 2015.
Results: The herringbones in groups A and C have a typical frequency drift direction and a propagation direction along the frequency. Their frequency drift rates correspond to those of type III bursts and previously studied herringbones. Group B has a more complex spatial distribution, with two distinct sources separated by 50 arcsec and no clear spatial propagation with frequency. One of the herringbones in group B was found to have an exceptionally large frequency drift rate.
Conclusions: The characteristics derived from imaging spectroscopy suggest that the studied herringbones originate from different processes. Herringbone groups A and C most likely originate from single-direction beam electrons, while group B may be explained by counterstreaming beam electrons.

Identification and extraction of type II and III radio bursts based on YOLOv7

Solar radio bursts (SRBs) are extreme space weather events characterized by intense solar radio emissions that are closely related to solar flares. They represent signatures of the same underlying processes that are responsible for well-documented solar phenomena such as sunspots, solar flares, and coronal mass ejections (CMEs). The study of SRBs holds significant importance as it provides a means to monitor and predict solar flares and CMEs, enhancing our ability to forecast potential impacts on Earth's communications and satellites. Typically, SRBs below several hundred megahertz can be categorized into five types (I-V), with type II and type III bursts being the most prevalent. This study introduces a novel approach based on the YOLOv7 model for the detection and classification of type II and type III SRBs. The proposed method effectively identifies and classifies various SRB types, achieving a mean average precision accuracy of 73.5%. A trained neural network was deployed for SRB detection in the Chashan Broadband Solar radio spectrograph at meter wavelength (CBSm) data, enabling the extraction of valuable SRB information for subsequent research. This demonstrates that even when we are dealing with extensive datasets, this method can automatically recognize outbursts and extract pertinent physical information. Although our experiments with the CBSm dataset currently rely on the daily spectrum, further advancements in CBSm backend data processing techniques are expected to enable near-real-time burst detection, which is a powerful tool for accurately assessing and analyzing SRBs, and significantly contribute to the field of space weather forecasting and protective measures. Furthermore, the applicability of this method to other stations within the Chinese Meridian Project II (e.g., Mingantu Spectral Radioheliograph and Daocheng Solar Radio Telescope) enhances the capability of space weather data fusion and model development. Therefore, this research represents a substantial contribution to the domain of space weather research, offering a valuable tool for the detection and classification of SRBs and thereby improving our ability to predict and mitigate the impacts of extreme space weather events on Earth's technology and infrastructure.

Observation of solar radio burst events from Mars orbit with the Shallow Radar instrument

Context. Multispacecraft and multiwavelength observations of solar eruptions, such as flares and coronal mass ejections, are essential to understanding the complex processes behind these events. The study of solar burst events in the radio frequency spectrum has relied almost exclusively on data from ground-based observations and a few dedicated heliophysics missions such as STEREO or Wind.
Aims: By reanalysing existing data from the Mars Reconnaissance Orbiter (MRO) Shallow Radar (SHARAD) instrument, a Martian planetary radar sounder, we discovered the instrument was also capable of detecting solar radio bursts and that it was able to do so with unprecedented resolution for a space-based solar instrument. In this study, we aim to demonstrate the reliability and value of SHARAD as a new solar radio observatory.
Methods: We characterised the sensitivity of the instrument to type III solar radio bursts through a statistical analysis of correlated observations using STEREO and Wind as references. Using 38 correlated detections, we established the conditions under which SHARAD can observe solar bursts in terms of acquisition geometry. As an example of scientific application, we also present the first analysis of type III characteristic times at high resolution beyond 1 AU.
Results: A simple logistic model based purely on geometrical acquisition parameters can predict burst show versus no-show in SHARAD data with an accuracy of 79.2%, demonstrating the reliability of the instrument in detecting solar bursts and laying the foundation for using SHARAD as a solar radio observatory. The extremely high resolution of the instrument, both in temporal and frequency directions; its bandwidth; and its position in the Solar System enable SHARAD to make significant contributions to heliophysics. Notably, it could provide data on plasma processes on the site of the burst generation and along the propagation path of associated fast electron beams.

Efficiency of solar microflares in accelerating electrons when rooted in a sunspot

Context. The spectral shape of the X-ray emission in solar flares varies with the event size, with small flares generally exhibiting softer spectra than large events, indicative of a relatively lower number of accelerated electrons at higher energies.
Aims: We investigate two microflares of GOES classes A9 and C1 (after background subtraction) observed by STIX onboard Solar Orbiter with exceptionally strong nonthermal emission. We complement the hard X-ray imaging and spectral analysis by STIX with co-temporal observations in the (E)UV and visual range by AIA and HMI to investigate what makes these microflares so efficient in high-energy particle acceleration.
Methods: We made a preselection of events in the STIX flare catalog based on the ratio of the thermal to nonthermal quicklook X-ray emission. The STIX spectrogram science data were used to perform spectral fitting to identify the non- thermal and thermal components. The STIX X-ray images were reconstructed to analyze the spatial distribution of the precipitating electrons and the hard X-ray emission they produce. The EUV images from SDO/AIA and SDO/HMI LOS magnetograms were analyzed to better understand the magnetic environment and the chromospheric and coronal response. For the A9 event, EOVSA microwave observations were available, allowing for image reconstruction in the radio domain.
Results: We performed case studies of two microflares observed by STIX on October 11, 2021 and November 10, 2022, which showed unusually hard microflare X-ray spectra with power-law indices of the electron flux distributions of = (2.98 0.25) and = (4.08 0.23), during their non-thermal peaks and photon energies up to 76 keV and 50 keV, respectively. For both events under study, we found that one footpoint is located within a sunspot covering areas with mean magnetic flux densities in excess of 1500 G, suggesting that the hard electron spectra are caused by the strong magnetic fields the flare loops are rooted in. Additionally, we revisited a previously published unusually hard RHESSI microflare and found that in this event, there was also one flare kernel located within a sunspot, which corroborates the result from the two hard STIX microflares under study in this work.
Conclusions: The characteristics of the strong photospheric magnetic fields inside the sunspot umbrae and penumbrae where flare loops are rooted play an important role in the generation of exceptionally hard X-ray spectra in these microflares.

Movies associated to Figs. 5 and 9 are available at https://www.aanda.org

Connecting remote and in situ observations of shock-accelerated electrons associated with a coronal mass ejection

Context. One of the most prominent sources for energetic particles in our Solar System are huge eruptions of magnetised plasma from the Sun, known as coronal mass ejections (CMEs), which usually drive shocks that accelerate charged particles up to relativistic energies. In particular, energetic electron beams can generate radio bursts through the plasma emission mechanism, for example, type II and accompanying herringbone bursts.
Aims: In this work, we investigate the acceleration location, escape, and propagation directions of various electron beams in the solar corona and compare them to the arrival of electrons at spacecraft.
Methods: To track energetic electron beams, we used a synthesis of remote and direct observations combined with coronal modeling. Remote observations include ground-based radio observations from the Nanay Radioheliograph (NRH) combined with space-based extreme- ultraviolet and white-light observations from Solar Dynamics Observatory (SDO), Solar Terrestrial Relations Observatory (STEREO), and Solar Orbiter (SolO). We also used direct observations of energetic electrons from the STEREO and Wind spacecraft. These observations were then combined with a three-dimensional (3D) representation of the electron acceleration locations, including the results of magneto-hydrodynamic models of the solar corona. This representation was subsequently used to investigate the origin of electrons observed remotely at the Sun and their link to in situ electrons.
Results: We observed a type II radio burst followed by herringbone bursts that show single-frequency movement through time in NRH images. The movement of the type II burst and herringbone radio sources seems to be influenced by regions in the corona where the CME is more capable of driving a shock. We found two clear distinct regions where electrons are accelerated in the low corona and we found spectral differences between the radio emission generated in these regions. We also found similar inferred injection times of near-relativistic electrons at spacecraft to the emission time of the type II and herringbone bursts. However, only the herringbone bursts propagate in a direction where the shock encounters open magnetic field lines that are likely to be magnetically connected to the same spacecraft.
Conclusions: Our results indicate that if the in situ electrons are indeed shock-accelerated, the most likely origin of the in situ electrons arriving first is located near the acceleration site of herringbone electrons. This is the only region during the early evolution of the shock where there is clear evidence of electron acceleration and an intersection of the shock with open field lines, which can be directly connected to the observing spacecraft.

Movies are available at https://www.aanda.org

The Non-Thermal Radio Emissions of the Solar Transition Region and the Proposal of an Observational Regime

The transition region is a very thin but most peculiar layer in the solar atmosphere located between the solar chromosphere and the corona. It is a key region for understanding coronal heating, solar eruption triggers, and the origin of solar winds. Here, almost all physical parameters (density, temperature, and magnetic fields) have the maximum gradient. Therefore, this region should be highly dynamic, including fast energy releasing and transporting, plasma heating, and particle accelerating. The physical processes can be categorized into two classes: thermal and non-thermal processes. Thermal processes can be observed at ultraviolet (UV) and extreme ultraviolet (EUV) wavelengths via multi-wavelength images. Non-thermal processes accelerate non- thermal electrons and produce radio emissions via the gyrosynchrotron mechanism resulting from the interaction between the non-thermal electrons and magnetic fields. The frequency range spans from several GHz to beyond 100 GHz, in great number of bursts with narrowband, millisecond lifetime, rapid frequency drifting rates, and being referred to as transition region small-scale microwave bursts (TR-SMBs). This work proposes a new type of Solar Ultra-wide Broadband Millimeter-wave Spectrometer (SUBMS) that can be used to observe TR-SMBs. From SUBMS observations, we can derive rich dynamic information about the transition region, such as information about non-thermal energy release and propagation, the flows of plasma and energetic particles, the magnetic fields and their variations, the generation and transportation of various waves, and the formation and evolution of the source regions of solar eruptions. Such an instrument can actually detect the non- thermal signals in the transition region during no flare as well as the eruptive high-energy processes during solar flares.

Quantifying chaos and randomness in magnetar bursts

In this study, we explore the dynamical stability of magnetar bursts within the context of the chaos-randomness phase space for the first time, aiming to uncover unique behaviours compared to various astrophysical transients, including fast radio bursts (FRBs). We analyse burst energy time series data from active magnetar sources SGR J1550-5418 and SGR J1935+2154, focusing on burst arrival time and energy differences between consecutive events. We find a distinct separation in the time domain, where magnetar bursts exhibit significantly lower randomness compared to FRBs, solar flares, and earthquakes, with a slightly higher degree of chaos. In the energy domain, magnetar bursts exhibit a broad consistency with other phenomena, primarily due to the wide distribution of chaos-randomness observed across different bursts and sources. Intriguingly, contrary to expectations from the FRB- magnetar connection, the arrival time patterns of magnetar bursts in our analysis do not exhibit significant proximity to repeating FRBs in the chaos-randomness plane. This finding may challenge the hypothesis that FRBs are associated with typical magnetar bursts but indirectly supports the evidence that FRBs may primarily be linked to special magnetar bursts like peculiar X-ray bursts from SGR J1935+2154 observed simultaneously with Galactic FRB 200428.

Experiments with a Three-Element Radio Interferometer in 12 GHz

In order to attract students to the radio universe, we have constructed a three-element radio interferometer in the National Youth Space Center, Goheung, Korea. It consists of three 1.8 m off-axis parabola antennas with driving systems, sideband separation receivers operating in 12 GHz, a narrow band digitizer, and correlation software. We have used as many commercial products as possible to reduce development costs. The maximum separation of 20 m gives an angular resolution of ~4', and the shortest baseline of 3.8 m prevents a serious missing flux. Fringes are detected for several radio sources, including the sun and Cas A. After a rough relative calibration, we have derived visibilities for the sun, whose amplitudes are decreasing for longer baselines. We have made a solar image using the visibility amplitudes and closure phases, referring to the 17 GHz image by Nobeyama Radioheliograph. Developing a flexible real-time correlator seems most crucial if this kind of the system is to be used for more rigorous scientific studies.

Reconstruction of global ionospheric TEC maps from IRI-2020 model based on deep learning method

The Total Electron Content (TEC) computed from ionospheric models is a widely used parameter for characterizing the morphological structure of the ionosphere. The global TEC maps from empirical models, like the International Reference Ionosphere (IRI) model, have limited accuracy compared to those calculated by dual-frequency measurements from the global navigation satellite systems (GNSS). We have developed a reconstructed IRI TEC model for generating high-precision global TEC maps based on a deep learning method. For this, we have collected 48,204 pairs of global TEC maps from the IRI-2020 model and Global Ionosphere Maps (GIM) model with 2-h time resolution from 2009 to 2019 covering the whole solar cycle 24. The daily solar radio flux (F_10.7), sunspot number (SSN), Dst, and Kp indices are also introduced as input features to train the model. We have investigated the optimum combination of the input parameters for the reconstructed TEC model and compared the performance of the model during the years with high and low solar activity levels. Results show that the reconstructed TEC model with F_10.7 and Kp features has a better performance compared to that considering all solar and geomagnetic indices. The global TEC maps predicted from our model are much more consistent with the corresponding TEC maps from the GIM model than those from the IRI-2020 model. Especially, the large-scale equatorial ionospheric anomaly (EIA) crests and the pronounced enhancement of TEC are well predicted by the reconstructed TEC model. From statistical metrics, the accuracy of the reconstructed TEC model increased by 40.8% during the high solar activity year 2015 and 43.0% during the low solar activity year 2018 compared with the IRI-2020 model. The prediction performance of the reconstructed TEC model also shows better accuracy during the storm periods.

Dependence of Annual Asymmetry in NmF2 on Local Time

Based on the global empirical model of the median of the critical frequency of the F2 layer (SDMF2), the properties of diurnal variations in the annual asymmetry in the maximal concentration NmF2 of the F2 layer for different values of the solar activity index F are analyzed. The AI index characterizing the relative difference in NmF2 averaged over all latitudes and longitudes between January and July at a given local time is used as a parameter of this asymmetry. It is found that a semidiurnal mode prevails in the diurnal variations in the AI index with maxima at the daytime and nighttime hours. The daytime maximum of the AI index hardly depends at all on the solar activity level. The nighttime AI maximum decreases with an increase in solar activity. The daytime and nighttime maxima in AI almost coincide by the amplitude when AI = 1617% for low solar activity. The difference in the solar radio emission flux between January and July due to the ellipticity of Earth's orbit relative to the Sun contributes substantially into the AI index at all hours of the day. On average, it is 34% and could reach 5% for low solar activity during the nighttime hours. The difference in the AI index for low and high solar activity according to the International Reference Ionosphere model IRI (with the URSI and even more with the CCIR coefficients) is overestimated with respect to the SDMF2 model at almost all hours of the day, apparently due to limited amount of the experimental data used to obtain the CCIR and URSI coefficients, especially over oceans.

Predicting the Arrival Time of an Interplanetary Shock Based on DSRT Spectrum Observations for the Corresponding Type II Radio Burst and a Blast Wave Theory

Since fast head-on coronal mass ejections and their associated shocks represent potential hazards to the space environment of the Earth and even other planets, forecasting the arrival time of the corresponding interplanetary shock is a priority in space weather research and prediction. Based on the radio spectrum observations of the 16-element array of the Daocheng Solar Radio Telescope (DSRT), the flagship instrument of the Meridian Project of China, during its construction, this study determines the initial shock speed of a type II solar radio burst on 2022 April 17 from its drifting speed in the spectrum. Assuming that the shock travels at a steady speed during the piston-driven phase (determined from the X-ray flux of the associated flare) and then propagates through interplanetary space as a blast wave, we estimate the propagation and arrival time of the corresponding shock at the orbit of the Solar Terrestrial Relations Observatory-A (STEREO-A). The prediction shows that the shock will reach STEREO-A at 14:31:57 UT on 2022 April 19. The STEREO-A satellite detected an interplanetary shock at 13:52:12 UT on the same day. The discrepancy between the predicted and observed arrival time of the shock is only 0.66 hr. The purpose of this paper is to establish a general method for predicting the shock's propagation and arrival time from this example, which will be utilized to predict more events in the future based on the observations of ground-based solar radio spectrometers or telescopes like DSRT.

Implications for an Independent Small-scale Dynamo

Clette recently showed that F _10.7 systematically approaches a quiet Sun daily value of 67 solar flux units (sfu) at solar minima as the number of spotless days on the Sun increases. Previously, a floor of 2.8 nT had been proposed for the solar wind (SW) magnetic field strength (B). F _10.7, which closely tracks the Sun's unsigned photospheric magnetic flux, and SW B exhibit different relationships to their floors at 11 yr solar minima during the last 50 yr. While F _10.7 approaches 67 sfu at each minimum, the corresponding SW B is offset above 2.8 nT by an amount approximately proportional to the solar polar field strengthwhich varied by a factor of 2.5 during this interval. This difference is substantiated by 130 yr of reconstructed F _10.7 (via the range of the diurnal variation of the East- component (rY) of the geomagnetic field) and SW B (based on the interdiurnal variability geomagnetic activity index). For the last 60 yr, the contribution of the slow SW to SW B has exhibited a floor-like behavior at 2 nT, in contrast to the contributions of coronal mass ejections and high-speed streams that vary with the solar cycle. These observations, as well as recent SW studies based on Parker Solar Probe and Solar Dynamics Observatory data, suggest that (1) the Sun has a small-scale turbulent dynamo that is independent of the 11 yr sunspot cycle; and (2) the small-scale magnetic fields generated by this nonvarying turbulent dynamo maintain a constant open flux carried to the heliosphere by the Sun's floor-like slow SW.

Adiabatic Spectrum of Radio Emission of Plasma Clouds, Emitted by the Sun During Solar Flares, and Inhomogeneities of the Spectrum of Radio Emission of Clouds

It is known that so-called solar flares systematically occur in the area of sunspots. They are accompanied by radiation in almost all frequency ranges and sometimes by the emission of hot plasma. Observations on the RATAN-600 radio telescope have shown that the radio emission spectrum of plasma clouds heated to values of the order of 10⁶ K erupted from the solar flare region turned out to be adiabatic. The high correlation of the inhomogeneities of the radio emission spectra of active formation over a group of sunspots indicates the stable presence of recombination radio lines in the radiation of active formation. However, the radio emission spectra of hot plasma clouds ejected from the region of solar flares occurring in this group of spots do not show any correlation.

Radio signatures and in-situ observations

Coronal Mass Ejections (CMEs) are large-scale releases of hot plasma, to which the magnetic field is frozen-in. If the CMEs are faster than the local magnetosonic velocity in the solar wind, they create shock waves as they travel through the corona and Interplanetary (IP) space. Shock waves driven by CMEs can accelerate Solar Energetic Particles (SEPs). Both phenomena involve the acceleration of electrons, which can be observed as electromagnetic radiation and plasma radiation. This doctoral thesis presents analyses of the presence and propagation of accelerated electrons in the IP medium. By utilizing the observations of multiple science satellites, such as STEREO-A, STEREO-B, and Wind, we get a comprehensive picture of the accelerated particles and solar radio bursts, at various wavelengths. We can use this information to determine the origin of the eruptions, their directivity, and connections to other solar events. In particular, the role of shock waves in the acceleration of relativistic electrons is the subject of this research. Earlier studies have already confirmed the role of shock waves in the acceleration of electrons to keV energies using radio bursts, but for the higher energies, the research is still in progress.

As a result of observations by many space instruments we now have a 3D view of the Sun, particularly in the analysis of type IV radio bursts in multi- spacecraft radio dynamic spectra. The directivity of radio bursts, i.e., being seen only toward a certain direction, can be explained either by absorption in the surrounding medium or by obstruction of dense plasma region, even by the solar disk itself. The presence of dense plasma regions like solar streamers, in directions where no radiation is visible, strengthens this conclusion. Type II radio bursts can be associated with the interaction of streamers and shock waves. Our analysis of three separate type IV radio bursts revealed that their radiation was not visible toward directions where type II radio bursts were observed. The eruptions were generated by the same active region on three different days, and the location of the eruption region on the Sun changed from the disk center to the solar limb. The directivity of the type IV radio bursts could therefore be explained as absorption in the type II burst regions, as the shock fronts contain higher-density plasma.

In the study of isolated type II radio bursts, i.e., when separated from other bursts in time and also in frequency, we found that contrary to the previous studies, the majority of these radio bursts were associated with shocks that were created near the CME leading fronts. The analysis suggests the necessity of special coronal conditions, to form this subgroup of low-frequency type II radio bursts.

The creation of relativistic electrons in IP shocks led to the investigation of whether these shocks can continue to accelerate electrons up to one Astronomical Unit (AU). Using in-situ observations of the electron flux, SEP events, and associated Energetic Storm Particle (ESP) occurrences, we identified nine cases observed by High Energy Telescope (HET) onboard STEREO where MeV electrons showed a significant increase associated with shocks driven by fast speed Interplanetary CMEs (ICMEs). We also found that such events were rare at a distance of 1 AU. The research suggests the necessity to make observations with satellites orbiting closer to the Sun, such as the Parker Solar Probe and Solar Orbiter, so that we can find out how electrons are accelerated in IP shocks. Finally, in-situ observations show clear signatures of local acceleration of electrons during the passage of the shock wave or the sheath region of the ICME during the ESP event.

Characterization and performance modeling of non-imaging solar X-ray spectrometers and their use in analyzing solar eruptions

The Sun, besides being by far the most important source of energy for all life on the Earth, also exhibits various forms of intense magnetic activity. The most spectacular are the eruptive phenomena such as solar flares and coronal mass ejections (CMEs). Solar flares are abrupt localized explosions of energy, where increased radiation is unleashed throughout the whole electromagnetic spectrum from radio to gamma-rays, while CMEs are huge eruptions of plasma and magnetic field from the solar corona. Flares and CMEs can occur independently, but they often occur simultaneously and originate from the destabilization of the same magnetic structure in the solar atmosphere. These phenomena are also the primary drivers of space weather within the heliosphere. The increased X-ray and EUV radiation during solar flares can cause ionization in the upper atmosphere, which can disrupt long range radio communications and the geomagnetic storms caused by interactions between CMEs and the Earth's magnetic field can be very damaging to electrical grid systems and pipeline networks on the ground. Solar flares and CMEs are also often accompanied by solar energetic particle events, which can be damaging to the satellite electronics and pose a radiation hazard in space and on-board aircraft at cruising altitude.

Solar eruptions originate from the magnetic structures in the Sun's hot and tenuous corona. The electromagnetic emission of the coronal plasma is mainly in EUV and soft X-ray bands, which are very efficiently absorbed by the Earth's atmosphere. Monitoring of solar eruptions must therefore be performed in space using satellite based EUV and X-ray observatories. In this thesis I present an overview about the missions and instrumentation that are currently or have been previously used for studying and monitoring the solar and stellar coronae. The emphasis is on non-imaging X-ray spectrometers based on solid state detectors that utilize high- purity silicon photodiodes. These instruments are capable of monitoring the whole solar corona simultaneously in a wide dynamic range of X-ray fluxes, from the levels of quiescent Sun to strong X-class solar flares with high energy resolution of <200 eV at 6 keV.

Two non-imaging solar X-ray spectrometers based on high-purity Si detectors are described in detail. They are the X-ray detection system of the Solar Intensity X-ray and particle Spectrometer (SIXS) on-board BepiColombo mission to Mercury and the X-ray Flux Monitor for CubeSats (XFM-CS), which has been used in the successful in-orbit demonstration of XFM technology on-board the SUNSTORM-1 CubeSat at low Earth orbit. The basics of operation of these instruments, their detectors and readout electronics are described. I also present a performance model for modeling the sensitivity, energy resolution and background of non- imaging solid state X-ray spectrometers, which was used to optimize the designs of SIXS and XFM-CS detectors. The characterization of SIXS and XFM-CS instruments and evaluation of their scientific performance is presented in detail. Included are procedures for both ground and in- flight characterization as well as the descriptions of the science data processing pipelines that are used to calibrate the raw solar X-ray data. The results of cross calibration of SIXS and XFM-CS instruments against the GOES X-ray sensors are also presented.

The use of high- resolution spectral data of solar X-rays in space weather forecasting is discussed. Time resolved spectroscopic analysis was performed to a sample of 16 M and X class solar flares observed with XFM-CS in an effort to identify spectral features associated with CMEs accompanying the flares. The results were compared against the simultaneous EUV images from the Solar Dynamics Observatory and coronagraph images from SOHO and STEREO-A. The results show that associated CMEs reduce the plasma temperature and density inside the flaring magnetic structures, which can be used for early detection of CMEs as well as for rough estimation of their mass when combined with independent observations of the flaring magnetic structures. It was also tentatively found that flare associated CMEs delay the stabilization of the elemental fractionation of the plasma to the normal levels in closed loop corona during the decay phases of flares and that the strength of this effect appears to correlate with the CME lift-off time. The additional information about the CME properties complements the information that can be derived from coronagraph imaging and can be useful in improving our understanding of the processes responsible for flares and CMEs, and can also improve the timeliness and accuracy of space weather forecasts.

Interaction of global electron content with the Sun and solar wind during intense geomagnetic storms

Assessment of solar and solar wind parameters driving the ionosphere model is essential for prediction of the ionospheric weather. In the present paper impact of the different solar, interplanetary and geomagnetic parameters on the global electron content (GEC) during intense space weather storms is investigated. Hourly GEC values are calculated from JPL global maps of total electron content GIM-TEC from 1995 to 2023. The sample comprises 90 intense storms from 1995 to 2023 associated with monthly peak of the weighted accumulation of the geomagnetic Apo(, t) index exceeding 90 nT. The 27 day weighted accumulation of the solar sunspot numbers SSN2(), solar radio flux F10.7(), the solar hydrogen emission Lyman_() and the composite magnesium MgII() indices are explored as precursors of GEC enhancements. As distinct from the positive ionosphere storm, the solar wind speed Vsw, the solar wind electric field Ey, merging electric field Em and Apo(, t) indices proved to be effective as potential drivers of the negative GEC depletion. The positive and negative dGEC deviations from hourly GEC are produced by subtracting a quiet reference GECav averaged during 24h prior the storm normalized by GECav. The hourly storm profiles Vsw(t), Em(t), Ey(t), Apo(, t), Dst(t), GEC(t) and dGEC(t) were reduced by method of superposed epochs. The zero epoch t₀ = 0 was taken at the peak Apo*(, t₀) and the storm time lasted for 48h from -12h prior t₀ and 35h afterwards. The best correlation of the positive storm dGECp amplitude is obtained with MgII() and the negative storm dGECn with E_m* and Apo*(, t₀) which are used to derive characteristics of five key points of storm-time dGEC(t) model: 1 - onset of the storm profile t₁ = t(dGECp); 2 - the amplitude dGECp_max and its time t₂(dGECp_max); 3 - the time of transition t₃(dGEC = 0) from the positive to negative () GEC storm; 4 - minimum negative disturbance dGECn_min and its time t₄(dGECn_min), 5 - the end of the storm profile t₅(dGECn). Analytical model of dGEC(t) is derived with Epstein step functions fitting 5 key points. Deviations dGEC(t) are inverted to GEC(t) using quiet reference pre- storm GECav. The model is validated for three intense storms on 26-28 February, 23-25 March and 23-25 April 2023. The results show improvement of dGEC forecast with RMS error reduced from 45 to 80% compared to results produced by the international reference ionosphere-plasmasphere model IRI-Plas.

Evolution and Flare Activity of Carrington-Class Solar Active Region NOAA 13664 and its Impact on the Earth

We have analyzed the temporal and spatial evolution and the flare activity of the active region (AR) NOAA 13664 and its impact on the Earth. The large group of sunspots that formed it definitely belonged to the Carrington class. The region appeared in the southern hemisphere of the solar disk on 2024 May 1. The number of sunspots was growing rapidly and its area increased from 40 to 2400 millionths of the solar hemisphere. The AR had a complex multipolar configuration of the magnetic field beginning on May 7. On May 8, solar flares of intensity X1.0, M8.7 and M9.9 took place in it, which caused coronal mass ejections (CMEs). These CMEs reached the Earth on May 10, causing strong and extreme geomagnetic storms with bright and very long-lasting auroras. The event was classified as a G5 geomagnetic storm, making it the most intense storm since 2003. On May 9-11, flares of intensity X2.3, X1.5, X4, and X5.8 occurred, each of which caused a CME. The radiation from the X5.8 flare caused a deep shortwave radio blackout over the Pacific Ocean. On May 14, an X8.7-class flare occurred on the western limb of the Sun, the most powerful in solar cycle 25 at that time. The CME that formed caused a short-wave radio blackout over America.

On May 9 during observations at the Ernest Gurtovenko solar horizontal telescope of the Main Astronomical Observatory of the National Academy of Sciences of Ukraine the X2.3 flare spectrograms in its main phase were obtained.

The active region 13664 passed beyond the solar disk and returned on May 29. It has been renumbered as NOAA 13697. On May 31 and June 1 AR produced three X-flares: X1.1, X1.4, and X1.0. Each of them formed CMEs, which reduced the power of shortwave transmissions at all frequencies below 30 MHz. Radiation from M9.8-class flare on June 8 ionized the upper part of the Earth's atmosphere, causing a deep shortwave radio blackout in the western Pacific Ocean.

On June 24 AR13664 returned again. This was its 3rd trip across the solar disk. It was renamed as NOAA 13723. Although the sunspot region was already fragmented to a fraction of its former size, its magnetic component continued to produce powerful solar flares. On June 23, M9.3-class flare occurred in AR, CME from which caused a moderate shortwave radio blackout in Western Europe and Africa.

In total, AR13664 produced 198 C-class, 87 M-class, and 17 X-class flares during its three passes across the Sun's disk.

By studying in detail the evolution of this hyperactive region NOAA 13664 and its impact on Earth, we are improving our ability to predict solar activity and warn of the extreme space weather events it causes.

Diagnostics of Solar Proton Events and Coronal Shock Waves by the Parameters of Solar Radio Bursts of Type II and IV

This paper presents the results of a study of the relationship between solar cosmic rays (SCR) and coronal shock waves (CSW) with the parameters of solar microwave continuum radio bursts of type IV (-bursts), as well as with the parameters of type II radio bursts. A total of 349 solar proton events (SPE) were analyzed for the period from 03-02-1986 to 12-02-2018. For the analysis, we used original records of solar radio emission at 8 fixed frequencies in the range of 245-15400 MHz based on data from the Radio Solar Telescope Network (RSTN), original records of dynamic spectra from the SRS (Solar Radio Spectrograph) in the range of 25-180 MHz, tabular data for the velocity of coronal shock waves, as well as original records of the SCR proton flux intensity with proton energies E_p in the range of >1-100 MeV based on data from the GOES series devices.

It has been previously shown that for most proton events there is a strong relationship between the SCR proton flux and the parameters of type IV continuum microwave bursts, which indicates a dominant role of the SCR acceleration process in the flare region. However, as a result of recent detailed studies of the fine structure of type II radio bursts, a strong relationship was found between the intensity of the mid-relativistic SCR proton flux and certain parameters of type II radio bursts in the 25-180 MHz range. The presence of a strong relationship between the SCR proton flux and the parameters of type II radio bursts indicates an important role of SCR proton acceleration at the fronts of coronal shock waves. A fairly strong relationship was also found between the velocity of coronal shock waves and the parameters of type IV microwave bursts, which definitely indicates that coronal shock waves are associated with solar flares.

Detection of long-lasting aurora-like radio emission above a sunspot

Auroral radio emissions in planetary magnetospheres typically feature highly polarized, intense radio bursts, usually attributed to electron cyclotron maser emission from energetic electrons in the planetary polar region that features a converging magnetic field. Similar bursts have been observed in magnetically active low-mass stars and brown dwarfs, often prompting analogous interpretations. Here we report observations of long-lasting solar radio bursts with high brightness temperature, wide bandwidth and high circular polarization fraction akin to these auroral and exo-auroral radio emissions, albeit two to three orders of magnitude weaker than those on certain low-mass stars. Spatially, spectrally and temporally resolved analysis suggests that the source is located above a sunspot where a strong, converging magnetic field is present. The source morphology and frequency dispersion are consistent with electron cyclotron maser emission due to precipitating energetic electrons produced by recurring nearby flares. Our findings offer new insights into the origin of such intense solar radio bursts and may provide an alternative explanation for aurora-like radio emissions on other flare stars with large starspots.

Solar radio bursts impact on the International GNSS Service Network during Solar Cycle 24

Solar radio bursts (SRB) are a known source of noise for Global Navigation Satellite Systems (GNSS) such as GPS or Galileo. They can degrade the carrier-to-noise ratio of satellite signals, thereby diminishing system performance and, in severe cases, causing total service outages. Although a small amount of particularly intense events have already been studied in detail, the commonness and intensity of SRBs that could potentially impair GNSS performance remain uncertain. This study broadens the scope beyond merely extreme SRBs, studying the impact of SRBs on GNSS throughout Solar Cycle 24. Solar 1.4 GHz observations from the Radio Solar Telescope Network are used to find the 20 most intense SRBs at that frequency. The impact of each SRB is then evaluated in terms of GNSS signal strength decrease, reduction in the number of available satellites, and precision degradation. The results show that at the GPS L1 frequency only one event presented extended service degradation, while at the L2 frequency, minimum operational requirements were not met by at least one station during seven of the SRBs. Only a modest correlation between performance degradation and SRB intensity is found. In particular, it is reported how some mild SRBs affected satellite signals while others almost ten times more intense went unnoticed. The fundamental role that the SRB circular polarization plays in these discrepancies is shown with new 1.4 GHz circular polarization observations from the SMOS satellite. The different responses of GNSS receivers to SRBs depending on the receiver manufacturer are also explored.

Ionospheric Response to Solar EUV Radiation Variations Using GOLD Observations and the CTIPe Model

To understand the global response of thermospheric-ionospheric (TI) parameters to variations in solar irradiance measurements from the Global-Scale Observations of the Limb and Disk (GOLD) ultraviolet imaging spectrograph, solar radio flux F10.7, predictions from the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model, and International Global Navigation Satellite System Service total electron content maps (TEC) have been used. Various parameters such as GOLD O/N₂, O₂, and the nighttime peak electron density (Nmax) have been compared with the CTIPe model simulations. The GOLD observed Nmax shows a number of significant features including a winter anomaly and an equatorial ionization anomaly. The comparison with solar proxies showed that the GOLD Q_EUV correlates very well with the EUV observations compared to the F10.7 index. The study also examined the relationship between the solar proxies and Nmax on different time scales and found that Nmax responded significantly to Q_EUV at both medium- and long-term timescales. Furthermore, a low correlation between Nmax in the equatorial region and solar proxies was found. A delayed ionospheric TEC response against solar flux variations within the 27-day solar rotation was investigated. This ionospheric delay of TEC with respect to solar flux was observed to be less than 1 day, which was reproduced in model simulations. The current study has shown that the GOLD observations can be used to investigate the delayed ionospheric response and to gain a better understanding of the influence of solar activity on the TI system.

Initial results from multi-beam observations of pulsars and solar transient with the digital beamformer for the Gauribidanur pulsar system

Recently a prototype for pulsar observations at low radio frequencies (RFs) (<100 MHz) using log-periodic dipole antennas (LPDAs) in the Gauribidanur Radio Observatory (77E14N) near Bangalore, India, was commissioned. The aforementioned system is currently being augmented (i) to directly digitize the RF signals from the individual antennas and (ii) with a digital beamformer to simultaneously observe different regions of the sky present within the primary "beam" of the LPDA used in the array. Our initial results indicate that co-temporal observations of a known pulsar along with the Sun using two different beams could be used to calibrate the dynamic spectrum of the solar radio transients. This is important because the calibration of the latter in observations with the conventional solar radio spectrographs is difficult.

Design and FPGA Implementation of an Adaptive Narrowband Interference Suppression Filter

The sensitivity and spectral performance of radio telescopes are increasingly affected by deteriorating electromagnetic environments. Therefore, developing a real-time interference elimination algorithm is urgent for future radio observations. In this article, we developed an improved interference suppression filter algorithm named the improved least mean square (IMLMS). Unlike traditional algorithms, the IMLMS algorithm uses an exponential function varying with error to adjust the step size. The input signal is normalized according to its own average energy so that it can not only satisfy the slow change in the step size at an error near zero but also reduce the influence of the input signal size on the algorithm. The improvement degree of the target interference ratio and the interference suppression degree are used to evaluate the performance of the IMLMS filter. Compared with traditional algorithms, the IMLMS algorithm has a better target interference ratio and interference suppression. The IMLMS filter is implemented in a Xilinx UltraScale KU115 FPGA, and the design of the IMLMS filter is optimized to meet the real-time observation requirements of a radio telescope with a 6-m aperture antenna at the Chashan observatory. The timing performance reaches 187.5 MHz. Compared to existing design and implementation methods, the IMLMS adaptive filter has high timing performance and solves the problem of interference suppression that requires high timing performance in fields such as radar and solar radio. Th e IMLMS filter is applied to the Chashan radio telescope for real-time observation, which further verifies the practicability of the IMLMS algorithm.

Triple Voltage Threshold Approximate Cache for Energy Harvesting Nonvolatile Processors

Energy harvesting is considered to be a substitute for batteries in many modern systems. Systems based on energy harvesting receive environmental energies from sources, such as sun, radio frequency, wind, vibration, etc, and convert them to electrical energy to be used by the capacitor of the system or feed the cyber-physical system (CPS) system directly. Despite its advantages, energy harvesting comes with some limitations, such as the instability of the received energy, which means that the energy may not be received for a moment due to environmental conditions during energy harvesting. Therefore, due to not receiving enough energy, the system function may face problems, which can lead to system shutdown and data loss. To prevent program execution interruption caused by frequent power interruptions in systems based on energy harvesting, these systems use nonvolatile processor (NVP). Saving the state in the NVP is done through nonvolatile registers and memories that can hold the contents until the power is restored. However, systems based on energy harvesting and NVPs also have challenges, such as frequent backups' energy consumption, slow program forward progress, and loss of data. In this article, we propose TVTAC, a framework for energy harvesting-based NVP CPS systems. TVTAC modifies conventional NVP's cache architecture to efficiently work with newly introduced operational mode to prevent unnecessary backup operations. Furthermore, TVTAC is equipped with an NVP's specific approximation unit that controls the approximation knobs during the approximate data cache accesses in order to save more energy. The simulation results show that TVTAC improves forward progress by 28% in the best case and 12% on average, compared to similar methods. From an energy consumption perspective, TVTAC reduces energy consumption by 43.5% in the best case and 28.5% on average.

Weak Solar Radio Bursts from the Solar Wind Acceleration Region Observed by the Parker Solar Probe and Its Probable Emission Mechanism

The Parker Solar Probe (PSP) provides us with an unprecedentedly close approach to the observation of the Sun and hence the possibility of directly understanding the elementary process that occurs on the kinetic scale of particles' collective interaction in solar coronal plasmas. We report a type of weak solar radio burst (SRB) that was detected by PSP when it passed a low-density magnetic channel during its second encounter phase. These weak SRBs have a low starting frequency of ~20 MHz and a narrow frequency range from a few tens of MHz to a few hundred kHz. Their dynamic spectra display a strongly evolving feature of the intermediate relative drift rate decreasing rapidly from above 0.01 s^-1 to below 0.01 s^-1. Analyses based on common empirical models of solar coronal plasmas indicate that these weak SRBs originate from a heliocentric distance of ~1.1-6.1 R _S (the solar radius), a typical solar wind acceleration region with a low- plasma, and that their sources have a typical motion velocity of ~v _A (Alfvn velocity) obviously lower than that of the fast electrons required to effectively excite SRBs. We propose that solitary kinetic Alfvn waves with kinetic scales could be responsible for the generation of these small-scale weak SRBs, called solitary wave radiation.

Opening a New Frontier in Solar Physics

Solar radio emissions provide several unique diagnostics to estimate different physical parameters of the solar corona, which are otherwise simply inaccessible. However, imaging the highly dynamic solar coronal emissions spanning a large range of angular scales at radio wavelengths is extremely challenging. At gigahertz frequencies, MeerKAT radio telescope is possibly globally the best-suited instrument at present for providing high-fidelity spectroscopic snapshot solar images. Here, we present the first published spectroscopic images of the Sun made using the observations with MeerKAT in the 880-1670 MHz band. This work demonstrates the high fidelity of spectroscopic snapshot MeerKAT solar images through a comparison with simulated radio images at MeerKAT frequencies. The observed images show extremely good morphological similarities with the simulated images. Our analysis shows that below ~900 MHz MeerKAT images can recover essentially the entire flux density from the large angular-scale solar disk. Not surprisingly, at higher frequencies, the missing flux density can be as large as ~50%. However, it can potentially be estimated and corrected for. We believe once solar observation with MeerKAT is commissioned, it will enable a host of novel studies, open the door to a large unexplored phase space with significant discovery potential, and also pave the way for solar science with the upcoming Square Kilometre Array-Mid telescope, of which MeerKAT is a precursor.

Spatial Localization and Correlation with Solar Flare Intensity

We present a comprehensive study of type III radio bursts and their association with solar flares of magnitude M1.0 and larger, as observed by four widely separated spacecraft (Parker Solar Probe, Solar Orbiter, STEREO-A, and Wind). Our main focus is the introduction and validation of two methods for localizing radio bursts using the available multispacecraft data. The first method utilizes intensity fitting with a circular Gaussian distribution, while the second method is based on the time arrival of radio bursts. We demonstrate the effectiveness of these methods through the analysis of a single type III burst event and compare their results with the traditional radio triangulation technique. Furthermore, we conduct a statistical study of 17 type III bursts associated with M- and X-class solar flares in years 2020-2022. Our findings suggest a possible correlation between solar flare intensities and longitudes, with east limb flares tending to be weaker than west limb flares. We also observe a systematic drift of radio burst longitudes toward the east, potentially explained by a poleward component of the local density gradient. Our results suggest a strong correlation between solar flare intensities and radio burst properties, enhancing our understanding of the relationship between solar flares and type III radio bursts.

High-harmonic Plasma Emission Induced by Electron Beams in Weakly Magnetized Plasmas

Electromagnetic radiation at higher harmonics of the plasma frequency ( ~ n _pe, n > 2) has been occasionally observed in type II and type III solar radio bursts, yet the underlying mechanism remains undetermined. Here we present two-dimensional fully kinetic electromagnetic particle-in-cell simulations with high spectral resolution to investigate the beam-driven plasma emission process in weakly magnetized plasmas of typical coronal conditions. We focused on the generation mechanisms of high-harmonic emission. We found that a larger beam velocity (u _d) favors the generation of the higher-harmonic emission. The emissions grow later for higher harmonics and decrease in intensity by ~2 orders of magnitude for each jump of the harmonic number. The second and third harmonic (H₂ and H₃) emissions get closer in intensity with larger u _d. We also show that (1) the H₃ emission is mainly generated via the coalescence of the H₂ emission with the Langmuir waves, i.e., H₂ + L H₃, wherein the coalescence with the forward-propagating beam-Langmuir wave leads to the forward-propagating H₃, and coalescence with the backward- propagating Langmuir wave leads to the backward-propagating H₃; and (2) the H₄ emission mainly arises from the coalescence of the H₃ emission with the forward- (backward-)propagating Langmuir wave, in terms of H₃ + L H₄.

Enhancing Triangulation of Interplanetary Type III Bursts through Wavevector Correction

Interplanetary Type III bursts, generated by relativistic electron beams at solar flare reconnection sites, are explored through an investigation of 152 instances observed by the Solar Terrestrial Relations Observatory mission. This study reveals that the absolute values of the wavevector deviations from the Sun-spacecraft line are statistically 3.72 and 2.10 larger than predicted by the density model, assuming fundamental and harmonic emission, respectively. Through Monte Carlo simulations, we examine the impact of scattering by density inhomogeneities on the apparent locations of radio emissions in the interplanetary medium. The findings indicate that relative density fluctuations of 0.40 can account for the observed angular shift, a conclusion supported by the multiple flux-tube solar wind model, which confirms the presence of such magnitude of relative perpendicular density fluctuations in the solar wind. We propose a wavevector correction that incorporates this effect to enhance the triangulation of interplanetary Type III bursts, demonstrating that radio triangulation, with this correction, can reliably track electron beams in the interplanetary medium.

Periodic-chaotic alternating regimes during a fast solar metric-radio pulsation event

In the magnetically dominated corona, loop-like structures are expected to undergo pulsating events which are highly structured in time and classified into different types from strictly periodic to irregular. In the case of periodic pulsations, they differ in their characteristic period: from tens of minutes (associated with global large-scale structure oscillations) to milliseconds (fast oscillations associated with a small local scale). These pulsations are strongly related to the basic physical properties of the solar corona and the evolution of magnetic loop-like structures that make up the entire solar atmosphere. This contribution describes a real-world scenario where a route to chaos and reverse processes take place. We examine the dynamic characteristics of a group of a train of pulsations at metric waveband solar radio emission as it evolves in time. The associated time series was recorded with the radio polarimeter of the INAF Trieste Astronomical Observatory, Basovizza Station on April 17, 2002, with a resolution time of 10 ms. Possible scenarios and mechanisms for the observed deterministic-chaos alternating regimes are discussed.

A correlation, artificial neural network, and spectral analysis

The objective of this study was to examine the relationship between cosmic rays (CR) and various solar activity parameters, including the interplanetary magnetic field (B), solar radio emission flux at 10.7 cm (F10.7), solar wind speed (V), solar wind density (N), solar wind pressure (P), and the variance of the interplanetary magnetic field vector (F). The CR data collected from 2007 to 2013 were obtained using a muon detector situated at King Abdulaziz University (KAAU) in Jeddah, Saudi Arabia, with a rigidity cutoff of 14.8 GV. To explore the correlation between the CR data and the selected parameters, three distinct methods were employed: regression analysis, artificial neural network (ANN), and power spectral analysis.

Regression analysis revealed significant positive correlations between CR muons and plasma pressure (R = 0.1, p < 0.05), as well as solar wind speed (R = 0.1, p < 0.05). On the other hand, negative correlations were observed between CRs and radio flux (R = 0.75, p < 0.05), as well as interplanetary magnetic field (R = 0.1, p < 0.05). However, no significant correlations were found between CRs and plasma density or the variance of the interplanetary magnetic field vector (p > 0.05). Furthermore, multivariable correlation analyses were conducted to explore the relationship between CR muons and the considered parameters, resulting in only marginal improvement.

For comparison purposes, data from the King Abdulaziz City for Science and Technology (KACST) muon detector in Riyadh, Saudi Arabia (with a rigidity cutoff of 14.4 GV), as well as CR data from Oulu NM (with a rigidity cutoff of 0.3 GV), were utilized for the same period as the KAAU data. The correlation results indicated that the relationships between the CR data from the three detectors and some of the considered parameters were consistent. However, the strength of the correlations and the magnitudes of the slopes exhibited variations.

ANN procedures were also employed to compare the efficiency of neural network analysis in forecasting CR muons based on solar activity parameters. The Multilayer Perceptron (MLP) Module trained using the backpropagation learning algorithm was utilized to build a neural network model and assess its accuracy. The results indicated that the ANN method provided slightly more accurate predictions compared to regression analysis.

Finally, Fast Fourier Transform (FFT) analyses were conducted to examine the presence of periodicities between the CRs and the variables under consideration. The analysis revealed several shared periodicities, including cycles spanning 270-277 days, 146-158 days, 89-95 days, 66-73 days, and 21-31 days.

Empirical forecasting models for peak intensities of energetic storm particles at 1 AU

We have investigated the dependence of the peak intensities of energetic storm particles (ESPs) on various parameters characterising the coronal mass ejections (CMEs) and associated phenomena. The aim of this study is to suggest empirical models for forecasting the peak intensities of ESP events at 1 AU based on solar and interplanetary (IP) space observations.

For this study we searched for the associations of front-side full and partial halo CMEs with linear speeds > 400 km s^-1during the years 1996-2015 with IP shocks at 1 AU and ESP events observed near the time when the shock passes the observer. We found 88 CME-driven IP shocks associated with ESP events at proton energy range 5.0-7.2 MeV (nominal energy 6.0 MeV) and 59 shocks at the energy range 15.1-21.9 MeV (nominal energy 18.2 MeV). At these two energies 71 % and 68 % of the ESP events were associated with solar energetic particle (SEP) events, 85 % and 84 % were associated with decametric-hectometric (DH) type II radio bursts while 67 % and 66 % were associated with both.

For each CME - shock pair we calculated the predicted shock transit speed (V_TR) by using the method of Belov et al. (2022) and used this as the primary parameter in the investigation. We performed correlation analyses between the logarithm of the peak intensities of the ESP events (log₁₀ [I_ESP^peak ]) and the solar parameters related to the CMEs, solar flares, IP shocks, SEP events, and type II radio bursts. When using a single explanatory variable, we found best correlation coefficients for V_TR (0.68 0.05 and 0.71 0.06), the CME space speed (V_CME^space) (0.59 0.05 and 0.68 0.07), and the logarithm of SEP peak intensity (log₁₀ [I_SEP^peak ]) (0.55 0.08 and 0.70 0.08) at 6.0 and 18.2 MeV, respectively. Weak to moderate correlations were found for the logarithm of the soft X-ray flux (log₁₀ [SXRF]) and the logarithm of the duration of DH type II radio burst (log₁₀ [D_TII ]).

Using linear combinations of two or more variables improved the correlations. The best two-variable combination explaining log₁₀ [I_ESP^peak ] was V_TR combined with log₁₀ [I_SEP^peak ] and the best three- and four-variable combinations also included these two parameters. We found two methods for forecasting ESP peak intensities, one of which can be used for long lead time and the other for medium lead time forecasting. For long lead time forecasting V_TR , V_CME^space and log₁₀ [SXRF] are used. The correlation coefficients between the calculated and observed log₁₀ [I_ESP^peak ] were 0.71 0.05 at 6.0 MeV and 0.74 0.06 at 18.2 MeV. This method only depends on the coronagraph and X-ray observations at the Sun. For medium lead time forecasting the four parameters used are V_TR , log₁₀ [I_SEP^peak], V_CME^space (or log₁₀ [SXRF]), and log₁₀ [D_TII ]. The correlation coefficients were 0.80 0.04 at 6.0 MeV and 0.84 0.05 at 18.2 MeV. Coronagraph observations at the Sun and solar energetic particle and DH type II burst measurements in IP space are required for this method. The medium lead time forecasting provides an average warning time of 30 16 h.

Deep Learning LSTM-based approaches for 10.7 cm solar radio flux forecasting up to 45-days

Accurate forecasting of F10.7 index is important on short, medium and long-term timescales since F10.7 is an excellent proxy of solar activity and it plays an important role within the Space Weather framework. The analysis of the signatures of transient solar radio emission and its prediction are a challenging task as the underpinning physical processes are typically nonlinear, non-stationary and chaotic. In this paper we want to present three Deep Learning approaches for the daily forecasting of the adjusted F10.7 solar radio flux up to 45 days, using a family of Long Short Term Memory (LSTM) based models. We investigated two novel hybrid architectures: the LSTM model used in combination with Fast Iterative Filtering as decomposition algorithm (FIF-LSTM) and a method based on Multi-Head-Attention architecture (FIF-LSTM-MHA). FIF is a robust decomposition signal method very suitable for analyzing non- linear and non-stationary time series and it is used to separate the original time series into different oscillation components according to frequency, derived without leaving the time domain before to be fed into the neural network. The Attention mechanism is able to keep track of long-term dependencies in data sequences and improve the computational efficiency of the prediction model by reducing the effect of irrelevant information, mimicking human attention and selecting the most critical input. Our comparative analysis evaluated the models' performance for different time lags and solar activity levels. The results indicated that the hybrid models achieve better performance than the LSTM model for mid-range F10.7 predictions while the LSTM achieves better performance within the first few time lags. FIF-LSTM-MHA gives more promising output for longer forecasts since it tends to smooth the prediction curve due to the peculiarity of the Attention module to discard less relevant features of the time series and highlight the global trend.

Particle-in-cell simulations of electron-positron cyclotron maser forming pulsar radio zebras

Context. The microwave radio dynamic spectra of the Crab pulsar interpulse contain fine structures represented via narrowband quasiharmonic stripes. The pattern significantly constrains any potential emission mechanism. Similar to the zebra patterns observed, for example, in type IV solar radio bursts or decameter and kilometer Jupiter radio emission, the double plasma resonance (DPR) effect of the cyclotron maser instability may allow for interpretion of observations of pulsar radio zebras.
Aims: We provide insight at kinetic microscales of the zebra structures in pulsar radio emissions originating close to or beyond the light cylinder.
Methods: We present electromagnetic relativistic particle-in-cell (PIC) simulations of the electron-positron cyclotron maser for cyclotron frequency smaller than the plasma frequency. In four distinct simulation cycles, we focused on the effects of varying the plasma parameters on the instability growth rate and saturation energy. The physical parameters were the ratio between the plasma and cyclotron frequency, the density ratio of the "hot" loss-cone to the "cold" background plasma, the loss- cone characteristic velocity, and comparison with electron-proton plasma.
Results: In contrast to the results obtained from electron-proton plasma simulations (for example, in solar system plasmas), we find that the pulsar electron-positron maser instability does not generate distinguishable X and Z modes. On the contrary, a singular electromagnetic XZ mode was generated in all studied configurations close to or above the plasma frequency. The highest instability growth rates were obtained for the simulations with integer plasma-to-cyclotron frequency ratios. The instability is most efficient for plasma with characteristic loss-cone velocity in the range v_th = 0.2 0.3c. For low density ratios, the highest peak of the XZ mode is at double the frequency of the highest peak of the Bernstein modes, indicating that the radio emission is produced by a coalescence of two Bernstein modes with the same frequency and opposite wave numbers. Our estimate of the radiative flux generated from the simulation is up to 30 mJy from an area of 100 km² for an observer at 1 kpc distance without the inclusion of relativistic beaming effects, which may account for multiple orders of magnitude.

Magnetic Properties of Source Regions of CMEs and DH Type II Radio Bursts

The Sun is a dynamic star that exhibits various phenomena, including solar flares, coronal mass ejections (CMEs), and Type II radio bursts. CMEs are large-scale eruptions of plasma and magnetic field from the Sun that can disrupt the interplanetary medium and the Earth's magnetic field. Type II radio bursts are radio emissions associated with shocks generated by the CMEs. Only a few CMEs are associated with Type II radio bursts and the reasons for the absence of these bursts are still under debate. The magnetic properties of source active regions (ARs) from where CMEs with and without decameter-hectometer (DH) Type II radio bursts originate are investigated. Relations between the speed of CMEs and the source region properties are also obtained for these two groups of events (with and without radio bursts). The data from the Solar Dynamics Observatory (SDO) and the Radio and Plasma Wave (WAVES) Experiment on board the Wind spacecraft and the CMEs observed by the Solar and Heliospheric Observatory (SOHO) mission for the period of 2010 - 2014 in Solar Cycle 24 are utilized for this study. The statistical properties (like range, mean, median, and standard deviation) of source AR magnetic properties and the speed of the CMEs associated with DH Type II radio bursts (first group called radio loud) are found to be higher than those of CMEs without DH Type II radio bursts (second group called radio quiet). In addition, we found a positive correlation between the magnetic properties of the source AR and the speed of the CMEs with DH Type II radio bursts, but it is absent for events without DH Type II bursts. We also found that the probability of CME-streamer interaction is higher for the first group than for the second group, which shows a strong relation between the CME-streamer interaction and Type II bursts. These results reveal distinct magnetic characteristics in the source region for radio loud and radio quiet CMEs.

Analysis of Two Selected Solar Events in 2011 and 2015 With Mars Express Radio Occultation Data

The temporal behavior of the Martian ionosphere is highly variable due to various dynamic processes including space weather events. Here, we study the effect of solar flares and coronal mass ejections (CMEs) on the Martian ionosphere for two selected solar events in 2011 and 2015, using the publicly available Mars EXpress (MEX) radio occultation (RO) data (MaRS). We developed a data processing software that converts the calibrated Radio Occultation (RO) Doppler data to scientifically valuable atmospheric profiles. Using this software and previously unexplored MaRS observations, the variations in ionospheric parameters (electron density profiles and total electron content (TEC)) are calculated in order to evaluate the ionospheric changes due to solar flares and CMEs. The RO measurements mostly available 1-4 days apart from the peak events, showed no evident change in the TEC nor in the shape of electron density profiles except for a possible gradual decrease in altitude of M2 (main layer) peak density following the arrival of CMEs. To better quantify the effect of solar events on electron density profiles, RO observations near the time of arrival of solar flares and CMEs are crucial. This can be achieved by frequent RO measurements by various Mars orbiters including spacecraft-to-spacecraft measurements assisted by multi-instrument monitoring of the ionosphere.

A Novel Low-frequency Radio Astronomical Observation Array (1 90 MHz) and its First Light

The extremely low frequency (f < 40 MHz) is a very important frequency band for modern radio astronomy observations. It is also a key frequency band for solar radio bursts, planetary radio bursts, fast radio bursts detected in the lunar space electromagnetic environment, and the Earth's middle and upper atmosphere with low dispersion values. In this frequency band, the solar stellar activity, the early state of the universe, and the radiation characteristics of the planetary magnetosphere and plasma layer can be explored. Since there are few observations with effective spatial resolution in the extremely low frequency, it is highly possible to discover unknown astronomical phenomena on such a band in the future. In conjunction with low frequency radio observation on the far side of the Moon, we initially set up a novel low-frequency radio array in the Qitai station of Xinjiang Astronomical Observatory deep in Tianshan Mountains, Xinjiang, China on 2021 August 23. The array covers an operating frequency range of 1 ~ 90 MHz with a sensitivity of -78 dBm/125kHz, a dynamic range of 72 dB, and a typical gain value of 6 dBi, which can realize unattended all-weather observations. The two antennas due south of the Qitai Low- Frequency Radio Array were put into trial observations on 2021 May 28, and the very quiet electromagnetic environment of the station has been confirmed. So far, many solar radio bursts and other foreign signals have been detected. The results show that this novel low frequency radio array has the advantages of good performance, strong direction, and high antenna efficiency. It can play a unique role in Solar Cycle 25, and has a potential value in prospective collaborative observation between the Earth and space for extremely low frequency radio astronomy.

Application to early great solar flares

Large solar flares occasionally trigger significant space-weather disturbances that affect the technological infrastructures of modern civilization, and therefore require further investigation. Although these solar flares have been monitored by satellite observations since the 1970s, large solar flares occur only infrequently and restrict systematic statistical research owing to data limitations. However, Toyokawa Observatory has operated solar radio observations at low frequencies (at 3.75 and 9.4 GHz) since 1951 and captured the early great flares as solar radio bursts. To estimate the magnitudes of flares that occurred before the start of solar X-ray (SXR) observations with the Geostationary Operational Environmental Satellite (GOES) satellites, we show the relationship between microwave fluxes at 3.75 and 9.4 GHz and X-ray fluxes of flares that occurred after 1988. In total, we explored 341 solar flares observed with the Nobeyama Radio Polarimeters and Toyokawa Observatory from 1988-2014 and compared them with the SXR observations recorded by the GOES satellites. The correlation coefficient was approximately 0.7. Therefore, the GOES X-ray class can be estimated from the peak flux at 3.75 and 9.4 GHz with a large variance and an error of factor of 3 (1). Thus, for the first time, we quantitatively estimated the light curves of two early solar flares observed in 1956 February by the Toyokawa solar radio observations using the relationship between SXR thermal radiation and microwave nonthermal radiation (Neupert, 1968, ApJ, 153, 59).

Fast radio bursts trigger aftershocks resembling earthquakes, but not solar flares

The production mechanism of repeating fast radio bursts (FRBs) is still a mystery, and correlations between burst occurrence times and energies may provide important clues to elucidate it. While time correlation studies of FRBs have been mainly performed using wait time distributions, here we report the results of a correlation function analysis of repeating FRBs in the 2D space of time and energy. We analyse nearly 7,000 bursts reported in the literature for the three most active sources of FRB 20121102A, 20201124A, and 20220912A, and find the following characteristics that are universal in the three sources. A clear power-law signal of the correlation function is seen, extending to the typical burst duration (~ 10 msec) towards shorter time intervals (t). The correlation function indicates that every single burst has about a 10-60 per cent chance of producing an aftershock at a rate decaying by a power law as (t)^-p with p = 1.5-2.5, like the Omori-Utsu law of earthquakes. The correlated aftershock rate is stable regardless of source activity changes, and there is no correlation between emitted energy and t. We demonstrate that all these properties are quantitatively common to earthquakes, but different from solar flares in many aspects, by applying the same analysis method for the data on these phenomena. These results suggest that repeater FRBs are a phenomenon in which energy stored in rigid neutron star crusts is released by seismic activity. This may provide a new opportunity for future studies to explore the physical properties of the neutron star crust.

Using Partial Solar Eclipse for the 14-Metre Radio Telescope Calibration

A partial solar eclipse occurred on 25 October 2022, in the central and northern parts of Europe. The partial solar eclipse was observed at Aalto University Metshovi Radio Observatory, Finland at the radio wavelength of 8 mm (37 GHz). In Finland, the magnitude of the partial solar eclipse was 62.7 %. Solar radio maps at the time cadence of about 9 minutes were observed over the whole eclipse. The solar eclipse observations could be used for instrument calibration purposes. This paper investigates the solar brightness temperature, the limb brightening effect, the height of the chromosphere and the location of radio brightening using the aforementioned partial solar eclipse observations.

We got the confirmation that our earlier results are consistent, e.g., the solar brightness temperature matches with 8100 K 300 K. It was also possible to detect limb brightening effect. However, the prevailing solar activity might have distorted the final conclusions. The Moon should operate as a focusing element and the location of radio brightenings could be defined more carefully than in the normal conditions. We investigated this feature. Our results are in some parts unexpected and need further investigations.

Pre-impulsive and Impulsive Phases of the Sub-Terahertz Flare of March 28, 2022

Properties of the solar radio spectrum, as well as the temporal profiles of flare emission, indicate the thermal nature of the sub-terahertz (sub-THz) component observed as the growth of radio emission in the frequency range of 1001000 GHz. The sub-THz flare onset can be ahead of the impulsive phase for several minutes. However, the origin of the pre- impulsive and impulsive sub-THz emission remains unclear. The present work is devoted to a detailed analysis of the M4.0 X-class solar flare observed on March 28, 2022 with the Bauman Moscow State Technical University Radio Telescope RT-7.5 at 93 GHz. We supply these data with multiwavelength solar observations in the X-ray (GOES, GBM/Fermi), extreme ultraviolet (AIA/SDO), and microwave ranges. The differential emission measure (DEM) responsible for EUV emission is determined by solving the inverse problem based on the AIA/SDO data. Using the DEM and assuming a thermal free-free emission mechanism in pre-impulsive and impulsive phases, we calculated the millimeter emission flux of coronal plasma of the flare source, which turned out to be much smaller than the observed values. We concluded that electrons accelerated in the corona and heat fluxes from the coronal loop top cannot be responsible for heating the sub-THz emission source located in the transition region and upper chromosphere. A possible origin of chromospheric heating in the pre-impulsive phase of the solar flare is discussed.

A Statistical Model of CME Acceleration

An algorithm for automatic approximation of the time dependence x(t) is built for the observed coordinate of the coronal mass ejection (CME) front from the admissible starting point to the first appearance in the field of view of the LASCO coronagraph and further, up to a heliocentric distance of ~25 solar radii (R_S). In the region from the starting point to the first appearance of the CME, two sections are assumed, with uniform (impulsive) acceleration and with uniform motion; then, the motion is approximated by observations. At the beginning of the approximation, either the CME start time is found through the appearance of certain frequencies of radio emissions (RSTN data) and type II and IV radio emissions (sequence characteristics are determined by machine learning), or the start time is determined by averaging over the allowable takeoff area; then the polynomial-ballistic model is optimized. The first and second derivatives x(t) determine the speed and acceleration of the CME at any point of its trajectory. Such an algorithm is necessary to obtain the most accurate kinematic characteristics of CMEs, which can allow one to study the physical, spatial, and temporal relationships between flares and CMEs in all their diversity. Widely used approximation techniques simplify the real CME trajectories x(t), thereby possibly discarding important features of the CME kinematics and flare development in the posteruptive phase. The algorithm was trained and tested on 17 solar flares and associated CMEs, which are known for their powerful proton events with proton energies greater than 300 MeV. The rate of the first occurrence of CMEs turned out to be different from the average rate given in the LASCO catalog, which is important for estimating the energy of flares and CMEs. In 7 out of 17 events, there was acceleration only in the impulsive phase (and then deceleration), while acceleration in the impulsive and posteruptive phases occurred in 10 events. In 4 out of 17 events, CME velocities greater than 2000 km/s were reached at a distance of 20R_S. The accuracy of determining the kinematic characteristics of CMEs can be improved by using additional observations, for example, SDO AIA in the September 10, 2017 event.

On the Relationship of Solar Energetic Particles with Metric Type II Radio Bursts

According to the data from the GOES satellites and the SRS solar radio spectrograph, 112 proton events were analyzed for the time interval from November 24, 2000 to December 20, 2014, accompanied by an increase in the intensity of solar energetic particles (protons) I_p with energies E_p > 1850 MeV and type II radio bursts in the range of 25180 MHz. A correlation was found for the proton intensity I_p with the intensity of type II radio bursts I_i and the frequency drift rate V. The correlation coefficients between I_p and I_i, as well as between I_p and V, reach approximately 0.82 and 0.67, respectively. The correlation decreases quite sharply at E_p > 100 MeV. Nonthermal electrons responsible for type II radio bursts and energetic protons are generated at the fronts of the same shock waves, while the acceleration of relativistic particles with E_p > 100 MeV is determined by nonthermal processes in the region of flare energy release.

Quick Event During the Decay Phase of the Microwave Emission of a Flare on May 22, 2021

This paper presents the results of a study of a short event that occurred during the decay phase of a C6.0 circular flare (GOES). The event was simultaneously observed by the Irkutsk Incoherent Scatter Radar at 161 MHz and by the Siberian Radioheliograph (SRH) at 5.6 GHz. The SRH data were obtained at a single frequency, but with a high time resolution (0.2 s). This made it possible to localize the microwave source and carry out its temporal profile using imaging photometry. The similarity of the time profiles at both frequencies and the close values of the periods of the quasi-periodic oscillations seen on both profiles indicate a close relationship between the emeission in the meter and microwave ranges. According to the images at 5.6 GHz, the burst source was located near the circular ribbon of the flare, but was not a part of it. The event was folowed by an eruption that manifested itself in the meter range as the Type II radio burst at frequencies above 200 MHz, while a burst at 161 MHz demonstrated the emission of the Type III radio burst. An analysis of the three-dimensional structure of the magnetic field, reconstructed on the basis of vector magnetograms, showed that the event source was located on relatively open field lines, which were above the dome structure of the circular flare. The active region topology near the microwave source of the event suggests the following scenario based on the expansion of the dome structure of the circular flare due to gas dynamic processes. This caused an impact on relatively open field lines, which resulted to eruption, acceleration of electrons, and their propagation along open field lines. It is shown that the event was triggered by a flare, despite the absence of a direct spatial connection between the burst and flare ribbons.

Sub-Terahertz Radiation Features from the Flare of Proxima Cen

A comparative analysis of stellar and solar flares at millimeter wavelength range has been carried out using the red dwarf Proxima Centauri as an example. The main attention is paid to the results of ALMA observations, which show that the flux of sub-THz radiation from stellar flares is an order of magnitude greater than that for solar flares, decreases with radiation frequency, and has a degree of linear polarization reaching tens of percent. We showed that the synchrotron mechanism is responsible for the sub-THz radiation, which assumes the acceleration of electrons to ultrarelativistic energies. The observed relationship between sub-THz, radio, and X-ray emission from stellar flares is discussed. The hypothesis about the important role of flare energy release in the dense layers of the stellar atmosphere is substantiated.

Ground-Based Observations of the Sun for Space Weather Forecasting

The possibilities for organizing a solar activity observation service for space weather forecasting are considered. At this stage, the most promising approach is the establishment of a ground-based observation network. This network should include solar magnetographs for observing large-scale magnetic fields of the Sun and patrol optical telescopes for detecting coronal mass ejections and solar flares. Magnetographic observations provide data for assessing recurrent solar wind streams. Patrol telescopes operating in continuous mode make it possible to detect eruptive moments and determine parameters of coronal mass ejections at the initial stage. The network can be complemented by other types of observations in radio and optical bands. The article discusses the composition of observation tools as well as methods and models of forecasting.

numerical modeling

Solar flares, as strong explosions on the Sun's surface, are well known driving agents that severely affect the near-Earth environment, producing additional ionization within the sunlit Earth's atmospheric layers. X-ray solar flares can be classified regarding their effects on the lower ionosphere and its electron density profile. The focus of this research is on the study of disturbances induced by X-ray solar flares in order to predict the impact of possible weak solar events. In this paper we examined solar activity of lower intensity by conducting numerical modeling using several models and based on data obtained by very low frequency radio signals and from the Geostationary Operational Environmental Satellite (GOES) database on solar X-ray radiation.

data and modeling

Solar flares are among main extraterrestrial events that are well known to severely affect both space weather and near-Earth surrounding conditions. Under incident X-ray solar flare radiation, ionospheric plasma undergoes perturbations clearly and distinctly observable on ground-based monitoring systems' recordings. Mid-latitude lower ionosphere under influence of energetic solar flare events was examined by employing different numerical modeling procedures, including Machine Learning, relying on ground-based Very Low Frequency (VLF) radio signal recordings from Belgrade VLF database and solar soft X-ray irradiance satellite measurements taken from Geostationary Operational Environmental Satellite (GOES) database.

Expansion of the Soft X-ray Source and ``Magnetic Detonation'' in Solar Flares

The detection of radio emission from solar flares at frequencies below ${}2$ GHz allows the upper limits for the characteristic size of the soft X-ray (SXR) source $L(t)$ to be estimated under the assumption that the density $n(t)$ is determined by the plasma frequency $_p$. If the SXR source with a higher density is inside the radio source, then the size of the SXR source will be $L(t)<(EM(t)/2n(t)²)^{1/3}$, where $EM(t)$ is the emission measure. For three flares (C7.2 on December 22, 2009, M2.9 on July 6, 2012, and X1.1 on July 6, 2012) we calculate the expansion speeds of the SXR source $V(t) dL(t)/dt$, which are compared with the estimates of the sound speed and the Alfvn speed. By ``magnetic detonation'' we mean the process of the propagation of magnetic reconnection with a supersonic speed in eruptive flares. Magnetic detonation and the succeeding coronal mass ejection (CME) were realized in the December 22, 2009 C7.2 and July 6, 2012 X1.1 flares, in which supersonic and super- Alfvn speeds were reached if the density of the SXR source was lower than $2.1 10⁹$ and $7.4 10⁸$ cm${}^{-3}$ ($_p<410$ and ${<}245$ MHz), respectively. There were no magnetic detonation and CME in the July 6, 2012 M2.9 flare, whose radio emission frequencies were only above 1415 MHz ($n>2.5 10^{10}$ cm${}^{-3}$). For magnetic detonation in the July 6, 2012 X1.1 flare we have estimated the magnetic field strength, the reconnection electric field strength, the plasma flow, and the CME mass.

Solar Radio Burst Detection Based on the MobileViT-SSDLite Lightweight Model

Real-time detection of solar radio bursts is crucial in solar physics research and space weather forecasting. However, current research on the automatic detection of solar radio bursts is limited to identifying the presence or absence of solar radio bursts or recognizing only a single type of burst, such as type II or III. Furthermore, existing methods cannot learn spectral and temporal features and often suffer from the drawbacks of large network models, resulting in slow speeds. This paper proposes an automatic recognition and localization method based on a lightweight object detection model for solar radio burst events. We collected observation data from e-CALLISTO and established a data set containing type II, III, IV, and V solar radio bursts. To address the real-time requirements of practical applications and consider the temporal and frequency domain information of spectrogram images, we improved a vision transformer with a self-attention mechanism and adopted a lightweight model for detection. The experimental results demonstrate that our proposed method achieves an average precision at a 50% intersection-over-union threshold of 78.2% and a recall rate of 92% on the established solar radio burst data set. Additionally, the model operates at a detection speed of 54.8 frames s^-1, where a frame refers to a spectral image with a duration of 15 minutes, enabling efficient automated detection and localization of type II, III, IV, and V solar radio bursts.

The Effect of the Parametric Decay Instability on the Morphology of Coronal Type III Radio Bursts

The nonlinear evolution of Alfvn waves in the solar corona leads to the generation of Alfvnic turbulence. This description of the Alfvn waves involves parametric instabilities where the parent wave decays into slow mode waves giving rise to density fluctuations. These density fluctuations, in turn, play a crucial role in the modulation of the dynamic spectrum of type III radio bursts, which are observed at the fundamental of local plasma frequency and are sensitive to the local density. During observations of such radio bursts, fine structures are detected across different temporal ranges. In this study, we examine density fluctuations generated through the parametric decay instability (PDI) of Alfvn waves as a mechanism to generate striations in the dynamic spectrum of type III radio bursts using magnetohydrodynamic simulations of the solar corona. An Alfvn wave is injected into the quiet solar wind by perturbing the transverse magnetic field and velocity components, which subsequently undergo the PDI instability. The type III burst is modeled as a fast-moving radiation source that samples the background solar wind as it propagates to emit radio waves. We find the simulated dynamic spectrum to contain striations directly affected by the multiscale density fluctuations in the wind.

Solar Orbiter Observations of a Match Made in the Heliosphere

Solar Orbiter's four in situ instruments have recorded numerous energetic electron events at heliocentric distances between 0.5 and 1 au. We analyze energetic electron fluxes, spectra, pitch-angle distributions, associated Langmuir waves, and type III solar radio bursts for three events to understand what causes modifications in the electron flux and identify the origin and characteristics of features observed in the electron spectrum. We investigate what electron beam properties and solar wind conditions are associated with Langmuir wave growth and spectral breaks in the electron peak flux as a function of energy. We observe velocity dispersion and quasilinear relaxation in the electron flux caused by the resonant wave-particle interactions in the deca-keV range, at the energies at which we observe breaks in the electron spectrum, cotemporal with the local generation of Langmuir waves. We show, via the evolution of the electron flux at the time of the event, that these interactions are responsible for the spectral signatures observed around 10 and 50 keV, confirming the results of simulations by Kontar and Reid. These signatures are independent of pitch-angle scattering. Our findings highlight the importance of using overlapping FOVs when working with data from different sensors. In this work, we exploit observations from all in situ instruments to address, for the first time, how the energetic electron flux is modified by the beam-plasma interactions and results in specific feature appearing in the local spectrum. Our results, corroborated with numerical simulations, can be extended to a wider range of heliocentric distances.

Scale Invariance in Gamma-Ray Flares of the Sun and 3C 454.3

Using the gamma-ray flare samples of the Sun and 3C 454.3 observed by the Fermi telescope, we investigate the statistical properties of sizes including fluence (energy), peak flux (luminosity), duration time, and waiting time in this work. We find that the cumulative distribution of the fluctuations of these sizes follow well the Tsallis q-Gaussian function. The obtained q values from q-Gaussian distribution remain stable around 2 without any significant change, implying that there is a scale invariance structure in gamma-ray flares of the Sun and 3C 454.3. This scale invariance characteristics of the Sun and 3C 454.3 indicated by q values are also comparable to those of earthquakes, soft gamma repeaters, fast radio burst (FRB 20121102), and X-ray flares of gamma- ray bursts. On top of that, we verify the relationship between q values and the power-law indices from the size frequency distributions, which is expressed as q = ( + 2)/. These statistical findings could be well explained within the physical framework of a self-organizing criticality system.

Solar Radio Spectro-polarimeter (50-500 MHz). I. Design, Development, and Characterization of a Cross-polarized, Log-periodic Dipole Antenna

The Zeeman effect has been routinely used to image and quantify the solar photospheric magnetic field (B). Such a direct measuring technique is not yet available for the corona (Lin et al. 2004). Since almost all transient nonthermal radio emissions from the corona are either partially or fully circularly polarized, observing their polarization signatures over broad frequency ranges would be of help to estimate B as a function of heliocentric height. This article aims to describe the design and development of a Cross-polarized Log-Periodic Dipole Antenna (CLPDA), an integral part of a radio spectro-polarimeter, which works in the 50-500 MHz frequency-range and to explain the tests that were carried out to characterize it. The above frequency range corresponds to a heliocentric height range 1.03 < r < 2.5 R (R = photospheric radius), wherein the numerous coronal nonthermal transients associated with space-weather effects are observed to originate. The CLPDA is used to determine the strength and sense of polarization of the received radio signal. The uncertainty involved in the determination depends on the polarization-isolation (PI) between the two orthogonal components of a CLPDA. Some of the recent advancements made in the antenna design concepts at high frequencies (~GHz) were adopted to reduce the PI at low frequencies (~MHz). Throughout the above frequency range, the CLPDA has a gain, return loss, and PI of 6.6 dBi, -10 dB, and -27 dB, respectively. The average PI of the CLPDA varies from -30 to -24 dB over an azimuthal angle range 0 to 45 within which the observations are performed regularly.

A Comparative Study of Ground-level Enhancement Events of Solar Energetic Particles

Major solar eruptions can accelerate protons up to relativistic energies. Solar relativistic ions arriving at 1 au may cause a solar particle event detectable by the worldwide network of neutron monitors (NMs), a ground-level enhancement (GLE) event. Using the newly computed NM yield function, we have fitted the 15 historic GLEs. Moments of the fitted proton distributions are used for the analysis. Profiles of the proton net flux are very diverse, while some profiles are similar. For this study, we select two events with similar time profiles, GLE 60 (2001 April 15) and GLE 65 (2003 October 28), and ask what makes these GLEs similar. We compare the GLEs with their progenitor solar flares and coronal mass ejections (CMEs). We find a close relationship between the rise and peak of the GLE, on the one hand, and the solar flare and the metric radio emissions from extended coronal sources at the base of the CME, on the other hand. The GLE decay time, the rate of the proton spectrum evolution, and the CME speed are proportional to the duration of the soft X-ray flare. We compare the two GLEs with GLE 59 (2000 July 14) analyzed by Klein et al. and with the deka-MeV nucleon^-1 proton and helium data from the ERNE instrument on the Solar and Heliospheric Observatory spacecraft. The comparison indicates that a single solar eruption can produce more than one component of solar energetic particles, differently contributing at different energies and locations.

The physics of solar spectral imaging observations in dm-cm wavelengths and the application on space weather

Recently, several new solar radio telescopes have been put into operation and provided spectral-imaging observations with much higher resolutions in decimeter (dm) and centimeter (cm) wavelengths. These telescopes include the Mingantu Spectral Radioheliograph (MUSER, at frequencies of 0.415 GHz), the Expanded Owens Valley Solar Array (EOVSA, at frequencies of 118 GHz), and the Siberian Radio Heliograph (SRH, at frequencies of 324 GHz). These observations offer unprecedented opportunities to study solar physics and space weather, especially to diagnose the coronal magnetic fields, reveal the basic nature of solar eruptions and the related non-thermal energy release, particle accelerations and propagation, and the related emission mechanisms. These results might be the important input to the space weather modeling for predicting the occurrence of disastrous powerful space weather events. In order to provide meaningful reference for other solar physicists and space weather researchers, this paper mainly focus on discussing the potential scientific problems of solar radio spectral- imaging observations in dm-cm wavelengths and its possible applications in the field of space weather. These results will provide a helpful reference for colleagues to make full use of the latest and future observation data obtained from the above solar radio telescopes.

Modelling of foF2 using artificial neural network over Equatorial Ionization Anomaly (EIA) region stations

An ANN based model has been suggested for foF2 values over EIA region stations during 19582008. The developed model predicts foF2 with a root mean square error of less than 1 MHz for the unseen testing data. The model may be used for practical purposes over Asian longitude sector, particularly for Pakistan.

Performance of a locally adapted NeQuick-2 model during high solar activity over the Brazilian equatorial and low-latitude region

The Brazilian equatorial and low-latitude regions are subject to various dynamic and electrodynamics processes. As a result, ionospheric modeling over this region remains a challenge. In this article, we present the results of first observation of data ingestion into the climatological model, NeQuick during both quiet and disturbed conditions over Brazil. The variation of the daily F10.7 solar radio flux, the main driver of the NeQuick model, strongly influences its performance in both space and time, especially during high solar activity. With data ingestion, using the local level of ionization, NeQuick's performance can be improved. We developed an algorithm to obtain the local effective ionization parameters (Az1 and Az2) using a single station, in the equatorial trough and low-latitude regions, which are subsequently used in the NeQuick to reproduce vTEC at co-located stations. The model's input (effective ionization level) was obtained when the modeled vTEC best fits the measured vTEC at the reference stations Maraba (5.35 S, 49.11 S, dip lat.: 3.06 S; MABA) and Ourinhos (22.93 S, 49.88 S, dip lat.: 17.42 S; OURI). Statistical results show that the model's performance greatly improves after data ingestion, reproducing vTEC at all latitudes close to the reference stations in 2014. We found that NeQuick improved by 71 %, 74 %, 83 %, and 69 % after ingestion during the storm periods of 1721 February, 1014 April, 610 June, and 2327, December in the low-latitude region at SJSP. Using the Az1 values obtained at MABA and Az3 at SALU during July 2014, NeQuick reproduces the critical frequency of the F2 layer with a percentage improvement of approximately 20 % and 37 % respectively.

Coronal diagnostics of solar type III radio bursts using LOFAR and PSP observations

Context. Solar type III radio bursts are common phenomena, recognized as the result of accelerated electron beams propagating through the solar corona. These bursts are of particular interest as they provide valuable information about the magnetic field and plasma conditions in the corona, which are difficult to measure directly.
Aims: This study aims to investigate the ambiguous source and the underlying physical processes of the type III radio bursts that occurred on April 3, 2019, through the utilization of multi-wavelength observations from the Low- Frequency Array (LOFAR) radio telescope and the Parker Solar Probe (PSP) space mission, as well as incorporating results from a Potential Field Source Surface (PFSS) and magnetohydrodynamic (MHD) models. The primary goal is to identify the spatial and temporal characteristics of the radio sources, as well as the plasma conditions along their trajectories.
Methods: We applied data preprocessing techniques to combine high- and low-frequency observations from LOFAR and PSP between 2.6 kHz and 80 MHz. We then extracted information on the frequency drift and speed of the accelerated electron beams from the dynamic spectra. Additionally, we used LOFAR interferometric observations to image the sources of the radio emission at multiple frequencies and determine their locations and kinematics in the corona. Lastly, we analyzed the plasma parameters and magnetic field along the trajectories of the radio sources using PFSS and MHD model results.
Results: We present several notable findings related to type III radio bursts. Firstly, through our automated implementation, we were able to effectively identify and characterize 9 type III radio bursts in the LOFAR-PSP combined dynamic spectrum and 16 type III bursts in the LOFAR dynamic spectrum. We found that the frequency drift for the detected type III bursts in the combined spectrum ranges between 0.24 and 4 MHz s¹, while the speeds of the electron beams range between 0.013 and 0.12 C. Secondly, our imaging observations show that the electrons responsible for these bursts originate from the same source and within a short time frame of fewer than 30 min. Finally, our analysis provides informative insights into the physical conditions along the path of the electron beams. For instance, we found that the plasma density obtained from the magnetohydrodynamic algorithm outside a sphere (MAS) model is significantly lower than the expected theoretical density.

Source positions of an interplanetary type III radio burst and anisotropic radio-wave scattering

Interplanetary solar radio type III bursts provide the means to remotely study and track energetic electrons propagating in the interplanetary medium. Due to the lack of direct radio source imaging, several methods have been developed to determine the source positions from space-based observations. Moreover, none of the methods consider the propagation effects of anisotropic radio-wave scattering, which would strongly distort the trajectory of radio waves, delay their arrival times, and affect their apparent characteristics. We investigate the source positions and directivity of an interplanetary type III burst simultaneously observed by Parker Solar Probe, Solar Orbiter, STEREO, and Wind and we compare the results of applying the intensity fit and timing methods with ray-tracing simulations of radio-wave propagation with anisotropic density fluctuations. The simulation calculates the trajectories of the rays, their time profiles at different viewing sites, and the apparent characteristics for various density fluctuation parameters. The results indicate that the observed source positions are displaced away from the locations where emission is produced, and their deduced radial distances are larger than expected from density models. This suggests that the apparent position is affected by anisotropic radio-wave scattering, which leads to an apparent position at a larger heliocentric distance from the Sun. The methods to determine the source positions may underestimate the apparent positions if they do not consider the path of radio-wave propagation and incomplete scattering at a viewing site close to the intrinsic source position.

Solar Flare Prediction and Feature Selection Using a Light-Gradient-Boosting Machine Algorithm

Solar flares are among the most severe space-weather phenomena, and they have the capacity to generate radiation storms and radio disruptions on Earth. The accurate prediction of solar-flare events remains a significant challenge, requiring continuous monitoring and identification of specific features that can aid in forecasting this phenomenon, particularly for different classes of solar flares. In this study, we aim to forecast C- and M-Class solar flares utilising a machine-learning algorithm, namely the Light Gradient Boosting Machine. We have utilised a dataset spanning nine years, obtained from the Space- weather Helioseismic and Magnetic Imager Active Region Patches (SHARP), with a temporal resolution of 1 h. A total of 37 flare features were considered in our analysis, comprising of 25 active-region parameters and 12 flare-history features. To address the issue of class imbalance in solar-flare data, we employed the Synthetic Minority Over-sampling Technique (SMOTE). We used two labelling approaches in our study: a fixed 24-h window label and a varying window that considers the changing nature of solar activity. Then, the developed machine-learning algorithm was trained and tested using forecast-verification metrics, with an emphasis on evaluating the true skill statistic (TSS). Furthermore, we implemented a feature-selection algorithm to determine the most significant features from the pool of 37 features that could distinguish between flaring and non-flaring active regions. We found that utilising a limited set of useful features resulted in improved prediction performance. For the 24-h prediction window, we achieved a TSS of 0.63 (0.69) and an accuracy of 0.90 (0.97) for C- (M)-Class solar flares.

The Brightness Temperature and Magnetic Field of Active Regions

Solar radio observations provide a powerful diagnostic of the physical conditions of the solar atmosphere over a wide range of heights. In this paper, we report regular solar mapping made at the X-band (8.1 - 9.2 GHz) with the Arecibo 12-m radio telescope, covering a period between 13 December 2021 and 9 April 2023. This has demonstrated its potential for identifying active regions and tracking their brightness-temperature changes. The X-band results are discussed along with the near- simultaneous datasets available from space- and ground-based observations. A comparison of magnetic properties of active regions with their emission characteristics indicates that the X-band brightness temperature provides better information of the magnetic-field strength associated with the emission and a brightness temperature in excess of 13 000 K allows us to infer the possibility of intense flares (i.e., M1 class) and coronal mass ejections. The `latitude-time' distribution of the brightness temperature reveals the three-dimensional evolution of quiet regions on the Sun, coronal holes, and eruptive sites, over many solar rotations in the ascending phase of the current Solar Cycle 25.

The Correlation between Ionospheric Electron Density Variations Derived from Swarm Satellite Observations and Seismic Activity at the Australian-Pacific Tectonic Plate Boundary

Swarm electron density (Ne) observations from the Langmuir probe (LP) can detect ionospheric disturbances at the altitude of a satellite. Along-track satellite observations provide a large number of very short observations of different places in the ionosphere, where Ne is disturbed. Moreover, different perturbations occupy various Ne signal frequencies. Therefore, such short signals are more recognizable in two dimensions, where aside from their change in time, we can observe their diversity in the frequency domain. Spectral analysis is an essential tool applied here, as it enables signal decomposition and the recognition of composite patterns of Ne disturbances that occupy different frequencies. This study shows a high-resolution application of short-term Fourier transform (STFT) to Swarm Ne observations in the Papua New Guinea region in the vicinity of earthquakes, tsunamis, and related general seismic activity. The system of tectonic plate junctions, including the Pacific-Australian boundary, is located orthogonally to Swarm track footprints. The selected wavelengths of seismically induced ionospheric disturbances detected via Swarm are compared with the three sets of three-month records of seismic activity: in the winter solstice of 2016/2017, when seismic activity was highest, and in the summer solstice and vernal equinox of 2016, which were calmer. Moreover, more Swarm data records are analyzed at the same latitudes for validation purposes, in a place where there are no tectonic plate boundaries that are orthogonal to the Swarm orbital footprint. Additional validation is supplied through Swarm Ne observations from completely different latitudes, where the Swarm orbital footprint orthogonally crosses a different subducting plate boundary. Aside from the seismic energy, the solar radio flux (F10.7), equatorial plasma bubbles (EPBs), and geomagnetic ap and Dst indices are also reviewed here. Their influence on the ionospheric Ne is also found in Swarm observations. Finally, the Pearson correlation coefficient (PCC), applied to the pairs of 3-month time series created from Swarm Ne variations, seismic energy, ap, Dst, and F10.7, summarizes the graphical inspection of mutual correlations. It points to the predominant correlation of Swarm Ne disturbances with seismicity, especially during nighttime. We show that most of the Ne disturbances at a selected wavelength of 300 km correlate more with seismicity than with geomagnetic and solar indices. Therefore, Swarm LP can be assessed as being capable of observing the lithosphere-atmosphere-ionosphere coupling (LAIC) from the orbit.

Unifying VLF Transmitter and Sferic Modeling Efforts Via Tomography

We demonstrate a methodology for utilizing measurements from very low frequency (VLF, 330 kHz) transmitters and lightning emissions to produce 3D lower electron density maps, and apply it to multiple geophysical disturbances. The D-region lower ionosphere (6090 km) forms the upper boundary of the Earth-ionosphere waveguide which allows VLF radio waves to propagate to global distances. Measurements of these signals have, in many prior studies, been used to infer path-average electron density profiles within the D region. Historically, researchers have focused on either measurements of VLF transmitters or radio atmospherics (sferics) from lightning. In this work, we build on recently published methods for each and present a method to unify the two approaches via tomography. The output of the tomographic inversion produces maps of electron density over a large portion of the United States and Gulf of Mexico. To illustrate the benefits of this unified approach, daytime and nighttime maps are compared between a sferic-only model and the new approach suggested here. We apply the model to characterize two geophysical disturbances: solar flares and lower ionospheric changes associated with thunderstorms.

A New Index to Describe the Relationship between Solar Extreme Ultraviolet Variation and Solar Activity

In this paper, a new solar activity index based on a novel disturbance extraction method, the spectral whitening method (SWM), is introduced to process the solar EUV data on the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. Our research suggests that the spatial information derived by SWM can well reflect the location of disturbance extraction, which is consistent with the location of the solar active region. It indicates that the disturbance extraction is effective. From AIA 094 to AIA 335 , SWM results are strongly correlated with solar radio flux F107 and the sunspot number (SSN), especially at AIA 211 , where the correlation coefficient reaches the maximum, while at AIA 1600 and AIA 1700 there are no detectable correlations. The proposed new solar activity index, ${J}_{P}\left(\mathrm{AIA}\right)$ , has the following characteristics: (1) the new index can reflect the main variations of F107 and SSN, indicating that the index is valid; (2) the new index has higher temporal resolution, which is more conducive to the more detailed study of solar activities on short timescales; (3) the new index reveals that the solar atmosphere still has significant variability during solar minimum characterized by low F107 and SSN; (4) the new index can be used in conjunction with the new magnetospheric and ionospheric indices, which are also derived by SWM to deepen the understanding of the causal chain of space weather and promote the improvement of forecasting capabilities.

Improved Electron Density Model for Solar Wind

We present a comprehensive analysis of electron density measurements in the solar wind using quasi-thermal noise (QTN) spectroscopy applied to data from the first 15 encounters of the Parker Solar Probe mission (2018 November-2023 March). Our methodology involves the identification of the plasma line frequency and the calculation of plasma density based on in situ measurements. By analyzing over 2.1 million data points, we derive a power-law model for electron density as a function of radial distance from the Sun in the range of 13 to 50 R : n _e(r) = (343,466 19921) r ^(-1.870.11). This model provides improved estimates for localizing interplanetary solar radio bursts. Moreover, obtained electron densities can be used for calibrating particle instruments on board the Parker Solar Probe. We discuss its limitations and potential for further refinement with additional Parker Solar Probe encounters.

Separating the effects of earthside and far side solar events. A case study

On 8 November 2013 a halo-type coronal mass ejection (CME) was observed, together with flares and type II radio bursts, but the association between the flares, radio bursts, and the CME was not clear. Our aim is to identify the origin of the CME and its direction of propagation, and to exclude features that were not connected to it. On the Earth-facing side, a GOES C5.7 class flare occurred close to the estimated CME launch time, followed by an X1.1 class flare. The latter flare was associated with an EUV wave and metric type II bursts. On the far side of the Sun, a filament eruption, EUV dimmings, and ejected CME loops were observed by imaging instruments onboard the Solar TErrestrial RElations Observatory (STEREO) spacecraft that were viewing the backside of the Sun. The STEREO radio instruments observed an interplanetary (IP) type II radio burst at decameter-hectometric wavelengths, which was not observed by the radio instrument onboard the Wind spacecraft located at L1 near Earth. We show that the halo CME originated from the eruption on the far side of the Sun, and that the IP type II burst was created by a shock wave ahead of the halo CME. The radio burst remained unobserved from the earthside, even at heliocentric source heights larger than 9 solar radii. During the CME propagation, the X-class flare eruption caused a small plasmoid ejection earthward, the material of which was superposed on the earlier CME structures observed in projection. The estimated heights of the metric type II burst match well with the EUV wave launched by the X-class flare. As this radio emission did not continue to lower frequencies, we conclude that the shock wave did not propagate any further. Either the shock driver died out, as a blast wave, or the driver speed no longer exceeded the local Alfven speed.

Low latitude ionospheric TEC response to the solar flares during the peak of solar cycle 24

We describe the newly discovered response of differential vertical total electron content (DVTEC) in the low-equatorial latitude ionosphere to the solar flares during the peak of solar cycle 24 i.e., the year 2014. GPS TEC data for the existing investigation has been acquired from the international GNSS services network station in Bangalore, India (geomagnetic latitude 4.58N). For the year 2014, we inspected five X-class solar flares, forty-nine M-class solar flares, and three hundred ninety-six C-class solar flares in the current study. To ascertain the relationship between DVTEC and the X-ray and EUV fluxes, the two main ionizing radiation fluxes during flare events are used. Additionally, to identify electron density change in the ionosphere due to solar flares, we used two well-known methodologies, the baseline method and the mean method. The baseline method provides a better correlation between DVTEC and X-ray for X-, M-, and C-class flares. For M- and C- class flares, both methods do not show any meaningful correlation. The Solar disc location effect shows an enhanced (0.573) correlation between DVTEC and X-ray*cos (CMD) for the X-class flares, whereas for M- and C-class flares, no significant change was observed, which indicates feeble or no CMD effect for lower class solar flares while it is opposite for X-class flares. Here we suggest that the high solar activity, and location of the observation station to the EIA trough region might be the possible reason for the observed results.

Uncertainties of the 30-408 MHz Galactic emission as a calibration source for radio detectors in astroparticle physics

Context. Arrays of radio antennas have proven to be successful in astroparticle physics with the observation of extensive air showers initiated by high-energy cosmic rays in the Earth's atmosphere. Accurate determination of the energy scale of the primary particles' energies requires an absolute calibration of the radio antennas for which, in recent years, the utilization of the Galactic emission as a reference source has emerged as a potential standard.
Aims: To apply the "Galactic calibration" a proper estimation of the systematic uncertainties on the prediction of the Galactic emission from sky models is necessary, which we aim to quantify on a global level and for the specific cases of selected radio arrays. We further aim to determine the influence of additional natural radio sources on the Galactic calibration.
Methods: We compared seven different sky models that predict the full-sky Galactic emission in the frequency range from 30 to 408 MHz. We made an inventory of the reference maps on which they rely and used the output of the models to determine their global level of agreement. We subsequently took typical sky exposures and the frequency bands of selected radio arrays into account and repeated the comparison for each of them. Finally, we studied and discuss the relative influence of the quiet Sun, the ionosphere, and Jupiter.
Results: We find a systematic uncertainty of 14.3% on the predicted power from the Galactic emission, which scales to approximately half of that value as the uncertainty on the determination of the energy of cosmic particles. When looking at the selected radio arrays, the uncertainty on the predicted power varies between 11.7% and 21.5%. The influence of the quiet Sun turns out to be insignificant at the lowest frequencies but increases to a relative contribution of ~30% around 400 MHz.

Element Abundances in Impulsive Solar Energetic-Particle Events

Impulsive solar energetic-particle (SEP) events were first distinguished as the streaming electrons that produce type III radio bursts as distinct from shock-induced type II bursts. They were then observed as the surprisingly enhanced 3He-rich SEP events, which were also found to have element enhancements rising smoothly with the mass-to-charge ratio A/Q through the elements, even up to Pb. These impulsive SEPs have been found to originate during magnetic reconnection in solar jets where open magnetic field lines allow energetic particles to escape. In contrast, impulsive solar flares are produced when similar reconnection involves closed field lines where energetic ions are trapped on closed loops and dissipate their energy as X-rays, -rays, and heat. Abundance enhancements that are power laws in A/Q can be used to determine Q values and hence the coronal source temperature in the events. Results show no evidence of heating, implying reconnection and ion acceleration occur early, rapidly, and at low density. Proton and He excesses that contribute their own power law may identify events with reacceleration of SEPs by shock waves driven by accompanying fast, narrow coronal mass ejections (CMEs) in many of the stronger jets.

Relativistic coronal mass ejections from magnetars

We study dynamics of relativistic coronal mass ejections (CMEs), from launching by shearing of foot-points (either slowly - the 'Solar flare' paradigm, or suddenly - the 'star quake' paradigm), to propagation in the preceding magnetar wind. For slow shear, most of the energy injected into the CME is first spent on the work done on breaking through the overlaying magnetic field. At later stages, sufficiently powerful CMEs may lead to the 'detonation' of a CME and opening of the magnetosphere beyond some equipartition radius r_eq, where the decreasing energy of the CME becomes larger than the decreasing external magnetospheric energy. Post-CME magnetosphere relaxes via the formation of a plasmoid-mediated current sheet, initially at ~r_eq, and slowly reaching the light cylinder. Both the location of the foot-point shear and the global magnetospheric configuration affect the frequent/weak versus rare/powerful CME dichotomy - to produce powerful flares, the slow shear should be limited to field lines that close in near the star. After the creation of a topologically disconnected flux tube, the tube quickly (at ~ the light cylinder) comes into force- balance with the preceding wind and is passively advected/frozen in the wind afterward. For fast shear (a local rotational glitch), the resulting large amplitude Alfvn waves lead to the opening of the magnetosphere (which later recovers similarly to the slow shear case). At distances much larger than the light cylinder, the resulting shear Alfvn waves propagate through the wind non-dissipatively.

On the detection of a solar radio burst event that occurred on 28 August 2022 and its effect on GNSS signals as observed by ionospheric scintillation monitors distributed over the American sector

As part of an effort to observe and study ionospheric disturbances and their effects on radio signals used by Global Navigation Satellite Systems (GNSS), alternative low-cost GNSS-based ionospheric scintillation and total electron content (TEC) monitors have been deployed over the American sector. During an inspection of the observations made on 28 August 2022, we found increases in the amplitude scintillation index (S₄) reported by the monitors for the period between approximately 17:45 UT and 18:20 UT. The distributed, dual-frequency observations made by the sensors allowed us to determine that the increases in S₄ were not caused by ionospheric irregularities. Instead, they resulted from Carrier-to-Noise (C/No) variations caused by a solar radio burst (SRB) event that followed the occurrence of two M-class X-ray solar flares and a Halo coronal mass ejection. The measurements also allowed us to quantify the impact of the SRB on GNSS signals. The observations show that the SRB caused maximum C/No fadings of about 8 dB-Hz (12 dB-Hz) on L1 ~ 1.6 GHz (L2 ~ 1.2 GHz) for signals observed by the monitor in Dallas for which the solar zenith angle was minimum (~24.4) during the SRB. Calculations using observations made by the distributed monitors also show excellent agreement for estimates of the maximum (vertical equivalent) C/No fadings in both L1 and L2. The calculations show maximum fadings of 9 dB-Hz for L1 and of 13 dB-Hz for L2. Finally, the results exemplify the usefulness of low-cost monitors for studies beyond those associated with ionospheric irregularities and scintillation.

Automatic Detection of Type II Solar Radio Burst by Using 1-D Convolution Neutral Network

Type II solar radio bursts show frequency drifts from high to low over time. They have been known as a signature of coronal shock associated with Coronal Mass Ejections (CMEs) and/or flares, which cause an abrupt change in the space environment near the Earth (space weather). Therefore, early detection of type II bursts is important for forecasting of space weather. In this study, we develop a deep-learning (DL) model for the automatic detection of type II bursts. For this purpose, we adopted a 1-D Convolution Neutral Network (CNN) as it is well-suited for processing spatiotemporal information within the applied data set. We utilized a total of 286 radio burst spectrum images obtained by Hiraiso Radio Spectrograph (HiRAS) from 1991 and 2012, along with 231 spectrum images without the bursts from 2009 to 2015, to recognizes type II bursts. The burst types were labeled manually according to their spectra features in an answer table. Subsequently, we applied the 1-D CNN technique to the spectrum images using two filter windows with different size along time axis. To develop the DL model, we randomly selected 412 spectrum images (80%) for training and validation. The train history shows that both train and validation losses drop rapidly, while train and validation accuracies increased within approximately 100 epoches. For evaluation of the model's performance, we used 105 test images (20%) and employed a contingence table. It is found that false alarm ratio (FAR) and critical success index (CSI) were 0.14 and 0.83, respectively. Furthermore, we confirmed above result by adopting five-fold cross-validation method, in which we re-sampled five groups randomly. The estimated mean FAR and CSI of the five groups were 0.05 and 0.87, respectively. For experimental purposes, we applied our proposed model to 85 HiRAS type II radio bursts listed in the NGDC catalogue from 2009 to 2016 and 184 quiet (no bursts) spectrum images before and after the type I bursts. As a result, our model successfully detected 79 events (93%) of type II events. This results demonstrates, for the first time, that the 1-D CNN algorithm is useful for detecting type II bursts.

Investigation of the nature and parameters of wave processes in the flare region of the solar atmosphere

The paper presents the results of simultaneous observations of fluctuations of the integral radio emission flux in the solar flare region generated in the transition layer ( = 10.7 cm) and the lower corona ( = 27.8 cm). The observations were carried out with the RT-12 radio telescope of the Institute of the Ionosphere of the Republic of Kazakhstan on June 7, 2011, October 25, 2013, and February 20, 2014. By measuring the time profiles between quasi-periodic radio emission intensity fluctuations, it was found that for periods of 20350 seconds, propagation of perturbations from the chromosphere to the corona is always observed with a time delay of 20100 seconds. Using a number of models for the active Sun, estimates were made of the propagation velocities of disturbances of 2002500 km/s and the ranges of the detected periods. It is shown that they correspond to the speed of Alfven and fast magnetoacoustic waves. Thus, we conclude that in our study, the quasi-periodicity in the flare region of the Sun is associated with MHD oscillations.

Electron Cyclotron Maser Emission in Solar Radio Bursts

Radio bursts are ubiquitous in the cosmic plasma. Solar radio emission mainly comes from the outer atmosphere of the sun. It is an induced radiation phenomenon generated by the interaction between energetic electrons and solar atmospheric plasma. Different dynamic spectra of solar radio bursts (SRBs) contain physical information of the plasma structure and state in the radiation source region. Therefore, the radiative mechanism of radio bursts has always been the object of research. There are two kinds of coherent radiation mechanisms related to solar radio bursts: one is the plasma radiation mechanism based on electron Langmuir frequency; the other is the electron cyclotron maser (ECM) radiation mechanism based on the electron cyclotron frequency. Although these two radiation mechanisms were proposed almost at the same time, based on the understanding of the coronal environment and the ECM mechanism at that time, the ECM radiation mechanism did encounter some difficulties in explaining SRBs. Until 1979, Wu & Lee introduced the relativistic effect and used the ECM radiation to explain the earth's Auroral Kilometric Radiation (AKR). Since then, the ECM emission has attracted wide attention. Considering some difficulties in applying the ECM emission mechanism to SRBs, we proposed a series of modified models in recent years. Firstly, the cutoff in the energy spectrum of the power-law electrons can effectively drive the ECM instability without relying on the anisotropic distribution of electron velocity. Secondly, considering the influence of Alfvn wave perturbations which are prevalent in space and celestial plasmas, a self-consistent ECM emission mechanism excited by energetic electron beams is developed. On this basis, this paper summarizes the application of the ECM emission mechanism in traditional SRB phenomena from type I to V and microwave SRBs in recent years.

The Calibration of the 35-40 GHz Solar Radio Spectrometer with the New Moon and a Noise Source

Calibrating solar radio flux has always been a concern in the solar community. Previously, fluxes were calibrated by matching load or the new Moon for relative calibration, and at times with the assistance of other stations' data. Moreover, the frequency coverage seldom exceeded 26 GHz. This paper reports the upgraded and calibrated Chashan Broadband Solar millimeter spectrometer (CBS) working from 35 to 40 GHz at the Chashan Solar Observatory (CSO). Initially, the calibration of the solar radiation brightness temperature is accomplished using the new Moon as the definitive source. Subsequently, the 35-40 GHz standard flux is achieved by establishing the correlation between the solar radio flux, brightness temperature, and frequency. Finally, the calibration of the solar radio flux is implemented by utilizing a constant temperature- controlled noise source as a reference. The calibration in 2023 February and March reveals that the solar brightness temperature is 11,636 K at 37.25 GHz with a standard deviation (STD) of 652 K. The solar radio flux's intensity is ~3000-4000 solar flux units (SFU) in the range of 35-40 GHz with a consistency bias of 5.3%. The system sensitivity is about ~5-8 SFU by a rough evaluation, a noise factor of about 200 K, and the coefficient of variation of the system transmission slope of 6.5% @ 12 hr at 37.25 GHz. It is expected that the upgraded CBS will capture more activity during the upcoming solar cycle.

An Anisotropic Density Turbulence Model from the Sun to 1 au Derived from Radio Observations

Solar radio bursts are strongly affected by radio-wave scattering on density inhomogeneities, changing their observed time characteristics, sizes, and positions. The same turbulence causes angular broadening and scintillation of galactic and extragalactic compact radio sources observed through the solar atmosphere. Using large-scale simulations of radio-wave transport, the characteristics of anisotropic density turbulence from 0.1 R to 1 au are explored. For the first time, a profile of heliospheric density fluctuations is deduced that accounts for the properties of extrasolar radio sources, solar radio bursts, and in situ density fluctuation measurements in the solar wind at 1 au. The radial profile of the spectrum-weighted mean wavenumber of density fluctuations (a quantity proportional to the scattering rate of radio waves) is found to have a broad maximum at around (4-7) R , where the slow solar wind becomes supersonic. The level of density fluctuations at the inner scale (which is consistent with the proton resonance scale) decreases with heliocentric distance as $\langle \delta {{n}_{i}}^{2}\rangle (r)\simeq 2\times {10}^{7}\,{\left(r/{R}_{\odot }-1\right)}^{-3.7}$ cm^-6. Due to scattering, the apparent positions of solar burst sources observed at frequencies between 0.1 and 300 MHz are computed to be essentially cospatial and to have comparable sizes, for both fundamental and harmonic emission. Anisotropic scattering is found to account for the shortest solar radio burst decay times observed, and the required wavenumber anisotropy is q /q = 0.25-0.4, depending on whether fundamental or harmonic emission is involved. The deduced radio-wave scattering rate paves the way to quantify intrinsic solar radio burst characteristics.

X-Ray Flares to Energetic (E > 10 MeV) Particle Events

Energetic particle environments are an important factor for the viability of life on exoplanets surrounding flare stars. In the heliosphere, large gradual solar energetic (E > 10 MeV) particle (SEP) events are produced by shocks from fast coronal mass ejections (CMEs). Extensive observations of solar X-ray flares, CMEs, and SEP events can provide guidance for flare star models of stellar energetic particle (StEP) events, for which stellar flares, but only rarely the associated CMEs, are observed. Comparing an extensive list of peak fluxes, timescales, and peak temperatures of 585 M3.0 solar X-ray flares with the occurrence of associated SEP events of peak flux Ip > 1.4 proton flux units, enhanced with proxy decametric-hectometric type II radio bursts, we determine guidelines for StEP event outcomes, given only stellar X-ray flare inputs. Longer timescales and lower peak temperatures of X-ray flares with a given peak X-ray flux Fp are more favorable for occurrence of associated SEP/StEP events, which, however, are only a minority of all solar flare outcomes. Most solar flares do not result in SEP events, invalidating scaling laws between stellar flares, CMEs, and StEP events. We discuss recent observations and models of the flare-CME relationship and suggest that StEP intensities Ip may often be overestimated.

Characteristics of the Accelerated Electrons Moving along the Loop Derived from Cyclical Microwave Brightenings at the Footpoints

Many particles are accelerated during solar flares. To understand the acceleration and propagation processes of electrons, we require the pitch-angle distributions of the particles. The pitch angle of accelerated electrons has been estimated from the propagation velocity of a nonthermal microwave source archived in Nobeyama Radioheliograph data. We analyzed a flare event (an M-class flare on 2014 October 22) showing cyclical microwave brightenings at the two footpoint regions. Assuming that the brightenings were caused by the accelerated electrons, we approximated the velocity parallel to the magnetic field of the accelerated electrons as ~7.7 10⁴ and 9.0 10⁴ km s ^-1. The estimated pitch angle of the accelerated electrons is 69-80 and the size of the loss cone at the footpoint (estimated from the magnetic field strength in the nonlinear force-free field model) is approximately 43. Most of the accelerated electrons could be reflected at the footpoint region. This feature can be interpreted as brightenings produced by bouncing motion of the accelerated electrons.

A novel short-term radio flux trend prediction model based on deep learning

Solar radio flux is an important indicator of solar activity and solar UV burst. Accurate prediction of solar radio flux plays a crucial role in preventing and mitigating the impact of solar activity on human productivity. We propose a novel approach for the first time to predict short-term radio flux trends using a bidirectional long short-term memory (BLSTM) network. This approach aims to address the unique characteristics of temporality and nonlinearity observed in solar radio flux data. Our model takes into account various frequency characteristics that impact radio flux. This allows it to learn temporal patterns within the data, ultimately enabling accurate predictions of radio flux for the next 30 minutes. The proposed method is experimentally applied to the radio flux dataset of the US Radio Solar Telescope Network (RSTN). The results show that, in most frequency bands, the BLSTM model exhibits superior prediction accuracy and greater sensitivity to peak responses compared to the LSTM model, LSTM-Attention (LSTM-A) model, BLSTM-Attention (BLSTM-A) model, and persistence model (PM). Consequently, the BLSTM model is better equipped to accurately forecast changes in radio flux for the next 30 minutes.

Solar source longitudinal dependence of SEPs and their association with solar flares and radio-loud CMEs

In this article we examine the occurrence probability of 104 major (Ip, at energies > 10 MeV) solar energetic particle events (SEPs) and their peak intensity dependence on source longitude and on the characteristics of solar flares/radio-loud (RL) coronal mass ejections (CMEs) during the period November 1997December 2014. We classified them into three sets of events based on the source longitude (L) of the associated solar flares: i) eastern side events (30E < L 90E), ii) disk center (30E L 30W), and iii) western side events (30W < L 90W). On average, the mean rise time and duration of SEPs are significantly larger for eastern side events (2168 min and 4.16 days, respectively) than the disk center (1338 min and 2.65 days, respectively) and western side events (662 min and 2.42 days, respectively). The mean peak intensity of SEPs from the disk center (10^3.56 pfu) is found to be greater than that of SEPs from the western (10^2.88 pfu) and eastern (10^2.53 pfu) sides, respectively. The western side events (54%) have significantly higher occurrence probability than the disk center (34%) and eastern side (12%). While there is no significant difference in most of the properties of solar flares and RL CMEs, the eastern side associated RL CMEs are highly decelerated (29.37 m s²) than the disk center (17.44 m s²) and western side (9.09 m s²) events. The relationship between peak intensity of SEPs and peak flux of solar flares shows that the correlation coefficients (cc) decrease from the eastern side to the western side: 0.65 (eastern), 0.51 (disk), and 0.35 (western). Interestingly, we found that there is a good correlation between the peak intensity of major SEPs and the speed of the CMEs (cc=0.75) for disk center. From this study, we have concluded that gradualness depends on source longitude and increases from western to eastern side. It is also inferred that the relationship of peak intensity of SEP events with solar flare flux/CME speed is strongly dependent on source longitude.

Understanding the radio emission from Eridani

Some solar-type stars are known to present faint, time-variable radio continuum emission whose nature is not clearly established. We report on Jansky Very Large Array observations of the nearby star Eridani at 10.0 and 33.0 GHz. We find that this star has flux density variations on scales down to days, hours, and minutes. On 2020 April 15 it exhibited a radio pulse at 10.0 GHz with a total duration of about 20 min and a peak four times larger than the plateau of 40 Jy present in that epoch. We were able to model the time behavior of this radio pulse in terms of the radiation from shocks ramming into the stellar wind. Such shocks can be produced by the wind interaction of violently expanding gas heated suddenly by energetic electrons from a stellar flare, similar to the observed solar flares. Because of the large temperature needed in the working surface to produce the observed emission, this has to be nonthermal. It could be gyrosynchrotron or synchrotron emission. Unfortunately, the spectral index or polarization measurements from the radio pulse do not have a high enough signal-to-noise ratio to allow us to determine its nature.

ARTEMIS-IV/JLS and NRH Observations

Radio bursts provide important diagnostics of energetic phenomena of the Sun. In particular, bursts in decimetric and metric wavelengths probe the physical conditions and the energy release processes in the low corona as well as their association with heliospheric phenomena. The advent of spectral radio data with high time and high frequency resolution has provided a wealth of information on phenomena of short duration and narrow bandwidth. Of particular value are spectral data combined with imaging observations at specific frequencies. In this work we briefly review the results of a series of observations comprised from high-sensitivity, low-noise dynamic spectra obtained with the acousto- optic analyzer (SAO) of the ARTEMIS-IV/JLS solar radiospectrograph, in conjunction with high time-resolution images from the Nanay Radioheliograph (NRH). Our studies include fine structures embedded in type-IV burst continua (mostly narrow-band "spikes" and intermediate drift "fiber" bursts) and spike-like structures detected near the front of type-II bursts. The implications of the observational results to theoretical models are discussed.

Release Episodes of Electrons and Protons in Solar Energetic Particle Events

We analyzed a sample of 21 solar energetic particle (SEP) events with clear signatures in both near-relativistic electrons and high-energy protons spanning over 2.5 solar cycles from 1997 to 2016. We employed velocity dispersion analysis (VDA) for protons and fractional VDA (FVDA) for electrons, as well as time shifting analysis (TSA) in order to identify the solar release times (SRTs) of the electrons. We found that, for the majority of the events (62%), a simultaneous release was observed, while, for 14% of the events, electrons were released later than protons (i.e., delayed electrons); for 24% of the events, the opposite result was found (i.e., delayed protons). We found that the path length (L) traveled by the protons and electrons was not related to the aforementioned categorization. Moreover, we show that, in the case of simultaneous SEP events, protons and electrons are being released in close connection to type III and type II bursts, while the opposite is the case for delayed events. In addition, we demonstrate that, for the simultaneous events, both the proton and the electron release are established in heights < 5RS and that, especially for the well-connected simultaneous events, there is a co-occurrence of the type II burst with the release time of the particles.

Statistical analysis of microflares as observed by the 48 GHz spectropolarimeter

Radio observations of weak events are one of the promising methods for studying energy release and non-thermal processes in the solar corona. The development of instrumental capabilities allows for radio observations of weak transient coronal events, such as quasi-stationary brightenings and weak flares of X-ray class B and below, which were previously inaccessible for analysis. We have measured the spectral parameters of microwave radiation for thirty weak solar flares with X-ray classes ranging from A to C1.5, using observations from the Badary Broadband Microwave Spectropolarimeter (BBMS). The spectra indicate that plasma heating is caused by the appearance of non-thermal electron fluxes, which can be detected by bursts of microwave radiation, predominantly with an amplitude ~56 solar flux units (SFU) at 45 GHz frequencies. One solar flux unit (SFU) of radio emission is equal to 1022 W/(mHz). The range of low-frequency spectrum growth indices f varies widely from =0.3 to 15. The distribution of high-frequency decay indices is similar to the distributions of regular flares. One of the explanations for the appearance of large f values is the Razin effect, which can influence the shape of the gyrosynchrotron spectrum during the generation of bursts in dense plasma under relatively weak magnetic fields. We have detected two events in which the appearance of non- thermal electrons led to the generation of narrowband bursts at frequencies near the double plasma frequency. SRH test trials have shown the potential for measuring the structure of flare sources with fluxes of the order of 1 SFU, indicating the high diagnostic potential of the radioheliograph for detecting acceleration processes in weak flare events and their localization in active regions.

applications and comparison

Solar activity affects the heliosphere in different ways. Variations in particles and radiation that impact the Earth's atmosphere, climate, and human activities often in disruptive ways. Consequently, the ability to forecast solar activity across different temporal scales is gaining increasing significance. In this study, we present predictions for solar cycle 25 of three solar activity indicators: the core-to-wing ratio of Mg II at 280 nm, the solar radio flux at 10.7 cmwidely recognized proxies for solar UV emissionand the total solar irradiance, a natural driver of Earth's climate. Our predictions show a very good agreement with measurements of these activity indicators acquired during the ascending phase of solar cycle 25, representing the most recent data available at the time of writing.

Solar radio observations in Trieste contributed to avert a nuclear war in 1967

Solar Radio Astronomy was started in Trieste by Margherita Hack in the second half of the Sixties as an innovative and young field of research. After the declassification in 2016, the science community could learn that the solar radio astronomical observations at 239 MHz carried out in May 1967 at the Trieste Astronomical Observatory helped to avert a nuclear war. This was an ante litteram SpaceWeather application relevant to the importance of Solar Radio Weather monitoring to detect potential radio frequency interferences with radio communications. In this work, we describe the solar radio event that was about to start a nuclear war by emphasising the role played by the Trieste radio observations in contributing to the correct scientific interpretation of the strong radio interferences su ered by the US Ballistic Missile EarlyWarning System radars in 1967, i.e., that they were of natural origin and not due to a malicious action of an adversary. This event marked the beginning of Solar Radio Weather operations in Trieste thanks to the open mind of Margherita Hack, and made it possible for the solar radio emission monitoring in Trieste to gain the scientific and applied role recognised worldwide.

The Effect of Solar Flares on HF Radio Communications over Turkey

This study investigates the effect of solar flares on absorption of high frequency (HF) radio signals over Turkey. For this purpose, the highest affected frequency (HAF) values by 1 dB absorption due to solar Xray flux over Turkey were analyzed for different phases of solar flare, different local times (LT), different solar flare classes and different days. The HAF and HAF values were calculated from an empirical model using X-ray flux data with 1-min resolution measured by the Geostationary Operational Environmental Satellite-15 (GOES-15) and solar zenith angle data. The increase in X-ray flux density during the ascending phase of the solar flare causes a sudden and large increase in HAF values. During this phase of flare, the HAF has a logarithmic relationship with X-ray flux values. The HAF reaches its maximum values at the solar flare peak. During the descending phase of solar flare, the HAF values gradually decrease as X-ray flux density decrease. The local time has a significant effect on HF absorption. The greatest increase in HAF values occurs around noon. Comparisons between solar flare classes show that the HAF values increases significantly as the solar flare density increases. For different days of year, the value of HAF increases with decreasing solar zenith angles and the mean HAF has a linear relationship with the values of mean solar zenith angle. The results of this study are important because it is the first attempt to examine the effect of solar flares on HF absorption over Turkey.

The Flare Emission of the May 4, 2022 Event and Its Millimeter Component

Based on observations at the RT-7.5 radio telescope of the Bauman Moscow State Technical University at a wavelength of 3.2 mm (93 GHz), along with other ground-based and space instruments (Siberian Radioheliograph, Solar Dynamics Observatory (SDO), Metshovi Radio Observatory), the origin of millimeter radiation from X-ray class M 5.7 SOL2022-05-04T08:45 solar flare was investigated. An analysis of the time profiles of radiation in the X-ray and radio ranges showed that the millimeter emission source is not associated with hot (5 10⁵10⁷ K) coronal plasma. This is also evidenced by the estimate of the sub-THz flux from radiating hot plasma according to the AIA/SDO data, which turned out to be much less than the observed values. Indications of the development of thermal instability in flare ultraviolet loops were obtained. The relationship between the millimeter emission of the flare and the heat source in the solar chromosphere has been substantiated.

A New 6-15 GHz Solar Radio Observation System

In this study, we have developed a centimeter-band solar radio telescope covering the 6-15 GHz frequency band. The radio telescope has the outstanding advantages of a large instantaneous sampling bandwidth and wide frequency coverage. As a new solar radio telescope, its time resolution reaches a very high level of 0.26 ms at a frequency resolution of 3 MHz, which is very conducive to observing the fine structure of radio burst signals. In terms of the structure design, the system employs a 3 m diameter parabolic antenna to receive solar radio signals. The antenna has high gain and good directivity, and the pointing accuracy reaches 0.02, which ensures the ability to accurately track the Sun in real time. In the analog signal processing module, the combination of radio frequency direct acquisition and down conversion is used to reduce the interference caused by multiple spectrum shifts. Regarding the digital receiver, a digital receiving module with high sampling rate and acquisition resolution is used for data acquisition and processing, which ensures that the observation system can obtain observation data with high time and frequency resolutions and real-time data processing. During the trial operation of the system, solar radio bursts have been observed many times, and these observations have been supported by similar international observation equipment. According to a data comparison, the data obtained by our observation system are more precise. At present, equipment calibration methods are being improved and constructed to obtain more accurate observation data.

Fundamental-Harmonic Pairs of Interplanetary Type III Radio Bursts

Type III radio bursts are not only the most intense but also the most frequently observed solar radio bursts. However, a number of their defining features remain poorly understood. Observational limitations, such as a lack of sufficient spectral and temporal resolution, have hindered a full comprehension of the emission process, especially in the hectokilometric wavelengths. Of particular difficulty is the ability to detect the harmonics of type III radio bursts. Here we report the first detailed observations of type III fundamental-harmonic pairs in the hectokilometric wavelengths, observed by the Parker Solar Probe. We present a statistical analysis of the spectral characteristics and polarization measurements of the fundamental-harmonic pairs. Additionally, we quantify various characteristics of the fundamental- harmonic pairs, such as the time delay and time profile asymmetry. Our report concludes that fundamental-harmonic pairs constitute a majority of all type III radio bursts observed during close encounters when the probe is in close proximity to the source region and propagation effects are less pronounced.

Cold Solar Flares. I. Microwave Domain

We identify a set of ~100 "cold" solar flares and perform a statistical analysis of them in the microwave range. Cold flares are characterized by a weak thermal response relative to nonthermal emission. This work is a follow-up of a previous statistical study of cold flares, which focused on hard X-ray emission to quantify the flare nonthermal component. Here, we focus on the microwave emission. The thermal response is evaluated by the soft X-ray emission measured by the GOES X-ray sensors. We obtain spectral parameters of the flare gyrosynchrotron emission and reveal patterns of their temporal evolution. The main results of the previous statistical study are confirmed: as compared to a "mean" flare, the cold flares have shorter durations, higher spectral peak frequencies, and harder spectral indices above the spectral peak. Nonetheless, there are some cold flares with moderate and low peak frequencies. In the majority of cold flares, we find evidence of the Razin effect in the microwave spectra, indicative of rather dense flaring loops. We discuss the results in the context of the electron acceleration efficiency.

Suprathermal Electron Transport and Electron Beam Formation in the Solar Corona

Electron beams that are commonly observed in the corona were discovered to be associated with solar flares. These "coronal" electron beams are found 300 Mm above the acceleration region and have velocities ranging from 0.1c up to 0.6c. However, the mechanism for producing these beams remains unclear. In this paper, we use kinetic transport theory to investigate how isotropic suprathermal energetic electrons escaping from the acceleration region of flares are transported upwardly along the magnetic field lines of flares to develop coronal electron beams. We find that magnetic focusing can suppress the diffusion of Coulomb collisions and background turbulence and sharply collimate the suprathermal electron distribution into beams with the observed velocity within the observed distance. A higher bulk velocity is produced if energetic electrons have harder energy spectra or travel along a more rapidly expanding coronal magnetic field. By modeling the observed velocity and location distributions of coronal electron beams, we predict that the temperature of acceleration regions ranges from 5 10⁶ to 2 10⁷ K. Our model also indicates that the acceleration region may have a boundary where the temperature abruptly decreases so that the electron beam velocity can become more than triple (even up to 10 times) the background thermal velocity and produce the coronal type III radio bursts.

An Unsupervised Machine Learning-based Algorithm for Detecting Weak Impulsive Narrowband Quiet Sun Emissions and Characterizing Their Morphology

The solar corona is extremely dynamic. Every leap in observational capabilities has been accompanied by unexpected revelations of complex dynamic processes. The ever more sensitive instruments now allow us to probe events with increasingly weaker energetics. A recent leap in the low-frequency radio solar imaging ability has led to the discovery of a new class of emissions, namely weak impulsive narrowband quiet Sun emissions (WINQSEs). They are hypothesized to be the radio signatures of coronal nanoflares and could potentially have a bearing on the long standing coronal heating problem. In view of the significance of this discovery, this work has been followed up by multiple independent studies. These include detecting WINQSEs in multiple data sets, using independent detection techniques and software pipelines, and looking for their counterparts at other wavelengths. This work focuses on investigating morphological properties of WINQSEs and also improves upon the methodology used for detecting WINQSEs in earlier works. We present a machine learning-based algorithm to detect WINQSEs, classify them based on their morphology, and model the isolated ones using 2D Gaussians. We subject multiple data sets to this algorithm to test its veracity. Interestingly, despite the expectations of their arising from intrinsically compact sources, WINQSEs tend to be resolved in our observations. We propose that this angular broadening arises due to coronal scattering. Hence, WINQSEs can provide ubiquitous and ever- present diagnostic of coronal scattering (and, in turn, coronal turbulence) in the quiet Sun regions, which has not been possible until date.

The Solar Origin of an In Situ Type III Radio Burst Event

Solar type III radio bursts are generated by beams of energetic electrons that travel along open magnetic field lines through the corona and into interplanetary space. However, understanding the source of these electrons and how they escape into interplanetary space remains an outstanding topic. Here we report multi-instrument, multiperspective observations of an interplanetary type III radio burst event shortly after the second perihelion of the Parker Solar Probe (PSP). This event was associated with a solar jet that produced an impulsive microwave burst event recorded by the Expanded Owens Valley Solar Array. The type III burst event also coincided with the detection of enhanced in situ energetic electrons recorded by both PSP at 0.37 au and WIND at 1 au, which were located very closely on the Parker spiral longitudinally. The close timing association and magnetic connectivity suggest that the in situ energetic electrons originated from the jet's magnetic reconnection region. Intriguingly, microwave imaging spectroscopy results suggest that the escaping energetic electrons were injected into a large opening angle of about 90, which is at least nine times broader than the apparent width of the jet spire. Our findings provide an interpretation for the previously reported, longitudinally broad spatial distribution of flare locations associated with prompt energetic electron events and have important implications for understanding the origin and distribution of energetic electrons in interplanetary space.

Turbulence in Sources of Decimetric Flare Continua

Decimetric continua are commonly observed during long-lasting solar flares. Their frequency boundaries vary with time. We studied frequency boundary variations using the power spectrum analysis. Analyzing five decimetric continua, we found that their power spectra have a power-law form with the power-law index close to the Kolmogorov turbulence index -5/3. The same power index was also found in the power spectra of radio flux variations at frequencies in the range of the frequency boundary variations. Moreover, these frequency boundary variations were highly correlated with the radio flux ones. We interpret these results to be due to turbulent density variations in the reconnection plasma outflow to the termination shock formed above flare loops. In three cases of decimetric continua, we estimated the level of the plasma density turbulence to be 7.6 - 11.2% of the mean plasma density. We think that the analysis of variations of decimetric continua can be used in studies of the plasma turbulence in solar flares.

Decametric Solar Radio Spectrometer Based on 4-element Beamforming Array and Initial Observational Results

The dynamic spectral observation at decametric wavelength is important to study solar radio physics and space weather. However, the observing system is difficult to observe with high sensitivity at this band due to the fact that the system temperature is dominated by the sky background noise and the antenna is difficult to design with high gain. An effective solution to improve the sensitivity is constructing an antenna array based on the beamforming method. Accordingly, we develop a decametric solar radio spectrometer system based on a 4-element beamforming array. The system consists of four antennas, an 8-channel analog receiver and a digital receiver. We use the true time delay to implement the beamformer and the classical FFT method to perform spectrum analysis in the digital receiver. Operating at a frequency range of 25-65 MHz with dual-circular polarizations, the system provides high resolution dynamic spectrum with spectral resolution of ~12 kHz and temporal resolution of ~5.3 ms (typical). Tens of solar radio bursts have been observed successfully during the period of the trial observation, demonstrating the system's ability to detect fine structures with high spectral and temporal resolution. In this article, we present the design, implementation, and initial observational results of the decametric solar radio spectrometer system in detail.

Kappa-distributed electrons

In space plasmas, electron populations exhibit non-equilibrium velocity distributions with high-energy tails that are reproduced by the Kappa power-laws and contrast with the Maxwellian distributions often used in theoretical and numerical analyses. In this work, we investigate typical electron beam-plasma systems and show the influence of Kappa tails on the linear dispersion and stability spectra of Langmuir-beam waves. The most common scenarios invoke instabilities of Langmuir waves at the origin of radio emissions in solar flares and interplanetary shocks. However, the parametric domain of these instabilities is narrow (i.e., energetic beams but with very low density, n b / n e 10 - 3), making their analytical and numerical characterization not straightforward, while the approximations used may lead to inconclusive results. Here, we provide exact numerical solutions of the Langmuir-beam mode, which distinguish from the classical ones (unaffected by the beam), and also from electron beam modes destabilized by more energetic and/or denser beams. Langmuir-beam solutions are only slightly modified by the Kappa distribution of the beam component, due to its very low density. However, if the main (core) population is Kappa distributed, the instability of the Langmuir-beam mode is strongly inhibited, if not suppressed. New analytical solutions are derived taking into account the more or less resonant involvement of the electron core and beam populations. As a result, the analytical solutions show an improved match with the exact solutions, making them applicable in advanced modeling of weak (weakly nonlinear) turbulence.

Operational prediction of solar flares using a transformer-based framework

Solar flares are explosions on the Sun. They happen when energy stored in magnetic fields around solar active regions (ARs) is suddenly released. Solar flares and accompanied coronal mass ejections are sources of space weather, which negatively affects a variety of technologies at or near Earth, ranging from blocking high-frequency radio waves used for radio communication to degrading power grid operations. Monitoring and providing early and accurate prediction of solar flares is therefore crucial for preparedness and disaster risk management. In this article, we present a transformer-based framework, named SolarFlareNet, for predicting whether an AR would produce a -class flare within the next 24 to 72 h. We consider three classes, namely the M5.0 class, the M class and the C class, and build three transformers separately, each corresponding to a class. Each transformer is used to make predictions of its corresponding -class flares. The crux of our approach is to model data samples in an AR as time series and to use transformers to capture the temporal dynamics of the data samples. Each data sample consists of magnetic parameters taken from Space-weather HMI Active Region Patches (SHARP) and related data products. We survey flare events that occurred from May 2010 to December 2022 using the Geostationary Operational Environmental Satellite X-ray flare catalogs provided by the National Centers for Environmental Information (NCEI), and build a database of flares with identified ARs in the NCEI flare catalogs. This flare database is used to construct labels of the data samples suitable for machine learning. We further extend the deterministic approach to a calibration-based probabilistic forecasting method. The SolarFlareNet system is fully operational and is capable of making near real-time predictions of solar flares on the Web.

Dependence of the Annual Asymmetry Local Index for the NmF2 Median on Solar Activity

Based on the medians of the F 2-layer maximum electron concentration NmF 2 for two ionospheric stations, Boulder and Hobart, in 19632013, an analysis of the dependence of the annual asymmetry local index R at noon on solar activity has been performed, where the R index is the January/July ratio of the summary NmF2 concentration for this pair of stations. Solar activity indices averaged over 81 days are used: F_obs is the solar radio emission flux at a wavelength of 10.7 cm measured by ground-based radio telescopes and F_adj is the value of F_obs reduced to a fixed distance from the Sun of one astronomical unit. It is found that the regression equations that manifest the NmF2 medians dependence on F_obs make it possible to obtain the annual asymmetry index R for a fixed value of F_obs with allowance for a substitution of F_obs by cF_obs in these regression equations, the c coefficient is equal to 1.03 and 0.97 for January and July, respectively. The c = 1 case corresponds to neglecting the annual asymmetry in the F_obs index due to the ellipticity of the Earth's orbit. For the c = 1 version, the R index increases with a solar activity increase from 1.2 under low activity up to almost 1.4 under high activity. An additional allowance for the annual asymmetry in F_obs leads to an increase in the R index by approximately 0.1 almost independently of the solar activity level. Apparently, this conclusion is being drawn for the first time. The F_adj index also makes it possible to obtain a correct estimate of the R index, because the annual asymmetry in the solar radiation flux is indirectly taken into account via the experimental values of NmF 2.

Evidences for Two-Stage Magnetic Reconnection during the Impulsive Phase

We present an analysis of the pre-limb eruptive X4.9 solar flare on February 25, 2014, by means of which we confirm a hypothesis of the two- stage energy release corresponding to two magnetic reconnection regimes in the flare impulsive phase. This flare is selected, firstly, because of its morphological peculiarities suggesting the presence of the two energy release stages. Secondly, the flare was very suitably located near the solar limb and it was well-observed by many instruments. We performed an analysis of multiwavelength observational data of this flare region to find a connection between changes of the photospheric magnetic field, morphology of hard and soft X-ray sources, dynamics of the photospheric optical emission sources, metric radio bursts, and kinematics of an eruptive structure. The simultaneous usage of the line- of-sight and vector Helioseismic Magnetic Imager (HMI) magnetograms allowed us to trace magnetic field changes during the flare impulsive phase with high temporal resolution. HMI filtergrams allowed to trace displacement of the photospheric emission sources, associated with the magnetic reconnection, with very high temporal resolution up to 2 s. Using all observational results, we argue that the found flare stages are characterized by the following magnetic reconnection regimes. The first stage is predominantly characterized by the three-dimensional zipping reconnection in the strong sheared magnetic field assuming the tether-cutting geometry. The second stage corresponds to the so-called "standard" model of eruptive flares with the quasi-two-dimensional reconnection below the eruptive flux-rope. All observational peculiarities of these two stages are discussed in details.

Observando la cromosfera solar en el infrarrojo

The solar chromosphere has historically been studied from spectral lines in the visible and UV, notably H, CaII, MgII and Ly. Observations at long UV wavelengths (304, 1600 and 1700 ) from space have been recently added. However, the chromosphere can also be studied in the infrared (IR), both in the continuum as in the lines. Studies in this spectral band, which by definition extends from 1 m to 1 mm, are scarce and recent, and its advantages having been little explored. In this work we present a review of what has been done and detail how much can be done with ground-based instruments. Argentina has a set of unique telescopes for the observation of the chromosphere, some with more than 20 years of operation and in process of renovation, others recently installed and still some in development. The panorama is very encouraging and allows to anticipate a strong international cooperation with other ground and space facilities.

Acceleration, Transport, Heating, and Energy Budget

Solar flares are driven by the release of free magnetic energy and its conversion to other forms of energy-kinetic, thermal, and nonthermal. Quantification of partitions between these energy components and their evolution is needed to understand the solar flare phenomenon including nonthermal particle acceleration, transport, and escape as well as the thermal plasma heating and cooling. The challenge of remote-sensing diagnostics is that the data are taken with finite spatial resolution and suffer from line-of-sight (LOS) ambiguity including cases when different flaring loops overlap and project one over the other. Here, we address this challenge by devising a data-constrained evolving 3D model of a multiloop SOL2014-02-16T064620 solar flare of GOES class C1.5. Specifically, we employed a 3D magnetic model validated earlier for a single time frame and extended it to cover the entire flare evolution. For each time frame we adjusted the distributions of the thermal plasma and nonthermal electrons in the model so that the observables synthesized from the model matched the observations. Once the evolving model had been validated in this way, we computed and investigated the evolving energy components and other relevant parameters by integrating over the model volume. This approach removes the LOS ambiguity and permits us to disentangle contributions from the overlapping loops. It reveals new facets of electron acceleration and transport as well as of the heating and cooling of the flare plasma in 3D. We find signatures of substantial direct heating of the flare plasma not associated with the energy loss of nonthermal electrons.

Fine Structures of Radio Bursts from Flare Star AD Leo with FAST Observations

Radio bursts from nearby active M-dwarfs have been frequently reported and extensively studied in solar or planetary paradigms. Whereas, their substructures or fine structures remain rarely explored despite their potential significance in diagnosing the plasma and magnetic field properties of the star. Such studies in the past have been limited by the sensitivity of radio telescopes. Here we report the inspiring results from the high time-resolution observations of a known flare star AD Leo with the Five-hundred-meter Aperture Spherical radio Telescope. We detected many radio bursts in the 2 days of observations with fine structures in the form of numerous millisecond-scale sub-bursts. Sub- bursts on the first day display stripe-like shapes with nearly uniform frequency drift rates, which are possibly stellar analogs to Jovian S-bursts. Sub-bursts on the second day, however, reveal a different blob-like shape with random occurrence patterns and are akin to solar radio spikes. The new observational results suggest that the intense emission from AD Leo is driven by electron cyclotron maser instability, which may be related to stellar flares or interactions with a planetary companion.

Characterizing the Spectral Structure of Weak Impulsive Narrowband Quiet Sun Emissions

Weak Impulsive Narrowband Quiet Sun Emissions (WINQSEs) are a newly discovered class of radio emission from the solar corona. These emissions are characterized by their extremely impulsive, narrowband, and ubiquitous nature. We have systematically been working on their detailed characterization, including their strengths, morphologies, temporal characteristics, energies, etc. This work is the next step in this series and focuses on the spectral nature of WINQSEs. Given that their strength is only a few percent of the background solar emission, we have adopted an extremely conservative approach to reliably identify WINQSES. Only a handful of WINQSEs meet all of our stringent criteria. Their flux densities lie in the 20-50 Jy range and they have compact morphologies. For the first time, we estimate their bandwidths and find them to be less than 700 kHz, consistent with expectations based on earlier observations. Interestingly, we also find similarities between the spectral nature of WINQSEs and the solar radio spikes. This is consistent with our hypothesis that the WINQSEs are the weaker cousins of the type III radio bursts and are likely to be the low-frequency radio counterparts of the nanoflares, originally hypothesized as a possible explanation for coronal heating.

First results

The newly installed CALLISTO spectrometer, hosted by the Department of Space Environment, Institute of Basic and Applied Sciences- EJUST, commenced operation on August 14, 2021. The system contains a cross dipole long-wavelength array antenna with high sensitivity to monitor solar radio transients. Its antenna was strategically positioned and appeared to be in the center of the CALLISTO network of spectrometers. Moreover, in the northern section of Africa, the Egypt-Alexandria CALLISTO and ALGERIA-CRAAG stations are the only ones operating. There are no stations in the West African region, while stations in the eastern part of Africa are not working. Thus, Egypt- Alexandria station serves as a reference for other stations within the e-CALLISTO network. Despite the low solar activity, the instrument detected several solar radio bursts not limited to type II, type III, and type V. A vigorous case study was conducted on two selected radio burst events to validate the authenticity of the recorded events. Other solar radio stations at different geographical locations recorded all the radio bursts detected by the spectrometer. The case study included brief analyses that indicated a type II radio burst observed on October 09, 2021, between 06:30 and 07:00 UT, was associated with an M1.6 solar flare located at N18E08 within NOAA-AR 2882 and a CME with a shock front speed of 978 km/s. However, the type III radio burst is neither CME nor solar flare associated. These analyses examine the instrument's capacity to provide real-time solar radio transient data 24 h a day to mitigate the challenges of data gaps faced in the African continent. Hence, the instrument has become an integral part of space weather monitoring and forecasting over the region and other parts of the globe.

New Insights from Imaging Spectroscopy of Solar Radio Emission

Newly available high-resolution imaging of solar radio emission at many closely spaced frequencies and times provides new physical insight into the processes, structure, and dynamics of the solar atmosphere. The observational advances have spurred renewed interest in topics dating from the early days of solar radio astronomy and have led to considerable advances in our knowledge. Highlights of recent advances include the following: Quantitatively measuring the dynamic magnetic field strength, particle acceleration, and hot thermal plasma at the heart of solar flares and hinting at the processes that relate them; Resolving in space and time the energization and transport of electrons in a wide range of contexts; Mapping the magnetized thermal plasma structure of the solar chromosphere and corona over a substantial range of heights in active and quiet regions of the Sun.This review explains why solar radio imaging spectroscopy is so powerful, describes the body of recent results, and outlines the future work needed to fully realize its potential. The application of radio imaging spectroscopy to stars and planets is also briefly reviewed.

Modulation depth of the gyrosynchrotron emission as an identifier of fundamental sausage modes

Aims: We aim to study the intensity, the modulation depth, and the mean modulation depth of the gyrosynchrotron (GS) radiation as a function of the frequency and the line of sight (LOS) in fast sausage modes.
Methods: By solving the 2.5D magnetohydrodynamics (MHD) ideal equations of a straight coronal loop considering the chromosphere and with typical flaring plasma parameters we analyse the wavelet transform of the density and the GS emission for different radio frequencies and different spatial resolutions, given impulsive and general perturbations with energies in the microflare range.
Results: A wavelet analysis performed over the GS radiation emission showed that a fast fundamental sausage mode of 7 s with a first harmonic mode of 3 s developed, for all the initial energy perturbations used. For both the high spatial resolution (central pixel integration) and the low spatial resolution (entire loop integration), the larger the radio frequency, the larger the modulation depth. However, high- and low-resolution integrations differ in that the larger the LOS angle with respect to the loop axis, the larger and smaller the modulation depth, respectively.
Conclusions: Fast MHD modes triggered by instantaneous energy depositions of the order of a microflare energy are able to reproduce deep intensity modulation depths in radio emission as observed in solar events. As the trends of the GS emission previously obtained for a linear and forced oscillation remain present when analysing a more general context, considering the chromosphere and where the sausage mode is triggered by an impulsive, non-linear perturbation, it seems that the behaviour found can be used as observational identifiers of the presence of sausage modes with respect to other quasi-periodic pulsation features. It can be inferred from this that finite-amplitude sausage modes have the potential to generate the observed deep modulation depths.

A Case Study over Haikou Station

This paper introduces a novel technique that uses observation data from GNSS to estimate the ionospheric vertical total electron content (VTEC) using the Kriging-Kalman method. The technique provides a method to validate the accuracy of the Ionospheric VTEC analysis within the Equatorial Ionization anomaly region. The technique developed uses GNSS VTEC alongside solar parameters, such as solar radio flux (F10.7 cm), Disturbance Storm Time (Dst) and other data, and Long Short Term Memory (LSTM) Networks to predict the occurrence time of the ionospheric equatorial anomaly and ionospheric VTEC changes. The LSTM method was applied to GNSS data from Haikou Station. A comparison of this technique with the neural network (NN) model and International Reference Ionosphere model shows that the LSTM outperforms all of them at VTEC estimation and prediction. The results, which are based on the root mean square error (RMSE) between GNSS VTEC and GIM VTEC outside the equatorial anomaly region, was 1.42 TECU, and the results of GNSS VTEC and VTEC from Beidou geostationary orbit satellite, which lies inside the equatorial ionization anomaly region, was 1.92 TECU. The method developed can be used in VTEC prediction and estimation in real time space operations.

The investigation of a portable solar telescope with a high gain metamaterial antenna for F10.7 radio flux measurement

A high gain and high aperture efficiency metamaterial (MTM) antenna is applied to a solar telescope in this paper. First, a portable solar telescope including the MTM antenna and a receiving system is presented. Next, the theory of the MTM antenna is proposed and analyzed based on the ray-tracing model. The designed MTM antenna is composed of a dual circularly polarized Fabry-Prot resonant antenna (FPRA) and four phase correction metasurfaces (PCMs). The proposed PCMs act as the reflection surface and the phase correction surface at the same time. Every PCM consists of 2 18 optimized artificial magnetic conductor (AMC) units. To solve the parallel incidence and narrow bandwidth problems of AMC units, a nonuniform partially reflective surface is designed. Compared with traditional FPRA, the proposed MTM antenna has an increase in peak gain of 37.5% and an aperture efficiency of 11.4%. Then, a receiving system composed of the receiver, equatorial mount, data acquisition module, and display module is presented for solar radio signal processing. Finally, the designed MTM antenna and solar telescope are simulated and measured. A good agreement between the simulation and measurement is observed and can be used to verify this design.

Predictions of solar activity cycles 25 and 26 using non-linear autoregressive exogenous neural networks

This study presents new prediction models of the 11-yr solar activity cycles (SC) 25 and 26 based on multiple activity indicator parameters. The developed models are based on the use of non-linear autoregressive exogenous (NARX) neural network approach. The training period of the NARX model is from July 1749 to December 2019. The considered activity indicator parameters are the monthly sunspot number time series (SSN), the flare occurence frequency, the 10.7-cm solar radio flux, and the total solar irradiance (TSI). The neural network models are fed by these parameters independently and the prediction results are compared and verified. The obtained training, validation, and prediction results show that our models are accurate with an accuracy of about 90 per cent in the prediction of peak activity values. The current models produce the dual-peak maximum (Gnevyshev gap) very well. Based on the obtained results, the expected solar peaks in terms of SSN (monthly averaged smoothed) of the solar cycles 25 and 26 are R_SSN = 116.6 (February 2025) and R_SSN = 113.25 (October 2036), respectively. The expected time durations of SC 25 and SC 26 cycles are 9.2 and 11 yr, respectively. The activity levels of SC 25 and 26 are expected to be very close and similar to or weaker than SC 24. This suggests that these two cycles are at the minimum level of the Gleissberg cycle. A comparison with other reported studies shows that our results based on the NARX model are in good agreement.

Investigating the effect of large solar flares on the ionosphere based on novel Digisonde data comparing three different methods

Increased solar radiation during solar flare events can cause additional ionization and enhanced absorption of the electromagnetic (EM) waves in the ionosphere leading to partial or even total radio fade-outs. In this study, the ionospheric response to large solar flares has been investigated using the ionosonde data from Juliusruh (54.63 N, 13.37 E), Prhonice (49.98 N, 14.55 E) and San Vito (40.6 N, 17.8 E) Digisonde (DPS-4D) stations. We studied the effect of 13 intense (>C4.8) solar flares that occurred between 06:00 and 16:30 (UT, daytime LT = UT+1 h) from 04 to 10 September 2017 using three different methods. A novel method based on the amplitude data of the measured EM waves is used to calculate and investigate the relative absorption changes (compared to quiet period) occurring during the flares. The amplitude data are compared with the variation of the fmin parameter (fmin, the minimum measured frequency, it is considered as a qualitative proxy for the "non-deviative" radio wave absorption). Furthermore, the signal-to- noise ratio (SNR) measured by the Digisondes was used as well to quantify and characterize the fade-out events and the ionospheric absorption. In order to compare the three different methods, residuals have been defined for all parameters, which provide the percentage changes compared to the selected reference periods. Total and partial radio fade-outs, increased values (+0.4%318%) of the fmin parameter, and +20%1400% amplitude changes (measured at 2.5 and 4 MHz) were experienced during and after the investigated flares. Generally, the observed changes depended on the intensity, solar zenith angle and duration of the flare events. Although the three different methods have their own advantages/disadvantages and their limitations, the combination of them seems to be an efficient approach to monitor the ionospheric response to solar flares.

Research Progress of the Heliospheric Radio Emissions

The heliospheric radio emissions are the strongest radio emissions phenomenon in the solar system, with a radiation power of at least 10¹³ W, which can provide important physical information of high energy electron beam and magnetic plasma structure near the heliospheric boundary. Since the first detection by the Voyager spacecraft in 1983, those radio emissions have widely and continuously attracted much attention from researchers. There are generally two types of the heliospheric radio emissions: instantaneous or drifting emission with relatively high frequency, and continuous emission or non-drifting emission with relatively low frequency. Usually, both types of emissions start from about 2 kHz. For the drifting emission, it has the characteristic of drifting toward high frequency, the drifting rate is about 1-3 kHz/yr, the frequency range is 1.8-3.6 kHz, and the duration is about 100-300 days. For the non-drifting emission, it has no obvious frequency drift, the frequency range is 1.8-2.6 kHz, and the duration is about 3 yr. It is generally believed that the heliospheric radio emissions are related to shock. In this paper, the possible source region of the radio emissions, the emission mechanisms, and the source of shock related to the emissions are introduced. Furthermore, the existing scientific problems and the future perspectives on the research of heliospheric radio emissions are discussed.

Study of Solar Flares in Centimeter Range by Dynamical Methods

The article investigates oscillatory processes in solar flares based on observational data obtained by a 12-meter radio telescope at a frequency f=3 GHz belonging to the Ionosphere Institute of the Kazakhstan Republic. The results of a brief review of methods for processing non- stationary time series suggest that the requirement of adaptability is also important. It is shown that this possibility is provided by the method proposed by Norden Huang. For clarity, the method of implementing EMD (Empirical Mode Decomposition) - the method of decomposing signals into functions (modes) will be considered using the example of a digital signal array x(t). In order to obtain more realistic results, the type of series was determined using fractal analysis. It is shown that at this stage, for the reliability of obtaining quasi-periodic pulsations (QPPs), it is necessary to carry out visual control and compare the results obtained by different methods

A Two-element Interferometer for Millimeter-wave Solar Flare Observations

In this paper, we present the design and implementation of a two-element interferometer operating in the millimeter-wave band (39.5-40 GHz) for observing solar radio emissions through nulling interference. The system is composed of two 50 cm aperture Cassegrain antennas installed on a common equatorial mount, with a separation of 230 wavelengths. The cross-correlation of the received signals effectively cancels out the quiet solar component of the high flux density (~3000 sfu) that reduces the detection limit due to atmospheric fluctuations. The system performance is as follows: the noise factor of the analog front end in the observation band is less than 2.1 dB, system sensitivity is approximately 12.4 K (~34 sfu) with an integration time constant of 0.1 ms (default), the frequency resolution is 153 kHz, and the dynamic range is 30 dB. Through actual testing, the nulling interferometer observes a quiet Sun with a low level of output fluctuations (up to 50 sfu) and has a significantly lower radiation flux variability (up to 190 sfu) than an equivalent single-antenna system, even under thick cloud cover. As a result, this new design can effectively improve observation sensitivity by reducing the impact of atmospheric and system fluctuations during observation.

Data-constrained Solar Modeling with GX Simulator

To facilitate the study of solar flares and active regions, we have created a modeling framework, the freely distributed GX Simulator IDL package, that combines 3D magnetic and plasma structures with thermal and nonthermal models of the chromosphere, transition region, and corona. Its object-based modular architecture, which runs on Windows, Mac, and Unix/Linux platforms, offers the ability to either import 3D density and temperature distribution models, or to assign numerically defined coronal or chromospheric temperatures and densities, or their distributions, to each individual voxel. GX Simulator can apply parametric heating models involving average properties of the magnetic field lines crossing a given voxel, as well as compute and investigate the spatial and spectral properties of radio, (sub)millimeter, EUV, and X-ray emissions calculated from the model, and quantitatively compare them with observations. The package includes a fully automatic model production pipeline that, based on minimal users input, downloads the required SDO/HMI vector magnetic field data, performs potential or nonlinear force-free field extrapolations, populates the magnetic field skeleton with parameterized heated plasma coronal models that assume either steady-state or impulsive plasma heating, and generates non-LTE density and temperature distribution models of the chromosphere that are constrained by photospheric measurements. The standardized models produced by this pipeline may be further customized through specialized IDL scripts, or a set of interactive tools provided by the graphical user interface. Here, we describe the GX Simulator framework and its applications.

Morphology of Solar Type II Bursts Caused by Shock Propagation through Turbulent and Inhomogeneous Coronal Plasma

Type II solar bursts are radio signatures of shock waves in the solar corona driven by solar flares or coronal mass ejections (CMEs). Therefore, these bursts present complex spectral morphologies in solar dynamic spectra. Here, we report meter-decameter radio observations of a type II burst on 2014 July 25 made with the Ukrainian radio telescopes UTR-2 (8.25-33 MHz) and GURT (8.25-78 MHz). The burst demonstrates fundamental and harmonic components, band splitting, a herringbone structure, and a spectral break. These specific spectral features, observed jointly in a single type II burst, are rarely detected. To contribute to our understanding of such puzzling type II events, we carried out a detailed analysis of the recorded type II dynamic spectrum. In particular, the herringbone pattern has been exploited to study electron density turbulence in the solar corona. We calculated the power spectral densities of the flux variations in selected herringbones. The spectral index is in the range of = -1.69 to -2.00 with an average value of -1.897, which is slightly higher than the Kolmogorov spectral index of -5/3 for fully developed turbulence. We also recognized that the second type II burst consists of three drifting lanes. The lane onset times coincide with the spectral break in the first type II burst. We regard that the CME/shock passage through a streamer caused the spectral break and triggered the multilane type II radio emission. Thus, we support one of the proposed scenarios for type II burst occurrence as being the result of CME/shock-streamer interaction.

A 7 Day Multiwavelength Flare Campaign on AU Mic. I. High-time-resolution Light Curves and the Thermal Empirical Neupert Effect

We present light curves and flares from a 7 day, multiwavelength observational campaign of AU Mic, a young and active dM1e star with exoplanets and a debris disk. We report on 73 unique flares between the X-ray to optical data. We use high-time-resolution near-UV (NUV) photometry and soft X-ray (SXR) data from the X-ray Multi-Mirror Mission to study the empirical Neupert effect, which correlates the gradual and impulsive phase flaring emissions. We find that 65% (30 of 46) flares do not follow the Neupert effect, which is 3 times more excursions than seen in solar flares, and propose a four-part Neupert effect classification (Neupert, quasi-Neupert, non-Neupert types I and II) to explain the multiwavelength responses. While the SXR emission generally lags behind the NUV as expected from the chromospheric evaporation flare models, the Neupert effect is more prevalent in larger, more impulsive flares. Preliminary flaring rate analysis with X-ray and U-band data suggests that previously estimated energy ratios hold for a collection of flares observed over the same time period, but not necessarily for an individual, multiwavelength flare. These results imply that one model cannot explain all stellar flares and care should be taken when extrapolating between wavelength regimes. Future work will expand wavelength coverage using radio data to constrain the nonthermal empirical and theoretical Neupert effects to better refine models and bridge the gap between stellar and solar flare physics.

Type II radio bursts and their association with coronal mass ejections in solar cycles 23 and 24

Context. Meter-wavelength type II solar radio bursts are thought to be the signatures of shock-accelerated electrons in the corona. Studying these bursts can give information about the initial kinematics, dynamics, and energetics of coronal mass ejections (CMEs) in the absence of white-light observations.
Aims: We investigate the occurrence of type II bursts in solar cycles 23 and 24 and their association with CMEs. We also explore whether type II bursts might occur in the absence of a CME.
Methods: We performed a statistical analysis of type II bursts that occurred between 200 and 25 MHz in solar cycles 23 and 24 and determined the temporal association of these radio bursts with CMEs. We categorized the CMEs based on their linear speed and angular width and studied the distribution of type II bursts with fast (500 km s¹), slow (< 500 km s¹), wide (60), and narrow (< 60) CMEs. We explored the dependence of type II bursts occurrence on the phases of the solar cycle.
Results: Our analysis shows that during solar cycles 23 and 24, 768 and 435 type II bursts occurred, respectively. Of these, 79% were associated with CMEs in solar cycle 23, and 95% were associated with CMEs in solar cycle 24. However, only 4% and 3% of the total number of CMEs were accompanied by type II bursts in solar cycle 23 and 24, respectively. Most of the type II bursts in both cycles were related to fast and wide CMEs (48%). We also determined the typical drift rate and duration for type II bursts, which is 0.06 MHz s¹ and 9 min. Our results suggest that type II bursts dominate at heights 1.7 2.3 0.3 R. A clear majority have an onset height around 1.7 0.3 R assuming the four- fold Newkirk model.
Conclusions: The results indicate that most of the type II bursts had a white-light CME counterpart, but a few type II bursts lacked a clear CME association. There were more CMEs in cycle 24 than in cycle 23. However, cycle 24 contained fewer type II radio bursts than cycle 23. The onset heights of type II bursts and their association with wide CMEs reported in this study indicate that the early lateral expansion of CMEs may play a key role in the generation of these radio bursts.

A type II solar radio burst without a coronal mass ejection

Context. The Sun produces the most powerful explosions in the Solar System, solar flares, which can also be accompanied by large eruptions of magnetised plasma, coronal mass ejections (CMEs). These processes can accelerate electron beams up to relativistic energies through magnetic reconnection processes during solar flares and CME-driven shocks. Energetic electron beams can in turn generate radio bursts through the plasma emission mechanism. CME shocks, in particular, are usually associated with type II solar radio bursts.
Aims: However, on a few occasions, type II bursts have been reported to occur either in the absence of CMEs or shown to be more likely related with the flaring process. It is currently an open question as to how a shock generating type II bursts forms without the occurrence of a CME eruption. Here, we aim to determine the physical mechanism responsible for a type II burst that occurs in the absence of a CME.
Methods: By using radio imaging from the Nanay Radioheliograph, combined with observations from the Solar Dynamics Observatory and the Solar Terrestrial Relations Observatory spacecraft, we investigate the origin of a type II radio burst that appears to have no temporal association with a white-light CME.
Results: We identify a typical type II radio burst with band-split structure that is associated with a C-class solar flare. The type II burst source is located above the flaring active region and ahead of disturbed coronal loops observed in extreme-ultraviolet (EUV) images. The type II burst is also preceded by type III radio bursts, some of which are in fact J bursts, indicating that accelerated electron beams do not all escape along open field lines. The type II sources show single-frequency movement towards the flaring active region. The type II burst is located ahead of a faint EUV front propagating through the corona.
Conclusions: Since there is no CME detection, a shock wave is most likely generated by the flaring process or the bulk plasma motions associated with a failed eruption. The EUV front observed is likely a freely propagating wave that expands into surrounding regions. The EUV front propagates at an initial speed of approximately 450 km s¹ and it is likely to steepen into a shock wave in a region of low Alfvn speed as determined from magneto-hydrodynamic modelling of the corona.

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Multiple injections of energetic electrons associated with the flare and CME event on 9 October 2021

Context. We study the solar energetic particle (SEP) event observed on 9 October 2021 by multiple spacecraft, including Solar Orbiter. The event was associated with an M1.6 flare, a coronal mass ejection, and a shock wave. During the event, high-energy protons and electrons were recorded by multiple instruments located within a narrow longitudinal cone.
Aims: An interesting aspect of the event was the multi-stage particle energisation during the flare impulsive phase and also what appears to be a separate phase of electron acceleration detected at Solar Orbiter after the flare maximum. We aim to investigate and identify the multiple sources of energetic electron acceleration.
Methods: We utilised SEP electron observations from the Energetic Particle Detector (EPD) and hard X-ray (HXR) observations from the Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter, in combination with radio observations at a broad frequency range. We focused on establishing an association between the energetic electrons and the different HXR and radio emissions associated with the multiple acceleration episodes.
Results: We find that the flare was able to accelerate electrons for at least 20 min during the non-thermal phase, observed in the form of five discrete HXR pulses. We also show evidence that the shock wave contributed to the electron acceleration during and after the impulsive flare phase. The detailed analysis of EPD electron data shows that there was a time difference in the release of low- and high-energy electrons, with the high-energy release delayed. Also, the observed electron anisotropy characteristics suggest a different connectivity during the two phases of acceleration.

deARCE Method

We present in detail an automatic radio-burst detection system, based on the AlexNet convolutional neural network, for use with any kind of solar spectrogram. A full methodology for model training, performance evaluation, and feedback to the model generator has been developed with special emphasis on i) robustness tests against stochastic and overfitting effects, ii) specific metrics adapted to the unbalanced nature of the solar-burst scenario, iii) tunable parameters for probability-threshold optimization, and iv) burst-coincidence cross match among e-Callisto stations and with external observatories (NOAA- SWPC). The resulting neural network configuration has been designed to accept data from observatories other than e-Callisto, either ground- or spacecraft-based. Typical False Negative and False Positive Scores in single-observatory mode are, respectively, in the 10 - 16% and 6 - 8% ranges, which improve further in cross-match mode. This mode includes new services (deARCE, Xmatch) allowing the end-user to check at a glance if a solar radio burst has taken place with a high level of confidence.

Slowly drifting radio fibres

The fine structure in the continuum radiation of type IV solar radio bursts is very rich in various structures (zebra structure, fibres, pulsations, spikes, etc.). So far, however, no attention has been paid to the isolated fibres that occasionally appear in the metre and decimetre ranges. Here we give and discuss examples of the dynamic spectra of such bursts obtained many years ago (in the metre waveband from 1969) as well in recent years (in the decimetre and microwave wavebands from 2003-2013). Isolated fibres are observed mostly in the decimetre range (although there are examples in both the metre and microwave ranges), and they reveal a number of features of classical fibre bursts. As a generation mechanism for such fibres, the process of interaction of whistlers with Langmuir plasmons was suggested. An analysis of conditions for the realization of this process in solar magnetic arch structures and its efficiency was carried out. Estimates of the intensity of low-frequency turbulence (whistlers) and magnetic field strength in the solar corona were obtained using the data of radio fibres.

Jupiter's equatorial X-ray emissions over two solar cycles

Jupiter's disc is bright in X-rays as H₂ molecules in the atmosphere are very effective at scattering solar X-rays. K-shell fluorescence from carbon atoms in atmospheric methane is thought to also provide a minor contribution. XMM-Newton has now observed Jupiter over a span of nearly two solar cycles from 2003 to 2021, offering the opportunity to determine whether Jupiter's disc emissions are driven by solar activity or not. We compare the count rates of X-rays of energies 0.2-10.0, 0.2-2.0, 2.1-5.0, and 5.1-10.0 keV from the planet's equatorial region, with the sunspot number and F_10.7 adjusted solar radio flux. The respective Pearson's correlation coefficients for both are 0.88/0.84, 0.86/0.83, 0.40/0.34, and 0.29/0.22 for each energy demonstrating that the low-energy X-ray disc emissions are indeed controlled by the Sun's activity. This relationship is less clear for the higher energy emissions, raising questions around the source of these emissions.

Deciphering Faint Gyrosynchrotron Emission from a Coronal Mass Ejection Using Spectropolarimetric Radio Imaging

Measurements of the plasma parameters of coronal mass ejections (CMEs), particularly the magnetic field and nonthermal electron population entrained in the CME plasma, are crucial to understand their propagation, evolution, and geo-effectiveness. Spectral modeling of gyrosynchrotron (GS) emission from CME plasma has been regarded as one of the most promising remote-sensing techniques for estimating spatially resolved CME plasma parameters. Imaging the very low flux density CME GS emission in close proximity to the Sun with orders of magnitude higher flux density has, however, proven to be rather challenging. This challenge has only recently been met using the high dynamic range imaging capability of the Murchison Widefield Array (MWA). Although routine detection of GS is now within reach, the challenge has shifted to constraining the large number of free parameters in GS models, a few of which are degenerate, using the limited number of spectral points at which the observations are typically available. These degeneracies can be broken using polarimetric imaging. For the first time, we demonstrate this using our recently developed capability of high-fidelity polarimetric imaging on the data from the MWA. We show that spectropolarimetric imaging, even when only sensitive upper limits on circularly polarization flux density are available, is not only able to break the degeneracies but also yields tighter constraints on the plasma parameters of key interest than possible with total intensity spectroscopic imaging alone.

A Continued Study

In this paper, we reanalyze the X1.7 class limb flare that occurred on 2013 May 13 (SOL2013-05-13T01:56 UT), concentrating on the energy- releasing process using microwave observations mainly made by Nobeyama and X-ray observations made by RHESSI. The analysis was carried out in the context of EUV observations made by the Atmospheric Imaging Assembly on board Solar Dynamics Observatory. First, we complement the initiation process by showing that the initiation occurred together with material falling from a large-scale overlying prominence, a signature of drainage instability. The usual downward and upward motions of the microwave and X-ray sources are observed from their evolution. However, the microwave source's height shows a recurrent decrease and increase during its overall upward motion; it shows a kind of recurrent contraction and expansion. The time period of the recurrent contraction and expansion corresponds to the period of post-contraction oscillation of EUV loops, and the oscillatory motions are closely correlated with four microwave/hard X-ray peaks that unusually increased nonthermal emission levels by several times. X-ray spectra get hardened during the oscillation. In addition, the rapid contraction of magnetic loops located on the outside of the erupting flux rope occurs 5 minutes after the onset of the flare, showing that the contraction of the peripheral magnetic loops is more likely due to the vortex and sink flows generated by an upward erupting magnetic flux rope rather than a coronal implosion. The results can provide more insight into the physics of dynamic coronal magnetic field and particle acceleration during solar flares.

Imaging Preflare Broadband Pulsations in the Decimetric-metric Wavelengths

Preflare activities contain critical information about the precursors and causes of solar eruptions. Here we investigate the characteristics and origin of a group of broadband pulsations (BBPs) in the decimetric- metric wavelengths that took place during the preflare stage of the M7.1 flare on 2011 September 24. The event was recorded by multiple solar instruments, including the Nanay Radioheliograh, that measure the properties of the radio source. The BBPs started ~24 minutes before the flare onset, extending from <360 to above 800 MHz without a discernible spectral drift. The BBPs consisted of two stages. During the first stage, the main source remained stationary, and during the second stage, it moved outward along with a steepening extreme-ultraviolet (EUV) wave driven by the eruption of a high-temperature structure. In both stages, we observe frequent EUV brightenings and jets originating from the flare region. During the second stage, the BBPs became more frequent and stronger in general, and the polarization level gradually increased from <20% to >60% in the right-handed sense. These observations indicate that the steepening EUV wave is important to the BBPs during the second stage, while the preflare reconnections causing the jets and EUV brightenings are important in both stages. This is the first time that such a strong association of an EUV wave with BBPs is reported. We suggest a scenario in which reconnection occurs together with a shock that sweeps across the loops as the cause of the BBPs.

Analysis of the Possibilities of Short-Term Prediction of Geomagnetic Perturbations from Observations of Coronal Mass Ejections at the BSA LPI Radio Telescope

From April 2021 to October 2022, in the monitoring data obtained daily at the Big Scanning Antenna radio telescope (BSA LPI), 11 events were identified for which X-ray flares in the solar corona were followed by magnetic storms on Earth. Interplanetary scintillation monitoring data were considered together with data on solar flare activity and a simple kinematic model of ejection propagation. Based on the estimated ejection velocity between the Sun and the probed region, under the assumption of a constant velocity, the time of arrival of the ejection to the Earth was calculated. Of the 11 events considered, 7 are associated with solitary flares followed by a coronal mass ejection (CME) and 4 are more complex and possibly associated with corotating perturbations or a superposition of corotating and flare perturbations. For the entire set of events, the average time of the real onset of a magnetic storm after the time predicted by the model was 3.6 h and the average time between the onset of scintillation enhancement and the onset of a magnetic storm was 20.1 h. For events associated with solitary flares, the magnetic storm began, on average, 0.8 hours after the predicted time and 15.6 hours after the onset of scintillation enhancement. The delay of magnetic storms with respect to the predicted time is apparently related to the deceleration of the ejection between the probed region of the solar wind and the Earth's orbit.

Langmuir waves associated with magnetic holes in the solar wind

Context. Langmuir waves (electrostatic waves near the electron plasma frequency) are often observed in the solar wind and may play a role in the energy dissipation of electrons. The largest amplitude Langmuir waves are typically associated with type II and III solar radio bursts and planetary foreshocks. In addition, Langmuir waves not related to radio bursts occur in the solar wind, but their source is not well understood. Langmuir waves have been observed inside isolated magnetic holes, suggesting that magnetic holes play an important role in the generation of Langmuir waves.
Aims: We provide the statistical distribution of Langmuir waves in the solar wind at different heliocentric distances. In particular, we investigate the relationship between magnetic holes and Langmuir waves. We identify possible source regions of Langmuir waves in the solar wind, other than radio bursts, by analyzing the local plasma conditions.
Methods: We analyzed data from Solar Orbiter's Radio and Plasma Waves (RPW) and Magnetometer (MAG) instruments. We used the triggered electric field snapshots and onboard statistical data (STAT) of the Time Domain Sampler (TDS) of RPW to identify Langmuir waves and investigate their properties. The plasma densities were derived from the spacecraft potential estimated by RPW. The MAG data were used to monitor the background magnetic field and detect magnetic holes, which are defined as regions with an isolated decrease in |B| of 50% or more compared to the background level. The statistical analysis was performed on data from 2020 to 2021, comprising heliocentric distances between 0.5 AU and 1 AU.
Results: We show that 78% of the Langmuir waves in the solar wind not connected to radio bursts occur in regions of local magnetic field depletions, including the regions classified as isolated magnetic holes. We also show that the Langmuir waves occur more frequently inside magnetic holes than in any other region in the solar wind, which indicates that magnetic holes are important source regions of solar wind Langmuir waves. We find that Langmuir waves associated with magnetic holes in the solar wind typically have lower amplitudes than those associated with radio bursts.

Improved Type III solar radio burst detection using congruent deep learning models

Solar flares are energetic events in the solar atmosphere that are often linked with solar radio bursts (SRBs). SRBs are observed at metric to decametric wavelengths and are classified into five spectral classes (Type I-V) based on their signature in dynamic spectra. The automatic detection and classification of SRBs is a challenge due to their heterogeneous form. Near-real time detection and classification of SRBs has become a necessity in recent years due to large data rates generated by advanced radio telescopes such as the LOw Frequency ARray (LOFAR). For this study, we implemented congruent deep learning models to automatically detect and classify Type III SRBs. We generated simulated Type III SRBs, which were comparable to Type IIIs seen in real observations, using a deep learning method known as the generative adversarial network (GAN). This simulated data were combined with observations from LOFAR to produce a training set that was used to train an object detection model known as you only look once (YOLOv2). Using this congruent deep learning model system, we can accurately detect Type III SRBs at a mean Average Precision (mAP) value of 77.71%.

Evidence for a northward-shifted relic field

Context. Solar and heliospheric parameters occasionally depict notable differences between the northern and southern solar hemisphere. Although the hemispheric asymmetries of some heliospheric parameters vary systematically with the Hale cycle, this has not been found to be commonly valid for solar parameters. Also, no verified physical mechanism exists that can explain possible systematic hemispheric asymmetries.
Aims: We use a novel method of high heliolatitudinal vantage points to increase the fraction of one hemisphere in solar 10.7 cm radio fluxes and sunspot numbers. We aim to explore the possibility that solar radio fluxes and sunspot numbers, the two most fundamental solar parameters, depict systematic, possibly mutually similar patterns in their hemispheric activities during the last 75 yr.
Methods: We used three different sets of time intervals with increasing mean heliographic latitude and calculated corresponding hemispheric high- latitude radio fluxes and sunspot numbers. We also normalized these fluxes by yearly means in order to study the variation of fluxes in the two hemispheres over the whole 75 yr time interval.
Results: We find that cycle-maximum radio fluxes and sunspot numbers in each odd solar cycle (19, 21, 23) are larger at northern high latitudes than at southern high latitudes, while maximum fluxes and numbers in all even cycles (18, 20, 22 24) are larger at southern high latitudes than at northern high latitudes. This alternation indicates a new form of systematic, Hale-cycle-related variation in solar activity. Hemispheric differences at cycle maxima are 15% for radio flux and 23% for sunspot numbers, on average. The difference is largest during cycle 19 and smallest in cycle 24. Normalized radio fluxes depict a dominant Hale- cycle variation in both hemispheres, with an opposite phase and overall amplitude of about 5% in the north and 4% in the south. Thus, there is systematic Hale-cycle alternation in magnetic flux emergence in both hemispheres.
Conclusions: The hemispheric Hale cycle in flux emergence can be explained if there is a northward-directed relic magnetic field, which is slightly shifted northward. In that case, in odd cycles, the northern hemisphere is enhanced more than the southern hemisphere, and in even cycles, the northern hemisphere is reduced more than the southern hemisphere, establishing the observed hemispheric alternation. The temporal change of asymmetry during the seven cycles can be explained if the relic shift oscillates at the 210 yr Suess/deVries period, which also provides a physical cause to this periodicity. Gleissberg cycles are explained as off-equator excursions of the relic, each Gleissberg cycle forming one half of the full relic shift oscillation cycle. Having a relic field in the Sun also offers interesting possibilities for century-scale forecasting of solar activity.

The 17 April 2021 widespread solar energetic particle event

Context. A complex and long-lasting solar eruption on 17 April 2021 produced a widespread solar energetic particle (SEP) event that was observed by five longitudinally well-separated observers in the inner heliosphere that covered distances to the Sun from 0.42 to 1 au: BepiColombo, Parker Solar Probe, Solar Orbiter, STEREO A, and near-Earth spacecraft. The event was the second widespread SEP event detected in solar cycle 25, and it produced relativistic electrons and protons. It was associated with a long-lasting solar hard X-ray flare that showed multiple hard X-ray peaks over a duration of one hour. The event was further accompanied by a medium-fast coronal mass ejection (CME) with a speed of 880 km s^-1 that drove a shock, an extreme ultraviolet wave, and long-lasting and complex radio burst activity that showed four distinct type III burst groups over a period of 40 min.
Aims: We aim to understand the reason for the wide spread of elevated SEP intensities in the inner heliosphere as well as identify the underlying source regions of the observed energetic electrons and protons.
Methods: We applied a comprehensive multi-spacecraft analysis of remote-sensing observations and in situ measurements of the energetic particles and interplanetary context to attribute the SEP observations at the different locations to the various potential source regions at the Sun. We used an ENLIL simulation to characterize the complex interplanetary state and its role in the energetic particle transport. The magnetic connection between each spacecraft and the Sun was determined using ballistic backmapping in combination with potential field source surface extrapolations in the lower corona. Using also a reconstruction of the coronal shock front, we then determined the times when the shock establishes magnetic connections with the different observers. Radio observations were used to characterize the directivity of the four main injection episodes, which were then employed in a 2D SEP transport simulation to test the importance of these different injection episodes.
Results: A comprehensive timing analysis of the inferred solar injection times of the SEPs observed at each spacecraft suggests different source processes being important for the electron and proton events. Comparison among the characteristics and timing of the potential particle sources, such as the CME-driven shock or the flare, suggests a stronger shock contribution for the proton event and a more likely flare-related source for the electron event.
Conclusions: In contrast to earlier studies on widespread SEP events, we find that in this event an important ingredient for the wide SEP spread was the wide longitudinal range of about 110 covered by distinct SEP injections, which is also supported by our SEP transport modeling.

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Diagnostics of Flare Loop Parameters in Shrinkage and Ascent Stages Using Radio, X-ray, and UV Emission

We propose the diagnostics of plasma parameters in flare loops using the data of multi-wavelength observations in both shrinkage and expansion phases of the loops. The approach of a flare loop as an equivalent electric circuit is applied. We show that depending on plasma loop parameters, the shrinkage may be accompanied by an increase in the electric current in the loop rather than a decrease. The number density, temperature, electric current, radius, loop-top altitude, and loop volume are determined for the flare events on 16 April 2002 and 24 August 2002.

Paired quasi-periodic pulsations of hard X-ray emission in a solar flare

We investigate high time resolution data obtained by the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) during the flare event on 2022 April 21 at 01:52 UT. Several subpeaks with durations of 46s have been detected in the hard X-ray precursor phase, and the key feature is that they appear in pairs and seem like double-peak structures. These subpeaks are rarely observed in hard X-ray band and confirmed by the microwave obtained by Nobeyama Radio Polarimeters (NoRP) and Radio Solar Telescope Network (RSTN). While an exponential function can describe the continuum component of the time profile from the precursor to part of the impulsive phase. The periods of quasi-periodic pulsations (QPPs) are detected to be about 7.3 and 12.8s for the precursor and impulsive phase, respectively, with at least 95% confidence level. The paired QPPs are assumed to be double-peak QPPs and then the scenario of current loop coalescence model is found to be in good agreement with our observation. The precursor phase can be interpreted as the oscillating coalescence of two islands, while the impulsive phase can be interpreted as more islands to coalesce one by one to form larger islands.

Investigation on the Formation of Herringbone Structure in Type II Solar Radio Bursts

We report detailed observation of the "herringbone" of a Type II solar radio burst that occurred on 2010 November 3rd. Data from the Space Weather Prediction Center, National Oceanic and Atmospheric Administration, e-CALLISTO, and Nanay RadioHeliograph are analyzed. We determine the brightness temperature and degree of circular polarization of the "herringbone" burst. Correlations between the physical parameters and the "herringbone" are examined. Based on the relationship, this is the first study that suggested this "herringbone" was generated through fundamental plasma.

A novel spectrogram visual security encryption algorithm based on block compressed sensing and five-dimensional chaotic system

Based on block compressed sensing theory, combined with a five- dimensional chaotic system, we propose and analyze a novel spectrogram visual security encryption algorithm. This research is devoted to solving the compression, encryption and steganography problems of spectrograms involving large data volumes and high complexity. First, the discrete wavelet transform is applied to the spectrogram to generate the coefficient matrix. Then, block compressed sensing is applied to compress and preencrypt the spectrogram. Second, we design a new five- dimensional chaotic system. Then, several typical evaluation methods, such as the phase diagram, Lyapunov exponent, bifurcation diagram and sample entropy, are applied to deeply analyze the chaotic behavior and dynamic performance of the system. Moreover, the corresponding Simulink model has been built, which proves the realizability of the chaotic system. Importantly, the measurement matrix required for compressed sensing is constructed by the chaotic sequence. Third, dynamic Josephus scrambling and annular diffusion are performed on the secret image to obtain the cipher image. Finally, an improved least significant bit embedding method and alpha channel synchronous embedding are designed to obtain a steganographic image with visual security properties. To make the initial keys of each image completely different from other images, the required keys are produced using the SHA-256 algorithm. The experimental results confirm that the visual security cryptosystem designed in this study has better compression performance, visual security and reconstruction quality. Furthermore, it is able to effectively defend against a variety of conventional attack methods, such as statistical attacks and entropy attacks.

RFI flagging in solar and space weather low frequency radio observations

Radio spectroscopy provides a unique inspection perspective for solar and space weather research, which can reveal the plasma and energetic electron information in the solar corona and inner heliosphere. However, radio-frequency interference (RFI) from human activities affects sensitive radio telescopes, and significantly affects the quality of observation. Thus, RFI detection and mitigation for the observations is necessary to obtain high quality science-ready data. The flagging of RFI is particularly challenging for the solar and space weather observations at low frequency, because the solar radio bursts can be brighter than the RFI, and may show similar temporal behaviour. In this work, we investigate RFI flagging methods for solar and space weather observations, including a strategy for AOLAGGER, and a novel method that makes use of a morphology convolution. These algorithms can effectively flag RFI while preserving solar radio bursts.

Research on Calibration Sources for a 35-40 GHz Millimeter-Wave Solar Radio Observation System

The microwave spectra in the optically thin regime provide unique information about the solar corona magnetic field; thus, a millimeter- wave solar radio observation system (35-40-GHz) in the Chashan observatory is newly established to obtain relevant data. Herein, we design and develop a blackbody calibration source employing a metal cone array and absorbing patch to calibrate the system and obtain accurate solar observation data for analysis and decision-making. The reflectivity of the calibration source is simulated and measured to characterize its emissivity. The reflectivity is obtained by the finite- integeral technique (FIT) method under various fitting conditions, i.e., the thickness of the absorbent patches and aspect ratios of the cone. The temperature gradient of cones is determined by the finite-element method (FEM). A $5\times $ 5 array microwave blackbody was developed based on a cone with a low scattering coefficient and excellent temperature uniformity. To reduce the influence of spurious signals, the time-domain gate technique is used to test the reflectivity of the target. The results demonstrate that the emissivity of the blackbody is higher than 0.9995 in the 35-40-GHz band. The temperature difference between the cone top and the cone bottom is 0.20 C.

Understanding the Relationship between Solar Coronal Abundances and F10.7 cm Radio Emission

Sun-as-a-star coronal plasma composition, derived from full-Sun spectra, and the F10.7 radio flux (2.8 GHz) have been shown to be highly correlated (r = 0.88) during solar cycle 24. However, this correlation becomes nonlinear during increased solar magnetic activity. Here we use cotemporal, high spatial resolution, multiwavelength images of the Sun to investigate the underlying causes of the nonlinearity between coronal composition (FIP bias) and F10.7 solar index correlation. Using the Karl G. Jansky Very Large Array, Hinode/EIS (EUV Imaging Spectrometer), and the Solar Dynamics Observatory, we observed a small active region, AR 12759, throughout the solar atmosphere from the photosphere to the corona. The results of this study show that the magnetic field strength (flux density) in active regions plays an important role in the variability of coronal abundances, and it is likely the main contributing factor to this nonlinearity during increased solar activity. Coronal abundances above cool sunspots are lower than in dispersed magnetic plage regions. Strong magnetic concentrations are associated with stronger F10.7 cm gyroresonance emission. Considering that as the solar cycle moves from minimum to maximum, the sizes of sunspots and their field strength increase with the gyroresonance component, the distinctly different tendencies of radio emission and coronal abundances in the vicinity of sunspots is the likely cause of saturation of Sun-as-a-star coronal abundances during solar maximum, while the F10.7 index remains well correlated with the sunspot number and other magnetic field proxies.

Machine learning based approach for modeling and forecasting of GPS-TEC during diverse solar phase periods

The ionosphere is a dispersive medium and it is the most important source of error affecting radio communications. As a result, precise improvements in ionospheric Total Electron Content (TEC) predictions and early warning alerts are critical for space-based radio communication and navigation systems services. In the present study, we examined the Global Positioning System (GPS)-derived TEC observations of Bangalore station (Geog. Lat. 13.02 N and Geog. Long. 77.57 E) during the 24th solar cycle, which extends the 11 years from 2009 to 2019. Based on Solar Radio Flux (sfu) and seasonal features, machine learning frameworks have been developed to forecast monthly/hourly GPS-TEC values. Motivated by the preliminary results, the problem of traditional approaches for predicting ionospheric GPS-TEC values and improvements using the machine learning approach is discussed. The Kernel extreme learning machine (KELM) model has characteristics of accuracy and good generalization performance compared with the traditional method of Holt- Winters (HW) and (Auto Regressive Moving Average (ARMA). The experimental results show that the statistical validations of the KELM model are superior to the other statistical models, with average Mean Absolute Error (MAE) and Root Mean Square Error values, which is of 0.94 and 1.305 TECU (KELM), 1.70 and 2.29 TECU (ARMA) and 2.91 and 3.86 TECU (HW) during different phases of the solar cycle. In addition, the proposed model is validated by comparison with global ionospheric models (International Reference Ionosphere (IRI) -2016 and Global Ionosphere Map (GIM). The results show that the proposed approach is superior to other alternatives using prediction accuracy. This paper outlines a machine learning framework for space weather predictions and recommends KELM classifier could be used for other query functions in Active Learning (AL) practice approaches and other datasets of the Global Navigation Satellite System (GNSS).

Research Progress on Interplanetary Type III Radio Bursts Based on PSP Observations

Compared to solar radio bursts, interplanetary (IP) radio bursts with lower radiation frequencies are generally believed to originate in the interplanetary space far from the low corona. The cutoff by Earth's ionosphere makes the ground-based observations for it impossible. Parker Solar Probe (PSP), launched by the National Aeronautics and Space Administration (NASA) to explore the Sun with the closer heliocentric distance than any previous spacecraft, provides an opportunity to study the low-frequency radio burst. The radio spectrometer it carries can observe radio radiation in the frequency range of 10 kHz--19.17 MHz. PSP can approach the radiation source region of the IP type III radio burst. Therefore, it has an unprecedented advantage to observe the interplanetary radio bursts. This paper reviews the studies about type III radio bursts observed by PSP so far. These studies include the occurrence rate, polarization, scattering, cutoff frequency, possible radiation mechanisms, and related radiation source regions of radio bursts and so on. Finally, future research prospects are discussed.

Research Progress of the Heliospheric Radio Emissions

The heliospheric radio emissions are the strongest radio emissions phenomenon in the solar system, with a radiation power of at least 10$^{13}$ W, which can provide important physical information of high energy electron beam and magnetic plasma structure near the heliospheric boundary. Since the first detection by the Voyager spacecraft in 1983, those radio emissions have widely and continuously attracted much attention from researchers. There are generally two types of the heliospheric radio emissions: instantaneous or drifting emission with relatively high frequency, and continuous emission or non-drifting emission with relatively low frequency. Usually, both types of emissions start from about 2 kHz. For the drifting emission, it has the characteristic of drifting to high frequency, the drifting rate is about 1--3 kHz/yr, the frequency range is 1.8--3.6 kHz, and the duration is about 100--300 days. For the non-drifting emission, it has no obvious frequency drift, the frequency range is 1.8--2.6 kHz, and the duration is about 3 yr. It is generally believed that the heliospheric radio emissions are related to shock. In this paper, the possible source region of the radio emissions, the emission mechanisms, and the source of shock related to the emissions are introduced. Furthermore, the existing scientific problems and the future perspectives on the research of heliospheric radio emissions are discussed.

Electron Cyclotron Maser Emission in Solar Radio Bursts

Preface

Over 99.9% of the baryonic matter in the universe is plasma, hence plasma astrophysics is an important branch of modern astrophysics, which provides the sound foundation for us to understand the formation, evolution, and various eruptive phenomena in astrophysical systems. The 14 papers in this Special Issue systematically introduce the research progress in the solar and heliospheric plasmas made by the Plasma Astrophysics group in Purple Mountain Observatory, with the aim to help readers get a glimpse of the research frontiers and remaining questions.

Adiabatic Radio Emission Spectrum of the Sun's Coronal Holes

Coronal holes on the Sun have been observed at individual frequencies for quite a long time in the wavelength range from radio to X-rays. Observations in a wide range of radio frequencies are carried out with the RATAN-600 radio telescope. An analysis of long-term spectral observations of the RATAN-600 radio telescope showed that the emission spectrum of coronal holes is radically different from the spectrum of active formations above sunspots, but, differing markedly from the spectrum of the quiet Sun, it also has similarities with it. It has been established that the radio emission of coronal holes has an adiabatic spectrum and does not contain noticeable coherent radiation, that is, recombination radio lines and lines of the fine structure of hydrogen and other elements.

Comparison of chromospheric diagnostics in a 3D model atmosphere. H linewidth and millimetre continua

Context. The H line, one of the most studied chromospheric diagnostics, is a tracer of magnetic field structures, while the intensity of its line core provides an estimate of the mass density. The interpretation of H observations is complicated by deviations from local thermodynamic equilibrium (LTE) or instantaneous statistical equilibrium conditions. Meanwhile, millimetre (mm) continuum radiation is formed in LTE, and therefore the brightness temperatures from Atacama Large Millimetre- submillimetre Array (ALMA) observations provide a complementary view of the activity and the thermal structure of stellar atmospheres. These two diagnostics together can provide insights into the physical properties of stellar atmospheres, such as their temperature stratification, magnetic structures, and mass density distribution.
Aims: In this paper, we present a comparative study between synthetic continuum brightness temperature maps at mm wavelengths (0.3 mm to 8.5 mm) and the width of the H 6565 line.
Methods: We used the 3D radiative- transfer codes Multi3D and Advanced Radiative Transfer (ART) to calculate synthetic spectra for the H line and the mm continua, respectively, from an enhanced network atmosphere model with non- equilibrium hydrogen ionisation generated with the state-of-the-art 3D radiation magnetohydrodynamics (rMHD) code Bifrost. We use a Gaussian point spread function (PSF) to simulate the effect of ALMA's limited spatial resolution and calculate the H versus mm continuum correlations and slopes of scatter plots for the original and degraded resolution of the whole box, quiet sun, and enhanced network patches separately.
Results: The H linewidth and mm brightness temperatures are highly correlated and the correlation is highest at a wavelength of 0.8 mm, that is, in ALMA Band 7. The correlation systematically increases with decreasing resolution. On the other hand, the slopes decrease with increasing wavelength. The degradation of resolution does not have a significant impact on the calculated slopes.
Conclusions: With decreasing spatial resolution, the standard deviations of the observables, H linewidth, and brightness temperatures decrease and the correlations between them increase, but the slopes do not change significantly. These relations may therefore prove useful in calibrating the mm continuum maps observed with ALMA.

Statistical study of type III bursts and associated HXR emissions

Context. Flare-accelerated electrons may produce closely temporarily related hard X-ray (HXR) emission while interacting with the dense solar atmosphere and radio type III bursts when propagating from the low corona to the interplanetary medium. The link between these emissions has been studied in previous studies. We present here new results on the correlation between the number and spectrum of HXR-producing electrons and the type III characteristics (flux, starting frequency).
Aims: The aim of this study is to extend the results from previous statistical studies of radio type III bursts and associated HXR emissions: in particular, to determine what kind of correlation, if any, exists between the HXR-emitting electron numbers and the radio flux, as well as whether any correlations between the electron numbers or energy spectra are deduced from associated HXR emissions and type III starting (stopping) frequencies.
Methods: This study is based on thirteen years of data between 2002 and 2014. We shortlisted 200 events with a close temporal association between HXR emissions and radio type III bursts in the 450-150 MHz range. We used X-ray flare observations from RHESSI and Fermi/GBM to calculate the number of electrons giving rise to the observed X-ray flux and observations from the Nanay Radioheliograph to calculate the peak radio flux at different frequencies in the 450-150 MHz range. Under the assumption of thick-target emissions, the number of HXR-producing electrons and their energy spectra were computed. The correlation between electron numbers, power-law indices, and the peak radio fluxes at different frequencies were analysed as well as potential correlations between the electron numbers and starting frequency of the radio burst. Bootstrap analysis for the correlation coefficients was performed to quantify the statistical significance of the fit.
Results: The correlation between the number of HXR electrons and the peak flux of the type III emission decreases with increasing frequency. This correlation is larger when considering the electron number above 20 keV rather than the electron number above 10 keV. A weak anti- correlation is also found between the absolute value of the electron spectral index and the peak radio flux at 228 MHz. A rough correlation is found between the HXR-producing electron number above 20 keV and the type III starting frequency. This correlation is smaller if the electron number above 10 keV is considered. All the results are discussed in the framework of results from previous studies and in the context of numerical simulations of bump-in-tail instabilities and subsequent radio emissions.

Quasi-Periodic Pulsations in an M-Class Solar Flare

We have studied the quasi-periodic pulsations (QPPs) of the M2.3 flare that occurred in the active region NOAA 12172 on 23 September 2014. Through the fast Fourier transform (FFT) method, we decompose the flare light curves into fast- and slowly-varying components, and the cut-off threshold is 100 s. We find that the QPPs have a period of 40 s at soft X-ray (SXR), hard X-ray (HXR), radio and ultraviolet (UV). Based on the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO), we find that the QPPs take place at the same time interval as the flare ribbon separation, and that the QPPs seem to originate from the flare ribbons. Our observations tend to support the mechanism of the periodic nonthermal electron injection during the flare eruption.

What to Do When the F10.7 Goes Out?

The solar radio flux at 10.7 cm, known as F10.7, is a critical operational space weather index. However, without a clear backup, any interruption to the service can result in substantial errors in model outputs. In this paper we show the impact of one such outage in March 2022 on the models TIE-GCM and NeQuick, and present a number of alternative solutions that could be used for future outages. The analysis is extended to the F10.7 time series since 1951 and the approach resulting in the smallest reconstruction error of F10.7 uses the solar radio flux observations at alternative wavelengths (the best giving a percentage error of 3.1%). Alternatively, use of Sunspot Number, a regular, robust alternative observation, results in a mean percentage error of 8.2% and is also a reliable fallback solution. Additionally, analysis of the error on the use of the conversion between the 12-month rolling sunspot number (R12) and its conversion to F10.7 is included.

Research on interference and noise reduction technology of solar radio observation system

The dynamic spectrum of solar radio burst is a very important tool to study the characteristics of solar radio burst. However, due to the influence of the instruments noise of the solar radio telescope, external interference, the change of ambient temperature, absorption of clouds and so on, the sensitivity of the observation system will be reduced. In particular, the weak solar radio burst signal is easily submerged by various interference signals. In order to improve the sensitivity of solar radio telescope to observe solar radio burst signals and remove narrow-band interference signals, a spectrum data processing algorithm of spectral subtraction method based on entropy and energy threshold values (SSM-EE) is proposed in this article. There are multiple processing steps of SSM-EE. The first step is to improve the signal-to-noise ratio (SNR) through accumulating and averaging the spectrum data. The second step is to eliminate narrowband interference. The spectral kurtosis algorithm is used to calculate the spectral kurtosis of the narrowband interference signal, and then whether to smooth it is determined according to whether it is greater than the threshold. The third step is to establish a noise model. Calculating the power spectrum entropy and energy entropy of each frame of the processed spectrum data, judge whether it is a noise signal by judging the relationship between the power spectrum entropy and energy entropy and the corresponding threshold. The update model for noise floor is used to obtain the real-time noise floor data. The last step is to eliminate the background noise through spectral subtraction. The new spectrum data are obtained by subtraction method between the original spectrum data and the noise floor data. After a large number of data simulation and verification of the actual observation data, it shows that the method proposed in this article has good practical value.

Microwave response to kink oscillations of a plasma slab

The modulation of the intensity of microwave emission from a plasma slab caused by a standing linear kink fast magnetoacoustic wave is considered. The slab is stretched along a straight magnetic field, and can represent, for example, a current sheet in a flaring active region in corona of the Sun, or a streamer or pseudostreamer stalk. The plasma density is non-uniform in the perpendicular direction and described by a symmetric Epstein profile. The plasma parameter is taken to be zero, which is a good approximation for solar coronal active regions. The microwave emission is caused by mildly relativistic electrons which occupy a layer within the oscillating slab and radiate via the gyrosynchrotron (GS) mechanism. Light curves of the microwave emission were simulated in the optically thin part of the GS spectrum, and their typical Fourier spectra were analysed. It is shown that the microwave response to a linear kink magnetohydrodynamic wave is non-linear. It is found that, while the microwave light curves at the node oscillate with the same frequency as the frequency of the perturbing kink mode, the frequency of the microwave oscillations at the antinode is two times higher than the kink oscillation frequency. Gradual transformation the one type of the light curves to another occurs when sliding from the node to the antinode. This result does not depend on the width of the GS-emitting layer inside the oscillating slab. This finding should be considered in the interpretation of microwave quasi-periodic pulsations in solar and stellar flares.

The frequency ratio and time delay of solar radio emissions with fundamental and harmonic components

Solar radio bursts generated through the plasma emission mechanism produce radiation near the local plasma frequency (fundamental emission) and double the plasma frequency (harmonic). While the theoretical ratio of these two frequencies is close to 2, simultaneous observations give ratios ranging from 1.6 to 2, suggesting either a ratio different from 2, a delay of the fundamental emission, or both. To address this long- standing question, we conducted high-frequency, high-time resolution imaging spectroscopy of type III and type J bursts with fine structures for both the fundamental and harmonic components with LOFAR between 30 and 80 MHz. The short-lived and narrow frequency-band fine structures observed simultaneously at fundamental and harmonic frequencies give a frequency ratio of 1.66 and 1.73, similar to previous observations. However, frequency-time cross-correlations suggest a frequency ratio of 1.99 and 1.95 with a time delay between the F and H emissions of 1.00 and 1.67 s, respectively for each event. Hence, simultaneous frequency ratio measurements different from 2 are caused by the delay of the fundamental emission. Among the processes causing fundamental emission delays, anisotropic radio-wave scattering is dominant. Moreover, the levels of anisotropy and density fluctuations reproducing the delay of fundamental emissions are consistent with those required to simulate the source size and duration of fundamental emissions. Using these simulations we are able to, for the first time, provide quantitative estimates of the delay time of the fundamental emissions caused by radio-wave propagation effects at multiple frequencies, which can be used in future studies.

Radio Measurements of Coronal Magnetic Fields in Fan-Spine Configurations on the Sun

Recent interest of solar physicists in the analysis of the coronal mass ejections and circular solar flares in fan-spine magnetic configurations (FSCs) necessitates measurements of the corresponding coronal magnetic fields. A dominant sunspot with the circumjacent magnetic flux of opposite polarity produces a specific coronal region of the quasi- transverse (QT-) propagation of microwaves. We make use of the theory of QT-propagation to evaluate the strengths of coronal magnetic fields in the active regions NOAA 11579, 12242, and 12488 while they are non- flaring. Microwave polarization changes were observed with the RATAN-600 radio telescope, the Siberian Solar Radio Telescope, and the Nobeyama Radioheliograph. Changes in the sign of circular radio polarization provide the strengths of coronal fields in a QT-region if the coronal plasma density N_e and the length scale of magnetic field divergence L_d are known. We evaluate the length scale by means of the potential-field source-surface (PFSS) model and the coronal density from the Gaussian inversion of the differential emission measure (from the Solar Dynamics Observatory observational data), obtaining N_e L_d = (0.46-0.64)10¹⁰ m². The resulting coronal fields of 1.410³ T and 2.3410³ T are attributed to the heights of 100 Mm and 50.2 Mm. We discuss the validity and consistency of the involved values to draw conclusions on the feasibility of coronal radio magnetography of FSCs.

The Predicition of Solar Flares Using Millimeter Radio Brightenings

Solar activity could have significant impacts on various Earth and near- Earth space systems, such as satellite communication and power grid systems. The prediction of solar activity and active solar events plays a major role when preparing for these disturbances. Various satellite- based instruments constantly observe the Sun. However, only a few ground-based solar instruments could provide versatile enough information for the space weather prediction. Metshovi Radio Observatory of Aalto University (Finland) has a unique collection of millimetre (8 mm) solar radio maps over the past 40 years, and even denser observational solar radio data catalogue since 2011. About 75-80 % days yearly are covered nowadays. This gives opportunity to make statistical estimation of solar flare occurrence based on solar radio maps. In this study, we had 2253 days when both solar radio map and GOES (Geostationary Operational Environmental Satellites) classified solar flare were observed. In this work, we used solar flare classification done by the Space Weather Centre (SWC) of the National Oceanic and Atmospheric Administration (NOAA). The data were observed between 1 January 2011 and 12 September 2022. Our study shows that the maximum intensity of radio brightenings is a good indicator to tell which kind of GOES classified solar flare could be expected to happen. The article presents that intense radio brightening is needed to produce a certain GOES classified solar flare.

Validation of F2-layer critical frequency variations in the ionosphere with radio observations of solar bursts

The high-frequency (HF) ionospheric cutoff restricts the possibilities of radio astronomical observations from the Earth, but on the other hand, it allows one to estimate the F2-layer critical frequency of the ionosphere. This effect has been measured using a new active antenna meant for receiving cosmic radio emission in the frequency range 1-40 MHz. The instrument was implemented near the Ukrainian T-shape Radiotelescope, second modification (UTR-2) and serves as a prototype of the HF antenna for a future radio array on the farside of the Moon. We detected directly a storm of solar radio bursts on May 22, 2021 and observed clearly their cutoff due to the ionosphere. For comparison our analysis of the experimental data was supplemented by simultaneous measurements from ionosondes located close to the UTR-2. The distance between the closest ionosonde near Zmiiv (Kharkiv region, Ukraine) and the radio astronomical antenna used in the experiment is 46.5 km. Results obtained from the solar radio records are consistent with the ionosonde data. We show that such an antenna implementation provides us with new opportunities to study the F2-layer critical frequency of the ionosphere.

Fine Structures of Type-IV Solar Radio Bursts Associated with Stationary and Moving Sources

Various types of fine structure in the continuum emission of type-IV radio bursts are considered as applied to different types of radiation sources, both stationary and moving. In the case of stationary sources, the origin of the fine structure is associated both with processes in individual magnetic loops (quasi-periodic acceleration and magnetohydrodynamic waves), and with large-scale processes associated with the propagation of magnetohydrodynamic disturbances, the formation of loop arcades, and processes of discrete acceleration of particles synchronous with them, causing the pulsating nature of radio emissions. For the case of a moving source, the generation mechanism largely depends on the magnetic structure of the source (an expanding magnetic arc or an isolated plasma cloud). In this case, the connection with coronal mass ejections and shock waves is also important. Secondary pulsations are explained by a magnetohydrodynamic fluctuation source in the form of a magnetic loop or cloud. The absence of other fine structures in the continuum of moving type-IV bursts may be due to the critical angle of the loss cone for the excitation of whistlers.

Dependence of NmF2 Local Annual Asymmetry Index on Local Time and Solar Activity

An analysis of the dependence of the local annual asymmetry R index on local time and solar activity has been performed for 19632013 based on the data of the medians of the maximum electron density of F2-layer NmF2 at the pair of the ionospheric stations BoulderHobart where the R index is the JanuaryJuly ratio of the total NmF2 concentration (for this pair of stations) at a fixed local time. As an indicator of solar activity for the median NmF2, the F index is used, i.e. the 81-day average value of the solar radio emission flux at a wavelength of 10.7 cm, which is centered on the middle of the given month. It was found that for the dependence of the R index on the local time LT, a semidiurnal mode prevails with maxima near noon and midnight and minima in the morning and evening. The lowest R values, R = 1, are observed at low solar activity in a narrow interval of 1919.5 LT. Annual asymmetry in the NmF2 median exists (R > 1) for all other hours of local time at any level of solar activity. Near noon the R index increases with solar activity with a tendency to saturation at a high level of this activity. Around midnight for the dependence of the R index on F there is a maximum near F = 140, above which R decreases with growth of F. High R index values at noon and midnight are mainly due to relatively high values of NmF2 in January in the Northern Hemisphere (local winter, Boulder) at noon and in the Southern Hemisphere (local summer, Hobart) at midnight.

The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar Flare

Solar flares are known to be prolific electron accelerators, yet identifying the mechanism(s) for such efficient electron acceleration in solar flare (and similar astrophysical settings) presents a major challenge. This is due in part to a lack of observational constraints related to conditions in the primary acceleration region itself. Accelerated electrons with energies above ~20 keV are revealed by hard X-ray (HXR) bremsstrahlung emission, while accelerated electrons with even higher energies manifest themselves through radio gyrosynchrotron emission. Here, we show, for a well-observed flare on 2017 September 10, that a combination of RHESSI HXR and and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) EUV observations provides a robust estimate of the fraction of the ambient electron population that is accelerated at a given time, with an upper limit of 10^-2 on the number density of nonthermal (20 keV) electrons, expressed as a fraction of the number density of ambient protons in the same volume. This upper limit is about 2 orders of magnitude lower than previously inferred from microwave observations of the same event. Our results strongly indicate that the fraction of accelerated electrons in the coronal region at any given time is relatively small but also that the overall duration of the HXR emission requires a steady resupply of electrons to the acceleration site. Simultaneous measurements of the instantaneous accelerated electron number density and the associated specific electron acceleration rate provide key constraints for a quantitative study of the mechanisms leading to electron acceleration in magnetic reconnection events.

Parametric study of the kinematic evolution of coronal mass ejection shock waves and their relation to flaring activity

Context. Coronal and interplanetary shock waves produced by coronal mass ejections (CMEs) are major drivers of space-weather phenomena, inducing major changes in the heliospheric radiation environment and directly perturbing the near-Earth environment, including its magnetosphere. A better understanding of how these shock waves evolve from the corona to the interplanetary medium can therefore contribute to improving nowcasting and forecasting of space weather. Early warnings from these shock waves can come from radio measurements as well as coronagraphic observations that can be exploited to characterise the dynamical evolution of these structures.
Aims: Our aim is to analyse the geometrical and kinematic properties of 32 CME shock waves derived from multi-point white-light and ultraviolet imagery taken by the Solar Dynamics Observatory (SDO), Solar and Heliospheric Observatory (SoHO), and Solar-Terrestrial Relations Observatory (STEREO) to improve our understanding of how shock waves evolve in 3D during the eruption of a CME. We use our catalogue to search for relations between the shock wave's kinematic properties and the flaring activity associated with the underlying genesis of the CME piston.
Methods: Past studies have shown that shock waves observed from multiple vantage points can be aptly reproduced geometrically by simple ellipsoids. The catalogue of reconstructed shock waves provides the time-dependent evolution of these ellipsoidal parameters. From these parameters, we deduced the lateral and radial expansion speeds of the shocks evolving over time. We compared these kinematic properties with those obtained from a single viewpoint by SoHO in order to evaluate projection effects. Finally, we examined the relationships between the shock wave and the associated flare when the latter was observed on the disc by considering the measurements of soft and hard X-rays.
Results: We find that at around 25 solar radii (R), the shape of a shock wave is very spherical, with a ratio between the lateral and radial dimensions (minor radii) remaining at around b/a 1.03 and a radial to lateral speed ratio (V_R/V_L)1.44. The CME starts to slow down a few tens of minutes after the first acceleration and then propagates at a nearly constant speed. We revisit past studies that show a relation between the CME speed and the soft X-ray emission of the flare measured by the Geostationary Operational Environmental Satellite (GOES) and extend them to higher flare intensities and shock speeds. The time lag between the peak of the flare and of the CME speed is up to a few tens of minutes. We find that for several well-observed shock onsets, a clear correlation is visible between the derivative of the soft X-ray flux and the acceleration of the shock wave.

Solar Radio Emissions and Ultralight Dark Matter

Ultralight axions and dark photons are well-motivated dark matter candidates. Inside the plasma, once the mass of ultralight dark matter candidates equals the plasma frequency, they can resonantly convert into electromagnetic waves, due to the coupling between the ultralight dark matter particles and the standard model photons. The converted electromagnetic waves are monochromatic. In this article, we review the development of using radio detectors to search for ultralight dark matter conversions in the solar corona and solar wind plasma.

Impacts of Extreme Space Weather Events on September 6th, 2017 on Ionosphere and Primary Cosmic Rays

The strongest X-class solar flare (SF) event in 24th solar cycle, X9.3, occurred on 6 September 2017, accompanied by earthward-directed coronal mass ejections (CMEs). Such space weather episodes are known to cause various threats to human activities ranging from radio communication and navigation disturbances including wave blackout to producing geomagnetic storms of different intensities. In this study, SFs' ionospheric impacts and effects of accompanied heliospheric disturbances on primary cosmic rays (CR) are investigated. This work offers the first detailed investigation of characteristics of these extreme events since they were inspected both from the perspective of their electromagnetic nature, through very low frequency (VLF) radio waves, and their corpuscular nature of CR by multi-instrumental approach. Aside data recorded by Belgrade VLF and CR stations, data from GOES and SOHO space probes were used for modeling and analysis. Conducted numerical simulations revealed a significant change of ionospheric parameters (sharpness and effective reflection height) and few orders of magnitude increase of electron density. We compared our findings with those existing in the literature regarding the ionospheric response and corresponding parameters. In addition, Forbush decrease (FD) magnitude, corrected for magnetospheric effect, derived from measurements, and one predicted from power exponents used to parametrize the shape of energetic proton fluence spectra at L1 were compared and found to be in good agreement. Presented findings could be useful for investigation of atmospheric plasma properties, particles' modeling, and prediction of extreme weather impacts on human activities.

sunspot oscillations in 3-6 GHz band

We present the first observations of spatially resolved oscillation sources obtained with the Siberian Radioheliograph at 3-6 GHz. We have found significant flux oscillations with periods of about 3, 5, and 13 min emitted from AR12833. The 3-min periodicity dominates at higher frequencies. It was found that the apparent level of oscillations depends on the active region location on the disc, and scales down towards the limbs. The oscillations were studied in detail during 1 h interval on 2021 June 19. We found that sources of 3-min oscillations were located above the umbra and their emission is extraordinary polarized. The 5- and 13-min periods were manifested in emission at lower frequencies, down to 2.8 GHz. Sources with 5-min periodicity were located near the umbra/penumbra boundary and in the pore region. Positions of sources with 13-min oscillations were different at 3.1 and 4.7 GHz. We found consistency between spatial location of the oscillation sources in radio and ultraviolet at 171 and 304 . There is significant correlation of signals in two ranges. Time delays between microwave oscillations increase as the frequency decreases, which can be explained by upward propagation of periodic disturbances. The localization of oscillation sources is probably related to magnetic structures with different wave cut-off frequencies at different heights.

Analyses for the Mechanism of Solar Radio Burst Interfering Satellite Navigation Signal and Influence Presentation.

Solar radio bursts are a potential interference factor in navigation systems. From the navigation signal model is derived in this paper, the paper analyzes the solar radio burst interference mechanism of navigation signal, the interference effects exist three effects, namely with the solar radio burst flux, receiver performance, and the Angle of the Sun-navigation antenna height, synthesize the proposed the influence of solar radio burst interfere with communication navigation equation. It is concluded that the total integrated power of the solar burst flow in the navigation communication frequency band is positively correlated with the decrease of the signal to noise ratio of the communication signal, and is affected by the modulation of the height Angle of the sun-antenna and the effective area of the antenna, and has a convolution relationship with the response function of the loop filter of the navigation receiver. Then, this paper further analyzed the GPS lock-out signal during the solar radio bursts on 28 October 2003, 6 December 2006 and 4 November 2015. It is found that the loss-rate of different receivers at the same site is different in the same event (receiver performances effect). The loss-of-lock rate of the same receiver at different latitudes in the same event is different (Sun-antenna height Angle effect). In addition, under the condition of non-uniform spectrum of L-band (12 GHz) solar radio bursts in the same event (solar radio burst flow distribution effect), the decrease of signal to noise ratio of L1 and L2 band communication signals is also different. Therefore, the analytical correctness of the above influence equation is verified by the observation characterization of the above three events.

10.7 cm10.7 cmSolar Full-disk Flare Forecasting Model Based on 10.7 cm Solar Radio Flux.

Solar flare is an important solar active phenomenon, which is manifested as electromagnetic radiant enhancement in almost all wave bands. The statistics indicate that the solar flare is positively associated with the solar active levels. In this paper, a method for predicting the probability of solar flare is established based on the statistical relationship between 10.7 cm flux and solar flare during 1975 to 2007. The forecasting model can be used to predict the probability of C, M and X class flares. During 2008 to 2016, the predicted errors of the model for C, M and X class flares are 0.113, 0.087 and 0.012, respectively, and the predicted skill scores are 0.250, 0.106 and 0.012, respectively. It means that our method has less predicted errors and more skill scores than the average model for predicting C, M and X class flares. During the period from 2008 to 2016, the predicted results of the model are similar to that of Space Environment Prediction Center in National Space Science Center. It indicates that the model is feasible in the actual space environment prediction.

Solar Radio Spikes and Type IIIb Striae Manifestations of Subsecond Electron Acceleration Triggered by a Coronal Mass Ejection

Understanding electron acceleration associated with magnetic energy release at subsecond scales presents major challenges in solar physics. Solar radio spikes observed as subsecond, narrow-bandwidth bursts with f/f ~ 10^-3-10^-2 are indicative of a subsecond evolution of the electron distribution. We present a statistical analysis of frequency- and time-resolved imaging of individual spikes and Type IIIb striae associated with a coronal mass ejection (CME). LOFAR imaging reveals that the cotemporal (<2 s) spike and striae intensity contours almost completely overlap. On average, both burst types have a similar source size with a fast expansion at millisecond scales. The radio source centroid velocities are often superluminal and independent of frequency over 30-45 MHz. The CME perturbs the field geometry, leading to increased spike emission likely due to frequent magnetic reconnection. As the field restores itself toward the prior configuration, the observed sky-plane emission locations drift to increased heights over tens of minutes. Combined with previous observations above 1 GHz, the average decay time and source size estimates follow a ~1/f dependence over three decades in frequency, similar to radio-wave scattering predictions. Both time and spatial characteristics of the bursts between 30 and 70 MHz are consistent with radio-wave scattering with a strong anisotropy of the density fluctuation spectrum. Consequently, the site of the radio-wave emission does not correspond to the observed burst locations and implies acceleration and emission near the CME flank. The bandwidths suggest intrinsic emission source sizes <1 at 30 MHz and magnetic field strengths a factor of two larger than average in events that produce decameter spikes.

Automated detection and statistical study of solar radio spikes

The most typical observational features of solar radio spikes are their short duration and narrow bandwidth. We have improved the YOLOv5s network model for these characteristics by adding inclined bounding frames and attention and feature fusion mechanism modules. The decimeter- and meter-wavelength spikes observed by the Solar Broad-band Radio Spectrometer in Huairou and the Chashan Solar Radio Observatory spectrograph are used to carry out experiments, respectively. The results demonstrate that the AP value obtained by the improved network is 74%, which is almost 14% higher than the original network. The improved network detects 9709 (1379) decimeter- (meter-) wavelength spikes in two events with durations, bandwidths, relative bandwidths, and frequency-drift rates. The spikes at decimeter and meter wavelengths are again categorized based on their frequency-drift rates, such as positive, negative, and no measurable frequency-drift rates. We have carried out a statistical study on these categorized spikes. These statistical results and findings constrain solar radio spikes' formation.

Analysis of ionospheric TEC response to solar and geomagnetic activities at different solar activity stages

Studying the relationship of total electron content (TEC) to solar or geomagnetic activities at different solar activity stages can provide a reference for ionospheric modeling and prediction. On the basis of solar activity indices, geomagnetic activity parameters, and ionospheric TEC data at different solar activity stages, this study analyzes the overall variation relationships of solar and geomagnetic activities with ionospheric TEC, the characteristics of the quasi-27-day periodic oscillations of the three variables at different stages, and the delayed TEC response of solar activity by conducting correlation analysis, Butterworth band-pass filtering, Fourier transform, and time lag analysis. The following results are obtained. (1) TEC exhibits a significant linear relationship with solar activity at different solar activity stages. The correlation coefficients |R| are arranged as follows: |R|_EUV > |R|_F10.7 > |R|_{sunspot
number}. No significant linear relationship exists between TEC and geomagnetic activity parameters (|R| < 0.35). (2) TEC, solar activity indices, and geomagnetic activity parameters have a period of 10.5 years. The maximum amplitudes of the Fourier spectrum for TEC and solar activity indices are nearly 27 days and those of geomagnetic activity parameters are nearly 27 and 13.5 days. (3) The deviations of the quasi-27-day significant periodic oscillation of TEC and solar activity indices are consistent. (4) No evident relationship exists between the quasi-27-day periodic oscillation of TEC and geomagnetic activity parameters. (5) The delay time of TEC for the 10.7 cm solar radio flux and extreme ultraviolet is always consistent, whereas that for sunspot number varies at each stage.

The Sun at millimeter wavelengths. IV. Magnetohydrodynamic waves in small-scale bright features

Aims: We used solar observations of a plage-enhanced network with the Atacama Large Millimeter/sub-millimeter Array (ALMA) in Band 3 and Band 6, together with synthetic continuum maps from numerical simulations with Bifrost in the same bands, to carry out a detailed study of bright small-scale magnetic features.
Methods: We made use of an algorithm to automatically identify and trace bright features within the field of view (FoV) of the ALMA observations and the simulation. In particular, the algorithm recovers information of the time evolution of the shape, motion of the centre of gravity, temperature, and size for each feature. These quantities are used to determine the oscillatory properties of each feature utilising wavelets analysis.
Results: We found 193 and 293 features in the Bands 3 and 6 observations, respectively. In the degraded simulation, the total number of features were 24 for Band 3 and 204 for Band 6. In the original simulation, the total number of features were 36 for Band 3 and 392 for Band 6. Based on the simulation, we confirm the magnetic nature of the features. We have obtained average oscillation periods of 30-99 s for the temperature, 37-92 s for size, and 37-78 s for horizontal velocity. There are indications for the possible presence of transverse (kink) waves with average amplitude velocities of 2.1-5.0 km s¹. We find a predominant anti-phase behaviour between temperature and size oscillations suggesting that the variations of the bright features are caused by compressible fast-sausage magnetohydrodynamics (MHD) modes. For the first time to our knowledge, we estimated the flux of energy of the fast-sausage waves at the chromospheric heights sampled by ALMA as 453-1838 W m² for Band 3 and 3640-5485 W m² for Band 6.
Conclusions: We have identified MHD waves, both transverse (kink) and compressible sausage modes, in small-scale (magnetic) structures, independently, in both ALMA Band 3 and Band 6 observations, along with their corresponding synthetic images from simulations. The decrease of wave energy-flux with height (from Band 6 to Band 3) could possibly suggest energy dissipation at chromospheric heights, namely, wave heating, with the assumptions that the identified small-scale waves are typical at each band and they propagate upward through the chromosphere.

Temporal and spatial association between microwaves and type III bursts in the upper corona

One of the most important tasks in solar physics is the study of particles and energy transfer from the lower corona to the outer layers of the solar atmosphere. The most sensitive methods for detecting fluxes of non-thermal electrons in the solar atmosphere is observing their radio emission using modern large radioheliographs. We analyzed joint observations from the 13 April 2019 event observed by LOw-Frequency ARray (LOFAR) at meter wavelengths, and the Siberian Radio Heliograph (SRH) and the Badary Broadband Microwave Spectropolarimeter (BBMS) spectropolarimeter in microwaves performed at the time of the second PSP perihelion. During a period without signatures of non-thermal energy release in X-ray emission, numerous type III and/or type J bursts were observed. During the same two hours we observed soft X-ray brightenings and the appearance of weak microwave emission in an abnormally narrow band around 6 GHz. At these frequencies the increasing flux is well above the noise level, reaching 9 sfu. In the LOFAR dynamic spectrum of 5380 MHz a region is found that lasts about an hour whose emission is highly correlated with 6 GHz temporal profile. The flux peaks in the meter waves are well correlated with extreme UV (EUV) emission variations caused by repeated surges from the bright X-point. We argue that there is a common source of non-thermal electrons located in the tail of the active region, where two loop systems of very different sizes interacted. The frequencies of type III and/or type J bursts are in accordance with large loop heights around 400 Mm, obtained by the magnetic field reconstruction. The microwave coherent emission was generated in the low loops identified as bright X-ray points seen in soft X-ray and EUV images, produced by electrons with energies several tens of keV at about twice the plasma frequency.

Multi-Periodicity of High-Frequency Type III Bursts as a Signature of the Fragmented Magnetic Reconnection

Using the radio spectra of the 2 April 2022 eruptive flare, we analyze a group of highfrequency type III bursts by our new wavelet method. In this analysis, we found a multi-periodicity of these bursts that is interpreted by the electron beams accelerated in the fragmented magnetic reconnection in the rising magnetic rope. We propose that each period in these type III bursts is a result of the periodic interaction of sub- ropes formed in the rising magnetic rope. In each interaction, the period depends on the diameter of interacting sub-ropes and local Alfvn velocity. This interpretation is supported by detection of the specific EUV structure which was, according to our knowledge, observed for the first time. All proposed processes occur in the rising magnetic rope. Thus, this flare deviates from the standard flare model, where the main magnetic reconnection is located below the rising magnetic rope.

Parametric Evolution of Power-law Energy Spectra of Energetic Electrons in the Coronal Loops

Fast electron beams (FEBs) are one of the main products of various active events and are ubiquitous in solar, space and cosmic plasmas. They reveal themselves in hard X-ray and radio emissions. The observed characteristics of X-ray and radio emissions sensitively depend on the energy distribution of FEBs, which usually have a power-law energy spectrum. As FEBs travel in the solar atmosphere, their energy distribution can considerably vary due to the interaction with ambient plasmas. Tang et al. investigated the evolution of the energy spectrum of the FEBs traveling along a flare loop and discussed the possible effects on associated hard X-ray (HXR) and radio emissions. Considering the ubiquitous coronal loops in active regions, in the present paper, we investigate the parametric evolution of the energy spectra of FEBs when propagating along coronal loops. Here, we take the sunpot atmospheric model as an approximate coronal loop atmosphere model. The results show that the energy loss has an important impact on the cutoff behavior and energy spectra of FEBs when precipitating in a coronal loop with density ratio n _b/n _e = 0.01. The initially single power-law spectrum with a steepness cutoff can evolve into a more complex double power-law spectrum or two "knees" power-law spectrum with a flattened steepness cutoff behavior or saturation cutoff behavior. Our calculations also demonstrate that the energy spectrum evolution is not obvious if n _b/n ₀ = 0.001 as Tang et al. asserted. The present results are helpful for a more comprehensive understanding of the dynamic spectra of HXR and radio emissions from FEBs.

Prediction of TEC and Range Error using Low-latitude GPS Data during January to April 2022 Solar Flare Events

The effects of solar, geomagnetic, and ionospheric anomalies on satellite communication are inextricable. Range Error (RE) is the most common fault that is faced by the navigational receivers during solar flares. Since RE always depends on the Total Electron Content (TEC) available across the satellite ray path, a prediction model capable of predicting the TEC in advance will be an excellent deterrent during adverse space weather conditions. In this research, Cokriging based Surrogate Model (COKSM) is constructed for predicting the TEC variations that occurred during the month of January 2022 to April 2022 over Hyderabad region. The input data used in the construction of the model includes F10.7 radio flux, Sunspot number (SSN), Geomagnetic index Kp and Ap along with Vertical TEC (VTEC) data collected from Hyderabad station located in 17.31 N latitude and 78.55 E longitude. The data is collected in hourly averaged resolution for a period of 120 days covering January to April 2022. The variations in Ionospheric TEC due to solar flares and geomagnetic anomalies that occurred during the selected observation dates are principally analyzed in order to evaluate the prediction capability of the COKSM program during adverse conditions. The performance of the model is evaluated using metrics like Root Mean Square Percentage error (RMSPE), Correlation Coefficient (), CHI- Squared goodness of fit test and R-squared. The results that are plotted as a linear regression scatter plot clearly shows that with very small residuals the proposed prediction model is performing well for TEC prediction. The overall RE predicted by the model is within the scale of 112 meters. The error parameters calculated between true TEC and predicted TEC is found out to be in the scale of 0.88 to 5.06% for RMSPE, 0.9308 to 0.9981 for correlation coefficient, 4.97 to 107.94 (TECU) for chi squared and 0.78 to 0.98 (TECU) for R squared.

Spectroradiometry of the Solar Corona on the RATAN-600

Modern studies of solar radio emission are complicated by continuous power amplification and multifrequency external interference, which often completely overlap important frequency ranges. Many topical problems in solar radio astronomy require large effective areas of radio telescopes, high frequency and time resolutions, accurate spatial measurements, and a large dynamic range. It becomes relevant to change the concept of receiving recording equipment. This paper deals with topical problems of the physics of the solar corona in combination with optimal methods of observation with large instruments. The features and difficulties of combining high parametersdynamic, spatial, temporal, and frequency resolutionsare considered. The proposed solutions of the new-generation observation complex implement the possibilities of intelligent selection of registration conditions in a multioctave mode with multichannel over 8000 channels/GHz with temporary permission up to 8 ms/spectrum. A multiobject observation mode becomes available from powerful flaring objects to faint structures of various nature. High- speed data processing makes it possible to implement an online mode of interference elimination, which is based on a fast statistical analysis of the spectrum with the selection of non-Gaussian (interference) structures. Methods for high-speed analysis of large-volume data (the principal component analysis method) and their presentation to the user are proposed. Examples of the operation of the complex in the range of 1-3 GHz are given. The prospects of a new approach for multiobject radio astronomy observations in the implementation of the RATAN-600 tracking mode are considered: from recombination lines to wide-range spectra, from low-contrast fluctuations to fast changes in flares, etc.

Modeling Hadronic Gamma-Ray Emissions from Solar Flares and Prospects for Detecting Nonthermal Signatures from Protostars

We investigate gamma-ray emission in the impulsive phase of solar flares and the detectability of nonthermal signatures from protostellar flares. Energetic solar flares emit high-energy gamma rays of GeV energies, but their production mechanism and emission site are still unknown. Young stellar objects, including protostars, also exhibit luminous X-ray flares, but the triggering mechanism of the flaring activity is still unclear owing to the strong obscuration. Nonthermal signatures in millimeter/submillimeter and gamma-ray bands are useful to probe protostellar flares owing to their strong penetration power. We develop a nonthermal emission model of the impulsive phase of solar flares, where cosmic-ray protons accelerated at the termination shock produce high-energy gamma rays via hadronuclear interaction with the evaporation plasma. This model can reproduce gamma-ray data in the impulsive phase of a solar flare. We apply our model to protostellar flares and show that the Cherenkov Telescope Array will be able to detect gamma rays of TeV energies if particle acceleration in protostellar flares is efficient. Nonthermal electrons accelerated together with protons can emit strong millimeter and submillimeter signals via synchrotron radiation, whose power is consistent with the energetic millimeter/submillimeter transients observed from young stars. Future gamma-ray and millimeter/submillimeter observations from protostars, coordinated with a hard X-ray observation, will unravel the nonthermal particle production and triggering mechanism of protostellar flares.

Effects of Fully Relativistic Condition on Electron Cyclotron Maser Emission

The electron cyclotron maser (ECM) instability is a very important nonthermal radiation mechanism. It has been developed by proposing various electron distribution functions as well as the relativistic resonance condition, called the semirelativistic correction. Taking account of the relativistic effects of both the velocity distribution of energetic electrons and the resonance condition, called the fully relativistic correction, the present paper investigates the ECM instability driven by a power-law electron distribution with a low- energy cutoff. The results show that (1) both in the semirelativistic and fully relativistic cases, the growth rate and relative frequency bandwidth of ordinary (O) and extraordinary (X) modes show a positive correlation with cutoff energy E _c, i.e., the peak frequency decreases with increasing E _c; (2) the peak frequency ratio (H _peak/F _peak) of the harmonic and fundamental waves is always ~2; (3) compared with the semirelativistic case, the fully relativistic case has a larger growth rate (for both the O and X mode) and a smaller peak frequency (only for the O mode) for energy > 50 keV, and there is almost no difference at lower energy for the two cases; (4) the peak frequency of the X1 mode can be higher than its cutoff frequency in a strongly magnetized plasma, implying that the X1 mode emission may escape more easily for a higher E _c and stronger magnetic field. These results can be helpful for us to understand better the physics of radio bursts from the Sun and other objects.

Possible Application to Quasiperiodic Pulsations

Magnetohydrodynamic (MHD) waves are often invoked to interpret quasiperiodic pulsations (QPPs) in solar flares. We study the response of a straight flare loop to a kink-like velocity perturbation using three-dimensional MHD simulations and forward model the microwave emissions using the fast gyrosynchrotron code. Kink motions with two periodicities are simultaneously generated, with the long-period component (P _L = 57 s) being attributed to the radial fundamental kink mode and the short-period component (P _S = 5.8 s) to the first leaky kink mode. Forward modeling results show that the two-periodic oscillations are detectable in the microwave intensities for some lines of sight. Increasing the beam size to (1)² does not wipe out the microwave oscillations. We propose that the first leaky kink mode is a promising candidate mechanism to account for short-period QPPs. Radio telescopes with high spatial resolutions can help distinguish between this new mechanism and such customary interpretations as sausage modes.

Synchrotron Radiation by Galactic Cosmic-Ray Electrons

The quiet Sun, i.e., in its nonflaring state or nonflaring regions, emits thermal radiation from radio to ultraviolet. The quiet Sun also produces nonthermal radiation observed in gamma rays due to interactions of Galactic cosmic rays (GCRs) with the solar atmosphere and photons. We report on a new component: the synchrotron emission by GCR electrons in the solar magnetic field. To the best of our knowledge this is the first time this emission has been theoretically claimed and modeled. We find that the measured GCR electrons with energies from tens of GeV to a few TeV produce synchrotron emission in X-rays, which is a few orders of magnitude lower than current upper limits of the quiet Sun set by RHESSI and FOXSI, with no energy losses included. For a radially decreasing solar magnetic field we find the expected synchrotron intensity to be almost constant in the solar disk, to peak in the close proximity of the Sun, and to quickly drop away from the Sun. We also estimate the synchrotron emission from radio to gamma rays, and we compare it with current observations, especially with LOFAR. While it is negligible from radio to UV compared to the solar thermal radiation, this emission can potentially be observed at high energies with NuSTAR and more promising future FOXSI observations. This could potentially allow for constraining GCR densities and magnetic-field intensities at the Sun. This study provides a more complete description and a possible new way for understanding the quiet Sun and its environment.

Probing, Statistics, and Implications

A strong coronal magnetic field, when present, manifests itself as bright microwave sources at high frequencies produced by the gyroresonant (GR) emission mechanism in thermal coronal plasma. The highest frequency at which this emission is observed is proportional to the absolute value of the strongest coronal magnetic field on the line of sight. Although no coronal magnetic field larger than roughly 2000 G has been expected, recently a field at least 2 times larger has been reported. Here, we report on a search for and a statistical study of such strong coronal magnetic fields using high-frequency GR emission. A historic record of spatially resolved microwave observations at high frequencies, 17 and 34 GHz, is available from the Nobeyama RadioHeliograph for a period covering more than 20 yr (1995-2018). Here, we employ this data set to identify sources of bright GR emission at 34 GHz and perform a statistical analysis of the identified GR cases to quantify the strongest coronal magnetic fields during two solar cycles. We found that although active regions with a strong magnetic field are relatively rare (less than 1% of all active regions), they appear regularly on the Sun. These active regions are associated with prominent manifestations of solar activity.

Study of Radio Transients from the Quiet Sun during an Extremely Quiet Time

In this work, we study a class of recently discovered meter-wave solar transients referred to as Weak Impulsive Narrowband Quiet Sun Emission (WINQSEs). Their strength is a few percent of the quiet Sun background and is characterized by their very impulsive, narrowband, and ubiquitous presence in quiet Sun regions. Mondal et al. (2020) hypothesized that these emissions might be the radio counterparts of nanoflares, and their potential significance warrants detailed studies. Here we present an analysis of data from an extremely quiet time and with improved methodology over the previous work. As before, we detect numerous WINQSEs, which we have used for their further characterization. Their key properties, namely, their impulsive nature and ubiquitous presence in the quiet Sun, are observed in these data as well. Interestingly, we also find some of the observed properties to differ significantly from the earlier work. With this demonstration of routine detection of WINQSEs, we hope to engender interest in the larger community to build a deeper understanding of WINQSEs.

Imaging-spectroscopy of a band-split type II solar radio burst with the Murchison Widefield Array

Type II solar radio bursts are caused by magnetohydrodynamic (MHD) shocks driven by solar eruptive events such as coronal mass ejections (CMEs). Often, both fundamental and harmonic bands of type II bursts are split into sub-bands, which are generally believed to be coming from upstream and downstream regions of the shock; however, this explanation remains unconfirmed. Here, we present combined results from imaging analyses of type II radio burst band splitting and other fine structures observed by the Murchison Widefield Array (MWA) and extreme ultraviolet observations from Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) on 28 September 2014. The MWA provides imaging- spectroscopy in the range 80300 MHz with a time resolution of 0.5 s and frequency resolution of 40 kHz. Our analysis shows that the burst was caused by a piston-driven shock with a driver speed of 112 km s¹ and shock speed of 580 km s¹. We provide rare evidence that band splitting is caused by emission from multiple parts of the shock (as opposed to the upstream-downstream hypothesis). We also examine the small-scale motion of type II fine structure radio sources in MWA images, and suggest that this motion may arise because of radio propagation effects from coronal turbulence, and is not due to the physical motion of the shock location. We present a novel technique that uses imaging spectroscopy to directly determine the effective length scale of turbulent density perturbations, which is found to be 12 Mm. The study of the systematic and small-scale motion of fine structures may therefore provide a measure of turbulence in different regions of the shock and corona.

Differences in physical properties of coronal bright points and their ALMA counterparts within and outside coronal holes

Aims: This study investigates and compares the physical properties, such as intensity and area, of coronal bright points (CBPs) inside and outside of coronal holes (CHs) using the Atacama Large Millimeter/submillimeter Array (ALMA) and Solar Dynamics Observatory (SDO) observations.
Methods: The CBPs were analysed using the single-dish ALMA Band 6 observations, combined with extreme-ultraviolet (EUV) 193 filtergrams obtained by the Atmospheric Imaging Assembly (AIA) and magnetograms obtained by the Helioseismic and Magnetic Imager (HMI), both on board SDO. The CH boundaries were extracted from the SDO/AIA images using the Collection of Analysis Tools for Coronal Holes (CATCH) and CBPs were identified in the SDO/AIA, SDO/HMI, and ALMA data. Measurements of brightness and areas in both ALMA and SDO/AIA images were conducted for CBPs within CH boundaries and quiet Sun regions outside CHs. Two equal size CBP samples, one inside and one outside CHs, were randomly chosen and a statistical analysis was conducted. The statistical analysis was repeated 200 times using a bootstrap technique to eliminate the results based on pure coincidence.
Results: The boundaries of five selected CHs were extracted using CATCH and their physical properties were obtained. Statistical analysis of the measured physical CBP properties using two different methods resulted in a lower average intensity in the SDO/AIA data, or brightness temperature in the ALMA data, for CBPs within the boundaries of all five CHs. Depending on the CBP sample size, the difference in intensity for the SDO/AIA data, and brightness temperature for the ALMA data, between the CBPs inside and outside CHs ranged from between 2 and 4.5, showing a statistically significant difference between those two CBP groups. We also obtained CBP areas, where CBPs within the CH boundaries showed lower values for the measured areas, with the observed difference between the CBPs inside and outside CHs between 1 and 2 for the SDO/AIA data, and up to 3.5 for the ALMA data, indicating that CBP areas are also significantly different for the two CBP groups. We also found that, in comparison to the SDO/AIA data, the measured CBP properties in the ALMA data show a small brightness temperature difference and a higher area difference between the CBPs within and outside of CHs, possibly because of the modest spatial resolution of the ALMA images.
Conclusions: Given the measured properties of the CBPs, we conclude that the CBPs inside CHs tend to be less bright on average, but also smaller in comparison to those outside of CHs. This conclusion might point to the specific physical conditions and properties of the local CH region around a CBP limiting the maximum achievable intensity (temperature) and size of a CBP. The need for the interferometric ALMA data is also emphasised to get more precise physical CBP property measurements at chromospheric heights.

Identifying the energy release site in a solar microflare with a jet

Context. One of the main science questions of the Solar Orbiter and Parker Solar Probe missions deals with understanding how electrons in the lower solar corona are accelerated and how they subsequently access interplanetary space.
Aims: We aim to investigate the electron acceleration and energy release sites as well as the manner in which accelerated electrons access the interplanetary space in the case of the SOL2021-02-18T18:05 event, a GOES A8 class microflare associated with a coronal jet.
Methods: This study takes advantage of three different vantage points, Solar Orbiter, STEREO-A, and Earth, with observations drawn from eight different instruments, ranging from radio to X-ray. Multi-wavelength timing analysis combined with UV/EUV imagery and X-ray spectroscopy by Solar Orbiter/STIX (Spectrometer/Telescope for Imaging X-rays) is used to investigate the origin of the observed emission during different flare phases.
Results: The event under investigation satisfies the classical picture of the onset time of the acceleration of electrons coinciding with the jet and the radio type III bursts. This microflare features prominent hard X-ray (HXR) nonthermal emission down to at least 10 keV and a spectrum that is much harder than usual for a microflare with = 2.9 0.3. From Earth's vantage point, the microflare is seen near the limb, revealing the coronal energy release site above the flare loop in EUV, which, from STIX spectroscopic analysis, turns out to be hot (i.e., at roughly the same temperature of the flare). Moreover, this region is moving toward higher altitudes over time (30 km s¹). During the flare, the same region spatially coincides with the origin of the coronal jet. Three- dimensional (3D) stereoscopic reconstructions of the propagating jet highlight that the ejected plasma moves along a curved trajectory.
Conclusions: Within the framework of the interchange reconnection model, we conclude that the energy release site observed above-the-loop corresponds to the electron acceleration site, corroborating that interchange reconnection is a viable candidate for particle acceleration in the low corona on field lines open to interplanetary space.

Structured type III radio bursts observed in interplanetary space

Context. The last few decades have seen numerous studies dedicated to fine structures of type III radio bursts observed in the meter-decameter wavelengths. Most of the explanations of the structured radio emission involve the propagation of electron beams through the strongly inhomogeneous plasma in the low corona. To date, only a few type III bursts with fine structures, observed at hecto-kilometric wavelengths, have been reported.
Aims: We report here the existence of numerous structured type III radio bursts observed during the STEREO era by all three WAVES instruments on board STEREO A, B, and Wind. The aim of the study is to report and classify structured type III bursts, and to present the characteristics of their fine structures. The final goal is to try to understand the physical mechanism responsible for the generation of structured radio emission.
Methods: In this study we used data from all available spacecraft, specifically STEREO and Wind. We employed 1D density models to obtain the speed of the source of type III radio emission, the electron beam. We also performed a spectral analysis of the fine structures in order to compare their characteristics with the metric-decametric fine structures.
Results: The presented similarities of the type III fine structures in the metric to decametric and interplanetary wavelengths indicate that the physical processes responsible for the generation of structured type III radio bursts could be the same, at heights from the low corona to the interplanetary range. We show that the observed structuring and intermittent nature of the type III bursts can be explained by the variation in the level of density fluctuations, at different distances from the Sun.

Automatic detection of solar radio bursts in NenuFAR observations

Solar radio bursts are some of the brightest emissions at radio frequencies in the solar system. The emission mechanisms that generate these bursts offer a remote insight into physical processes in solar coronal plasma, while fine spectral features hint at its underlying turbulent nature. During radio noise storms many hundreds of solar radio bursts can occur over the course of a few hours. Identifying and classifying solar radio bursts is often done manually although a number of automatic algorithms have been produced for this purpose. The use of machine learning algorithms for image segmentation and classification is well established and has shown promising results in the case of identifying Type II and Type III solar radio bursts. Here we present the results of a convolutional neural network applied to dynamic spectra of NenuFAR solar observations. We highlight some initial success in segmenting radio bursts from the background spectra and outline the steps necessary for burst classification.

Five years of solar observations with LOFAR station in Baldy

We present exemplary observations of solar radio bursts collected in 2017 2021 with the use of LOFAR station PL612 located in Baldy (Poland), operating in local mode. In that period, the Sun was observed for 1190.3 hours over 235 days. We gathered high time (8 ms) and frequency (0.4 MHz) resolution spectra in the band 10 250 MHz. Despite the fact that this period can be characterized with relatively low solar activity, we managed to observe quite a large number of various bursts. Detailed analysis of these events will undoubtedly contribute to expanding our knowledge about physical properties of solar activity. Additionally, a single LOFAR station can be used to continuously monitor radio bursts in order to analyze the mechanisms affecting the so-called space weather.

A method for the automatic detection of solar type III radio bursts with Wind/Waves

Solar type III radio bursts have a characteristic signature in frequency-time dynamic spectrograms and provide important insight into the dynamics of the Sun and solar wind. Direct physical inferences of the source electrons or their propagation can be made, but radio observations typically contain multiple emission sources which must be accounted for. As well as this, the emission feature of interest must be selected, which can be problematic if done manually, particularly with a large amount of data. The WAVES instrument onboard the Wind spacecraft has accumulated almost thirty years of solar and terrestrial observations, with the majority from L1. Our previous work (Waters et al., 2021b) has focused on the extraction of terrestrial auroral kilometric radiation (AKR) from the Wind/WAVES data, and the removal of noise. In this study we use a similar concept, separating the radio sources by the measured signal variability during a spacecraft spin, but focusing on the parameter space containing interplanetary (IP) type III bursts below 1 MHz. Using an inverse threshold of the spin variability metric and a signal-to-noise ratio (SNR) threshold, the radio data is filtered to return mostly Type III emission above 200 kHz. Integrating the flux density between 500-1045 kHz and choosing a threshold of 1 109 W sr1 to select start times of Type III bursts between 1995 and 2021 yields the detection of 120560 events. Annual event occurrence shows good correlation with yearly-averaged sunspot number across the two solar cycles examined here (P=0.94). Evaluating classification statistics of the selection via visual examination of a random sample of 50 days of Wind data returns a true positive rate of 0.9 and a positive predictive value of 0.4, indicating that the majority of observed Type III bursts are returned. This technique thus provides an initial step to broad statistical examination of the physical properties of Type III bursts using observations from Wind/WAVES or other instruments.

a constellation of CubeSats around the Sun

One of the greatest challenge facing current space weather monitoring operations is forecasting the arrival of coronal mass ejections (CMEs) and Solar Energetic Particles (SEPs) within their Earth-Sun propagation timescales. Current campaigns mainly rely on extreme ultra-violet and white light observations to create forecasts, missing out many potential events that may be hazardous to Earth's infrastructure undetectable at these wavelengths. Here we introduce the SURROUND mission, a constellation of CubeSats each with identical radio spectrometers, and the results of the initial Phase-0 study for the concept. The main goal of SURROUND is to monitor and track solar radio bursts (SRBs), widely utilised as a useful diagnostic for space weather activity, and revolutionise current forecasting capabilities. The Phase-0 study concludes that SURROUND can achieve its mission objectives using 3 - 5 spacecraft using current technologies with feasible SEP and CME forecasting potential: a first for heliospheric monitors.

Solar/stellar atmospheric tomography with mm-radio snapshot spectroscopic imaging

Millimeter (mm) frequencies are primarily sensitive to thermal emission from layers across the stellar chromosphere up to the transition region, while metrewave (radio) frequencies probe the coronal heights. Together the mm and radio band spectroscopic snapshot imaging enables the tomographic exploration of the active atmospheric layers of the cool main-sequence stars (spectral type: FGKM), including our Sun. Sensitive modern mm and radio interferometers let us explore solar/stellar activity covering a range of energy scales at sub-second and sub-MHz resolution over wide operational bandwidths. The superior uv-coverage of these instruments facilitate high dynamic range imaging, letting us explore the morphological evolution of even energetically weak events on the Sun at fine spectro-temporal cadence. This article will introduce the current advancements, the data analysis challenges and available tools. The impact of these tools and novel data in field of solar/stellar research will be summarised with future prospects.

Auroral emissions and inner magnetospheric dynamics during Earth's response to the 28th October 2021 Coronal Mass Ejection

On 28th October 2021 the Sun released a large Coronal Mass Ejection (CME) in Earth's direction. An X1.0 class solar flare and a rare ground level enhancement (GLE) were observed, along with bright solar radio bursts. Here we examine data from the near-Earth environment to investigate the terrestrial response to this solar event, using newly accessible Wind Auroral Kilometric Radiation (AKR) observations. The CME arrival is tracked at ~1 AU using remote radio observations from Wind, along with in-situ interplanetary magnetic field (IMF) and solar wind measurements from OMNI. Geomagnetic activity is studied with SYM-H, SuperMAG and PC indices. The auroral response is monitored for the first time with Wind AKR observations from L1 (Lagrange point 1), UV auroral emissions and field-aligned current densities, exploring the AKR source location and inner magnetospheric dynamics. We thus quantify the timeline for solar windmagnetosphereionosphere coupling and address the visibility of AKR sources from Wind's position on the dayside at L1.

Deriving Large Coronal Magnetic Loop Parameters Using LOFAR J Burst Observations

Large coronal loops around one solar radius in altitude are an important connection between the solar wind and the low solar corona. However, their plasma properties are ill-defined, as standard X-ray and UV techniques are not suited to these low-density environments. Diagnostics from type J solar radio bursts at frequencies above 10 MHz are ideally suited to understand these coronal loops. Despite this, J-bursts are less frequently studied than their type III cousins, in part because the curvature of the coronal loop makes them unsuited for using standard coronal density models. We used LOw-Frequency-ARray (LOFAR) and Parker Solar Probe (PSP) solar radio dynamic spectrum to identify 27 type III bursts and 27 J-bursts during a solar radio noise storm observed on 10 April 2019. We found that their exciter velocities were similar, implying a common acceleration region that injects electrons along open and closed magnetic structures. We describe a novel technique to estimate the density model in coronal loops from J-burst dynamic spectra, finding typical loop apex altitudes around 1.3 solar radius. At this altitude, the average scale heights were 0.36 solar radius, the average temperature was around 1 MK, the average pressure was 0.7 mdyncm², and the average minimum magnetic field strength was 0.13 G. We discuss how these parameters compare with much smaller coronal loops.

Simulating Solar Radio Bursts Using Generative Adversarial Networks

Solar flares are one of the most extreme drivers of space weather in our solar system. The impulsive solar radio emission associated with a solar flare is known as a solar radio burst (SRB). They are generally studied in dynamic spectra and are classified into five major spectral classes, ranging from Type I to Type V, based on their form and frequency, and time duration. Due to their intricate characterisation, generating a training set for object-detection and classification models of such phenomena is a difficulty in machine learning. Current algorithms implement parametric modelling where the quantity, grouping, intensity, drift rate, heterogeneity, start-end frequency and start-end time of Type-III and Type-II radio bursts are all random. However, this model does not factor in the true shape or general features seen in real dynamic spectra observations of the Sun, which can be crucial when training classification or object-detection algorithms. In this research, we introduce a methodology named a Generative Adversarial Network (GAN) for generating realistic SRB simulations. By using real examples of Type-III and Type-II SRB data, we can train GANs to generate images almost comparable to real observed data. Furthermore, we evaluate the results of the generated model using human perception, then we compare and contrast the results using a metric known as the Frchet Inception Distance.

VLF Signal Variations Seen in the Bucharest Observatory Recordings

This paper presents a new instrument setup at the Bucharest Observatory of the Astronomical Institute of the Romanian Academy, as well as preliminary results using this setup. It consists of records of VLF (very low frequency) perturbations using a superSID instrument. During March and December 2022 various mounts and settings tests have been performed. Starting October we obtained satisfactory data. We report on detecting the signature of moderate to intense solar flares (M and Xclass). In this configuration, the Cclass solar flares are less distinguishable from the background noise.

Observations of Ionospheric Clutter at Near Equatorial High Frequency Radar Stations

The temporal variation of received clutter and noise at a pair of oceanographic high frequency radars (HFR) operating near the geomagnetic equator in the Republic of Palau is investigated. Oceanographic HFRs process range-gated Doppler spectra from groundwave signals that are backscattered from the ocean's surface to derive maps of ocean currents. The range performance of the radars exhibited a regular diurnal signal which is determined to be a result of both ionospheric clutter and noise. The increased Clutter plus Noise Floor (C+NF) decreases the Signal to Clutter plus Noise Ratio (SCNR) which, in turn, reduces the range and quality of ocean surface current measurement. Determining the nature and origin of this degradation is critical to QA/QC of existing HFR deployments as well as performance predictions of future installations. Nighttime impacts are most severe and negatively affect ocean surface current measurements as low SCNR is found to extend across the Doppler spectra at all ranges, challenging the ability of HFR to map the ocean surface current. Daytime degradation is less severe and presents itself in a way consistent with independent observations of ionospheric clutter, specifically the diurnal temporal pattern and range where the C+NF features occur. A timeseries analysis of SCNR and C+NF is pursued to understand this relationship using received range-dependent Doppler spectra and C+NF features using image segmentation techniques. Clutter plus noise features are classified into daytime, nighttime, and no-noise feature types. The diurnal structure and variability of these features are examined, and the occurrences of each feature type are calculated. The occurrences are compared with space weather indices including a measure of geomagnetic activity, namely the EE (Equatorial Electro Jet) index (determined from magnetometers measuring the earth's magnetic field), as well as solar impacts using the F10.7 solar radio clutter index to assess the relationship of ionospheric conditions with HFR ocean surface current measurement.

Progress in the Study of Decameter-Wavelength Solar Radio Emission with Ukrainian Radio Telescopes. Part 1. (Invited paper)

Subject and Purpose. Results are presented of the solar corona investigations performed with the world famous Ukrainian radio telescopes. The work has been aimed at offering a consistent review of recent achievements in observations of a variety of low-frequency radio emissions from the Sun. Methods and Methodology. The studies of the quiet (thermal) and sporadic (burst-like) radio emissions from the Sun have been car- ried out with the decameter-wavelength radio telescopes UTR-2, GURT and URAN-2. Specific features of the low-frequency solar radio emissions from a variety of sources are presented, with characterization of the optimized techniques that were applied in each case for evaluating physical parameters of the corona in the areas of decameter-wavelength radio wave generation. Results. The analysis of temporal, frequency and spatial characteristics of solar radio emissions has allowed suggesting a number of models for the coronal electron density distribution, and evaluating magnetic field strengths in the corona. Also, our experimental results have proven to be consistent with the observational data obtained in different frequency ranges and with the use of both ground based and space-borne instruments. Conclusions. The radio observations performed with Ukrainian radio telescopes have permitted studying, with high temporal, fre- quency and spatial resolutions, solar radio frequency emissions from various localized sources. Along with the large effective area and high sensitivity of the antennas, this permits application of a wide range of methods and tools aimed at detecting and analyzing solar bursts, of both strong and weak intensity, against the background of terrestrial interference of natural or artificial origin

Estimation of Solar Observations with the Five-hundred-meter Aperture Spherical Radio Telescope (FAST)

We present the estimation of solar observation with the Five-hundred- meter Aperture Spherical radio Telescope (FAST). For both the quiet Sun and the Sun with radio bursts, when pointing directly to the Sun, the total power received by FAST would be out of the safe operational range of the signal chain, even resulting in damage to the receiver. As a conclusion, the Sun should be kept at least ~2 away from the main beam during observations at ~1.25 GHz. The separation for lower frequency should be larger. For simplicity, the angular separation between the FAST beam and the Sun is suggested to be ~5 for observations at 200 MHz or higher bands.

Deciphering Radio Emission from Solar Coronal Mass Ejections using High-fidelity Spectropolarimetric Radio Imaging

Solar coronal mass ejections (CMEs) are large-scale expulsions of plasma and magnetic field from the Sun into the heliosphere and are the most important driver of space weather. The geo-effectiveness of a CME is primarily determined by its magnetic field strength and topology. The evolution of CMEs while propagating through the corona and the heliosphere complicates the prediction/extrapolation of the vector magnetic field of the CMEs near the Earth based essentially on photospheric measurements. Hence, the measurement of CME magnetic fields, both in the corona and heliosphere, is essential for improving space weather forecasting. Although CMEs are routinely observed by white-light coronagraphs, these observations cannot provide a direct measure of the CME-entrained vector magnetic fields. Observations at radio wavelengths, however, can provide several remote measurement tools for estimating both strength and topology of the CME magnetic fields. Among them, gyrosynchrotron (GS) emission produced by mildly- relativistic electrons trapped in CME magnetic fields is one of the promis- ing methods to estimate magnetic field strength and other plasma parameters of CMEs at lower and middle coronal heights. However, GS emissions from some parts of the CME are much fainter than the quiet Sun emission and require high dynamic range (DR) imaging for their detection. This thesis presents a state- of-the-art calibration and imaging algorithm capable of routinely producing high DR spectropolarimetric snapshot solar radio images using data from a new technology radio telescope, the Murchison Widefield Array (MWA). This allows us to detect much fainter GS emissions from CME plasma at much higher coronal heights than before. For the first time, robust circular polarization measurements have been jointly used with total intensity measurements to constrain the GS model parameters, which has significantly improved the robustness of the estimated GS model parameters. A piece of observational evidence is also found that the routinely used homogeneous and isotropic GS models may not always be sufficient to model the observations. In the future, with more sensitive measurements from the upcoming new generation telescopes and physics- based forward models, it should be possible to relax some of these assumptions and make this method more robust for estimating CME plasma parameters at coronal heights.

The Relationship of the Intensity of the SCR Proton Flux with the Parameters of Type II Solar Radio Bursts

112 solar proton events (SPEs) were processed for the period from November 24, 2000 to December 20, 2014, which were accompanied by type II radio bursts. For the analysis, used original records of solar radio emission from a solar radio spectrograph in the range of 25180 MHz, as well as original records of the flux intensity proton of solar cosmic rays (SCR) protons I_p with energy E_p in the range > 0.8-850 MeV according to data from the GOES series of devices. In this case, superimposed proton events were always separated and identified with the corresponding solar proton flares, and the maximum proton flux intensity I_p of superimposed proton events was determined from the level of the previous proton event.

Based on data from the solar radio spectrograph, regression models were obtained for 91 type II bursts that established the relationship between the frequency drift velocity V_i,j and the frequency of type II bursts f_i,j, and for 73 type II bursts it was possible to obtain regression models which established the relationship between the intensity of type II bursts I_i,j and type II burst frequency f_i,j in the range 25-180 MHz. Detailed studies have shown that the intensity of type II bursts I_i,j, as well as the frequency drift velocity V_i,j, strongly depends on the frequency of type II burst f_i,j and monotonically changes with time t_i along the harmonics of type II bursts.

As a result, the relationship between the maximum values of the SCR proton flux intensity I_p and the calculated values of the frequency drift velocity V_i,j and intensity of type II bursts I_i,j was investigated. A comparative analysis showed that the relationship between the intensity of the SCR proton flux I_p and the intensity of type II bursts I_i,j is much stronger than with the frequency drift velocity V_i,j, where the correlation coefficient r is 0.82 and 0.71, respectively, for protons with energies E_p > 30 MeV. The relationship between the proton flux intensity I_p and the frequency drift velocity V_i,j and the intensity of type II bursts I_i,j was also studied as a function of the proton energy E_p and the frequency f_i,j of type II radio bursts. It was shown that the strongest relationship between the intensity of the SCR proton flux I_p with the frequency drift velocity V_i and with the intensity of type II bursts I_i,j is observed with subrelativistic SCR protons with energies E_p in the range > 30-100 MeV and for type II radio bursts at a frequency f_i,j in the range 40-160 MHz.

Interferometric Observations of the Quiet Sun at Decameter Wavelengths Under Strong Radio Frequency Interference

Studies of the quiet Sun radio emission were carried out in a wide range of wavelengths from extremely short up to decameter ones. At the longest wavelengths, the measurements of angular sizes of the solar corona were previously carried out using the UTR-2 radio telescope in the scanning mode. We have developed a simple interferometric technique for measuring the angular diameter of an extended radio source. It uses a set of interferometers formed from the antenna sections of the north-south and east-west arms of the UTR-2 radio telescope to measure the size of the quiet Sun in the equatorial and polar directions. The first interferometric observations with this approach were carried out using the receivers and software of the URAN interferometers back in 2014. That study allowed us to determine equatorial and polar solar sizes at the fixed frequencies of 20 and 25 MHz. To expand the frequency range of the studies in the following observations, we used broadband digital DSPZ receivers in the correlation mode. However, in the daytime, broadband observations are complicated by radio frequency interference of various types, which often significantly exceed the level of wanted signals. To limit the effect of RFI, software has been developed that automatically detects and mitigates narrowband and impulse interference in a recorded signal. The paper describes the methods of RFI mitigation and criteria for the degree of signal clearing, which are used in this software. We also present the measurement results of the angular parameters of the quiet Sun radio emission, which were obtained by the interferometric method in the frequency range of 10 - 30 MHz. The observations were carried out during the minimum of solar activity in 2018 - 2020.

Self-organized criticality in solar GeV flares

The Sun emits significant flares in X-ray, ultraviolet, and radio wavelengths. It is thought to originate from the magnetic reconnection activity, which is capable of accelerating particles to high energies. The magnetic process can be described by the avalanche model of self- organized criticality (SOC), and it is evidenced by the observation. Here, we study the frequency distribution of fluence, peak flux, and duration time for solar GeV flares detected first by Fermi-Large Area Telescope. Their cumulative distributions show a power-law behaviour. The exponents are also consistent with those derived from the observations at low-energy bands, and follow the predictions of the fractal-diffuse SOC model. In the meantime, the waiting time shows power-law distribution, and agrees a non-stationary Poission process. We then explore the correlation between energy (fluence) and duration time using a two-variable regression analysis. The correlation is found to be $T_{\rm Duration} \propto F_{\rm GeV}^{0.38\pm 0.08}$ with the solar GeV flare sample, which is comparable to that of the solar X-ray flares and gamma-ray bursts (GRBs) and could be understood in an SOC model. These facts suggest that, similar to the physical process accounting for the X-ray emission of solar flares and prompt emission of GRBs, magnetic reconnection may still dominate the energy-release process and particle acceleration for solar flares at GeV energies.

Solar radio-frequency reflectivity and localization of FRB from solar reflection

The radiation of a fast radio burst (FRB) reflects from the Moon and Sun. If a reflection is detected, the time interval between the direct and reflected signals constrains the source to a narrow arc on the sky. If both Lunar and Solar reflections are detected these two arcs intersect, narrowly confining the location on the sky. A previous paper calculated reflection by the Moon. Here, we calculate the reflectivity of the Sun in the 'flat Sun' approximation as a function of angle of incidence and frequency. The reflectivity is high at low frequencies ($\lessapprox 100\,$MHz) and grazing incidence (angles 60), but exceeds 0.1 for frequencies $\lessapprox 80\,$ MHz at all angles. However, the intense thermal emission of the Solar corona likely precludes detection of the Solar reflection of even MJy Galactic bursts like FRB 200428.

Rediscovering the observations of solar prominences from 1906 to 1957 recorded at the Madrid Astronomical Observatory

The Madrid Astronomical Observatory implemented a solar observation program from 1876 to 1986. In addition to sunspots, the observers at this observatory recorded other solar features such as prominences. In this work, we have consulted the documentary sources of the Madrid Astronomical Observatory (the information is not digitally available), digitized the records of the observers on the annual number of prominences, and constructed a homogeneous series of the total and hemispheric annual number of prominences with heights of 25 and more for the period 1906-1957. To evaluate the quality of the data and assess their potential, we have compared the Madrid prominence series with the number of prominences recorded by the Astronomical Observatory of the University of Coimbra and other time series such as the sunspot number index, solar radio flux at 10.7 cm, and sunspot areas. We have also analyzed the hemispheric prominence numbers and the asymmetry index. We obtained the strongest correlation between Madrid and Coimbra prominence series (r = 0.7), whereas the correlations between the Madrid prominence series and the other solar activity time series are similar (r 0.6). In addition, we found that the correlation coefficient between the Madrid prominence series and the sunspot number is lower than that from the Coimbra prominence series and the sunspot number. We suggest that these differences are a consequence of the way prominences were counted in the Madrid Astronomical Observatory.

The effects of solar radio bursts on frequency bands utilised by the aviation industry in Sub-Saharan Africa

Solar radio bursts have been associated with a number of disruptions in avionic systems. The objective of this work is to develop solar radio burst interference thresholds which account for the technical specifications of aviation-related instrumentation, instrument operating frequencies as well as industry stipulated error tolerances. Solar radio bursts are suggested to be potentially hazardous when exceeding these calculated thresholds. Particular attention is paid to the radio altimeter, an important component in aviation safety. The thresholds suggested in this work for VHF communication, GPS navigation receivers and radio altimeter frequencies are; 10², 10³, and 10⁴ sfu respectively. Solar radio burst interference (for solar radio bursts above 10⁴ sfu) is shown to result in large errors (64-251 m) in the altitude estimates for the Frequency Modulated Continuous Waves (FMCW) radio altimeter simulated in this work.

Ionospheric scintillation characteristics from GPS observations over Malaysian region after the 2011 Valentine's day solar flare

Ionospheric scintillations due to plasma irregularities can severely affect the modern dynamic and technological systems whose operations rely on satellite-based navigation systems. We investigate the occurrence of ionospheric scintillation in the equatorial and low latitude region over Malaysia after the 2011 Valentine's Day solar flare. A network of three Global Ionospheric Scintillation and Total Electron Content Monitor (GISTM) GSV4004B receivers with increasing latitudes from the magnetic equator were used to monitor ionospheric TEC, rate of change of TEC index (ROTI), and amplitude (S4) as well as phase ( ) scintillation indices. The results show a simultaneous sudden rise in S4 and along with a significant depletion of TEC at all three locations. However, the largest enhancement of scintillation indices accompanying a substantial TEC depletion is observed at the farthest low latitude station (UNIMAS) from the equator with values around 0.5, 0.3 rad, and 1 TECU, respectively. The corresponding values at the near-equatorial station (Langkawi; 0.4, 0.2 rad, and 3 TECU) and intermediate station (UKM; 0.45, 0.3 rad, and 5 TECU) are examined along with ROTI variations, confirming the simultaneous occurrence of kilometer-scale and sub kilometer scale irregularities during 17 and 18 February 2011. The radiation effects of the solar flare on the ionosphere were prominently recognized at the local nighttime hours (around 14:00 to 17:00 UT) coinciding with the equatorial prereversal enhancement (PRE) time to seed the equatorial plasma bubbles (EPBs) enhancement that resulted in ionospheric irregularities over the low latitudes. The significant TEC depletion seen in the signals from selected GPS satellites (PRNs 11, 19, 23, and 32) suggests plausible degradation in the performance of GPS- based services over the Malaysian region. The study provides an effective understanding of the post-flare ionospheric irregularities during an episode of minor geomagnetic storm period and aligns with the efforts for mitigating the scintillation effects in space-based radio services over low latitudes.

The Sun Seen with the Atacama Large mm and sub-mm Array (ALMA) -First Results *

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Spatio-Temporal Comparisons of the Hydrogen-Alpha Line Width and ALMA 3 mm Brightness Temperature in the Weak Solar Network

Comparisons between the Atacama Large Millimeter/sub-millimeter Array (ALMA) 3mm emission and a range of optical and UV solar observations have found the strongest correspondence between the width of the hydrogen alpha line at 656.3nm and the 3mm brightness temperature. Previous studies on the oscillatory power of \pmode{}s using ALMA Band 3 and Band 6 data in the 3 to 5 minute period bandpass have found a confusing mix of results, with many reporting a complete lack of the \pmode{} enhancement typically found in other chromospheric observables. We study these issues using an extensive, publicly available coordinated data set targeting a region of weak network flux near disk center at time SOL2017-03-17T15:42-16:45. We focus on the Interferometric Bidimensional Spectropolarimeter (IBIS) H-alpha and ALMA 3mm data series. We confirm the strong correlation between the H-alpha line width and the 3mm brightness temperature, but find a bimodal relation between the two diagnostics, with a shallower slope of 7.4e-5\AA{}/K in cooler regions and steeper slope of 1.2e-4\AA{}/K in hotter regions. The origin of the bimodal distribution is unknown, but does hold for the duration of the observations. Both slopes are steeper than a previously reported value, but this is likely due to systematic differences in the analysis. We then calculate the oscillatory power in the H-alpha and 3mm data. The IBIS data clearly show the \pmode{} oscillations in spatially averaged power spectra while the ALMA data do not. However, when we remove IBIS data at times corresponding to the ALMA calibration windows, the spatially averaged power spectra for the two data series are nearly identical, with a Pearson correlation coefficient of 0.9895. Further, the power in the two bands remains strongly correlated when the spatial information is retained but the power is integrated over different temporal frequency bands. We therefore argue that the lack of observed \pmode{}s in the ALMA data may be predominantly due to spectral windowing induced by the timing and duration of the calibration observations. Finally, we find that spatial maps of oscillatory power at 3mm display the pattern of magnetic shadows and halos typically displayed by other chromospheric diagnostics.

Effects of Solar Extreme Ultraviolet Radiation on Thermospheric Neutral Density.

The Solar Extreme Ultraviolet (EUV) radiation is well known as the major source of energy of driving the Earth atmospheric motion and transformation. The 10.7 cm solar radio flux (F_10.7) instead of EUV measurement is often used in the thermospheric neutral density models, to calculate the thermosphere density and temperature. In this paper, the continuous measurements of EUV from SEM instruments onboard the SOHO satellite during 2001-2021 are directly compared with the densities derived from accelerometers onboard the CHAMP, GRACE-A and SWARM-C satellites, to reveal the essential relation between the solar EUV radiation and the thermosphere density. It is found that the correlation coefficients between the EUV data and the densities are obviously greater than those between the F_10.7 indexes and the densities, which proves that the solar EUV radiation is the dominant energy source of heating the thermosphere rather than the radio radiation. From the liner regression slopes at different local time, it is shown that the maximum value occurs at 15:00 LT (corresponding to 14:00-16:00 LT), while the minimum value at 01:00 LT (00:00-02:00 LT), which confirms that the EUV radiation derives the diurnal variation of the thermosphere. According to the statistical results from three satellites with different orbital altitudes, it is indicated that in the 350~500 km altitude region, the heating effect of the EUV radiation at lower altitude is more intense than that at higher altitude (considering absolute variation). Because of the high spatial resolution of the observation of three satellites, the difference in the effect of the EUV radiation at various local time and geographic latitudes can be concluded: the slope in sumer hemisphere, which is not described by the thermosphere models such as DTM2000 and NRLMSISE00 yet. In order to improve the modeling of the effect of the EUV radiation, the fitting method of six-order spherical harmonics is proposed. Compared with the formulas of the present models, the spherical harmonics are more valuable in improving the model construction or correction of the EUV effect and the thermosphere diurnal variation. At last, the physical mechanism of statistical results is discussed by considering the energy transmission and meridional circulation in the thermosphere.

High Dynamic Range Solar Radio Imaging Based on Deconvolution Using Prolate Spheroidal Wave Functions

When the synthetic aperture radio telescope is used to observe the Sun, faint sources can be revealed by accurately removing the bright extended sources. Moreover, high dynamic range imaging can be achieved. The inherent limitations of using image pixels as basis functions in the CLEAN algorithm commonly used in radio astronomy lead to poor results for modeling extended sources. In this paper, a deconvolution method based on Prolate Spheroidal Wave Functions (PSWF) is applied to solar radio imaging to overcome the limitations of the CLEAN algorithm. The optimal PSWF orthogonal basis is determined by the Region of Interest (ROI) in dirty images and UV coverage of the interferometric array. The PSWF orthogonal basis is applied to the deconvolution of the solar radio images observed by the uniform circular array to confirm its efficiency. Moreover, the performance of the CLEAN algorithm and the method using PSWF were quantitatively compared from two aspects including dynamic range and fidelity. The faint sources in the residual dirty images produced by deconvolution using PSWF orthogonal basis are closer to the true model. A higher dynamic range imaging can also be obtained by using PSWF.

Solar Coronal Density Turbulence and Magnetic Field Strength at the Source Regions of Two Successive Metric Type II Radio Bursts

We report spectral and polarimeter observations of two weak, low- frequency (85-60 MHz) solar coronal type II radio bursts that occurred on 2020 May 29 within a time interval 2 minutes. The bursts had fine structures, and were due to harmonic plasma emission. Our analysis indicates that the magnetohydrodynamic shocks responsible for the first and second type II bursts were generated by the leading edge (LE) of an extreme-ultraviolet flux rope/coronal mass ejection (CME) and interaction of its flank with a neighboring coronal structure, respectively. The CME deflected from the radial direction by 25 during propagation in the near-Sun corona. The estimated power spectral density and magnetic field strength (B) near the location of the first burst at heliocentric distance r 1.35 R are 2 10^-3 W²m and 1.8 G, respectively. The corresponding values for the second burst at the same r are 10^-3 W²m and 0.9 G. The significant spatial scales of the coronal turbulence at the location of the two type II bursts are 62-1 Mm. Our conclusions from the present work are that the turbulence and magnetic field strength in the coronal region near the CME LE are higher compared to the corresponding values close to its flank. The derived estimates of the two parameters correspond to the same r for both the CME LE and its flank, with a delay of 2 minutes for the latter.

The First Flare Observation with a New Solar Microwave Spectrometer Working in 35-40 GHz

The microwave spectrum contains valuable information about solar flares. Yet, the present spectral coverage is far from complete and broad data gaps exist above 20 GHz. Here we report the first flare (the X2.2 flare on 2022 April 20) observation of the newly built Chashan Broadband Solar millimeter spectrometer (CBS) working from 35 to 40 GHz. We use the CBS data of the new Moon to calibrate, and the simultaneous NoRP data at 35 GHz to cross-calibrate. The impulsive stage has three local peaks with the middle one being the strongest and the maximum flux density reaches ~9300 solar flux unit at 35-40 GHz. The spectral index of the CBS data ( _C) for the major peak is mostly positive, indicating the gyrosynchrotron turnover frequency ( _t) goes beyond 35-40 GHz. The frequency _t is smaller yet still larger than 20 GHz for most of the other two peaks according to the spectral fittings with NoRP-CBS data. The CBS index manifests the general rapid-hardening- then-softening trend for each peak and gradual hardening during the decay stage, agreeing with the fitted optically thin spectral index ( _tn) for _t < 35 GHz. In addition, the obtained turnover frequency ( _t) during the whole impulsive stage correlates well with the corresponding intensity (I _t) according to a power-law dependence ( ${I}_{t}\propto {\nu }_{t}^{4.8}$ ) with a correlation coefficient of 0.82. This agrees with earlier studies on flares with low turnover frequency (17 GHz), yet it is being reported for the first time for events with a high turnover frequency (20 GHz).

The 18-19 March 2022 series of ³He-rich events observed by Solar Orbiter at 0.36 au compared with EUV, X-ray, and radio observations

Context. During the first close perihelion pass of Solar Orbiter, a series of impulsive ³He-rich solar particle events was observed on 18-19 March 2022 from a distance of 0.36 au. In addition to the energetic particle, radio, and X-ray data from Solar Orbiter, the events were observed in radio and/or extreme ultraviolet by STEREO-A, SDO, Wind, and Parker Solar Probe.
Aims: Observations of the event series along with remote sensing of flaring and radio emission with only small timing delays due to the close distance allow the association with energetic particles to be determined with much higher accuracy than previously possible from 1 au.
Methods: By comparing the onsets of type-III bursts with the arrival of electrons of tens of keV at Solar Orbiter only a few minutes later, it can be seen that, overall, each of the more intense type-III bursts was associated with an electron and ion injection. Extreme ultraviolet data show that the times of the type-III bursts coincide with emission from a small (approximately Earth-sized) loop to the west of a nearby active region.
Results: The energetic particle spectra and abundances show typical properties of impulsive ³He-rich flares and, when combined with the remote sensing observations, establish that the particle-accelerating mechanism in this series of events operates near the solar surface in association with magnetic loops, and in the absence of other phenomena such as jets and small coronal mass ejections.

Movie associated to Fig. 4 is available at https://www.aanda.org.

Flares detected in ALMA single-dish images of the Sun

Context. The millimeter and submillimeter radiation of solar flares is poorly understood. Without spatial resolution, millimeter emission cannot be easily compared to flare emission in other wavelengths. Though the Atacama Large Millimeter-submillimeter Array (ALMA) offers sufficient resolution for the first time, ALMA cannot be used on demand to observe when a flare occurs, and when used as an interferometer, its field of view is smaller than an active region.
Aims: We used readily available large-scale single-dish ALMA observations of solar millimeter flares and compared them to well-known features observed in other wavelengths. The properties of these other flare emissions, correlating in space and time, could then be used to interpret the millimeter brightenings and vice versa. The aim is to obtain reliable associations limited by the time and space resolution of single-dish observations.
Methods: Ordinary interferometric ALMA observations require single-dish images of the full Sun for calibration. We collected such observations at 3 mm and 1 mm and searched for millimeter brightenings during times listed in a flare catalog.
Results: All of the flares left a signature in millimeter waves. We found five events with nine or more images that could be used for comparison in time and space. The millimeter brightenings are associated with a variety of flare features in cool (H, 304 ), intermediate (171 ), and hot (94 ) lines. In several cases, the millimeter brightening peaked at the footpoint of a hot flare loop. In other cases the peak of the millimeter brightening coincided with the top or footpoint of an active H filament. We also found correlations with post-flare loops and the tops of a hot loop. In some images, the millimeter radiation peaked at locations where no feature in the selected lines was found.
Conclusions: The wide field of view provided by the single-dish ALMA observations allowed for a complete overview of the flare activity in millimeter waves for the first time. The associated phenomena often changed in type and location during the flare. The variety of phenomena detected in these millimeter observations may explain the sometimes bewildering behavior of millimeter flare emissions previously observed without spatial resolution.

Deep solar ALMA neural network estimator for image refinement and estimates of small-scale dynamics

Context. The solar atmosphere is highly dynamic, and observing the small-scale features is valuable for interpretations of the underlying physical processes. The contrasts and magnitude of the observable signatures of small-scale features degrade as angular resolution decreases.
Aims: The estimates of the degradation associated with the observational angular resolution allows a more accurate analysis of the data.
Methods: High-cadence time-series of synthetic observable maps at = 1.25 mm were produced from three-dimensional magnetohydrodynamic Bifrost simulations of the solar atmosphere and degraded to the angular resolution corresponding to observational data with the Atacama Large Millimeter/sub-millimeter Array (ALMA). The deep solar ALMA neural network estimator (Deep-SANNE) is an artificial neural network trained to improve the resolution and contrast of solar observations. This is done by recognizing dynamic patterns in both the spatial and temporal domains of small-scale features at an angular resolution corresponding to observational data and correlated them to highly resolved nondegraded data from the magnetohydrodynamic simulations. A second simulation, previously never seen by Deep-SANNE, was used to validate the performance.
Results: Deep-SANNE provides maps of the estimated degradation of the brightness temperature across the field of view, which can be used to filter for locations that most probably show a high accuracy and as correction factors in order to construct refined images that show higher contrast and more accurate brightness temperatures than at the observational resolution. Deep-SANNE reveals more small-scale features in the data and achieves a good performance in estimating the excess temperature of brightening events with an average of 94.0% relative to the highly resolved data, compared to 43.7% at the observational resolution. By using the additional information of the temporal domain, Deep-SANNE can restore high contrasts better than a standard two-dimensional deconvolver technique. In addition, Deep-SANNE is applied on observational solar ALMA data, for which it also reveals eventual artifacts that were introduced during the image reconstruction process, in addition to improving the contrast. It is important to account for eventual artifacts in the analysis.
Conclusions: The Deep-SANNE estimates and refined images are useful for an analysis of small-scale and dynamic features. They can identify locations in the data with high accuracy for an in-depth analysis and allow a more meaningful interpretation of solar observations.

Using the slope of the brightness temperature continuum as a diagnostic tool for solar ALMA observations

Context. The intensity of radiation from the solar atmosphere at millimetre wavelengths is closely related to the plasma temperature, and the formation height of the radiation is wavelength dependent. It follows from this that the slope of the intensity continuum, or the brightness temperature continuum, samples the local gradient of the gas temperature of the sampled layers in the solar atmosphere.
Aims: We aim to show the added information and diagnostics potential of the solar atmosphere that comes with measuring the slope of the brightness temperature continuum.
Methods: We used solar observations from the Atacama Large Millimeter/sub-millimeter Array (ALMA) and estimated and predicted the slope using a numerical three-dimensional radiation- magnetohydrodynamic simulation. The slope was estimated by the differences between observables at wavelengths corresponding to different sub-bands at opposite sides of the ALMA receiver band 3 (2.8-3.2 mm) and band 6 (1.20-1.31 mm).
Results: The sign of the brightness temperature slope indicates temperature changes with increasing height at the sampled layers. A positive sign implies an increase in temperature, while a negative sign implies a temperature decrease. The differences in brightness temperature between the sub- bands across the field of view of the simulation typically span from 0.4 kK to 0.75 kK for band 3 and 0.2 kK to 0.3 kK at band 6. The network patches are dominated by large positive slopes, while the quiet- Sun region shows a mixture of positive and negative slopes. As the slope of the continuum is coupled to the small-scale dynamics, a negative slope is seen typically under quiet-Sun conditions as a result of propagating shock waves and the corresponding post-shock regions. The temporal evolution of the slopes can therefore be used to identify shocks. The observability of the slope of the brightness temperatures is estimated at bands 3 and 6 for different angular resolutions corresponding to ALMA observations. The simulations also show that the intensity of the radiation at bands 3 and 6 can simultaneously originate from several major components at different heights, which is strongly dependent on the small-scale dynamics and is seen in both quiet-Sun and network patches. Our in-depth analysis of selected shock waves that propagating upward in the atmosphere shows that the delay of shock signatures between two wavelengths (e.g., bands 6 and 3) does not necessarily reflect the propagation speed of the shock front, but might be cause by the rate of change in opacity of higher layers at these wavelengths.
Conclusions: The slope of the brightness temperature continuum sampled at different ALMA receiver sub-bands serves as an indicator of the slope of the local plasma temperature at the sampled heights in the atmosphere. This offers new diagnostic possibilities for measuring the underlying physical properties of small-scale dynamic features and thus contributes to the understanding of these features and the related transport of energy and heat in the chromosphere.

Interferometric imaging of the type IIIb and U radio bursts observed with LOFAR on 22 August 2017

Context. The Sun is the source of different types of radio bursts that are associated with solar flares, for example. Among the most frequently observed phenomena are type III solar bursts. Their radio images at low frequencies (below 100 MHz) are relatively poorly studied due to the limitations of legacy radio telescopes.
Aims: We study the general characteristics of types IIIb and U with stria structure solar radio bursts in the frequency range of 20-80 MHz, in particular the source size and evolution in different altitudes, as well as the velocity and energy of electron beams responsible for their generation.
Methods: In this work types IIIb and U with stria structure radio bursts are analyzed using data from the LOFAR telescope including dynamic spectra and imaging observations, as well as data taken in the X-ray range (GOES and RHESSI satellites) and in the extreme ultraviolet (SDO satellite).
Results: In this study we determined the source size limited by the actual shape of the contour at particular frequencies of type IIIb and U solar bursts in a relatively wide frequency band from 20 to 80 MHz. Two of the bursts seem to appear at roughly the same place in the studied active region and their source sizes are similar. It is different in the case of another burst, which seems to be related to another part of the magnetic field structure in this active region. The velocities of the electron beams responsible for the generation of the three bursts studied here were also found to be different.

a novel remote sensing approach for coronal loops

Type U radio bursts are impulsive coherent radio emissions produced by the Sun that indicate the presence of subrelativistic electron beams propagating along magnetic loops in the solar corona. In this work, we present the analysis of a type U radio burst that was exceptionally imaged on 2011 March 22 by the Nanay Radioheliograph (NRH) at three different frequencies (298.7, 327.0, and 360.8 MHz). Using a novel modelling approach, we show for the first time that the use of high- resolution radio heliograph images of type U radio bursts can be sufficient to both accurately reconstruct the 3D morphology of coronal loops (without recurring to triangulation techniques) and to fully constrain their physical parameters. At the same time, we can obtain unique information on the dynamics of the accelerated electron beams, which provides important clues as to the plasma mechanisms involved in their acceleration and as to why type U radio bursts are not observed as frequently as type III radio bursts. We finally present plausible explanations for a problematic aspect related to the apparent lack of association between the modeled loop as inferred from radio images and the extreme-ultraviolet (EUV) structures observed from space in the same coronal region.

The first gradual solar energetic particle event with an enhanced ³He abundance on Solar Orbiter

The origin of ³He abundance enhancements in gradual solar energetic particle (SEP) events remains largely unexplained. Two mechanisms have been suggested: the reacceleration of remnant flare material by coronal mass ejection (CME)-driven shocks in interplanetary space, and concomitant activity in the corona. We explore the first gradual SEP event with enhanced ³He abundance that was observed by Solar Orbiter. The event started on 2020 November 24 and was associated with a relatively fast halo CME. During the event, the spacecraft was at 0.9 au from the Sun. The event-averaged ³He/⁴He abundance ratio is 24 times higher than the coronal or solar wind value, and the timing of the ³He intensity was similar to that of other species. We inspected available imaging, radio observations, and the spacecraft magnetic connection to the CME source. The most probable cause of the enhanced ³He abundance apparently are residual ³He ions remaining from a preceding long period of ³He-rich SEPs on 2020 November 17-23.

Classification of Solar Radio Spectrum Based on Swin Transformer

Solar radio observation is a method used to study the Sun. It is very important for space weather early warning and solar physics research to automatically classify solar radio spectrums in real time and judge whether there is a solar radio burst. As the number of solar radio burst spectrums is small and uneven, this paper proposes a classification method for solar radio spectrums based on the Swin transformer. First, the method transfers the parameters of the pretrained model to the Swin transformer model. Then, the hidden layer weights of the Swin transformer are frozen, and the fully connected layer of the Swin transformer is trained on the target dataset. Finally, parameter tuning is performed. The experimental results show that the method can achieve a true positive rate of 100%, which is more accurate than previous methods. Moreover, the number of our model parameters is only 20 million, which is 80% lower than that of the traditional VGG16 convolutional neural network with more than 130 million parameters.

Self-Supervised Learning for Solar Radio Spectrum Classification

Solar radio observation is an important way to study the Sun. Solar radio bursts contain important information about solar activity. Therefore, real-time automatic detection and classification of solar radio bursts are of great value for subsequent solar physics research and space weather warnings. Traditional image classification methods based on deep learning often require considerable training data. To address insufficient solar radio spectrum images, transfer learning is generally used. However, the large difference between natural images and solar spectrum images has a large impact on the transfer learning effect. In this paper, we propose a self-supervised learning method for solar radio spectrum classification. Our method uses self-supervised training with a self-masking approach in natural language processing. Self-supervised learning is more conducive to learning the essential information about images compared with supervised methods, and it is more suitable for transfer learning. First, the method pre-trains using a large amount of other existing data. Then, the trained model is fine- tuned on the solar radio spectrum dataset. Experiments show that the method achieves a classification accuracy similar to that of convolutional neural networks and Transformer networks with supervised training.

Common Origin of Quasi-Periodic Pulsations in Microwave and Decimetric Solar Radio Bursts

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Storm-Time Relative Total Electron Content Modelling Using Machine Learning Techniques

Accurately predicting total electron content (TEC) during geomagnetic storms is still a challenging task for ionospheric models. In this work, a neural-network (NN)-based model is proposed which predicts relative TEC with respect to the preceding 27-day median TEC, during storm time for the European region (with longitudes 30W-50E and latitudes 32.5N-70N). The 27-day median TEC (referred to as median TEC), latitude, longitude, universal time, storm time, solar radio flux index F10.7, global storm index SYM-H and geomagnetic activity index Hp30 are used as inputs and the output of the network is the relative TEC. The relative TEC can be converted to the actual TEC knowing the median TEC. The median TEC is calculated at each grid point over the European region considering data from the last 27 days before the storm using global ionosphere maps (GIMs) from international GNSS service (IGS) sources. A storm event is defined when the storm time disturbance index Dst drops below 50 nanotesla. The model was trained with storm-time relative TEC data from the time period of 1998 until 2019 (2015 is excluded) and contains 365 storms. Unseen storm data from 33 storm events during 2015 and 2020 were used to test the model. The UQRG GIMs were used because of their high temporal resolution (15 min) compared to other products from different analysis centers. The NN-based model predictions show the seasonal behavior of the storms including positive and negative storm phases during winter and summer, respectively, and show a mixture of both phases during equinoxes. The model's performance was also compared with the Neustrelitz TEC model (NTCM) and the NN-based quiet-time TEC model, both developed at the German Aerospace Agency (DLR). The storm model has a root mean squared error (RMSE) of 3.38 TEC units (TECU), which is an improvement by 1.87 TECU compared to the NTCM, where an RMSE of 5.25 TECU was found. This improvement corresponds to a performance increase by 35.6%. The storm-time model outperforms the quiet-time model by 1.34 TECU, which corresponds to a performance increase by 28.4% from 4.72 to 3.38 TECU. The quiet-time model was trained with Carrington averaged TEC and, therefore, is ideal to be used as an input instead of the GIM derived 27-day median. We found an improvement by 0.8 TECU which corresponds to a performance increase by 17% from 4.72 to 3.92 TECU for the storm-time model using the quiet-time-model predicted TEC as an input compared to solely using the quiet-time model.

On some features of the solar proton event on 2021 October 28 - GLE73

In addition to several recent articles devoted to the rare event of ground-level enhancement of the solar relativistic proton flux observed on 2021 October 28 - GLE73, we study the 10-100 MeV solar energetic particle (SEP) component of this event. Based on the Geostationary Operational Environmental Satellite data for 26 GLEs recorded since 1986, we have formed a scatter plot displaying the ratio of the peak fluxes of the $ $10 MeV ($J_{10}$) and $ $100 MeV ($J_{100}$) protons and their energy spectra. Two extreme characteristics of the prompt component of the SEP-GLE73 event were revealed: (1) very small $J_{10}$ and $J_{100}$ proton fluxes and (2) a very hard energetic spectrum in the 10-100 MeV range. There are only two events with these characteristics similar to SEP-GLE73, namely GLE40 (1989 July 25) and GLE46 (1989 November 15). A correspondence was demonstrated between the hard frequency spectrum of microwave radio bursts of initiating flares and the hard SEP energy spectrum of these two and other GLEs. These results suggest that the flare magnetic reconnection both in the impulsive and post-eruption phases plays an important role in the acceleration of the SEP-GLE protons.

Complex Type II Solar Radio Event on 4 July 2022

On 4 July 2022, a complex low-frequency solar radio burst was observed in Metshovi Radio Observatory of Aalto University. The radio burst was observed at a frequency range between 20 and 80 MHz. In GOES (Geostationary Operational Environmental Satellite) class, the event was classified as C5.1. However, coronal mass ejection (CME) was not associated to this event. The observed radio burst was a long-lasting (~10 minutes) event, and it could be mainly classified as type II solar radio event. Also type III solar events were observed before long- lasting type II event. The event includes common frequency drifting emission structures, both fundamental and harmonic structures, but also rarely observed continuum-like or stationary structure. It is assumed that the continuum-like radio emission structure is originated from the stationary flare (coronal) loop, which was visible over the whole event. The drifting emission structure means accelerated electrons, which are produced by the shock related phenomena. The paper provides the observations from this event on radio wavelength, and also soft-X-ray regime and optical wavelength (AIA 171). In addition, a possible, simplified scenario is presented for forming the drifting and continuum solar radio emissions in type II solar burst.

Characterization and Climatological Modeling of Equatorial Ionization Anomaly (EIA) Crest Position

The Equatorial Ionization Anomaly (EIA) is characterized by strong ionospheric gradients that complicate ionospheric modeling and mitigation of ionospheric impact on Global Navigation Satellite System applications. In this study, the variation of EIA crest locations as a function of longitude and solar radio flux index F10.7 has been derived and modeled. Seven years (2013-2019) of ionospheric NmF2 and in-situ ion density data from Constellation Observing System for Meteorology Ionosphere and Climate and Swarm-A satellites have been used. We find that the distance between the northern and southern crest of EIA grows slightly with increase of solar flux. The position of the crests of EIA follows a wave number 4 structure with peak values at around 130, 0, and 100E. At the altitude of Swarm-A (460 km), the equatorial crests are found closer to the geomagnetic dip equator than they are at the peak density height hmF2 (310 km) which may be due to the spatial variations of geomagnetic field strength. Accordingly, two Crest Position climatological Models have been developed, one for the F2 Peak height (CPM_F2P) and the other one for Swarm-A height (CPM_SAH).

India's first radio telescope

India's first radio telescope was the Kalyan Array, a T-configuration solar grating array erected near Bombay (Mumbai) in 1965. This was then used until 1968 to monitor solar radio emission at 612 MHz. The antennas from this radio telescope originally formed the E-W solar grating interferometer invented by the Australian radio astronomer W.N. (Chris) Christiansen, and were sited at Potts Hill in Sydney, a field station Keywords: Govind Swarup, Potts Hill, W.N Christiansen, solar grating arrays, Kalyan Array, coronal emission, radio plages, Ooty Radio Telescope maintained by the CSIRO's Division of Radiophysics. In early 1953 Govind Swarup (along with R. Parthasarathy) went to Sydney on a 2-year Colombo Plan Fellowship, and was involved in reducing observations made by this array and a companion N-S grating array. Then, in 1954, Govind and Parthasarathy altered the array so that it could operate at 500 MHz, and they used it to examine the extent of limb- brightening at this frequency. When both arrays became surplus to requirements in March 1955 Govind convinced the Division of Radiophysics to donate the E-W array antennas to the Physical Research Laboratory in New Delhi. This eventually occurred, and after some delay and the transfer of the antennas to the Tata Institute of Fundamental Research in Bombay they were erected nearby at Kalyan. In this paper I review Govind Swarup's Sydney 'apprenticeship' and his substantial involvement with the Potts Hill E-W solar grating array; his subsequent return to India and intervals he then spent in the USA at Fort Davis (Harvard University) and Stanford University (where he completed a solar radio astronomy PhD); his return to India to found the Tata Institute's radio astronomy group; erection of the solar grating array at Kalyan, and the solar research that was carried out with it; and - finally - how the Kalyan Array was always perceived as a training facility and forerunner to the much more ambitious Ooty Radio Telescope.

Sunspot periodicity

The Schwabe (~11 yr) value for the annual sunspot number is sometimes uncritically applied to other measures of solar activity, direct and indirect, including the 10.7 cm radio flux, the inflow of galactic cosmic rays, solar flare frequency, terrestrial weather, and components of space climate, with the risk of a resulting loss of information. The ruling (Babcock) hypothesis and its derivatives link the sunspot cycle to dynamo processes mediated by differential solar rotation, but despite 60 years of observation and analysis the ~11 yr periodicity remains difficult to model; the possible contribution of planetary dynamics is undergoing a revival. The various solar sequences that genuinely display an~11 yr cycle stand to benefit from an understanding of its periodicity that goes beyond statistical kinship. The outcome could ironically prompt the demotion of sunspots from their dominant historical role in favour of other possible indicators of solar cyclicity, such as the solar wind flux and its isotopic signatures, even if they are less accessible.

Aeronomic and Dynamic Correction of the Global Model GTEC for Disturbed Conditions

An aeronomic and dynamic correction of the GTEC median global model of the total electron content for disturbed conditions (Ap 15 nT) is proposed. The GTEC global median model is developed for quiet conditions (Ap < 15 nT) as a function of the geographic coordinates, universal time UT, day of the year, and solar activity level (the solar radio emission flux F10.7). The model is based on a spherical harmonic analysis of the GIM-TEC Global Ionospheric Maps (19962019) provided by the Jet Propulsion Laboratory (NASA). The proposed GDMTEC global dynamic model (Global Dynamic Model of TEC) consists of the GTEC median model updated with several dynamic and aeronomic corrections related to formation of the main ionospheric trough, position of the auroral ionization maximum and changes of the thermospheric temperature and composition. The advantage of the proposed corrections of the median model is the independence of forecast of the data in real time from assimilation of the current observational data. Testing of the model for disturbed conditions shows an improvement of the forecast compared to the IRI-Plas ionospheric reference model.

Assessment of long-term impact of solar activity on the ionosphere over an African equatorial GNSS station

The level of solar activity and its influence on ionospheric variability has received a focal interest among the ionospheric community. This paper examines the impact of long-term solar activity on the ionosphere over an African equatorial global navigation satellite system (GNSS) station based on a large database of solar and ionospheric datasets. The ionospheric total electron content (TEC) periodic variations, climatology and its relation to solar activity are investigated through wavelet analyses. It is observed that the magnitude and intensity of TEC fluctuations measured by GNSS receivers vary substantially with solar radiation intensity. To probe the processes accountable for the TEC periodicities in the region, the wavelet analysis is undertaken by considering solar indices (solar radio flux and sunspot number) and observed TEC values. The results show a clear strong periodicity of 27 days in the ionospheric TEC, sunspots and F10.7 solar flux power spectra, associated with the sun's 27 days rotation period. The ultraviolet ray intensity and solar ionization were lower during the solar maximum period (2013 to 2014) of solar cycle 24 compared to the solar maximum period (2001 to 2002) of solar cycle 23, which is the key factor for driving the decreasing trend in TEC between 1999 and 2017. The ionospheric and solar climatology analysis indicates that ionospheric TEC behaviour could be a good indicator of long-term solar activity trend. A large mass of Africa lies within the equatorial and low latitudes making the ionosphere over the region being highly susceptible to low latitude electrodynamics. Results from this study would support the concerted efforts towards developing a reliable regional ionospheric forecasting model for technological applications relying on spaced-based navigation and satellite services.

Monitoring solar activity during 23/24 solar cycle minimum through VLF radio signals

Solar activity during solar minimum between 23^rd and 24^th solar cycle was inspected, based on solar X-ray radiation listings from Geostationary Operational Environmental Satellite (GOES) database. Periods of quiet solar conditions with low background X radiation are particularly favourable for analysis and exploration of low intensity solar flare events and their effects on lower ionosphere. Low intensity solar flare events were monitored and examined through Very Low Frequency (VLF) radio signals (3-30 kHz), recorded by the use of Absolute Phase and Amplitude Logger system (AbsPAL) operating at the Institute of Physics Belgrade, Serbia. For the purposes of numerical modeling of low ionospheric response to low class solar flare events, the Long Wave Propagation Capability (LWPC) software, based on Wait's theory, was applied. Main results are presented in this paper.

Connecting Loop-top and Footpoint Hard X-Ray Sources

The acceleration and transport of energetic electrons during solar flares is one of the outstanding topics in solar physics. Recent X-ray and radio imaging and spectroscopy observations have provided diagnostics of the distribution of nonthermal electrons and suggested that, in certain flare events, electrons are primarily accelerated in the loop top and likely experience trapping and/or scattering effects. By combining the focused particle transport equation with magnetohydrodynamic (MHD) simulations of solar flares, we present a macroscopic particle model that naturally incorporates electron acceleration and transport. Our simulation results indicate that physical processes such as turbulent pitch-angle scattering can have important impacts on both electron acceleration in the loop top and transport in the flare loop, and their influences are highly energy- dependent. A spatial-dependent turbulent scattering with enhancement in the loop top can enable both efficient electron acceleration to high energies and transport of abundant electrons to the footpoints. We further generate spatially resolved synthetic hard X-ray (HXR) emission images and spectra, revealing both the loop-top and footpoint HXR sources. Similar to the observations, we show that the footpoint HXR sources are brighter and harder than the loop-top HXR source. We suggest that the macroscopic particle model provides new insights into understanding the connection between the observed loop-top and footpoint nonthermal emission sources by combining the particle model with dynamically evolving MHD simulations of solar flares.

Multiple Regions of Nonthermal Quasiperiodic Pulsations during the Impulsive Phase of a Solar Flare

Flare-associated quasiperiodic pulsations (QPPs) in radio and X-ray wavelengths, particularly those related to nonthermal electrons, contain important information about the energy release and transport processes during flares. However, the paucity of spatially resolved observations of such QPPs with a fast time cadence has been an obstacle for us to further understand their physical nature. Here, we report observations of such a QPP event that occurred during the impulsive phase of a C1.8-class eruptive solar flare using radio imaging spectroscopy data from the Karl G. Jansky Very Large Array (VLA) and complementary X-ray imaging and spectroscopy data. The radio QPPs, observed by the VLA in the 1-2 GHz with a subsecond cadence, are shown as three spatially distinct sources with different physical characteristics. Two radio sources are located near the conjugate footpoints of the erupting magnetic flux rope with opposite senses of polarization. One of the sources displays a QPP behavior with a ~5 s period. The third radio source, located at the top of the postflare arcade, coincides with the location of an X-ray source and shares a similar period of ~25-45 s. We show that the two oppositely polarized radio sources are likely due to coherent electron cyclotron maser emission. On the other hand, the looptop QPP source, observed in both radio and X-rays, is consistent with incoherent gyrosynchrotron and bremsstrahlung emission, respectively. We conclude that the concurrent, but spatially distinct QPP sources must involve multiple mechanisms which operate in different magnetic loop systems and at different periods.

Determination of the correlation coefficient of selected short-periodic comets of the Jupiter family and solar activity

The paper presents the study of the dependence between photometric parameters of selected short-period comets of the Jupiter family and the activity of the Sun. As a quantity of solar activity, we used the sunspot area, the Wolf number, the annual mean solar radio flux, the solar flare index (full disk), and the annual mean AA-index solar activity. To study the correlation between cometary and solar activity the Dobrovolsky method was used. It has been found no direct correlation between the absolute stellar magnitude and the photometric parameter of comets with individual parameters of solar activity. Moreover, the correlation coefficients show that some comets are not associated with solar activity.

Shock-accelerated electrons during the fast expansion of a coronal mass ejection

Context. Some of of the most prominent sources for energetic particles in our Solar System are huge eruptions of magnetised plasma from the Sun called coronal mass ejections (CMEs), which usually drive shocks that accelerate charged particles up to relativistic energies. In particular, energetic electron beams can generate radio bursts through the plasma emission mechanism. The main types of bursts associated with CME shocks are type II and herringbone bursts. However, it is currently unknown where early accelerated electrons that produce metric type II bursts and herringbones propagate and when they escape the solar atmosphere.
Aims: Here, we investigate the acceleration location, escape, and propagation directions of electron beams during the early evolution of a strongly expanding CME-driven shock wave associated with herrinbgone bursts.
Methods: We used ground-based radio observations from the Nanay Radioheliograph combined with space-based extreme-ultraviolet and white-light observations from the Solar Dynamics Observatory and and the Solar Terrestrial Relations Observatory. We produced a three-dimensional (3D) representation of the electron acceleration locations which, combined with results from magneto-hydrodynamic (MHD) models of the solar corona, was used to investigate the origin of the herringbone bursts observed.
Results: Multiple herringbone bursts are found close to the CME flank in plane-of-sky images. Some of these herringbone bursts have unusual inverted J shapes and opposite drifting herringbones also show opposite senses of circular polarisation. By using a 3D approach combined with the radio properties of the observed bursts, we find evidence that the first radio emission in the CME eruption most likely originates from electrons that initially propagate in regions of low Alfvn speeds and along closed magnetic field lines forming a coronal streamer. The radio emission appears to propagate in the same direction as a coronal wave in three dimensions.
Conclusions: The CME appears to inevitably expand into a coronal streamer where it meets ideal conditions to generate a fast shock which, in turn, can accelerate electrons. However, at low coronal heights, the streamer consists of exclusively closed field lines indicating that the early accelerated electron beams do not escape. This is in contrast with electrons which, in later stages, escape the corona so that they are detected by spacecraft.

Movies are available at https://www.aanda.org

A Machine Learning-Based Method for Modeling TEC Regional Temporal-Spatial Map

In order to achieve the high-accuracy prediction of the total electron content (TEC) of the regional ionosphere for supporting the application of satellite navigation, positioning, measurement, and controlling, we proposed a modeling method based on machine learning (ML) and use this method to establish an empirical prediction model of TEC for parts of Europe. The model has three main characteristics: (1) The principal component analysis (PCA) is used to separate TEC's temporal and spatial variation characteristics and to establish its corresponding map, (2) the solar activity parameters of the 12-month mean flux of the solar radio waves at 10.7 cm (F10.712) and the 12-month mean sunspot number (R12) are introduced into the temporal map as independent variables to reflect the temporal variation characteristics of TEC, and (3) The modified Kriging spatial interpolation method is used to achieve the spatial reconstruction of TEC. Finally, the regression learning method is used to determine the coefficients and harmonic numbers of the model by using the root mean square error (RMSE) and its relative value (RRMSE) as the evaluation standard. Specially, the modeling process is easy to understand, and the determined model parameters are interpretable. The statistical results show that the monthly mean values of TEC predicted by the proposed model in this paper are highly consistent with the observed values curve of TEC, and the RRMSE of the predicted results is 12.76%. Furthermore, comparing the proposed model with the IRI model, it can be found that the prediction accuracy of TEC by the proposed model is much higher than that of the IRI model either with CCIR or URSI coefficients, and the improvement is 38.63% and 35.79%, respectively.

Long-Term Relationship Between foF2 From China Ionospheric Stations and Solar Activity During the 24th Solar Activity Cycle

This paper employs the complete data of six ionospheric observation stations in China in the 24th solar activity cycle and researches the variation of frequency of the F2 layer (foF2) and the relationship with the solar activity indices. We discover that the dependence of foF2 on the sunspot number (R) and the solar radio flux of 10.7 cm wavelength (F10.7) is strongest in mid-solar years, and weakest in high solar years. At the same time, the long-term variation trends of foF2, R and F10.7 in the complete 24th solar activity cycle are analyzed using the mutual information method in information theory. We utilize the linear and nonlinear methods to compare the effects of long term change relationship between foF2 and R during the rising and falling phases of solar activity. The findings reveal that the sunspot number is more suitable to depict the long-term trend of foF2 in China, and the relationship between them is better described by nonlinear quasi- polynomial. This result has important implications for improving foF2 forecasts and the prediction accuracy of some ionospheric models.

Prospects and challenges of numerical modelling of the Sun at millimetre wavelengths

The Atacama Large Millimeter/submillimeter Array (ALMA) offers new diagnostic possibilities that complement other commonly used diagnostics for the study of our Sun. In particular, ALMA's ability to serve as an essentially linear thermometer of the chromospheric gas at unprecedented spatial resolution at millimetre wavelengths and future polarisation measurements have great diagnostic potential. Solar ALMA observations are therefore expected to contribute significantly to answering long- standing questions about the structure, dynamics and energy balance of the outer layers of the solar atmosphere. In this regard, current and future ALMA data are also important for constraining and further developing numerical models of the solar atmosphere, which in turn are often vital for the interpretation of observations. The latter is particularly important given the Sun's highly intermittent and dynamic nature that involves a plethora of processes occurring over extended ranges in spatial and temporal scales. Realistic forward modelling of the Sun therefore requires time-dependent three-dimensional radiation magnetohydrodynamics that account for non-equilibrium effects and, typically as a separate step, detailed radiative transfer calculations, resulting in synthetic observables that can be compared to observations. Such artificial observations sometimes also account for instrumental and seeing effects, which, in addition to aiding the interpretation of observations, provide instructive tools for designing and optimising ALMA's solar observing modes. In the other direction, ALMA data in combination with other simultaneous observations enables the reconstruction of the solar atmospheric structure via data inversion techniques. This article highlights central aspects of the impact of ALMA for numerical modelling for the Sun, their potential and challenges, together with selected examples.

Rapid variations of Si IV spectra in a flare observed by interface region imaging spectrograph at a sub-second cadence

We report on observations of highly-varying Si IV 1402.77 line profiles observed with the Interface Region Imaging Spectrograph (IRIS) during the M-class flare from 2022 January 18 at an unprecedented 0.8 s cadence. Moment analysis of this line observed in flare ribbon kernels showed that the intensity, Doppler velocity, and non-thermal broadening exhibited variations with periods below 10 s. These variations were found to be correlated with properties of the Gaussian fit to a well- resolved secondary component of the line redshifted by up to 70 km/s, while the primary component was consistently observed near the rest wavelength of the line. A particularly high correlation was found between the non-thermal broadening of the line resulting from the moment analysis and the redshift of the secondary component. This means that the oscillatory enhancements in the line broadening were due to plasma flows (away from the observer) with varying properties. A simple de- projection of the Doppler velocities of the secondary component based on a three-dimensional reconstruction of flare loops rooted in the kernel suggests that the observed flows were caused by downflows and compatible with strong condensation flows recently predicted by numerical simulations. Furthermore, peaks of the intensity and the trends of Doppler velocity of the Gaussian fit to the secondary component (averaged in the ribbon) were found to correspond to one of the quasi- periodic pulsations (QPPs) detected during the event in the soft X-ray flux (as measured by the Geostationary Operational Environmental Satellite, GOES) and the microwave radio flux (as measured by the Expanded Owens Valley Solar Array, EOVSA). This result supports a scenario in which the QPPs were driven by repeated magnetic reconnection.

The "SPectrogram Analysis and Cataloguing Environment" (SPACE) Labelling Tool

The SPectrogram Analysis and Cataloguing Environment (SPACE) tool is an interactive python tool designed to label radio emission features of interest in a time-frequency map (called "dynamic spectrum"). The program uses Matplotlib's Polygon Selector widget to allow a user to select and edit an undefined number of vertices on top of the dynamic spectrum before closing the shape (polygon). Multiple polygons may be drawn on any spectrum, and the feature name along with the coordinates for each polygon vertex are saved into a ".json" file as per the "Time- Frequency Catalogue" (TFCat) format along with other data such as the feature id, observer name, and data units. This paper describes the first official stable release (version 2.0) of the tool.

Polarization Observations of a Split-band Type II Radio Burst from the Solar Corona

Using temporal observations of circular polarized harmonic plasma emission from a split-band type II solar radio burst at 80 MHz, we separately estimated the coronal magnetic field strengths (B) associated with the lower (L) and upper (U) frequency bands of the burst. The corresponding Stokes I and V data were obtained with the polarimeter operating at the above frequency in the Gauribidanur observatory. The burst was associated with a flare/coronal mass ejection on the solar disk. Simultaneous spectral observations with the spectrograph there in the frequency range 80-35 MHz helped to establish that the observed polarized emission was from the harmonic component of the burst. The B values corresponding to the polarized emission from the L and U bands at 80 MHz are B _L 1.2 G and B _U 2.4 G, respectively. The different values of B for the observed harmonic emission at the same frequency (80 MHz) from the two bands imply unambiguously that the corresponding fundamental emission at 40 MHz must have originated at different spatial locations. Two-dimensional radio imaging observations of the burst with the radioheliograph in the same observatory at 80 MHz indicate the same. As comparatively higher B is expected behind a propagating shock due to compression as well as the corresponding coronal regions being closer to the Sun, our results indicate that the sources of L- and U-band emission should be located ahead of and behind the associated coronal shock, respectively. These are useful to understand the pre- and postshock corona as well as locations of electron acceleration in a propagating shock.

First Millimeter Flares Detected from Eridani with the Atacama Large Millimeter/submillimeter Array

We report the detection of three large millimeter flaring events from the nearby Sun-like, Eridani, found in archival Atacama Large Millimeter/submillimeter Array (ALMA) 12 m and Atacama Compact Array observations at 1.33 mm taken from 2015 January 17 to 18 and 2016 October 24 to November 23, respectively. This is the first time that flares have been detected from a Sun-like star at millimeter wavelengths. The largest flare among our data was detected in the ALMA observations on 2015 January 17 from 20:09:10.4-21:02:49.3 UT with a peak flux density of 28 7 mJy and a duration of 9 s. The peak brightness of the largest flare is 3.4 0.9 10¹⁴ erg s^-1Hz^-1, a factor of >50 times brighter than the star's quiescent luminosity and >10 brighter than solar flares observed at comparable wavelengths. We find changes in the spectral index (F ) at the flare peak, with = 1.81 1.94 and a lower limit on the fractional linear polarization Q/I = 0.08 0.12. This positive spectral index is more similar to millimeter solar flares, differing from M-dwarf flares also detected at millimeter wavelengths that exhibit steeply negative spectral indices.

Radio Emission Variability of Supernova Remnants and a Possible Explanation of the Phenomenon

The flux densities and current spectra of plerions and combined supernova remnants (SNRs): 3C58, 3C144, G11.2-0.3, G21.5-0.9, and 3C396 are determined on an artificial moon (AM) flux scale. The radio emission variability of the objects on different time scales is studied by multiple measurements using the RT-32 radio telescope at the Svetloe Observatory, Institute of Applied Astronomy, Russian Academy of Sciences and by mutual comparisons of the published measurement data, which have been brought to a common system using the AM flux scale. The radio emission from the SNRs displays both slow evolutionary variations and rapid ones, nonstationary in time. The rapid variability of the SNRs has features that are similar to those observed during solar flares. This fact may testify to the identity of the physical mechanisms underlying the rapid variability of the radio emission from plerions and from the Sun. Field line reconnections are considered as such a common mechanism.

Magnetic Field and Density Models in the Zebra Source Region

Using the double-plasma resonance model of solar radio zebras, we analyze five models of the magnetic field and density in the zebra source region. We present analytical relations of zebra-stripe frequencies depending on the gyro-harmonic number. By fitting of observed zebra-stripe frequencies using model frequencies, we find that the determined gyro-harmonic number and corresponding magnetic field depend on the model used. We show that all previously analyzed zebras, where the absolute value of the difference between neighboring zebra- stripe frequencies increases with respect to increasing frequency, can be well fitted by the model with exponential dependencies of the magnetic field and density or by the model with smaller gradients of both of these variables. Although these models give different results, their more sophisticated versions give more similar results. We also present the models that can fit the zebras, if observed, where the absolute value of the difference between neighboring zebra-stripe frequencies decreases with respect to increasing frequency. We check all these models by a fitting of the zebra-stripe frequencies observed in the 21 June 2011 zebra event. In one model, although it reasonably describes the conditions in the atmosphere above the active region, the fit of the observed zebra-stripe frequencies could not be made.

Solar Radio-Burst Forecast Based on a Convolutional Neural Network

A solar radio burst is the enhancement of radio emission during the release of solar magnetic energy. It is an important indicator of the level of solar activity. In this paper, we propose a solar radio-burst forecast model that takes the Solar and Heliospheric Observatory (SOHO)/Michelson Doppler Imager (MDI) full-disk solar magnetograms as inputs. The model takes advantage of a Convolutional Neural Network (CNN) to automatically extract the effective feature information from the input images. Through multiple trainings, the relationship between the magnetic-field characteristics of the full-disk solar magnetograms and the solar radio bursts is established, so it is possible to predict the presence or absence of a solar radio burst on that day. The experimental results demonstrate that the forecast Accuracy of the proposed model is 0.875 0.007. The True Skill Statistic (TSS) is 0.723 0.026, and the Heidke Skill Score (HSS) is 0.713 0.019. These results indicate the strong reliability and wide applicability of the forecast model proposed in this paper. The proposed model is also used to predict solar type-II and type-III bursts, respectively. It is found that the prediction performance for type-III bursts is better than that of type-II bursts. The result is well explained from the differences of their formation mechanisms.

Solar Flare and Radio Burst Effects on GNSS Signals and the Ionosphere During September 2017

Strong solar flare events can occur even during the decay phase of the solar cycle. During these events concurrent increases in the X-ray and Enhanced UV (EUV) fluxes and solar radio bursts (SRBs) can be observed. The SRBs cover a large range of frequencies including the L band, giving rise to signal fades in the GNSS carrier-to-noise ratio and fluctuations in its amplitude and phase. The increases in the X-ray, UV, and EUV fluxes cause increase in the ionospheric D, E, and F region electron densities. The aim of this work is to analyze the effects in the GNSS signal, in the ionosphere and in the magnetic field H component of the X9.3 and X1.3 solar flares that occurred on 06 and 07 September 2017, respectively. Data from a network of six GNSS receivers, two magnetometers, and four Digisondes are used in the analysis. Fades of about 5 and 10 dB were observed in the signals of GNSS L1 and L2/L5 frequencies, respectively. Significant positioning errors, were observed for the strongest X9.3 flare. A sudden increase in Total Electron Content with the rates of 2.5-5.0 TECU/min was observed. An increase in the E layer density gave origin to an increase in the Equatorial Electrojet intensity, whose signatures were observed in the H component of two magnetometers. Another observed effect was the ionospheric D region density increase that caused disruption in the Digisonde signal. As a consequence of the described effects, GNSS receivers may fail to produce accurate navigation solution.

A New Position Calibration Method for MUSER Images

The Mingantu Spectral Radioheliograph (MUSER), a new generation of solar dedicated radio imaging-spectroscopic telescope, has realized high-time, high-angular, and high-frequency resolution imaging of the Sun over an ultra-broadband frequency range. Each pair of MUSER antennas measures the complex visibility in the aperture plane for each integration time and frequency channel. The corresponding radio image for each integration time and frequency channel is then obtained by inverse Fourier transformation of the visibility data. However, the phase of the complex visibility is severely corrupted by instrumental and propagation effects. Therefore, robust calibration procedures are vital in order to obtain high-fidelity radio images. While there are many calibration techniques available-e.g., using redundant baselines, observing standard cosmic sources, or fitting the solar disk-to correct the visibility data for the above-mentioned phase errors, MUSER is configured with non- redundant baselines and the solar disk structure cannot always be exploited. Therefore it is desirable to develop alternative calibration methods in addition to these available techniques whenever appropriate for MUSER to obtain reliable radio images. In the case where a point- like calibration source contains an unknown position error, we have for the first time derived a mathematical model to describe the problem and proposed an optimization method to calibrate this unknown error by studying the offset of the positions of radio images over a certain period of the time interval. Simulation experiments and actual observational data analyses indicate that this method is valid and feasible. For MUSER's practical data the calibrated position errors are within the spatial angular resolution of the instrument. This calibration method can also be used in other situations for radio aperture synthesis observations.

A Study on Non-coplanar Baseline Effects for Mingantu Spectral Radioheliograph

As a dedicated solar radioheliograph, the MingantU SpEctral RadioHeliograph (MUSER) has a maximum baseline of more than 3000 m and a frequency range of 400 MHz-15 GHz. According to the classical radio interferometry theory, the non-coplanar baseline effect (i.e., w-term effect) would be considered and calibrated for such a radio instrument. However, little previous literature made the qualitative or quantitative analyses on w-term effects of solar radioheliograph in-depth. This study proposes a complete quantitative analysis of w-term effects for the MUSER. After a brief introduction of the MUSER, we systematically investigate the baseline variations over a year and analyze the corresponding variations of w-term. We further studied the effects of the w-term in the imaging for the specified extended source, i.e., the Sun. We discussed the possible effects of the w-term, such as image distortion and so on. The simulated results show that the w-term is an essential and unavoidable issue for solar radio imaging with high spatial resolution.

Modulation of the solar microwave emission by sausage oscillations

The modulation of the microwave emission intensity from a flaring loop by a standing linear sausage fast magnetoacoustic wave is considered in terms of a straight plasma slab with the perpendicular Epstein profile of the plasma density, penetrated by a magnetic field. The emission is of the gyrosynchrotron (GS) nature, and is caused by mildly relativistic electrons that occupy a layer in the oscillating slab, i.e. the emitting and oscillating volumes do not coincide. It is shown that the microwave response to the linear sausage wave is highly non-linear. The degree of the non-linearity, defined as a ratio of the Fourier power of the second harmonic to the Fourier power of the principal harmonic, is found to depend on the combination of the width of the GS source and the viewing angle, and is different in the optically thick and optically thin parts of the microwave spectrum. This effect could be considered as a potential tool for diagnostics of the transverse scales of the regions filled in by the accelerated electrons.

Searching for stellar flares from low-mass stars using ASKAP and TESS

Solar radio emission at low frequencies (<1 GHz) can provide valuable information on processes driving flares and coronal mass ejections (CMEs). Radio emission has been detected from active M dwarf stars, suggestive of much higher levels of activity than previously thought. Observations of active M dwarfs at low frequencies can provide information on the emission mechanism for high energy flares and possible stellar CMEs. Here, we conducted two observations with the Australian Square Kilometre Array Pathfinder Telescope totalling 26 h and scheduled to overlap with the Transiting Exoplanet Survey Satellite Sector 36 field, utilizing the wide fields of view of both telescopes to search for multiple M dwarfs. We detected variable radio emission in Stokes I centred at 888 MHz from four known active M dwarfs. Two of these sources were also detected with Stokes V circular polarization. When examining the detected radio emission characteristics, we were not able to distinguish between the models for either electron cyclotron maser or gyrosynchrotron emission. These detections add to the growing number of M dwarfs observed with variable low-frequency emission.

Characteristic Features of the Magnetic and Ionospheric Storms on December 2124, 2016

Solar storms accompanied by solar flares, coronal mass ejections, and high-speed flows result in considerable disturbances in the Suninterplanetary mediummagnetosphereionosphereatmosphereEarth (internal geospheres) system. As a result, geospace storms with synergistically interacting magnetic, ionospheric, atmospheric, and electrical storms arise in our planet. Magnetic and ionospheric storms have been studied for a long time, but atmospheric storms and electrical storms have been studied considerably to a less extent. Geospace storms and their components exhibit significant variability. It may be asserted that no identical two storms exist. Therefore, a comprehensive study of each new geospace storm and its manifestations and features is an urgent scientific issue. This will contribute to a process of their adequate simulation and, in the long term, forecasting. The purpose of this article is to describe the observed features of the ionospheric and magnetic storms accompanying the geospace storm on December 2124, 2016. The state of the geomagnetic field has been observed via the fluxgate magnetometer located at the Magnetometer Observatory of the Karazin Kharkiv National University (4938' N, 3656' E). The dynamics of the ionospheric plasma has been monitored by a vertical incidence Doppler radar and a digisonde located at the Radio Physics Observatory of the Karazin Kharkiv National University (4938' N, 3620' E). The Doppler radar operate at 3.2 and 4.2 MHz; however, only measurements performed at 3.2 MHz are given below, since a frequency of 4.2 MHz turned out to be inefficient at nighttime when F2 layer critical frequency median f_{0 F2} 2 MHz, which prevented signal reflection from the ionosphere even at 3.2 MHz. Prior to the beginning of the magnetic storm on December 20, 2016, the level of the H and D components rarely exceeded 0.20.7 nT. The sudden commencement of a storm between 06:00 and 10:00 UTC virtually did not affect this level. During the second half of the day on December 21, 2016, the level of exhibited sporadic fluctuations increased from approximately 1 to 34 nT. During the next few days, up to December 25, 2016, their level showed variations mostly from approximately 1 nT to approximately 2 nT. Increases in the level were predominantly observed in the period from 05:00 to 15:00 UTC for the H component and from 10:00 to 20:00 UTC for the D component. The weak (power 20 GJ/s and energy approximately 0.45 PJ) geospace storm in the period of December 2124, 2016, was accompanied by a moderate positive ionospheric storm, as well as by three negative ionospheric storms, one of which was very strong, and the other two were strong and moderate. The geospace storm was accompanied by a moderate magnetic storm with an energy of approximately 2 PJ and a power of approximately 56 GW. The positive ionospheric storm barely affects the level of the signal reflected from the ionosphere, whereas the reflected signal may be very weak or totally absent during the negative ionospheric storms. The positive ionospheric storm has a substantial effect on the Doppler shift when the wave activity enhanced in the ionosphere. The relative amplitude of disturbances in the electron density increases from a few percent to approximately 50%, and their period increases from 612 to 40 min. It is impossible to follow wave activity during the negative ionospheric storms. In the course of a long magnetic storm, the level of D and H components in a subrange of 2001000 s of the period increased from 0.20.3 and 0.30.5 nT to 1.02.0 and 1.01.8 nT, respectively. In a subrange of 50200 s of the period, the corresponding levels increased from 0.30.5 and 0.30.5 nT to 0.51.0 and 1.52.0 nT, respectively. Within a subrange of 1050 s the period, the corresponding levels increased from 0.050.06 and 0.100.15 nT to 0.200.30 and 0.51.0 nT, respectively. Comparative studies of two geospace storms that occurred on December 2124, 2016, and March 2123, 2017, show that their ionospheric and magnetic effects are comparable, even if the storms are different.

The Solar Activity Index for the Critical Frequency of the E-Layer at Subauroral Latitudes

Based on the analysis of data from subauroral ionospheric stations during daytime hours with low geomagnetic activity, it was found that the index P = 0.5(F₁ + F₈₁) is the optimal solar activity index for the daily values of the E-layer critical frequency foE, where F₁ and F₈₁ are the solar radio flux at a wavelength of 10.7 cm on a given day and the average of this flux over 81 days. The standard deviations of the foE dependence on P are at their maximum for winter. The value in this season for the Salekhard and Lycksele stations, which are located at the Arctic Circle and near it, is significantly greater than for the Leningrad station. Substituting the P index into the IPG, IRI, or NeQuick models allows these models to be used for the calculation of daily foE values. Based on the preliminary analysis, it was found that the NeQuick model is more accurate than the IPG and IRI models for winter and equinoxes. For summer, these models have approximately the same accuracy with a slight advantage of the IPG model. For the Salekhard and Lycksele stations in winter at foE < 2 MHz, even the NeQuick model underestimates the foE values by approximately 0.2 MHz on average. The search for the causes of this property of the ionosphere requires special consideration.

Medium-Term Forecast of Solar Activity From Daily Data

Empirical ionospheric models reflect the dependence of key ionospheric characteristics on 12-month smoothed solar activity indices. These indices are determined with a delay of 6 months relative to the current time so the model implementation in real time is made with a forecast of solar activity, whose errors affect the accuracy of the ionospheric prediction. The 81-day smoothed proxy indices of solar activity can be used for driving the ionospheric model in real time including daily indices for the previous 40 days, an observation or forecast for the current day, and a forecast for the subsequent 40 days. In this paper, a method for predicting solar activity for 45 days (MSA45) is proposed, which is equally suitable for use with F10.7 solar radio emission flux indices and the SSN2 sunspot number. The model is based on the similarity of data in the current phase of the solar cycle with indices of solar activity in a relevant phase of the previous solar cycle. The model input parameters are the daily indices of solar activity F10.7 or SSN2 for the previous 45 days (d₄₅, , d₁), the solar cycle phase (d) for the current day, and the daily indices of solar activity for the subsequent 45 days (d₁, , d₄₅) in the corresponding phase of the previous solar cycle. The forecast of the sunspot number SSN2 for 45 days is made for the first time with an accuracy from 5.1 units for low solar activity to 23.1 units for high solar activity. A comparison of the forecast of F10.7 index of the MSA45 model with USAF-45DF forecast of this parameter and observational data show an improvement in the forecast accuracy from 15% at the solar activity maximum to 50% at the minimum solar activity.

DLITEAn inexpensive, deployable interferometer for solar radio burst observations

Solar radio bursts (SRBs) are brief periods of enhanced radio emission from the Sun. SRBs can provide unique insights into the plasma structure where emission occurs. SRBs can also provide critical information concerning space weather events such as coronal mass ejections or solar energetic particle events. Providing continuous monitoring of SRBs requires a full network of detectors continuously monitoring the Sun. A promising new network is being developed, employing a four-element interferometer called the Deployable Low-band Ionosphere and Transient Experiment (DLITE) array. DLITE, which operates in a 3040 MHz band, was specifically designed to probe the Earth's ionosphere using high resolution measurements (1.024-s temporal resolution, 16.276-kHz frequency resolution); however, this also makes DLITE a powerful new tool for providing detailed observations of SRBs at these frequencies. DLITE is particularly adept at detecting long-duration SRBs like Type II and Type IV bursts. DLITE provides high resolution SRB data that can complement ground-based networks like e-Callisto or space-based observations, e.g., from Wind/WAVES. As an inexpensive interferometer, DLITE has strong potential as an educational tool: DLITE can be used to study the ionosphere, SRBs, and even Jovian radio bursts. Future DLITE arrays could be enhanced by using the full 2080 MHz band accessible by the antennas and employing its millisecond time-resolution capability; this would improve DLITE's ability to track long-duration bursts, create the opportunity to study short-duration Type III bursts in detail, and, in particular, make the study of Type I bursts practical.

The Sun and Space Weather

The explosion of space weather research since the early 1990s has been partly fueled by the unprecedented, uniform, and extended observations of solar disturbances from space- and ground-based instruments. Coronal mass ejections (CMEs) from closed magnetic field regions and high-speed streams (HSS) from open-field regions on the Sun account for most of the disturbances relevant to space weather. The main consequences of CMEs and HSS are their ability to cause geomagnetic storms and accelerate particles. Particles accelerated by CME-driven shocks can pose danger to humans and their technological structures in space. Geomagnetic storms produced by CMEs and HSS-related stream interaction regions also result in particle energization inside the magnetosphere that can have severe impact on satellites operating in the magnetosphere. Solar flares are another aspect of solar magnetic energy release, mostly characterized by the sudden enhancement in electromagnetic emission at various wavelengthsfrom radio waves to gamma-rays. Flares are responsible for the sudden ionospheric disturbances and prompt perturbation of Earth's magnetic field known as magnetic crochet. Nonthermal electrons accelerated during flares can emit intense microwave radiation that can drown spacecraft and radar signals. This review article summarizes major milestones in understanding the connection between solar variability and space weather.

A Small Flare from Proxima Cen Observed in the Millimeter, Optical, and Soft X-Ray with Chandra and ALMA

We present millimeter, optical, and soft X-ray observations of a stellar flare with an energy squarely in the regime of typical X1 solar flares. The flare was observed from Proxima Cen on 2019 May 6 as part of a larger multi-wavelength flare monitoring campaign and was captured by Chandra, the Las Cumbres Observatory Global Telescope, the Irn du Pont Telescope at Las Campanas Observatory, and the Atacama Large Millimeter Array. Millimeter emission appears to be a common occurrence in small stellar flares that had gone undetected until recently, making it difficult to interpret these events within the current multi-wavelength picture of the flaring process. The May 6 event is the smallest stellar millimeter flare detected to date. We compare the relationship between the soft X-ray and millimeter emission to that observed in solar flares. The X-ray and optical flare energies of 10^{30.3 0.2} and 10^{28.9 0.1} erg, respectively, the coronal temperature of T = 11.0 2.1 MK, and the emission measure of 9.5 2.2 10⁴⁹ cm^-3 are consistent with M-X class solar flares. We find the soft X-ray and millimeter emission during quiescence are consistent with the Gdel-Benz relation, but not during the flare. The millimeter luminosity is >100 higher than that of an equivalent X1 solar flare and lasts only seconds instead of minutes as seen for solar flares.

Tracking a Beam of Electrons from the Low Solar Corona into Interplanetary Space with the Low Frequency Array, Parker Solar Probe, and 1 au Spacecraft

Type III radio bursts are the result of plasma emission from mildly relativistic electron beams propagating from the low solar corona into the heliosphere where they can eventually be detected in situ if they align with the location of a heliospheric spacecraft. Here we observe a type III radio burst from 0.1 to 16 MHz using the Parker Solar Probe (PSP) FIELDS Radio Frequency Spectrometer (RFS) and from 20 to 80 MHz using the Low Frequency Array (LOFAR). This event was not associated with any detectable flare activity but was part of an ongoing type III and noise storm that occurred during PSP encounter 2. A deprojection of the LOFAR radio sources into 3D space shows that the type III radio burst sources were located on open magnetic field from 1.6 to 3 R and originated from a near-equatorial active region around longitude E48. Combining PSP/RFS observations with WIND/WAVES and Solar Terrestrial Relations Observatory (STEREO) WAVES, we reconstruct the type III radio source trajectory in the heliosphere interior to PSP's position, assuming ecliptic confinement. An energetic electron enhancement is subsequently detected in situ at the STEREO A spacecraft at compatible times, although the onset and duration suggests the individual burst contributes a subset of the enhancement. This work shows relatively small-scale flux emergence in the corona can cause the injection of electron beams from the low corona into the heliosphere, without needing a strong solar flare. The complementary nature of combined ground and space-based radio observations, especially in the era of PSP, is also clearly highlighted by this study.

Influence of Fine Structures on Gyrosynchrotron Emission of Flare Loops Modulated by Sausage Modes

Sausage modes are a leading mechanism for interpreting short-period quasi-periodic pulsations (QPPs) of solar flares. Forward modeling their radio emission is crucial for identifying sausage modes observationally and for understanding their connections with QPPs. Using the numerical outputs from three-dimensional magnetohydrodynamic simulations, we forward model the gyrosynchrotron emission of flare loops modulated by sausage modes and examine the influence of fine structures of loops. The temporal evolution of the emission intensity is analyzed for an oblique line of sight crossing the loop center. We find that the low- and high- frequency intensities oscillate in phase in the periods of sausage modes for models with or without fine structures. For low-frequency emissions where the optically thick regime arises, the modulation magnitude of the intensity is dramatically reduced by the fine structures at some viewing angles. On the contrary, for high-frequency emissions where the optically thin regime holds, the effects of fine structures or the viewing angle are marginal. Our results show that the periodic intensity variations of sausage modes are not wiped out by fine structures, and that sausage modes remain a promising candidate mechanism for QPPs, even when the flare loops are fine-structured.

First Detection of Transverse Vertical Oscillation during the Expansion of Coronal Loops

In this Letter, we perform a detailed analysis of the M5.5 class eruptive flare occurring in active region 12,929 on 2022 January 20. The eruption of a hot channel generates a fast coronal mass ejection (CME) and a dome-shaped extreme-ultraviolet (EUV) wave at speeds of 740-860 km s^-1. The CME is associated with a type II radio burst, implying that the EUV wave is a fast-mode shock wave. During the impulsive phase, the flare shows quasi-periodic pulsations (QPPs) in EUV, hard X-ray, and radio wavelengths. The periods of QPPs range from 18 to 113 s, indicating that flare energy is released and nonthermal electrons are accelerated intermittently with multiple timescales. The interaction between the EUV wave and low-lying adjacent coronal loops (ACLs) results in contraction, expansion, and transverse vertical oscillation of ACLs. The speed of contraction in 171, 193, and 211 is higher than that in 304 . The periods of oscillation are 253 s and 275 s in 304 and 171 , respectively. A new scenario is proposed to explain the interaction. The equation that interprets the contraction and oscillation of the overlying coronal loops above a flare core can also interpret the expansion and oscillation of ACLs, suggesting that the two phenomena are the same in essence.

Long-term temperature and ozone response to natural drivers in the mesospheric region using 16 years (2005-2020) of TIMED/SABER observation data at 5-15N

The long-term mesospheric (60-100 km) temperature and ozone volume mixing ratio variability during the period of January 2005 to December 2020 were analyzed to obtain trends and their response to natural influences using the SABER, an instrument onboard the TIMED satellite to obtain observation data in the latitude range of 5N-15N. A wavelet analysis technique has been used to identify the dominant periodic oscillations in mesospheric temperature and ozone. Using the proxy data of F10.7, Nino 3.4, and zonal wind index (QBO at 30 hPa), the mesospheric response to natural drivers was investigated using a multiple linear regression technique. Among the three natural drivers, solar radio flux (F10.7) is the dominant contributor to mesospheric variability rather than ENSO and QBO. It influences negatively the lower mesosphere (60-80 km), and above 80 km, it responds positively in temperature (2.6 K), whereas ozone responds with a constant negative value (0.12ppmv) up to 80 km, and after it influences by a maximum positive value of 0.7 ppmv. At 80 km, the temperature and ozone respond in phase to all natural influences (F10.7, ENSO, and QBO), and are out of phase below and above 80 km. Both the temperature and the ozone reveal cooling trends (-0.85 K/decade and -0.12 ppmv/decade) of the the lower mesosphere (60-80 km) and are followed by the upper mesospheric (85-100 km) warming trends (1.25 K/decade and 0.27 ppmv/ decade) over the low latitudes. In general, natural influences affected the mesospheric temperature more strongly than the ozone volume mixing ratio. Our results are expected to be an updated and reliable estimation of the mesospheric temperature and ozone variability for the equatorial mesosphere.

Stochastic approach to Markovian interrelationship assessment of solar activity indices

This paper employs a discrete-time Markov chain (DTMC) stochastic process to investigate a state/event based Markovian interrelationship between various solar activity indices (SAI) (including 10.7 cm solar radio flux (SF_10.7), coronal index (CI), solar flare index (SFI) and total solar irradiance (TSI)) in relation to sunspot number (SSN). First, we applied the first order DTMC model as a first approximation to the total number of transitions between different states of SAI in order to estimate the probability of occurrence corresponding with each transition. Next, several DTMC descriptors like persistency, state dependency, stationarity, mean first passage time and entropy are derived from estimated transition probability matrices. These descriptors are very useful as they related to time series characteristics (like randomness, nature of cycles and predictability) within a stochastic dynamical system as well as crucial for checking the applicability of Markov chain method. Therefore, via the DTMC analysis and derived descriptors, this study found remarkable similarities in the formation of transition matrices and diagrams, significant 2-dimensional correlation values, robust self-communication behaviour among states, existence of dependent successive transitions and stationary nature of data throughout the space. Further, the resemblance in the average transit time from one state to another, probabilistically disordered symmetrical time series and existence of randomness in transition states has been observed. Therefore, results obtained in this paper provide a new insight to increase the level of knowledge of the possible linkage between underlying SAI that could be helpful in enhanced understanding of the potential future climate changes and other solar energy-related objectives.

Overexpansion-dominated coronal mass ejection formation and induced radio bursts

Aims: Coronal mass ejections (CMEs) are the most fascinating explosions in the Solar System. Their formation is still not fully understood, however.
Methods: We investigated a well-observed CME on 2021 May 7 that showed a typical three-component structure and was continuously observed from 0 to 3 R by a combination of SDO/AIA (0-1.3 R), PROBA2/SWAP (0-1.7 R), and MLSO/K-Cor (1.05-3 R). Furthermore, we compared the morphological discrepancy between the CME white-light bright core and the extreme-UV (EUV) blob. We finally explored the origin of various radio bursts that are closely related to the interaction of the CME overexpansion with a nearby streamer.
Results: An interesting finding is that the height increases of the CME leading front and of the bright core are dominated by the overexpansion during the CME formation. The aspect ratios of the CME bubble and bright core, quantifying the overexpansion, are found to decrease as the SO/STIX 4-10 keV and GOES 1-8 soft X-ray flux of the associated flare increases near the peaks. This indicates that the flare reconnection plays an important role in the first overexpansion. The CME bubble even undergoes a second overexpansion, although it is relatively weak, which is closely related to the compression with a nearby streamer and likely arises from an ideal magnetohydrodynamics process. Moreover, the CME EUV blob is found to be relatively lower and wider than the CME white-light bright core, which may correspond to the bottom part of the growing CME flux rope. The interaction between the CME and the streamer leads to two type II radio bursts, one that is drifting normally and another that is stationary, which are speculated to be induced by two different sources of the CME-driven shock front. The bidirectional electrons shown in series of C-shaped type III bursts suggest that the interchange reconnection is also involved during the interaction of the CME and streamer.

Movies associated to Figs. 1 and 2 are available at https://www.a anda.org

A Mini-Review

Plasma spectroscopy, employing the shapes of spectral lines of atoms and ions, historically started from astrophysical plasmas (thus, it was the crossing of the macro- and micro-worlds)-before finding broad applications to all kinds of laboratory plasmas intended for various important practical purposes. The present mini-review covers advances in understanding a variety of selected astrophysical plasmas achieved by the analysis of their emission or absorption in the visible, microwave, and radio ranges. It discusses the observational data and also touches upon the relevant advances in the theory of line shapes. The presentation is organized in order of the decreasing electron density $N_{e}$ of the astrophysical objects: from white dwarfs ($N_{e} ~\sim ~10^{17}$ cm$^{-3}$ ) to flare stars ($N_{e} ~\sim ~10^{15}$ cm$^{-3}$ ), solar flares ($N_{e} ~\sim ~10^{13}$ cm$^{-3}$ ), the quiet Sun ($N_{e} ~ \sim ~10^{11}$ cm$^{-3}$ ), the H II regions ($N_{e} ~\sim ~10^{4}$ cm$^{-3}$ ), and the recombination epoch of the early universe ($N_{e} ~\sim ~10^{4}$ cm$^{-3}$ ).

The Influence of Solar Activity on Snow Cover over the Qinghai-Tibet Plateau and Its Mechanism Analysis

Using global ocean vertical temperature anomaly data, we identified that a significant response of the sea temperature anomaly (STA) to the solar radio flux (SRF) exists. We found that the STA exhibited a significant correlation with Asian summer and winter precipitation, among which the response from the Qinghai-Tibet Plateau (the QTP) was particularly noticeable. Based on NCEP/NCAR reanalysis data, the latent heat flux (LHF) anomaly, which plays a key role in winter precipitation in China, especially over the QTP, showed a significant response to the SRF in the Pacific. The results demonstrated the bottom-up mechanism of impact of solar activity (SA) on the plateau snow through sea-air interaction. Meanwhile, a top-down mechanism was also present. When the SRF was high, the stratospheric temperature in the low and mid-latitudes increased and the temperature gradient pointed to the pole to strengthen the westerly wind in the mid-latitudes. The EP flux showed that atmospheric long waves in the high altitudes propagated downward from the stratosphere to the troposphere. A westerly (easterly) wind anomaly occurred in the south (north) of the QTP at 500 hPa and the snowfall rate over the QTP tended to increase. When the SRF was low, the situation was the opposite, and the snowfall rate tended to decrease. The model results confirmed that when total solar irradiance (TSI) became stronger (weaker), both of the solar radiation fluxes at the top of the atmosphere and the surface temperature over the QTP increased (decreased), the vertical updraft intensified (weakened), and the snowfall rate tended to increase (decrease) accordingly. These conclusions are helpful to deepen the understanding of SA's influence on the snow cover over the QTP.

Searching for Quasi-periodic Oscillations in Astrophysical Transients Using Gaussian Processes

Analyses of quasi-periodic oscillations (QPOs) are important to understanding the dynamic behavior in many astrophysical objects during transient events like gamma-ray bursts, solar flares, magnetar flares, and fast radio bursts. Astrophysicists often search for QPOs with frequency-domain methods such as (Lomb-Scargle) periodograms, which generally assume power-law models plus some excess around the QPO frequency. Time-series data can alternatively be investigated directly in the time domain using Gaussian process (GP) regression. While GP regression is computationally expensive in the general case, the properties of astrophysical data and models allow fast likelihood strategies. Heteroscedasticity and nonstationarity in data have been shown to cause bias in periodogram-based analyses. GPs can take account of these properties. Using GPs, we model QPOs as a stochastic process on top of a deterministic flare shape. Using Bayesian inference, we demonstrate how to infer GP hyperparameters and assign them physical meaning, such as the QPO frequency. We also perform model selection between QPOs and alternative models such as red noise and show that this can be used to reliably find QPOs. This method is easily applicable to a variety of different astrophysical data sets. We demonstrate the use of this method on a range of short transients: a gamma-ray burst, a magnetar flare, a magnetar giant flare, and simulated solar flare data.

Comparative analysis of type III solar radio bursts associated with solar particle events and its impact on space weather for solar cycle 23 & 24

We have analyzed type III-l radio bursts associated with SEP events (>10 MeV) during solar cycles 23 and 24 respectively. We have studied the properties of type III radio bursts and SEP events. We have also analyzed the correlation between the properties of type III-l bursts and SEPs. We found that the mean duration of type III-l bursts and their deviation for solar cycle 24 is higher. The log peak intensity of SEP and its deviation is higher for solar cycle 23, which suggests that the SEP events for solar cycle 23 are faster and long-duration events. From the correlations, we conclude that the type III-l radio burst properties for solar cycle 23 contribute more towards the SEP events as compared to solar cycle 24.

Propagation of transverse waves in the solar chromosphere probed at different heights with ALMA sub-bands

The Atacama Large Millimeter/sub-millimeter Array (ALMA) has provided us with an excellent diagnostic tool for studies of the dynamics of the Solar chromosphere, albeit through a single receiver band at one time presently. Each ALMA band consists of four sub-bands that are comprised of several spectral channels. To date, however, the spectral domain has been neglected in favour of ensuring optimal imaging, so that time- series observations have been mostly limited to full-band data products, thereby limiting studies to a single chromospheric layer. Here, we report the first observations of a dynamical event (i.e., wave propagation) for which the ALMA Band 3 data (centred at 3 mm; 100 GHz) is split into a lower and an upper sideband. In principle, this approach is aimed at mapping slightly different layers in the Solar atmosphere. The side-band data were reduced together with the Solar ALMA Pipeline (SoAP), resulting in time series of brightness-temperature maps for each side-band. Through a phase analysis of a magnetically quiet region, where purely acoustic waves are expected to dominate, the average height difference between the two side-bands is estimated as 73 16 km. Furthermore, we examined the propagation of transverse waves in small- scale bright structures by means of wavelet phase analysis between oscillations at the two atmospheric heights. We find 6% of the waves to be standing, while 54% and 46% of the remaining waves are propagating upwards and downwards, respectively, with absolute propagating speeds on the order of 96 km s¹, resulting in a mean energy flux of 3800 W m².

Diagnostics of the dynamics of the Langmuir spectrum based on radio emission during the 12 March 2015 solar radio burst

Aims: We investigate the dynamics of spectra of Langmuir waves in the plasma radiation of solar radio bursts.
Methods: We simulated the radio emission that is formed during the merging of Langmuir waves. The observed frequency spectra of radio bursts were fitted by the model spectrum of Langmuir waves.
Results: We determined shapes of the Langmuir wave spectra, consistent with the solar burst observed in the 0.8-2.0 GHz range on 12 March 2015 by the Ondejov radiospectrograph. We estimated the sizes of the corresponding radio source for different values of the energy density of Langmuir waves. We present the time evolution of the model Langmuir wave spectra at four instants. Finally, we explain a role of the induced scattering of Langmuir waves in the formation of their spectra.

Zebra Stripes with High Gyro-Harmonic Numbers

Solar radio zebras are used in the determination of the plasma density and magnetic field in solar flare plasmas. Analyzing observed zebra stripes and assuming their generation by the double-plasma resonance (DPR) instability, high values of the gyro-harmonic number are found. In some cases they exceed one hundred, in disagreement with the DPR growth rates computed up to now, which decrease with increasing gyro-harmonic number. We address the question of how zebras with high values of the gyro-harmonic numbers s are generated. For this purpose, we compute the growth rates of the DPR instability in a very broad range of s , considering a loss-cone -distribution of superthermal electrons and varying the loss-cone angle, electron energies, and background plasma temperature. We have numerically calculated the dispersion relations and the growth rates of the upper-hybrid waves and found that the growth rates increase with increasing gyro-harmonic numbers if the loss-cone angles are 80. The highest growth rates for these loss-cone angles are obtained for velocity v=0.15 c . The growth rates as a function of the gyro-harmonic number still show well distinct peaks, which correspond to zebra-stripe frequencies. The contrast between peak growth rates and surrounding growth rate levels increases as the index increases and the background temperature decreases. Zebras with high values of s can be generated in regions where loss-cone distributions of superthermal electrons with large loss-cone angles (80) are present. Furthermore, owing to the high values of s , the magnetic field is relatively weak and has a small spatial gradient in such regions.

Analysis of the Radio Solar Telescope Network's Noon Flux Observations Over Three Solar Cycles (1988-2020)

The Sun is a readily accessible source of wideband noise that can be used for the calibration of a wide variety of radio systems. Since 1979 the United States Air Force Radio Solar Telescope Network (RSTN) has performed solar flux observations around noon on eight frequencies between 245 and 15,400 MHz. These fluxes are disseminated by the National Ocean and Atmospheric Administration (NOAA) and are used by many radio and radar system operators to perform system calibration. Little analysis has been performed on the RSTN noon flux values. Here we review the calibration procedures of RSTN and perform statistical tests between the four sites comprising RSTN. For system operators who require precise system calibration the noted variations may be large enough to cast doubt on RSTN flux values for such calibration. However, for many communication systems a calibration within one or two dB is adequate. We discuss the reported variation both in linear and logarithmic terms. When it is possible to used smoothed data the inter-site variation becomes less of a problem, although the correlation between sites at Ku band (15,400 MHz) still appears excessively large. We update some simple empirical models for wideband solar flux based purely on the 10 cm flux at any time, as these have also been used in solar radio calibration procedures. We give practical advice on the use of solar flux for radio system calibration and make some recommendations that we believe could improve the accuracy and utility of solar radio flux reports.

A 3 Giga Sample Per Second 14-bit Digital Receiver with 9 GHz Input Bandwidth for Solar Radio Observation

A new digital receiver with excellent performances has been designed and developed for solar radio observation, which can receive the radio signal from direct current (DC) to 9 GHz in the direct acquisition way. On the digital receiver, the analog-to-digital converter (ADC) with 14-bit, two input channels and 3 Giga Samples per second (Gsps) are used to acquire observed signal, and the field-programmable-gate-array chip XCKU115 acts as the processing module. The new digital receiver can be used to directly sample the solar radio signals of frequency under 9 GHz. When receiving the solar radio signal above 9 GHz, the new digital receiver can save 1-2 stages of frequency down-conversion, and effectively improve many indexes of the solar radio observation system, i.e., the time resolution, analog front-end circuit, weight and volume of the analog circuit system. Compared with the digital receiver with sampling rate below 1 Gsps used in existing solar radio telescope, the new digital receiver reduces the frequency switching times of large bandwidth, which is beneficial to improving the frequency and time resolutions. The ADC sampling resolution of 14 bits, providing a large dynamic range, is very beneficial to observing smaller solar eruptions. This receiver, which would be used in the solar radio observation system, well meets the latest requirements with the resolutions of time (1 ms) and frequency (0.5 MHz) for fine observation of radio signals.

Diagnostics of energy release in solar flares with radio dynamic imaging spectroscopy

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Design of a high-resolution antenna array solar observing system for radio frequencies of 25-110 MHz

Spectral observations with high temporal and frequency resolution are of great significance for studying the fine structures of solar radio bursts. In addition, it is helpful to understand the physical processes of solar eruptions. In this paper, we present the design of a system to observe solar radio bursts with high temporal and frequency resolutions at frequencies of 25-110 MHz. To reduce the impact of analog devices and improve the system flexibility, we employ various digital signal processing methods to achieve the function of analog devices, such as polarisation synthesis and beamforming. The resourceful field programmable gate array is used to process radio signals. The system has a frequency resolution of $$ 30 kHz and a temporal resolution of up to 0.2 ms. The left/right circular polarisation signals can be simultaneously observed. At present, the system has been installed at Chashan Solar Observatory operated by the Institute of Space Science, Shandong University. The system is running well, multiple bursts have been observed, and relevant data have been obtained.

Characteristics of stripes-pattern radio-emission sources

An investigation of the generation mechanism for stripes-pattern radio spectra is important for an understanding of the dynamics of non-thermal electrons in several astronomical objects, including the Sun, Jupiter, and the Crab Pulsar. A new analytical study is carried out to identify the plasma characteristics of fiber- and zebra-pattern emission sources without an underlying density or magnetic model. The analysis demonstrates that the source region of the stripes emission is located underneath the reconnection point, where the ratio s of the instability growth rate to the electron gyrofrequency _c does not equal unity; that is, s = kv/_c 1. When |s| < 1, the plasma condition of the source region becomes kv < _p < _c, where _p is the plasma frequency, and the emission source is likely to produce a fiber radio burst. For |s| > 1, the plasma condition of the source region is _c < _p < kv, and the emission source is likely to produce zebra-pattern emission. This indicates that the magnetic field in the source region of zebra-pattern radio emission is weak and it is relatively high in the source region of fiber-pattern emission. An approach is applied to estimate the plasma parameters of a zebra-pattern emission source observed on 2011 June 21. The behaviour of the blasted medium, which is produced by magnetic reconnection, is investigated. The results show that the blasted medium propagates isothermally as a sausage-like wave for a short time during the emission. The study discusses the conditions for producing different types of striped radio emission and provides a simple computational approach that could be useful in a number of astronomical contexts.

Self-powered wearable sensors design considerations

Wearable sensors have been implemented widely to provide comfortable and continuous long-term monitoring in many applications. Minimal requirements on maintenance is a main characteristic of wearable sensors, but unfortunately, many of them are still powered by battery with limited capacity which need to be charged or replaced regularly. Energy harvesting technologies are applied to provide a reliable solution to this issue. This paper presents several design considerations for self-powered wearable sensors. Suitable energy sources are discussed, such as ambient energy sources (solar, radio frequency, and ultrasonic energy), human body energy (mechanical, piezoelectric, triboelectric, electromagnetic, electrostatic, and thermal energy). Moreover, power management integrated circuits, energy storage options, and the material selection and conditioning circuit of triboelectric nanogenerator are discussed. Five case studies utilizing different energy harvesting techniques are discussed and evaluated in terms of their system implementation and performance to provide some deeper understandings of wearable sensors.

The Relationship of the IG and T Ionospheric Indices to the Solar and Geomagnetic Activity Indices

An analysis of the relationship between the monthly mean ionospheric indices IG and T and the indices of solar (F107) and geomagnetic (Ap) activity based on the massif of these data within 19542020 interval is presented. F107 and Ap are the monthly mean flux of the solar radio emission at a wavelength of 10.7 cm and the planetary index of geomagnetic activity Ap, respectively. It is found that the index F = (F107₀ + F107₁)/2 provides a higher correlation to the ionospheric indices than the indices F107 over the given (F107₀) or previous (F107₁) months. The IG and T dependencies on F in the form of a second-degree polynomials make it possible to reproduce 96% of variations in IG and 98% of variations in T for the analyzed time interval. That is the reason that the additional contribution of Ap into IG and T is weak. Nevertheless, the contribution of Ap into T and IG depends on the time of the year: it is not significant for January and is significant for July. This property of the annual anomaly in the ionospheric parameters by the IG index, apparently, is discovered for the first time. In all considered cases, an increase in Ap leads to a decrease in T and IG, that is, to a mean (global) decrease in the median of the F2-layer maximum concentration and, under equal other conditions, such a decrease is more significant in July than in January. The properties of the dependencies of the IG and T indices on F and Ap are in many aspects similar, but the accuracy of these dependencies is higher for T than for IG.

A Publicly Available Multiobservatory Data Set of an Enhanced Network Patch from the Photosphere to the Corona

New instruments sensitive to chromospheric radiation at X-ray, UV, visible, IR, and submillimeter wavelengths have become available that significantly enhance our ability to understand the bidirectional flow of energy through the chromosphere. We describe the calibration, coalignment, initial results, and public release of a new data set combining a large number of these instruments to obtain multiwavelength photospheric, chromospheric, and coronal observations capable of improving our understanding of the connectivity between the photosphere and the corona via transient brightenings and wave signatures. The observations center on a bipolar region of enhanced-network magnetic flux near disk center on SOL2017-03-17T14:00-17:00. The comprehensive data set provides one of the most complete views to date of chromospheric activity related to small-scale brightenings in the corona and chromosphere. Our initial analysis shows a strong spatial correspondence between the areas of broadest width of the hydrogen- spectral line and the hottest temperatures observed in Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 radio data, with a linear coefficient of 6.12 10^-5/K. The correspondence persists for the duration of cotemporal observations (60 m). Numerous transient brightenings were observed in multiple data series. We highlight a single, well-observed transient brightening in a set of thin filamentary features with a duration of 20 minutes. The timing of the peak intensity transitions from the cooler (ALMA, 7000 K) to the hotter (XRT, 3 MK) data series.

Third and Fourth Harmonics of Electromagnetic Emissions by a Weak Beam in a Solar Wind Plasma with Random Density Fluctuations

Electromagnetic emissions ${{ \mathcal H }}_{3}$ and ${{ \mathcal H }}_{4}$ at the third and fourth harmonics of the plasma frequency _p were observed during the occurrence of type II and type III solar radio bursts. Two-dimensional particle-in-cell simulations are performed using a weak beam, high space and time resolutions, and a plasma with density fluctuations of a few percent, for parameters relevant to regions of type III bursts. For the first time, a detailed study of the different wave coalescence processes involved in the generation of ${{ \mathcal H }}_{3}$ and ${{ \mathcal H }}_{4}$ waves is presented and the impact of density fluctuations on the wave interaction mechanisms is demonstrated. Energy ratios between the second, third, and fourth harmonics ${{ \mathcal H }}_{2}$ , ${{ \mathcal H }}_{3}$ , and ${{ \mathcal H }}_{4}$ are consistent with space observations. It is shown that, in both homogeneous and inhomogeneous plasmas, the dominant processes generating ${{ \mathcal H }}_{3}$ ( ${{ \mathcal H }}_{4}$ ) are the coalescence of ${{ \mathcal H }}_{2}$ ( ${{ \mathcal H }}_{3}$ ) with a Langmuir wave, in spite of the random density fluctuations modifying the waves' resonance conditions by energy transport in the wavevector space and of the damping of Langmuir waves. The role of the backscattered (forward-propagating) Langmuir waves coming from the first (second) cascade of the electrostatic decay of beam-driven Langmuir waves is determinant in these processes. Understanding such wave coalescence mechanisms can provide indirect information on Langmuir and ion acoustic wave turbulence, the average level of density inhomogeneities, and suprathermal electron fluxes generated in solar wind regions where the harmonics manifest. Causes for the rarity of their observations are discussed.

A Comparative Statistical Study on Their Characteristics and Geoeffectiveness

Interplanetary sheaths and corotating interaction regions (CIRs), while having different solar sources, represent turbulent solar-wind plasma and magnetic field that can perturb the Earth's magnetosphere. We explore long-term solar-wind measurements upstream of the Earth during Solar Cycle 24, from January 2008 to December 2019, to compare their solar-cycle variation, characteristic features, and geoeffectiveness. Earth is found to be encountered by 2.6 times more CIRs (290) than sheaths (110) during this period. The sheath occurrence follows the F_10.7 solar radio-flux variation, with a cross-correlation coefficient (r_cc) of +0.71 at zero-year time lag. However, the CIR occurrence is more prominent during the solar cycle descending to minimum phases, reflected in r_cc values of 0.53 and +0.50 at time lags of 2 and +4 years, respectively, between the CIR occurrence and the F_10.7 solar flux. Both sheath and CIR are characterized by identical average plasma density and interplanetary magnetic-field (IMF) magnitude, and their fluctuations characterized by enhanced variance, and periodic variations of a few minutes to an hour. However, on average, the CIR has 12% higher plasma speed, 33% higher temperature, 20% stronger southward IMF component, 131% longer duration, and 158% longer radial extent than the sheath. The intensities of the auroral electrojet index [AE] and the symmetric ring- current index [SYM-H] are, respectively, 38% and 55% stronger during the CIR than the sheath, on average. The geoeffectiveness of the CIR is found to be significantly higher than the sheath. Among all CIRs (sheaths), 25% ( 14%) caused moderate storms (50 nT SYM-H >100 nT), and 5% ( 4%) caused intense storms (SYM-H 100 nT).

Continuum Imaging in the 18 - 26 GHz Range

We present a new solar radio imaging system implemented through the upgrade of the large single-dish telescopes of the Italian National Institute for Astrophysics (INAF), not originally conceived for solar observations.

Study of Fine Radio-Burst Structures (FRBS) Observed by the Mexican Array Radio Telescope (MEXART)

Solar events occur in several energy ranges and durations, with emissions involving a wide range of the electromagnetic spectrum. The present work reveals the instrumental capacity of the Mexican Array Radio Telescope (MEXART) to detect solar radio emissions in the VHF band. Particular attention is focused on intense, short-duration solar transient emissions in the form of fine radio-burst structures (FRBS), observed by MEXART at around 140 MHz. A Type-I noise storm event with metric FRBS corresponding to observations on 6 May 2019 is reported. The FRBS exhibited distinct durations within the range of 0.47 - 8.07 s, a mean value of 2.48 s, and intensities between 1.0 - 8.0 sfu, with the initial FRBS having a longer duration and greater peak intensity levels than the subsequent radio bursts. The time profile of the FRBS has an asymmetric structure consisting of an abrupt rise, a short-term maximum peak, and a slow decay phase with mean values of 1.90 and 2.50 s for the rise and decay times, respectively. Compared with other fast radio transients observed at higher frequencies, the longer duration of the FRBS suggests lack of interaction of an electron beam with its surrounding parcelled low-density plasma environment. The properties of the FRBS structures are discussed.

Diagnostic Functions of Solar Coronal Magnetic Fields from Radio Observations

In solar physics, it is a big challenge to measure the magnetic fields directly from observations in the upper solar atmosphere, including the chromosphere and corona. Radio observations are regarded as the most feasible approach to diagnose the magnetic field in solar chromosphere and corona. However, because of the complexity and diversity of the emission mechanisms, the previous studies have only presented the implicit diagnostic functions of the magnetic field for specific mechanism from solar radio observations. This work collected and sorted out all methods for diagnosing coronal magnetic field from solar radio observations, which are expressed as a set of explicit diagnostic functions. In particular, this work supplemented some important diagnostic methods missed in other reviews. This set of diagnostic functions can completely cover all regions of the solar chromosphere and corona, including the quiet region, active region and flaring source regions. At the same time, it also includes incoherent radiation such as bremsstrahlung emission of thermal plasma above the quiet region, cyclotron and gyro-synchrotron emissions of magnetized hot plasma and mildly relativistic nonthermal electrons above the active regions, as well as coherently plasma emission around flaring source regions. Using this set of diagnostic functions and the related broadband spectral solar radio imaging observations, we can derive the magnetic fields of almost all regions in the solar atmosphere, which may help us to make full use of the spectral imaging observations of the new generation solar radio telescopes (such as MUSER, EVOSA and the future FASR, etc.) to study the solar activities, and provide a reliable basis for the prediction of disastrous space weather events.

Wave Emission of Nonthermal Electron Beams Generated by Magnetic Reconnection

Magnetic reconnection in solar flares can efficiently generate nonthermal electron beams. The energetic electrons can, in turn, cause radio waves through microscopic plasma instabilities as they propagate through the ambient plasma along the magnetic field lines. We aim at investigating the wave emission caused by fast-moving electron beams with characteristic nonthermal electron velocity distribution functions (EVDFs) generated by kinetic magnetic reconnection: two-stream EVDFs along the separatrices and in the diffusion region, and perpendicular crescent-shaped EVDFs closer to the diffusion region. For this purpose, we utilized 2.5D fully kinetic Particle-In-Cell code simulations in this study. We found the following: (1) the two-stream EVDFs plus the background ions are unstable to electron/ion (streaming) instabilities, which cause ion-acoustic waves and Langmuir waves due to the net current. This can lead to multiple-harmonic plasma emission in the diffusion region and the separatrices of reconnection. (2) The perpendicular crescent-shaped EVDFs can cause multiple-harmonic electromagnetic electron cyclotron waves through the electron cyclotron maser instabilities in the diffusion region of reconnection. Our results are applicable to diagnose the plasma parameters, which are associated to magnetic reconnection in solar flares by means of radio wave observations.

Study of Time Evolution of Thermal and Nonthermal Emission from an M-class Solar Flare

We conduct a wide-band X-ray spectral analysis in the energy range of 1.5-100 keV to study the time evolution of the M7.6-class flare of 2016 July 23, with the Miniature X-ray Solar Spectrometer (MinXSS) CubeSat and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft. With the combination of MinXSS for soft X-rays and RHESSI for hard X-rays, a nonthermal component and three-temperature multithermal component-"cool" (T 3 MK), "hot" (T 15 MK), and "superhot" (T 30 MK)-were measured simultaneously. In addition, we successfully obtained the spectral evolution of the multithermal and nonthermal components with a 10 s cadence, which corresponds to the Alfvn timescale in the solar corona. We find that the emission measures of the cool and hot thermal components are drastically increasing more than hundreds of times and the superhot thermal component is gradually appearing after the peak of the nonthermal emission. We also study the microwave spectra obtained by the Nobeyama Radio Polarimeters, and we find that there is continuous gyrosynchrotron emission from mildly relativistic nonthermal electrons. In addition, we conducted a differential emission measure (DEM) analysis by using Atmospheric Imaging Assembly on board the Solar Dynamics Observatory and determined that the DEM of cool plasma increases within the flaring loop. We find that the cool and hot plasma components are associated with chromospheric evaporation. The superhot plasma component could be explained by the thermalization of the nonthermal electrons trapped in the flaring loop.

A case study of the M_w 7.8 Nepal earthquake

We propose a Multi-Network-based Hybrid Long Short Term Memory (N-LSTM) model for ionospheric anomaly detection to forecast highly irregular data of the ionospheric Total Electron Content (TEC). Previously purposed models were suffering from the problems of vanishing gradient, exploding gradient, uncertainty, and parameter bias. LSTM is used to overcome the vanishing and exploding gradient problems and the remaining two (uncertainty and parameter bias) are subjugated by N-LSTM. We have also discussed the implementation of the model by using the Mw 7.8 Nepal Earthquake (EQ) that occurred on April 25, 2015. The proposed model detects a significant negative anomaly about 14 days before the impending EQ. Furthermore, we find gradual increments in the TEC time- series, where the TEC values gradually increased, in terms of positive anomalies, for four continuous days (April 21-24, 2015) till the mainshock. Further analysis revealed that the nighttime TEC enhancements are more dominant than the daytimes, which persisted for 10 h (22:00-08:00 LT) in the absence of the solar flux, we named them the absolute seismic precursors. These gradual enhancements establish a perfect correlation with several atmospheric anomalies, reported in the previous studies, where the variations in different atmospheric parameters like Outgoing Longwave Radiation (OLR), air temperature, Ozone, and sub-ionospheric Very-Low Frequency (VLF), were reported for 4 consistent days till the EQ day. The N-LSTM result was compatible with a 30-day running median, which is the classical method of ionospheric anomaly detection. We also examined the planetary K-index (Kp), disturbance storm-time (Dst), solar radio flux (F10.7), and solar wind speed (V_SW) indices to check the possible attribution of space weather on the ionosphere during the analysis. The results showed that the anomalies of the ionospheric TEC were more dominantly caused by the EQ phenomenon than the space weather during the nighttime hours. Also, N-LSTM provided sufficient short-term prediction performance of the ionospheric TEC and anomaly detection.

Evidence of particle acceleration within and escape from the solar corona

Aims: We analyse particle, radio, and X-ray observations during the first relativistic proton event of solar cycle 25 detected on Earth. The aim is to gain insight into the relationship between relativistic solar particles detected in space and the processes of acceleration and propagation in solar eruptive events.
Methods: To this end, we used ground-based neutron monitor measurements of relativistic nucleons and space-borne measurements of electrons with similar speed to determine the arrival times of the first particles at 1 AU and to infer their solar release times. We compared the release times with the time histories of non-thermal electrons in the solar atmosphere and their escape to interplanetary space, as traced by radio spectra and X-ray light curves and images.
Results: Non-thermal electrons in the corona are found to be accelerated in different regions. Some are confined in closed magnetic structures expanding during the course of the event. Three episodes of electron escape to the interplanetary space are revealed by groups of decametric-to-kilometric type III bursts. The first group appears on the low-frequency side of a type II burst produced by a coronal shock wave. The two latter groups are accompanied at higher frequencies by bursts with rapid drifts to both lower and higher frequencies (forward- or reverse-drifting bursts). They are produced by electron beams that propagate both sunward and anti-sunward. The first relativistic electrons and nucleons observed near Earth are released with the third group of type III bursts, more than ten minutes after the first signatures of non-thermal electrons and of the formation of the shock wave in the corona. Although the eruptive active region is near the central meridian, several tens of degrees east of the footpoint of the nominal Parker spiral to the Earth, the kilometric spectrum of the type III bursts and the in situ detection of Langmuir waves demonstrate a direct magnetic connection between the L1 Lagrange point and the field lines onto which the electron beams are released at the Sun.
Conclusions: We interpret the forward- and reverse-drifting radio bursts as evidence of reconnection between the closed expanding magnetic structures of an erupting flux rope and ambient open magnetic field lines. We discuss the origin of relativistic particles near the Earth across two scenarios: (1) acceleration at the CME-driven shock as it intercepts interplanetary magnetic field lines rooted in the western solar hemisphere and (2) an alternative where the relativistic particles are initially confined in the erupting magnetic fields and get access to the open field lines to the Earth through these reconnection events.

Movie is available at https://www.aanda.org

Double peak quasi-periodic pulsations in a circular-ribbon flare

We study quasi-periodic pulsations (QPPs) during the impulsive phase of the C8.3 flare SOL2002-08-06T01:43. The shape of an extended 5.7 GHz source is similar to a tadpole with the head located above the region of a negative magnetic polarity, surrounded by positive polarity patches and with a remote tail source. The flare configuration includes bright extreme ultraviolet (EUV) ropes with footpoints near the boundary of the negative magnetic field region and it can be identified as a circular ribbon flare. We use simultaneous observations carried out by the Siberian Solar Radio Telescope at 5.7 GHz, the Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)/HXR, and the Transition Region and Coronal Explorer (TRACE) imaging in the extreme ultraviolet. The flare HXR emission is produced by a compact source located at the south periphery of the Negative Magnetic field Region (NMR). The QPPs are observed during a one-minute interval after the start of the impulsive phase, when this HXR source appeared. The remote source is detected on the variation maps of the of the brightness temperature at 17 GHz and is located at the end of tadpole tail about 60 arcsec eastward. More than a dozen cotemporal HXR and microwave pulses with timescales from 1.5 s up to about 8 s were observed in the flare kernel. At 5.7 GHz, the pulses are more prominent near the remote source where they are highly polarized and generated by the electron beams propagating from the flare kernel. The main tone of the QPP periodicity corresponds to the oscillations with a period of 8 s and is accompanied by the variations in the hardness of nonthermal electrons, that is, in the efficiency of the acceleration mechanism. The second intensity harmonic (about a 3-s period) appears due to a double peak structure of the QPP event. Such pulse shapes suggest oscillations of the current sheet during the loop coalescence as a modulation mechanism of the flare energy release.

Clusters of Solar Radio Spikes Modulated by Quasi-Periodic Pulsations in a Confined Flare

Spikes are typical radio bursts in solar flares, which are proposed to be the signal of energy release in the solar corona. The whole group of spikes always shows different spectral patterns in the dynamic spectrum. Here, we present a special new feature at 0.62 GHz in a confined flare. Each group of spikes is composed of many quasi-periodic sub-clusters, which are superposed on the broadband quasi-periodic pulsations (QPPs). The quasi-periodic cluster of spikes (QPSs) have very intense emissions, and each cluster includes tens of individual spikes. When the intensity of background pulsation is increased, the intensity, duration and bandwidth of the spike cluster are also enlarged. There are 21 groups of QPSs throughout the confined flare. The central frequency of the whole group shifts from 1.9 to 1.2 GHz, and the duration of each cluster shows a negative exponential decay pattern. We propose that nonthermal electron beams play a crucial role in emitting both pulsations and spikes. The tearing-mode oscillations of a confined flux rope produce periodic accelerated electron beams. These electron beams travel inside the closed magnetic structure to produce frequency drifting pulsations via plasma emission and scattered narrowband spikes by electron- cyclotron maser emission (ECME). The slow rise of flux rope makes the source region move upward, and thus, QPSs shift towards low frequency. We propose that the confined flux rope may provide the essential conditions for the formation of QPSs.

Duration and Fluence of Major Solar Energetic Particle (SEP) Events

To understand solar energetic particle (SEP) events and their acceleration processes, it is important to study the SEP properties, e.g. duration and fluence. In this work, we analyzed the temporal evolution of fluxes [cm² sr¹ s¹] of >10, >30, and >60 MeV protons and the temporal and spectral evolution of electromagnetic-radiation components for 34 major SEP events that include 13 ground-level enhancement (GLE)-SEP and 21 non-GLE-SEP events, and then determined their possible onset and end times [UT], their duration [hours], and fluence [cm² sr¹]. It is observed that the temporal fluxes of >30 MeV protons can sometimes be utilized for those of the fluxes of >10 MeV protons. Correspondence between SEP duration and fluence demonstrates the dependence of fluence on duration that helps distinguish the typical and atypical SEP events. For instance, for the >10 MeV protons, correspondence between the duration and fluence exhibited a weaker correlation (r 0.78; p <0.002) during the 13 GLE-SEPs than that (r 0.83; p <0.0001) during the 21 non-GLE-SEPs, revealing a few GLE-SEPs with disproportionate comparability. During the 13 GLE-SEPs, correspondence between the SEP duration and fluence for >30 MeV protons exhibited a stronger correlation (r 0.82; p <0.0006) than that (r 0.78; p <0.002) for the >10 MeV protons, indicating that the temporal window of >30 MeV protons is sometimes more appropriate for obtaining a reasonable duration of SEPs. Accordingly, when the temporal window of flux of >30 MeV protons is utilized for that of the >10 MeV protons, the correlation increased significantly (r 0.86; p <0.0002) during the 13 GLE-SEPs.

Solar radio emission as a disturbance of radiomobile networks

This paper analyses the effects of solar radio emissions in the radiomobile context, for the first time leveraging massive European Telecommunications Standards Institute (ETSI) 3rd Generation Partnership Program (3GPP) Minimization of Drive Test (MDT) radio measures produced by 4G LTE (Long Term Evolution) terminals and by 4G LTE Base Station cells. A method to study solar noise effects starting from radiomobile 3GPP standard MDT measures is illustrated and correlated with the excellent 10.7 cm (2800 MHz) indicator of solar activity (National Research Council Canada). The effects of solar disturbance on the LTE radio access network for mobile services are analysed, and possible countermeasures are presented from the perspective of radiomobile network evolution to 5G and 6G.

Relationship Between Solar Millimeter and Soft X-Ray Emissions

The connection between solar radio and soft X-ray emission has earlier been studied at various radio frequencies. For instance, the intensity peak times during solar flares have been compared between these two wavelength regimes. It has been reported that solar radio emission peaks before soft X-ray emission during a flare. However, opposite results have also been presented. In this study, we compare millimetre (8 mm) solar and soft X-ray emissions (0.5-4 and 1-8 ). The radio observations were made at Metshovi Radio Observatory of Aalto University in Finland between 2015 and 2019. The soft X-ray data were observed with GOES-15 (Geostationary Operational Environmental Satellite). The data show that the solar millimetre emission can peak either before or after soft X-ray peak emission. In this study, we present two different scenarios, which could explain the peaking time differences and behaviour. The first scenario proposes a tight connection between the millimetre (8 mm) and soft X-ray emissions, the second one is for cases where the emission mechanisms are more separate.

Driving Influences of the Doppler Flash Observed by SuperDARN HF Radars in Response to Solar Flares

Sudden enhancement in high-frequency absorption is a well-known impact of solar flare-driven Short-Wave Fadeout (SWF). Less understood, is a perturbation of the radio wave frequency as it traverses the ionosphere in the early stages of SWF, also known as the Doppler flash. Investigations have suggested two possible sources that might contribute to it's manifestation: first, enhancements of plasma density in the D-and lower E-regions; second, the lowering of the F-region reflection point. Our recent work investigated a solar flare event using first principles modeling and Super Dual Auroral Radar Network (SuperDARN) HF radar observations and found that change in the F-region refractive index is the primary driver of the Doppler flash. This study analyzes multiple solar flare events observed across different SuperDARN HF radars to determine how flare characteristics, properties of the traveling radio wave, and geophysical conditions impact the Doppler flash. In addition, we use incoherent scatter radar data and first- principles modeling to investigate physical mechanisms that drive the lowering of the F-region reflection points. We found, (a) on average, the change in E- and F-region refractive index is the primary driver of the Doppler flash, (b) solar zenith angle, ray's elevation angle, operating frequency, and location of the solar flare on the solar disk can alter the ionospheric regions of maximum contribution to the Doppler flash, (c) increased ionospheric Hall and Pedersen conductance causes a reduction of the daytime eastward electric field, and consequently reduces the vertical ion-drift in the lower and middle latitude ionosphere, which results in lowering of the F-region ray reflection point.

Similarities and Differences between Forbush Decreases Associated with Streams from Coronal Holes, Filament Ejections, and Ejections from Active Regions

The Forbush decreases for the period from 1997 to 2020 were studied based on data from the database on Forbush effects and interplanetary disturbances created and maintained at the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN). Using statistical methods, we compared the Forbush decreases associated with coronal mass ejections from active regions of the Sun accompanied by solar flares; filament ejections outside active regions; high-speed streams from coronal holes; and several sources. The difference between Forbush decreases was related to coronal mass ejections when magnetic clouds in the interplanetary disturbances near the Earth were observed or not observed. It has been shown that the distributions of most of parameters are asymmetric for the sporadic Forbush decreases; for the recurrent Forbush decreases, they are nearly symmetric. The strongest correlations between the parameters of Forbush decreases and interplanetary disturbances are observed in the group of coronal ejections from active regions that are accompanied by solar flares and have a structure of magnetic cloud.

A Genetic Algorithm to Model Solar Radio Active Regions From 3D Magnetic Field Extrapolations

In recent decades our understanding of solar active regions (ARs) has improved substantially due to observations made with better angular resolution and wider spectral coverage. While prior AR observations have shown that these structures were always brighter than the quiet Sun at centimeter wavelengths, recent observations at millimeter and submillimeter wavelengths have shown ARs with well defined dark umbrae. Given this new information, it is now necessary to update our understanding and models of the solar atmosphere in active regions. In this work, we present a data-constrained model of the AR solar atmosphere, in which we use brightness temperature measurements of NOAA 12470 at three radio frequencies: 17, 100 and 230 GHz. The observations at 17 GHz were made by the Nobeyama Radioheliograph (NoRH), while the observations at 100 and 230 GHz were obtained by the Atacama Large Millimeter/submillimeter Array (ALMA). Based on our model, which assumes that the radio emission originates from thermal free-free and gyroresonance processes, we calculate radio brightness temperature maps that can be compared with the observations. The magnetic field at distinct atmospheric heights was determined in our modelling process by force-free field extrapolation using photospheric magnetograms taken by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). In order to determine the best plasma temperature and density height profiles necessary to match the observations, the model uses a genetic algorithm that modifies a standard quiet Sun atmospheric model. Our results show that the height of the transition region (TR) of the modelled atmosphere varies with the type of region being modelled: for umbrae the TR is located at 1080 20 km above the solar surface; for penumbrae, the TR is located at 1800 50 km; and for bright regions outside sunspots, the TR is located at 2000 100 km. With these results, we find good agreement with the observed AR brightness temperature maps. Our modelled AR can be used to estimate the emission at frequencies without observational coverage.

The Algorithm

Coronal magnetic fields are well known to be one of the crucial parameters defining coronal physics and space weather. However, measuring the global coronal magnetic fields remains challenging. The polarization properties of coronal radio emissions are sensitive to coronal magnetic fields. While they can prove to be useful probes of coronal and heliospheric magnetic fields, their usage has been limited by technical and algorithmic challenges. We present a robust algorithm for precise polarization calibration and imaging of low-radio frequency solar observations and demonstrate it on data from the Murchison Widefield Array, a Square Kilometre Array (SKA) precursor. This algorithm is based on the Measurement Equation framework, which forms the basis of all modern radio interferometric calibration and imaging. It delivers high-dynamic-range and high-fidelity full-Stokes solar radio images with instrumental polarization leakages <1%, on par with general astronomical radio imaging, and represents the state of the art. Opening up this rewarding, yet unexplored, phase space will enable multiple novel science investigations and offer considerable discovery potential. Examples include detection of low-level circular polarization from thermal coronal emission to estimate large-scale quiescent coronal fields; polarization of faint gyrosynchrotron emissions from coronal mass ejections for robust estimation of plasma parameters; and detection of the first-ever linear polarization at these frequencies. This method has been developed with the SKA in mind and will enable a new era of high-fidelity spectropolarimetric snapshot solar imaging at low radio frequencies.

The Temporal and Spatial Behaviors of CME Occurrence Rate at Different Latitudes

The statistical study of the coronal mass ejections (CMEs) is a hot topic in solar physics. To further reveal the temporal and spatial behaviors of the CMEs at different latitudes and heights, we analyzed the correlation and phase relationships between the occurrence rate of CMEs, the coronal brightness index (CBI), and the 10.7 cm solar radio flux (F10.7). We found that the occurrence rate of the CMEs correlates with the CBI relatively stronger at high latitudes (60) than at low latitudes (50). At low latitudes, the occurrence rate of the CMEs correlates relatively weaker with the CBI than the F10.7. There is a relatively stronger correlation relationship between CMEs, the F10.7, and the CBI during Solar Cycle 24 (SC24) than Solar Cycle 23 (SC23). During SC23, the high-latitude CME occurrence rate lags behind the F10.7 by 3 months, and during SC24, the low-latitude CME occurrence rate leads the low-latitude CBI by 1 month. The correlation coefficient values turn out to be larger when the very faint CMEs are removed from the samples of the CDAW catalog. Based on our results, we may speculate that the source regions of the high/low-latitude CMEs may vary in height, and the process of magnetic energy accumulation and dissipation is from the lower to the upper atmosphere of the Sun. The temporal offsets between different indicators could help us better understand the physical processes responsible for the solar-terrestrial interactions.

Implications for Additional Plasma Heating Driving the Extreme-ultraviolet Late Phase of a Solar Flare with Microwave Imaging Spectroscopy

Extreme-ultraviolet late phase (ELP) refers to the second extreme- ultraviolet (EUV) radiation enhancement observed in certain solar flares, which usually occurs tens of minutes to several hours after the peak of soft X-ray emission. The coronal loop system that hosts the ELP emission is often different from the main flaring arcade, and the enhanced EUV emission therein may imply an additional heating process. However, the origin of the ELP remains rather unclear. Here we present the analysis of a C1.4 flare that features such an ELP, which is also observed in microwave wavelengths by the Expanded Owens Valley Solar Array. Similar to the case of the ELP, we find a gradual microwave enhancement that occurs about 3 minutes after the main impulsive phase microwave peaks. Radio sources coincide with both foot points of the ELP loops and spectral fits on the time-varying microwave spectra demonstrate a clear deviation of the electron distribution from the Maxwellian case, which could result from injected nonthermal electrons or nonuniform heating to the footpoint plasma. We further point out that the delayed microwave enhancement suggests the presence of an additional heating process, which could be responsible for the evaporation of heated plasma that fills the ELP loops, producing the prolonged ELP emission.

Circular Polarization Observations of Type II Solar Radio Bursts and the Coronal Magnetic Field

It is well known that magnetic field strength (B) in the solar corona can be calculated using the Alfvn Mach number (M _A) and Alfvn speed (v _A) of the magnetohydrodynamic shock waves associated with coronal type II radio bursts. We show that observations of weak circularly polarized emission associated with the harmonic component of the type II bursts provide independent and consistent estimates of B. For the coronal type II burst observed on 2021 October 9, we obtained B 1.5 G and 1.9 G at a heliocentric distance (r) of 1.8 R , using the above two techniques, respectively.

Harmonic Electron Cyclotron Maser Emission along the Coronal Loop

Efficient radiation at second and/or higher harmonics of _ce has been suggested to circumvent the escaping difficulty of the electron cyclotron maser emission mechanism when it is applied to solar radio bursts, such as spikes. In our earlier study, we developed a three-step numerical scheme to connect the dynamics of energetic electrons within a large-scale coronal loop structure with the microscale kinetic instability energized by the obtained nonthermal velocity distribution and found that direct and efficient harmonic X-mode (X2 for short) emission can be achieved due to the strip-like features of the distribution. That study only considered the radiation from the loop top at a specific time. Here we present the emission properties along the loop at different locations and timings. We found that, in accordance with our earlier results, few to several strip-like features can appear in all cases, and the first two strips play the major role in exciting X2 and Z (i.e., the slow extraordinary mode) that propagate quasi- perpendicularly. For the four sections along the loop, significant excitation of X2 is observed from the upper two sections, and the strongest emission is from the top section. In addition, significant excitation of Z is observed for all loop sections, while there is no significant emission of the fundamental X mode. The study provides new insight into coherent maser emission along the coronal loop structure during solar flares.

Imaging of the Quiet Sun in the Frequency Range of 20-80 MHz

Radio emission of the quiet Sun is considered to be due to thermal bremsstrahlung emission of the hot solar atmosphere. The properties of the quiet Sun in the microwave band have been well studied, and they can be well described by the spectrum of bremsstrahlung emission. In the meter-wave and decameter-wave bands, properties of the quiet Sun have rarely been studied due to the instrumental limitations. In this work, we use the LOw Frequency ARray telescope to perform high quality interferometric imaging spectroscopy observations of quiet Sun coronal emission at frequencies below 90 MHz. We present the brightness temperature spectrum and the size of the Sun in the frequency range of 20-80 MHz. We report on dark coronal regions with low brightness temperatures that persist with frequency. The brightness temperature spectrum of the quiet Sun is discussed and compared with the bremsstrahlung emission of a coronal model and previous quiet Sun observations.

Quasi-periodic Accelerations of Energetic Particles during a Solar Flare

We report the observation of nonstationary quasi-periodic pulsations (QPPs) in high-energy particles during the impulsive phase of an X4.8 flare on 2002 July 23 (SOL2002-07-23T00:35). The X4.8 flare was simultaneously measured by the Reuven Ramaty High Energy Solar Spectroscopic Imager, Nobeyama Radio Polarimeters, and Nobeyama Radioheliograph. The quasi-period of ~50 15 s, determined by the wavelet transform, is detected in the -ray line emission. Using the same method, a quasi-period of ~90 20 s is found in the -ray continuum, hard X-ray (HXR), and radio emissions during almost the same time. Our observations suggest that the flare QPPs should be associated with energetic ions and nonthermal electrons that are quasi-periodically accelerated by the repetitive magnetic reconnection. The different quasi-periods between the -ray line and continuum/HXR/radio emissions indicate an apparent difference in acceleration or propagation between energetic ions and nonthermal electrons of this solar flare.

Recombination Radio Lines of the Sun

The lines observed in the spectra of astronomical objects provide unique information about them. Currently, only one radio line has been detected in the spectrum of the Sun: a hydrogen fine structure line 2²P_3/2-2²S_1/2 at the frequency 9845 MHz (3.05 cm). It was also found that in the spectrum of solar active formations above sunspots, two more hydrogen fine structure lines are observed with high probability: 3²P_3/2-3²S_1/2 and 3²D_3/2-3²P_1/2 at the frequencies 2917 and 3237 MHz (10.28 and 9.27 cm). The analysis of long- term spectral observations of the Sun by the RATAN-600 radio telescope has shown that numerous recombination radio lines of hydrogen and other elements should be observed in the spectrum of solar active formations above sunspots.

First detection of metric emission from a solar surge

We report the first detection of metric radio emission from a surge, observed with the Nanay Radioheliograph (NRH), STEREO, and other instruments. The emission was observed during the late phase of the M9 complex event SOL2010-02-012T11:25:00, described in a previous publication. It was associated with a secondary energy release, also observed in STEREO 304 images, and there was no detectable soft X-ray emission. The triangulation of the STEREO images allowed for the identification of the surge with NRH sources near the central meridian. The radio emission of the surge occurred in two phases and consisted of two sources, one located near the base of the surge, apparently at or near the site of energy release, and another in the upper part of the surge; these were best visible in the frequency range of 445.0 to about 300 MHz, whereas a spectral component of a different nature was observed at lower frequencies. Sub-second time variations were detected in both sources during both phases, with a 0.2-0.3 s delay of the upper source with respect to the lower, suggesting superluminal velocities. This effect can be explained if the emission of the upper source was due to scattering of radiation from the source at the base of the surge. In addition, the radio emission showed signs of pulsations and spikes. We discuss possible emission mechanisms for the slow time variability component of the lower radio source. Gyrosynchrotron emission reproduced the characteristics of the observed total intensity spectrum at the start of the second phase of the event fairly well, but failed to reproduce the high degree of the observed circular polarization or the spectra at other instances. On the other hand, type IV-like plasma emission from the fundamental could explain the high polarization and the fine structure in the dynamic spectrum; moreover, it gives projected radio source positions on the plane of the sky, as seen from STEREO-A, near the base of the surge. Taking all the properties into consideration, we suggest that type IV-like plasma emission with a low- intensity gyrosynchrotron component is the most plausible mechanism.

Movie associated to Fig. A.2 is available at https://www.aanda.org

Solar Radio Bursts Associated with In Situ Detected Energetic Electrons in Solar Cycles 23 and 24

The first comprehensive analysis between the in situ detected solar energetic electrons (SEEs) from ACE/EPAM satellite and remotely observed radio signatures in solar cycles (SCs) 23 and 24 (19972019) is presented. The identified solar origin of the SEEs (in terms of solar flares, SFs, and coronal mass ejections, CMEs) is associated with solar radio emission of types II, III and IV, where possible. Occurrence rates are calculated as a function of the radio wavelength, from the low corona to the interplanetary space near Earth. The tendencies of the different burst appearances with respect to SC, helio-longitude, and SEE intensity are also demonstrated. The corresponding trends of the driver (in terms of median values of the SF class and CME projected speed) are also shown. A comparison with the respective results when using solar energetic protons is presented and discussed.

Kinematic Study of Radio-Loud CMEs Associated with Solar Flares and DH Type-II Radio Emissions During Solar Cycles 23 and 24

We have statistically analyzed 379 radio-loud (RL) CMEs and their associated flares during the period 1996 - 2019 covering both Solar Cycles (SC) 23 and 24. We classified them into two populations based on the observation period: i ) 235 events that belong to SC 23 (August 1996 - December 2008) and ii ) 144 events that belong to SC 24 (January 2009 - December 2019). For both cycles, the mean sky-plane speed, projection corrected speed (space speed), and initial acceleration of RL CMEs are found to be similar. Moreover, the average residual acceleration of RL CMEs in SC 24 (17.39 43.51 m s²) is twice lower than that of the RL CMEs in SC 23 (8.29 36.23 m s²), which means that the deceleration of RL CMEs in SC 24 is twice as fast as in SC 23. RL CMEs reach their peak speed at higher altitudes in SC 23 (1443 504 km s¹; 13.82 7.40 R) than SC 24 (1920 649 km s¹; 12.51 7.41 R). We also observed that the mean apparent widths of RL CMEs in SC 23 are less than in SC 24, which is statistically significant. SC 23 has a lower average CME nose height (3.85 R) at the start time of DH type-II bursts than that of SC 24 (3.46 R). The starting frequencies of DH type-II bursts associated with RL CMEs for SC 24 are significantly larger (formed at lower heights) than those of SC 23. We found that there is a good correlation between the drift rates and the midfrequencies of DH type-II radio bursts for both of these solar cycles (R² = 0.80, = 1.53). Most of the RL CMEs kinematics and their associated solar-flare properties are found to be similar for SC 23 and SC 24. The annual variations for the general population of CMEs are well consistent with the mean sunspot number but small variations in halo and RL CMEs are observed. We concluded that the reduced total pressure in the heliosphere for SC 24 enables RL CMEs to expand wider and decelerate faster, resulting in DH type-II radio emissions at lower heights than for SC 23.

Spectroscopy of Electric-Field Oscillations in the Solar Wind During the Passage of a Type III Radio Burst Using Observations Compared with Self-Similar Theory

We present a spectrum of electric-field oscillations observed in situ in the solar wind by the WAVES experiment on the Wind spacecraft during the passage of a type III solar radio burst. 19 frequencies of this spectrum are compared with recent predictions of a self-similar nonlinear theory of two-dimensional electron oscillations (Osherovich and Fainberg, 2018).

A novel solar radio spectrogram encryption algorithm based on parameter variable chaotic systems and DNA dynamic encoding

Considering that chaotic systems are highly sensitive to parameters, we design two new parameter variable chaotic systems by constructing parameter perturbation items. These systems are constructed using the state variables of the Liu chaotic system to perturb the parameters of the Lorenz and Chen chaotic systems and are called the Lorenz-Liu chaotic system (LLCS) and Chen-Liu chaotic system (CLCS), respectively. In particular, the parameter perturbation items constructed in this study are not periodic but rather chaotic signals and change in real time. Compared with the original systems, they exhibit more complex randomness and dynamic behaviors. In the proposed cryptosystem, which considers the concept of Deoxyribonucleic Acid (DNA), the solar radio spectrogram is dynamically encoded through the LLCS, and then, the CLCS is used to scramble and diffuse the decoding matrices. In addition, the algorithm uses the 256-bit Secure Hash Algorithm (SHA-256) to generate the initial keys, which enhances the algorithm's sensitivity to plaintext. Simulation results and security analysis show that the cryptosystem has a large key space and high key sensitivity, and can resist various attacks, such as differential attacks and chosen- plaintext attacks.

The Solar Cycle Clock

The Sun's variability is controlled by the progression and interaction of the magnetized systems that form the 22-year magnetic activity cycle (the "Hale Cycle") as they march from their origin at 55 latitude to the equator, over 19 years. We will discuss the end point of that progression, dubbed "terminator" events, and our means of diagnosing them. In this paper we expand on the Extended Solar Cycle framework to construct a new solar activity "clock" which maps all solar magnetic activity onto a single normalized epoch based on the terminations of Hale Magnetic Cycles. Defining phase 0*2 on this clock as the Terminators, then solar polar field reversals occur at 0.2*2, and the geomagnetically quiet intervals centered around solar minimum start at 0.6*2 and end at the terminator, thus lasting 40% of the cycle length. At this onset of quiescence, dubbed a "pre-terminator," the Sun shows a radical reduction in active region complexity and, like the terminator events, is associated with the time when the solar radio flux crosses F10.7 = 90 sfu. We use the terminator-based clock to illustrate a range of phenomena that further emphasize the strong interaction of the global-scale magnetic systems of the Hale Cycle: the vast majority, 96%, of all X-flares happen between the Terminator and pre-Terminator. In addition to the X-rays from violent flares, rapid changes in the number of energetic photonsEUV spectral emission from a hot corona and the F10.7 solar radio fluximpinging on the atmosphere are predictable from the Terminator-normalized unit cycle, which has implications for improving the fidelity of atmospheric modelling.

Statistical Analysis of Circular-ribbon Flares

Circular-ribbon flares (CFs) are a special type of solar flares owing to their particular magnetic topology. In this paper, we conducted a comprehensive statistical analysis of 134 CFs from 2011 September to 2017 June, including 4 B-class, 82 C-class, 40 M-class, and 8 X-class flares. The flares were observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory spacecraft. The physical properties of CFs are derived, including the location, area (A _CF), equivalent radius (r _CF) assuming a semispherical fan dome, lifetime ( _CF), and peak soft X-ray (SXR) flux in 1-8 . It is found that all CFs are located in active regions, with the latitudes between -30 and 30. The distributions of areas and lifetimes could be fitted with a lognormal function. There is a positive correlation between the lifetime and area. The peak SXR flux in 1-8 is well in accord with a power-law distribution with an index of -1.42. For the 134 CFs, 57% of them are accompanied by remote brightenings or ribbons. A positive correlation exists between the total length (L _RB) and average distance (D _RB) of remote brightenings. About 47% and 51% of the 134 CFs are related to type III radio bursts and jets, respectively. The association rates are independent of flare energies. About 38% of CFs are related to minifilament eruptions, and the association rates increase with flare classes. Only 28% of CFs are related to coronal mass ejections (CMEs), meaning that a majority of them are confined rather than eruptive events. There is a positive correlation between the CME speed and peak SXR flux in 1-8 , and faster CMEs tend to be wider.

Multi-instrument Comparative Study of Temperature, Number Density, and Emission Measure during the Precursor Phase of a Solar Flare

We present a multi-instrument study of the two precursor brightenings prior to the M6.5 flare (SOL2015-06-22T18:23) in the NOAA Active Region 12371, with a focus on the temperature (T), electron number density (n), and emission measure (EM). The data used in this study were obtained from four instruments with a variety of wavelengths, i.e., the Solar Dynamics Observatory's Atmospheric Imaging Assembly (AIA), in six extreme ultraviolet (EUV) passbands; the Expanded Owens Valley Solar Array (EOVSA) in microwave (MW); the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in hard X-rays (HXR); and the Geostationary Operational Environmental Satellite (GOES) in soft X-rays (SXR). We compare the temporal variations of T, n, and EM derived from the different data sets. Here are the key results. (1) GOES SXR and AIA EUV have almost identical EM variations (1.5-3 10⁴⁸ cm^-3) and very similar T variations, from 8 to 15 million Kelvin (MK). (2) Listed from highest to lowest, EOVSA MW provides the highest temperature variations (15-60 MK), followed by RHESSI HXR (10-24 MK), then GOES SXR and AIA EUV (8-15 MK). (3) The EM variation from the RHESSI HXR measurements is always less than the values from AIA EUV and GOES SXR by at most 20 times. The number density variation from EOVSA MW is greater than the value from AIA EUV by at most 100 times. The results quantitatively describe the differences in the thermal parameters at the precursor phase, as measured by different instruments operating at different wavelength regimes and for different emission mechanisms.

A first look at the submillimeter Sun with ALMA

We present the first full-disk solar images obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 7 (0.86 mm; 347 GHz). In spite of the low spatial resolution (21), several interesting results were obtained. During our observation, the sun was practically devoid of active regions. Quiet Sun structures on the disk are similar to those in Atmospheric Imaging Assembly images at 1600 and 304 , after the latter are smoothed to the ALMA resolution, as noted previously for Band 6 (1.26 mm) and Band 3 (3 mm) images; they are also similar to negative H images of equivalent resolution. Polar coronal holes, which are clearly seen in the 304 band and small H filaments, are not detectable at 0.86 mm. We computed the center-to-limb variation of the brightness temperature, T_b, in Band 7, as well as in Bands 6 and 3, which were obtained during the same campaign, and we combined them to a unique curve of T_b(log ₁₀₀), where ₁₀₀ is the cosine of the heliocentric angle reduced to 100 GHz. Assuming that the absolute calibration of the Band 3 commissioning observations is accurate, we deduced a brightness temperature at the center of the disk of 6085 K for Band 7, instead of the value of 5500 K, extrapolated from the recommended values for Bands 3 and 6. More importantly, the T_b(log ₁₀₀) curve flattens at large values of ₁₀₀, and so does the corresponding T_e(log ₁₀₀) at large ₁₀₀. This is probably an indication that we are approaching the temperature minimum.

Power distribution of oscillations in the atmosphere of a plage region. Joint observations with ALMA, IRIS, and SDO

Context. Joint observations of the Atacama Large Millimeter/Submillimeter Array (ALMA) with other solar observatories can provide a wealth of opportunities for understanding the coupling between different layers of the solar atmosphere.
Aims: We present a statistical analysis of the power distribution of oscillations in a plage region in active region NOAA AR12651, which was observed jointly with ALMA, the Interface Region Imaging Spectrograph (IRIS), and the Solar Dynamics Observatory (SDO).
Methods: We employ coordinated ALMA Band 6 (1.25 mm) brightness temperature maps, IRIS slit-jaw images in the 2796 passband, and observations in six passbands (1600 , 304 , 131 , 171 , 193 , and 211 ) from the Atmospheric Imaging Assembly (AIA) on board SDO. We perform Lomb-Scargle transforms to study the distribution of oscillation power by means of dominant period maps and power maps. We study the spatial association of oscillations through the atmosphere, with a focus on the correlation of the power distribution of ALMA oscillations with others.
Results: We do not observe any significant association of ALMA oscillations with IRIS and AIA oscillations. While the global behavior of the dominant ALMA oscillations shows a similarity with that of the transition region and coronal passbands of AIA, the ALMA dominant period maps and power maps do not show any correlation with those from the other passbands. The spatial distribution of dominant periods and power in different period intervals of ALMA oscillations is uncorrelated with those of any other passbands.
Conclusions: We speculate that the non-association of ALMA oscillations with those of IRIS and AIA is due to significant variations in the height of formation of the millimeter continuum observed by ALMA. Additionally, the fact that ALMA directly maps the brightness temperature, in contrast to the intensity observations by IRIS and AIA, can result in the very different intrinsic nature of the ALMA oscillations compared to the IRIS and AIA oscillations.

Heating of the solar chromosphere through current dissipation

Context. The solar chromosphere is heated to temperatures higher than predicted by radiative equilibrium. This excess heating is greater in active regions where the magnetic field is stronger.
Aims: We aim to investigate the magnetic topology associated with an area of enhanced millimeter (mm) brightness temperatures in a solar active region mapped by the Atacama Large Millimeter/submillimeter Array (ALMA) using spectropolarimetric co-observations with the 1-m Swedish Solar Telescope (SST).
Methods: We used Milne-Eddington inversions, nonlocal thermodynamic equilibrium (non-LTE) inversions, and a magnetohydrostatic extrapolation to obtain constraints on the three-dimensional (3D) stratification of temperature, magnetic field, and radiative energy losses. We compared the observations to a snapshot of a magnetohydrodynamics simulation and investigate the formation of the thermal continuum at 3 mm using contribution functions.
Results: We find enhanced heating rates in the upper chromosphere of up to 5 kW m², where small-scale emerging loops interact with the overlying magnetic canopy leading to current sheets as shown by the magnetic field extrapolation. Our estimates are about a factor of two higher than canonical values, but they are limited by the ALMA spatial resolution (1.2). Band 3 brightness temperatures reach about 10⁴ K in the region, and the transverse magnetic field strength inferred from the non-LTE inversions is on the order of 500 G in the chromosphere.
Conclusions: We are able to quantitatively reproduce many of the observed features including the integrated radiative losses in our numerical simulation. We conclude that the heating is caused by dissipation in current sheets. However, the simulation shows a complex stratification in the flux emergence region where distinct layers may contribute significantly to the emission in the mm continuum.

The movie is available at https://www.aanda.org

Simulations of solar radio zebras

Context. Solar radio zebras are used in diagnostics of solar flare plasmas and it is of great importance to construct accurate models to correctly characterize them.
Aims: We simulated two zebras to verify their double-plasma resonance (DPR) model.
Methods: In our zebra simulations, we used the DPR model in an expanding and compressing part of the loop as well as with the wave propagating along the loop.
Results: Using the DPR model in such a loop, we successfully simulated zebras from the 1 August 2010 and 21 June 2011 flares. We found that increasing the density or decreasing the magnetic field in the part of the loop, where zebra-stripe sources are located, the zebra stripes are shifted to higher frequencies and vice versa. In the case of the 21 June 2011 flare, we confirm that small deviations of zebra-stripe frequencies from their mean values can be explained by waves propagating along the loop. We also confirm high values for the gyro-harmonic number of zebra stripes. We explain an inconsistency in the wave velocities derived from the plasma parameters and from the frequency drift in combination with the density model of the solar atmosphere. Finally, we discuss the high values of the gyro-harmonic number found in the studied zebras.

Exploring the Circular Polarisation of Low-Frequency Solar Radio Bursts with LOFAR

The Sun is an active star that often produces numerous bursts of electromagnetic radiation at radio wavelengths. Low frequency radio bursts have recently been brought back to light with the advancement of novel radio interferometers. However, their polarisation properties have not yet been explored in detail, especially with the Low Frequency Array (LOFAR), due to difficulties in calibrating the data and accounting for instrumental leakage. Here, using a unique method to correct the polarisation observations, we explore the circular polarisation of different sub-types of solar type III radio bursts and a type I noise storm observed with LOFAR, which occurred during March-April 2019. We analysed six individual radio bursts from two different dates. We present the first Stokes V low frequency images of the Sun with LOFAR in tied-array mode observations. We find that the degree of circular polarisation for each of the selected bursts increases with frequency for fundamental emission, while this trend is either not clear or absent for harmonic emission. The type III bursts studied, that are part of a long-lasting type III storm, can have different senses of circular polarisation, occur at different locations and have different propagation directions. This indicates that the type III bursts forming a classical type III storm do not necessarily have a common origin, but instead they indicate the existence of multiple, possibly unrelated acceleration processes originating from solar minimum active regions.

Mitigation of Radio Frequency Interference in the Solar Radio Spectrum Based on Deep Learning

Radio frequency interference (RFI) may contaminate the signal received by solar radio telescopes. The existence of RFI in the solar radio spectrum affects the accuracy and efficiency of the extraction of burst parameters, which is related to the quality of scientific results and even the authenticity of conclusions. Therefore, it is necessary to carry out research on RFI recognition algorithms for solar radio data. This article aims to compare the recognition performance of six different deep-learning networks (FCN, Deconvnet, Segnet, Unet, Dual- Resunet, and DSC Based Dual-Resunet) on the RFI in solar radio spectra observed by the Chinese Solar Broadband Radio Spectrometer (SBRS). The accuracy and convergence speed in the training process, as well as various performance metrics in the test, indicate that the proposed DSC Based Dual-Resunet is the most suitable neural-network for this task and can achieve both performance and light weight. The RFI recognition accuracy of the DSC Based Dual-Resunet is close to Unet when there is no burst in the spectrum, but in the case of a burst DSC Based Dual-Resunet is obviously better than Unet in terms of RFI recognition. Moreover the model size and number of parameters are approximately 12.5% of those of Unet, and the amount of computation is 38% of that of Unet, which greatly improves the computation efficiency and is of great significance for the realization of the network on mobile hardware. It is promising for the large-scale application of RFI recognition for solar radio telescopes.

Detection of Solar Flares from the Analysis of Signal-to-Noise Ratio Recorded by Digisonde at Mid-Latitudes

This work proposes a new indirect method to detect the impact of solar flares on ionospheric sounding measurements, i.e., on the signal-to- noise ratio of ionospheric reflected radio signals. The method allows us to detect and characterize the ionospheric absorption of high-frequency radio waves as a product of these energetic events. The detection is based on the estimation of the quiet conditions signal-to-noise ratio (SNR) pattern of the month and the subsequent comparison of this pattern with the SNR for the analyzed day. The method has been tested by using data from Ebro Observatory ionospheric station (DPS4D, EB040), but it can be applied to any other ionospheric station. At EB040, it can provide observational data to the international Service of Rapid Magnetic Variations (SRMV) to help confirm Sfe (Solar Flare Effects). To set up the method, we considered a data set of 262 solar flares that occurred during 2011-2014 and were observed during daylight hours at EB040 (17 X-class, 124 M-class, and 121 C-class). This led to impose a threshold of -20 dB in the SNR for at least four consecutive frequencies to confirm that a solar flare took place. The method is particularly sensitive for the detection of X-class solar flares, performs quite well with M-class events, and is even able to detect some C-class flares with high solar altitude angles. Furthermore, we studied some constraints that affect the detection of solar flares from the analysis of GOES-15 hard X-ray flux data about the considered events. For each flare, we computed its solar altitude angle at the time of the ionospheric sounding to get an estimation of its geoeffective irradiance, which had an effect on the local ionosphere. We can confirm that the method of detection is more effective with flares that present a solar elevation angle higher than 18.94, a geoeffective hard X-ray irradiance above 3.30 10^-6 W/m², and a geoeffective hard X-ray radiant exposure higher than 1.61 10^-3 J/m², computed during the 5 min preceding the ionospheric sounding.

Probing the Solar Corona at High Temporal and Spatial Resolution with the Low Frequency Array

The solar corona is the outermost layer of the Sun's atmosphere. Advancements in radio astronomy over the last 50 years have revealed a number of radio phenomena which occur in the corona each with different temporal and spectral characteristics. Current generation interferometers such as the LOw Frequency ARray (LOFAR) give an unprecedented insight into the fine spatial, spectral and temporal structure of these radio bursts. Of particular interest are what are known as type III radio bursts, which are indicative of electrons propagating along open magnetic field lines in the solar corona. Observations of type III bursts allow for the remote sensing of the plasma at various heights in the corona due to the relation between emission at the plasma frequency and electron density. High spatial resolution observations of radio bursts give insight into the role of radio wave scattering on the observed source sizes while high temporal and spectral resolution observations can be used to determine the power density of electron density fluctuations in the corona. Key results of this thesis come from observations of solar radio emission at the highest temporal, spectral and spatial resolutions to date. Firstly, the REAL-time Transient Acquisition backend (REALTA) was developed and installed at the Irish LOFAR station (I-LOFAR) to record the raw voltages from the station at $5.12 \mu$s temporal resolution. This is among the most advanced data acquisition clusters installed at an international LOFAR station to date. First light observations from REALTA are shown, including a variety of solar radio bursts showcasing fine temporal and spectral structure. The installation of REALTA allows for observations of solar radio bursts at some of the highest temporal resolutions to date and is a key resource in investigating the fine temporal structure of solar radio emission. Secondly, a new technique was implemented to directly measure the size of radio bursts from their interferometric visibilities. This is the first time that such a technique has been used to study a type IIIb radio burst. Spectroscopic analysis of the fine frequency structure of a type IIIb burst is used to calculate an expected a source size of $3.18$~arcsec. The full width at half maximum height (FWHM) along the major and minor axes of the burst at 34.76~MHz is found to be $18.8$~$\pm~0.1$~arcmin and $10.2$~$\pm~0.1$~arcmin respectively. % at a plane of sky heliocentric distance of 1.75~R$_\odot$. The new fitting technique used in this analysis removes the need for interferometric imaging and as such, these results indicate that the large size observed for the type IIIb radio burst is due to radio wave scattering in the corona. It is also determined that this effect may be over estimated by previous tied array imaging observations.% These results suggest that the level of density fluctuations in the solar corona is the major cause of the scattering of radio waves, resulting in large source sizes. Finally, this technique was utilised to determine the size and shape of 29 type III bursts and compare them to predictions from state-of-the-art radio wave scattering simulations. It is found that these bursts have a mean size along the major and minor axis of FWHM\textsubscript{x} = 16.27~arcmin and FWHM\textsubscript{y} = 11.96~arcmin respectively. No trend of source size with respect to helioprojective longitude is found, which is in contrast to predictions from modelling of anisotropic scattering from a point source. I attribute this discrepancy to an intrinsic source size of type III bursts. These results underscore the necessity for thorough comparison between radio observations and radio wave scattering simulations before one can be used to infer information from the other. I highlight some future work that could be built upon the research presented in this thesis to further advance the knowledge of radio wave generation and propagation in the solar corona.

Geoneutrino Detection and Other Non-Solar Neutrino Physics Achievements of Borexino

The cosmic silence of the underground Gran Sasso laboratory together with the exceptional radio purity of the liquid scintillator, has allowed Borexino to investigate the radiogenic heating of the Earth's interior and to contribute to various fields of experimental neutrino astronomy. This contribution is aimed to summarize the results obtained by Borexino on geo-neutrinos and on possible extra-terrestrial sources of antineutrinos such as supernovae explosions and solar flares.

Pulsations of microwave emission from a solar flare in a twisted loop caused by intrinsic magnetohydrodynamic oscillations

We present results revealing microwave pulsations produced in a model of a flaring twisted solar coronal loop, without any external oscillatory driver. Two types of oscillations are identified: slowly decaying oscillations with a period of about 70-75 s and amplitude of about 5-10 per cent seen in loops both with and without energetic electrons, and oscillations with a period of about 40 s and amplitude of a few tens of per cent observed only in loops with energetic electrons for about 100 s after the onset of fast energy release. We interpret the longer-period oscillations as the result of a standing kink mode modulating the average magnetic field strength in the loop, whilst the short-period intermittent oscillations associated with energetic electrons are likely to be produced by fast variations of the electric field, which produces energetic electrons in this scenario. The slowly decaying oscillations can explain the quasi-periodic pulsations often observed in the flaring corona.

The Coupling of an EUV Coronal Wave and Ion Acceleration in a Fermi-LAT Behind-the-Limb Solar Flare

We present the Fermi-LAT observations of the behind-the-limb (BTL) flare of 2021 July 17 and the joint detection of this flare by STIX on board the Solar Orbiter. The separation between Earth and the Solar Orbiter was 99.2 at 05:00 UT, allowing STIX to have a front view of the flare. The location of the flare was S20E140 in Stonyhurst heliographic coordinates, making this the most distant behind-the-limb flare ever detected in >100 MeV gamma-rays. The LAT detection lasted for ~16 minutes, the peak flux was 3.6 0.8 (10^-5) ph cm^-2 s^-1 with a significance >15. A coronal wave was observed from both STEREO-A and SDO in extreme ultraviolet (EUV), with an onset on the visible disk in coincidence with the LAT onset. A complex type II radio burst was observed by GLOSS also in coincidence with the onset of the LAT emission, indicating the presence of a shock wave. We discuss the relation between the time derivative of the EUV wave intensity profile at 193 as observed by STEREO-A and the LAT flux to show that the appearance of the coronal wave at the visible disk and the acceleration of protons as traced by the observed >100 MeV gamma-ray emission are coupled. We also report how this coupling is present in the data from three other BTL flares detected by Fermi-LAT, suggesting that the protons driving the gamma-ray emission of BTL solar flares and the coronal wave share a common origin.

Density Turbulence and the Angular Broadening of Outer Heliospheric Radio Sources at High Latitudes and in the Ecliptic Plane

Density irregularities are responsible for the scattering of radio waves in the solar wind and astrophysical plasmas. These irregularities significantly affect the inferred physical properties of radio sources, such as size, direction, and intensity. We present here a theory of angular broadening due to the scattering of radio waves by density irregularities that improves the existing formalism used to investigate radio wave scattering in the outer heliosphere and the very local interstellar medium. The model includes an inner scale and both latitudinal and radial dependencies for the density fluctuation spectra and propagation paths for the radiation both near and out of the ecliptic plane. Based on the pickup-ion-mediated solar wind model (PUI model) of Zank et al., we estimate the turbulence and solar wind quantities for the high-latitude fast solar wind. The predictions include the density variance, inner/dissipation scale, velocity correlation length, mean magnetic field, and proton temperature. The density turbulence amplitude is estimated in two ways. First, a simple scaling technique is used to extend the theoretical predictions of the PUI model for the high-latitude wind beyond the heliospheric termination shock. Second, the solar wind and turbulence quantities are calculated near the ecliptic plane using plasma and magnetometer data from the Voyager 2 spacecraft over the period 1977-2018. Based on the turbulence models and observations, we calculate the scattering angle of the radio sources in the high-latitude and near-ecliptic wind. Finally, we compare the numerical results with the analytic predictions from Cairns and Armstrong et al. and the observed source sizes.

Plasma Emission versus Electron Cyclotron Maser Emission due to Power-law Energetic Electrons in Differently Magnetized Coronal Plasmas

By using self-consistent 2.5-dimensional particle-in-cell simulations, we study the excitation efficiency of electromagnetic waves by power-law energetic electrons with an anisotropic pitch-angle velocity distribution, which can simultaneously trigger the Langmuir and electron cyclotron maser instabilities, in differently magnetized coronal plasmas. It is found that the (transverse) electromagnetic waves can be excited much more efficiently in the case of strongly magnetized plasmas with _ce > _pe than that of weakly magnetized plasmas with _ce < _pe, where _ce and _pe are the electron cyclotron frequency and the electron plasma frequency, respectively. In particular, in a weakly magnetized plasma the electromagnetic wave is hardly excited effectively via the nonlinear coupling of Langmuir waves; although the Langmuir waves can be generated by the power-law energetic electrons, implying that the so-called plasma emission does not effectively work. These results can be helpful for us to better understand the physical mechanism of solar radio bursts.

a multi-wavelength analysis

In this paper, we present the evolution and speed expansion of a shock driven by a Coronal Mass Ejection (CME) that triggered a type II radio. We combine radio analysis with a broad range of observations taken by other ground-based and space borne observatories to track the evolution of this CME/shock from Sun to 1 AU. The Halo CME studied here occurred on 22 August 2015 at 07:12 UT with a linear speed of 547 km/s derived from white-light observations obtained by the SoHO/LASCO spacecraft. The CME and its associated flare (M1 Class at 06:39 UT) originated at active region 12403 located at S14E09 on the visible solar disk. Type II radio bursts drift with the shock propagation up from the solar surface and produce emissions at the fundamental and higher harmonic frequencies which can be seen in radio dynamic spectrograms. The speed of the shock derived from radio observations was consistent with white-light observations. We combined different models to estimate the evolution of the ambient coronal magnetic field at lower corona. We present evidence that this halo CME has been detected in interplanetary space near 1 AU as a smooth rotation in the interplanetary magnetic field suggesting the passage of a magnetic cloud but no shock detected in situ. We compared the CME arrival time with the arrival time obtained from the Empirical shock arrival model and drag-based model. We discuss the importance of these findings.

A deep learning approach to solar radio flux forecasting

The effect of atmospheric drag on spacecraft dynamics is considered one of the predominant sources of uncertainty in Low Earth Orbit. These effects are characterised in part by the atmospheric density, a quantity highly correlated to space weather. Current atmosphere models typically account for this through proxy indices such as the F10.7, but with variations in solar radio flux forecasts leading to significant orbit differences over just a few days, prediction of these quantities is a limiting factor in the accurate estimation of future drag conditions, and consequently orbital prediction. In this work, a novel deep residual architecture for univariate time series forecasting, N-BEATS, is employed for the prediction of the F10.7 solar proxy on the days-ahead timescales relevant to space operations. This untailored, pure deep learning approach has recently achieved state-of-the-art performance in time series forecasting competitions, outperforming well-established statistical, as well as statistical hybrid models, across a range of domains. The approach was found to be effective in single point forecasting up to 27-days ahead, and was additionally extended to produce forecast uncertainty estimates using deep ensembles. These forecasts were then compared to a persistence baseline and two operationally available forecasts: one statistical (provided by BGS, ESA), and one multi-flux neural network (by CLS, CNES). It was found that the N-BEATS model systematically outperformed the baseline and statistical approaches, and achieved an improved or similar performance to the multi-flux neural network approach despite only learning from a single variable.

The first ground-level enhancement of solar cycle 25 on 28 October 2021

Aims: The first relativistic solar proton event of solar cycle 25 was detected on 28 October 2021 by neutron monitors (NMs) on the ground and particle detectors on board spacecraft in near-Earth space. This is the first ground-level enhancement (GLE) of the current cycle. A detailed reconstruction of the NM response together with the identification of the solar eruption that generated these particles is investigated based on in situ and remote-sensing measurements.
Methods: In situ proton observations from a few MeV to 500 MeV were combined with the detection of a solar flare in soft X-rays, a coronal mass ejection, radio bursts, and extreme ultraviolet (EUV) observations to identify the solar origin of the GLE. Timing analysis was performed, and a relation to the solar sources was outlined.
Results: GLE73 reached a maximum particle rigidity of 2.4 GV and is associated with type III, type II, and type IV radio bursts and an EUV wave. A diversity of time profiles recorded by NMs was observed. This points to the event having an anisotropic nature. The peak flux at E > 10 MeV was only 30 pfu and remained at this level for several days. The release time of 1 GV particles was found to be 15:40 UT. GLE73 had a moderately hard rigidity spectrum at very high energies ( 5.5). Comparison of GLE73 to previous GLEs with similar solar drivers is performed.

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Type III radio bursts and excitation of Langmuir waves by energetic electrons

Context. Solar activity occurs not only in terms of the well-known 11-year Sun spot cycle but also in terms of short-lived phenomena as radio bursts. For instance, type III radio bursts are the most common phenomenon of this activity in the Sun's radio radiation. In dynamic radio spectra, they appear as short-lived stripes of enhanced radio emission rapidly drifting from high to low frequencies. They are regarded as the radio signature of beams of energetic electrons travelling along magnetic field lines in the corona. The radio emission is thought to be plasma emission, that is to say the radio emission happens near the electron plasma frequency and/or its harmonics. Plasma emission means, that energetic electrons excite Langmuir waves, which convert into radio waves.
Aims: Initially, energetic electrons are injected in a small region in the corona. Due to their spatio- temporal evolution, they develop a beam-like velocity distribution function (VDF), which is able to excite Langmuir waves. The aim of the paper is to study the spatio-temporal behaviour of the generation of Langmuir waves under coronal cirumstances and its effect on type III radio bursts.
Methods: The generation of Langmuir waves is treated by means of the Maxwell-Vlasov equations. The results are discussed by employing plasma parameters usually found in the corona, for instance at the 150 MHz level.
Results: The Langmuir waves associated with the type III bursts are not generated by a monoenergetic electron beam, but by a population of energetic electrons with a broad velocity distribution. Hence, the Langmuir waves are produced by different parts of the energetic electron population at different times and positions.
Conclusions: In the case of type III bursts, the velocities derived from their drift rates in dynamic radio spectra are not the velocities of electrons, which generate the onset of the type III burst at a given frequency. That can lead to an apparent accelerated motion of the type III radio burst source.

Excitation of Langmuir waves at shocks and solar type II radio bursts

Context. In the solar corona, shocks can be generated due to the pressure pulse of a flare and/or driven by a rising coronal mass ejection (CME). Coronal shock waves can be observed as solar type II radio bursts in the Sun's radio radiation. In dynamic radio spectra, they appear as stripes of an enhanced radio emission slowly drifting from high to low frequencies. The radio emission is thought to be plasma emission, that is to say the emission happens near the electron plasma frequency and/or its harmonics. Plasma emission means that energetic electrons excite Langmuir waves, which convert into radio waves via non- linear plasma processes. Thus, energetic electrons are necessary for plasma emission. In the case of type II radio bursts, the energetic electrons are considered to be shock accelerated.
Aims: Shock drift acceleration (SDA) is regarded as the mechanism for producing energetic electrons in the foreshock region. SDA delivers a shifted loss-cone velocity distribution function (VDF) for the energetic electrons. The aim of the paper is to study in which way and under which conditions a shifted loss-cone VDF of electrons excites Langmuir waves in an efficient way in the corona.
Methods: By means of the results of SDA, the shape of the resulted VDF was derived. It is a shifted loss-cone VDF showing both a loss-cone and a beam-like component. The growth rates for exciting Langmuir waves were calculated in the framework of Maxwell-Vlasov equations. The results are discussed by employing plasma and shock parameters usually found in the corona at the 25 MHz level.
Results: We have found that moderate coronal shocks with an Alfven-Mach number in the range 1.59 < M_A < 2.53 are able to accelerate electrons up to energies sufficient enough to excite Langmur waves, which convert into radio waves seen as solar type II radio bursts.

Dynamics and Characteristics of Waves in the Zebra Radio Source

We analyzed the 17 August 1998 zebra event and showed that some quasi- periodic oscillations modulate the zebra-stripe frequencies. We determined the period of these oscillations as P_n=2.01 0.03 (in numbers of zebra stripes) and as P_f=11.8 0.17 MHz. In the first part of the analyzed zebra, we found a stable density wave that slowly propagated with the frequency drift less than 0.4 MHz s¹. Then, a stationary density wave appeared followed by a transformation of the waves to ones with longer periods. These long- period waves were recorded before and after the time interval when no zebra stripes were observed. We interpreted these density waves as magnetosonic waves. We calculated their wavelength and propagating velocity, considering two types of density models of the solar atmosphere. We also estimated the characteristic density and magnetic- field strength as N 9.2 10⁸ cm¹ and B 0.73 G, respectively. We found similar velocities derived from drifts of the density wave and velocities calculated from the density and magnetic- field strength considering gyro-harmonic numbers of zebra stripes.

A New Multichannel Parallel Real-time FFT Algorithm for a Solar Radio Observation System Based on FPGA

The real-time fast Fourier transform (FFT) is the essential algorithm for signal processing in a solar radio receiver. However, field- programmable gate array (FPGA) computation resources have become the limitation of real-time processing of signals with increasing time and spectral resolutions. It is necessary to design a real-time parallel FFT algorithm with reduced resource occupation in the development of future receiving systems. In this paper, we developed a multichannel parallel FFT algorithm named the multichannel parallel real-time fast Fourier transform (MPR-FFT), which can greatly reduce FPGA resource occupation while increasing the real-time processing speed. In this algorithm, the 4L simultaneous N-point FFTs are first converted into L simultaneous 4N-point FFTs. Fusion processing is then performed to obtain the 4 L N-point spectrum. This method has been used in developing a solar radio spectrometer, which works in the frequency range of 0.5-15 GHz in the Chashan Observatory. In this spectrometer, 16 channel MPR-FFT with 8k-point data is realized in a Xilinx UltraScale KU115 FPGA. The MPR-FFT algorithm reduced the computational resources to a large extent compared to the Cooley-Tukey-based parallel FFT method; for instance, the Look- Up-Table, Look-Up-Table RAM, Flip-Flop, and Digital Signal Process slices were reduced by 37%, 50%, 17%, and 2.48%, respectively. Although the MPR-FFT consumes 14 block RAM resources more than the Cooley-Tukey- based parallel FFT, the MPR-FFT algorithm presents an overall reduction in resource usage.

Subterahertz radius and limb brightening of the Sun derived from SST and ALMA

Measurements of the radius and limb brightening of the Sun provide important information about the solar atmosphere structure and temperature. The solar radius increases as the observation at radio frequency decreases, indicating that each emission originates higher in the atmosphere. Thus, different layers of the solar atmosphere can be probed by observing at multiple wavelengths. In this work, we determined the average radius and limb brightening at 100, 212, 230, and 405 GHz, using data from the Solar Submillimeter Telescope and Atacama Large Millimeter/submillimeter Array's single-dish observations. For the first time, limb brightening values for frequencies of 212 and 405 GHz were estimated. At sub-THz frequencies, the observed limb brightening may affect the solar radius measurements. We use two different and well- known approaches to determine the radius: The half-power method and the inflection-point method. We investigate how the antenna beam size and the limb brightening level, LB, can affect the radius measurements using both methods. Our results showed that the inflection-point method is the least affected by these parameters, and should thus be used for solar radius estimates at radio wavelengths. The measured average radii are 968 3 arcsec (100 GHz), 963 3 arcsec (212 GHz), 963 2 arcsec (230 GHz), and 963 5 arcsec (405 GHz). Finally, we used forward modelling to estimate the ranges of LB of the solar disc resulting in 5-19 per cent (100 GHz), 2-12 per cent (212 GHz), 6-18 per cent (230 GHz), and 3-17 per cent (405 GHz). Both radius and limb brightening estimates agree with previous measurements reported in the literature.

Solar Energetic Particles Produced during Two Fast Coronal Mass Ejections

Two recent extremely fast coronal mass ejections (CMEs) are of particular interest. The first one originated from the southern hemisphere on 2021 October 28 and caused strong solar energetic particle (SEP) events over a wide longitude range from Earth, STEREO-A, to Mars. However, the other one, originating from the center of the Earth-viewed solar disk 5 days later, left weak SEP signatures in the heliosphere. Based on the white-light images of the CMEs from the Solar and Heliospheric Observatory (SOHO) and the Ahead Solar Terrestrial Relations Observatory (STEREO-A), in combination with the observations of the corresponding solar flares, radio bursts, and in situ magnetic fields and particles, we try to analyze the series of solar eruptions during October 28-November 2 as well as their correspondences with the in situ features. It is found that the difference in SEP features between the two CMEs is mainly due to (1) the seed particles probably supplied by associated flares and (2) the magnetic connection influenced by the preceding solar wind speed.

Eruption of the EUV Hot Channel from the Solar Limb and Associated Moving Type IV Radio Burst

Using the observations from the Solar Dynamics Observatory, we study an eruption of a hot-channel flux rope (FR) near the solar limb on 2015 February 9. The pre-eruptive structure is visible mainly in EUV 131 images, with two highly sheared loop structures. They undergo a slow rising motion and then reconnect to form an eruptive hot channel, as in the tether-cutting reconnection model. The J-shaped flare ribbons trace the footpoint of the FR that is identified as the hot channel. Initially, the hot channel is observed to rise slowly at 40 km s^-1, followed by an exponential rise from 22:55 UT at a coronal height of 87 2 Mm. Following the onset of the eruption at 23:00 UT, the flare reconnection then adds to the acceleration process of the coronal mass ejection (CME) within 3 R . Later on, the CME continues to accelerate at 8 m s^-2 during its propagation period. Further, the eruption also launched type II radio bursts, which were followed by type III and type IVm radio bursts. The start and end times of the type IVm burst correspond to the CME's core height of 1.5 and 6.1 R , respectively. Also, the spectral index is negative, suggesting that nonthermal electrons are trapped in the closed loop structure. Accompanied by this type IVm burst, this event is unique in the sense that the flare ribbons are very clearly observed together with the erupting hot channel, which strongly suggests that the hooked parts of the J-shaped flare ribbons outline the boundary of the erupting FR.

Langmuir-Slow Extraordinary Mode Magnetic Signature Observations with Parker Solar Probe

Radio emission from interplanetary shocks, planetary foreshocks, and some solar flares occurs in the so-called "plasma emission" framework. The generally accepted scenario begins with electrostatic Langmuir waves that are driven by a suprathermal electron beam on the Landau resonance. These Langmuir waves then mode-convert to freely propagating electromagnetic emissions at the local plasma frequency f _pe and/or its harmonic 2f _pe. However, the details of the physics of mode conversion are unclear, and so far the magnetic component of the plasma waves has not been definitively measured. Several spacecraft have measured quasi-monochromatic Langmuir or slow extraordinary modes (sometimes called z-modes) in the solar wind. These coherent waves are expected to have a weak magnetic component, which has never been observed in an unambiguous way. Here we report on the direct measurement of the magnetic signature of these waves using the Search Coil Magnetometer sensor of the Parker Solar Probe/FIELDS instrument. Using simulations of wave propagation in an inhomogeneous plasma, we show that the appearance of the magnetic component of the slow extraordinary mode is highly influenced by the presence of density inhomogeneities that occasionally cause the refractive index to drop to low values where the wave has strong electromagnetic properties.

Robust Absolute Solar Flux Density Calibration for the Murchison Widefield Array

Sensitive radio instruments are optimized for observing faint astronomical sources, and usually need to attenuate the received signal when observing the Sun. There are only a handful of flux density calibrators that can comfortably be observed with the same attenuation setup as the Sun. Additionally, for wide field-of-view (FoV) instruments like the Murchison Widefield Array (MWA) calibrator observations are generally done when the Sun is below the horizon, to avoid the contamination from solar emissions. These considerations imply that the usual radio interferometric approach to flux density calibration is not applicable for solar imaging. A novel technique, relying on a good sky model and detailed characterization of the MWA hardware, was developed for solar flux density calibration for MWA. Though successful, this technique is not general enough to be extended to the data from the extended configuration of the MWA Phase II. Here, we present a robust flux density calibration method for solar observations with MWA independent of the array configuration. We use different approaches-the serendipitous presence of strong sources; detection of numerous background sources using high dynamic range images in the FoV along with the Sun; and observations of strong flux density calibrators with and without the additional attenuation used for solar observations-to obtain the flux scaling parameters required for the flux density calibration. Using the present method, we have achieved an absolute flux density uncertainty ~10% for solar observations even in the absence of dedicated calibrator observations.

Trieste CALLISTO station setup and observations of solar radio bursts

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High-resolution observations with ARTEMIS/JLS and the NRH. IV. Imaging spectroscopy of spike-like structures near the front of type-II bursts

Context. Narrowband bursts (spikes) are very small duration and bandwidth bursts which appear on dynamic spectra from microwave to decametric frequencies. They are believed to be manifestations of small- scale energy release through magnetic reconnection.
Aims: We study the position of the spike-like structures relative to the front of type-II bursts and their role in the burst emission.
Methods: We used high-sensitivity, low-noise dynamic spectra obtained with the acousto-optic analyzer (SAO) of the ARTEMIS-JLS solar radiospectrograph, in conjunction with high-time-resolution images from the Nanay Radioheliograph (NRH) in order to study spike-like bursts near the front of a type-II radio burst recorded at the west limb during the November 3, 2003 extreme solar event. The spike-like emission in the dynamic spectrum was enhanced by means of high-pass-time filtering.
Results: We identified a number of spikes in the NRH images. Due to the lower temporal resolution of the NRH, multiple spikes detected in the dynamic spectrum appeared as single structures in the images. These spikes had an average size of 200 and their observed brightness temperature was 1.4 to 5.6 10⁹ K, providing a significant contribution to the emission of the type-II burst front. At variance with a previous study on the type-IV associated spikes, we found no systematic displacement between the spike emission and the emission between spikes. At 327.0 MHz, the type II emission was located about 0.3 R above the pre-existing continuum emission, which, in turn, was located 0.1 R above the western limb.
Conclusions: This study, combined with our previous results, indicates that the spike-like chains aligned along the type II burst MHD shock front are not a perturbation of the type II emission, as in the case of type IV spikes, but a manifestation of the type II emission itself. The preponderance of these chains, together with the lack of isolated structures or irregular clusters, points towards some form of small- scale magnetic reconnection, organized along the type-II propagating front.

This work is dedicated to the memory of Costas Caroubalos (1928-2021), founder of the ARTEMIS radiospectrograh.

Non-thermal electron velocity distribution functions due to 3D kinetic magnetic reconnection for solar coronal plasma conditions

Magnetic reconnection can convert magnetic energy into kinetic energy of non-thermal electron beams. Those accelerated electrons can, in turn, cause radio emission in astrophysical plasma environments, such as solar flares via micro-instabilities. The properties of the electron velocity distribution functions (EVDFs) of those non-thermal beams generated by reconnection are, however, still not well understood, in particular, properties that are necessary conditions for some relevant micro- instabilities. We aim at characterizing the EVDFs generated in 3D magnetic reconnection by means of fully kinetic particle-in-cell code simulations. In particular, our goal is to identify the possible sources of free energy offered by the generated EVDFs and their dependence on the strength of the guide field. By applying a machine learning algorithm on the EVDFs, we find that (1) electron beams with positive gradients in their 1D parallel (to the local magnetic field direction) velocity distribution functions are generated in both diffusion region and separatrices. (2) Electron beams with positive gradients in their perpendicular (to the local magnetic field direction) velocity distribution functions are observed in the diffusion region and outflow region near the reconnection midplane. In particular, perpendicular crescent-shaped EVDFs (in the perpendicular velocity space) are mainly observed in the diffusion region. (3) As the guide field strength increases, the number of locations with EVDFs featuring a perpendicular source of free energy significantly decreases. The formation of non- thermal electron beams in the field-aligned direction is mainly due to magnetized and adiabatic electrons, while in the direction perpendicular to the local magnetic field, it is attributed to unmagnetized electrons.

Comparing results of real-scale time MHD modeling with observational data for first flare M 1.9 in AR 10365

As shown in the first results of MHD simulations in the real scale of time, above the active region (AR) 10365, during the first flare M 1.9 (05/26/2003 05:34) at a height of 16-18 mm (lower corona), a singular line of magnetic field appears. The local maximum of the current density is situated on this singular line. The magnetic field in the vicinity of this singular line is the superposition of an X-type magnetic configuration and a divergent magnetic field. The accumulation of magnetic energy for solar flare with current sheet creation takes place near this singular line due to magnetic field deformation by disturbances in the X-type configuration in spite of the presence of overlaid diverging magnetic configuration. The magnetic configuration is so complicated that the singular line can be found only by using specially developed graphical system of search. The position of singular line coincides with position of source of flare radio emission at the frequency 17 GHz above AR 10365 measured by Nobeyama Radioheliograph (NoRH). Also, MHD simulation shows appearance of the singular line, in the vicinity of which X-type configuration dominates. However, apparently due to small disturbance, propagating from the photosphere, sufficient magnetic energy was not accumulated in this configuration, so the NoRH does not show the flare source of emission at the frequency 17 GHz in the place, where this singular line is situated.

Solar Microwave Emission Associated with Coronal Mass Ejections (CME)

The connection between Coronal Mass Ejections (CME) and radio burst has been discovered especially at lower frequencies (< 2 GHz). The aim of the study is to investigate possible connection between CMEs and variability of radio brightenings at 37 GHz (8 mm) within the time frame of four days. The millimetre radio observations have been made on RT-14 radio telescope at Metshovi Radio Observatory of Aalto University, Finland. In addition, 11.2 GHz (2.7 cm) total solar flux information is included in the analysis. The radio observations were made between March 2011 and September 2017, totally including 24 events. The results demonstrate that in most of the cases the radio brightening intensity achieves its maximum before CME occurs. Time of 11.2 GHz intensity appearance matches with time of CME appearance with difference of two to three hours. However, in most cases a maximum of 11.2 GHz intensity appears before CMEs. The study investigates a possibility of predicting CME appearance based on milli- and centimetre radio observations. The study also proposes a scenario connection between CMEs and solar microwave events.

Dependence of D-Region Perturbations of the Midlatitude Ionosphere on the Spectral Composition of the X-Ray Radiation of Solar Flares According to Experimental Data

An experimental study was made of the dependence of the electron-density perturbations in the Dregions of the midlatitude ionosphere during solar flares on the spectral composition of X-ray radiation in the wavelength range from 0.01 to 0.4 nm, i.e., outside the measurement range of the GOES satellite. For this, the variation in the brightness temperature of the emission of six flares in 20142017 were calculated, and the composition of their radiation was determined. It is shown that, although only a small percent of the total flash radiation energy is in the 0.010.2 nm wavelength range, it is the hard components that are the main factor leading to a change in the reflection height of very low frequency (VLF) radio waves. It is shown that the efficiency of solar flares significantly depends on the previous solar activity, not only for class-C flares but also for powerful class-M and -X flares.

Solar-Activity Index for the E-Layer Critical Frequency at Middle Latitudes

Based on an analysis of data from the midlatitude ionospheric stations, it is found that the P = 0.5(F₁ + F₈₁) index is an optimal solar-activity index for the daily values of the E-layer critical frequency foE, where F₁ and F₈₁ are the solar radio-emission flux at a wavelength of 10.7 cm on the given day and the value of this flux averaged over 81 days, respectively. The standard deviations of the foE dependence on P in the daytime hours are almost equal for winter and summer at a latitude of approximately 50 N. The value is maximal at lower latitudes for summer and at higher latitudes for winter. The introduction of the P index into the Institute of Applied Geophysics (IPG), international reference ionosphere (IRI), or NeQuick models makes it possible to use these models to calculate the daily foE values. Based on a preliminary analysis (noon, moderate solar activity), it is found that the accuracy of the foE calculation with these models is nearly the same. The strongest difference between the models is observed for summer; at latitudes of 5060 N, the IPG and NeQuick models are more accurate than the IRI model. The IRI model could be preferable for lower latitudes.

Features of the Behavior of Time Parameters of Forbush Decreases Associated with Different Types of Solar and Interplanetary Sources

Forbush decreases occurring from 1997 to 2017 (1055 events in total) have been analyzed with the use of a database of Forbush effects and interplanetary disturbances built and currently maintained by the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation of the Russian Academy of Sciences (IZMIRAN). Based on statistical methods, we compared the temporal evolution of Forbush decreases in events of four types: (1) those associated with coronal mass ejections from active regions and accompanied by solar flares; (2) those induced by interplanetary disturbances due to filament eruptions from areas beyond active regions; (3) those caused by high-speed streams from coronal holes; and (4) those associated with two or more sources of different types of disturbances. In the comparison, we used the following time parameters of the development of Forbush decreases: the time intervals from the event onset to the detection of the minimal density of cosmic rays, the maximal hourly decrease in the density of cosmic rays, the maximal equatorial anisotropy of cosmic rays, the maximal speed of the solar wind, the maximal strength of the interplanetary magnetic field, and the minimal Dst index. Analysis of the distributions of the time parameters and their intercorrelation has shown that there are substantial differences between the evolution scenarios of Forbush decreases in the four sets of examined events.

A Broadband Solar Radio Dynamic Spectrometer Working in the Millimeter-wave Band

Most solar radio telescopes operate below ~18 GHz and cannot realize a complete frequency coverage of the microwave spectrum, especially in the optically thin regime during solar bursts, which can provide unique information about the magnetic field in the burst area in the solar corona. Therefore, the development of high-frequency microwave observation equipment is demanded by the solar radio community. In this paper, we present a microwave spectrum observation system operating at 35-40 GHz. In this system, the solar radio signal is acquired by an 80 cm Cassegrain circularly polarized antenna, which is then downconverted and channelized by a 35-40 GHz analog front end. The processed signal is finally sent to the digital receiver to generate the microwave dynamic spectrum, which is transmitted by gigabit Ethernet transmission to a host computer. The system performance has been tested and obtained as follows: a noise figure of ~300 K, system linearity of >0.9999, time resolution of about 134 ms (default), and frequency resolution of 153 kHz. We further conduct calibration for this system and find that the observed Sun-Moon ratio is about 43.1-53.3 @ 35.25 GHz during the new Moon, and is quite close to the theoretical value. The coefficient of variation of the system is ~0.61% in a 9 hr test. The system has been designed, developed, and tested for over 1 yr in Chashan Solar Observatory and is expected to play an important role in the microwave burst study in the 25th solar cycle.

Simultaneous Observations of Chromospheric Evaporation and Condensation during a C-class Flare

We explored simultaneous observations of chromospheric evaporation and condensation during the impulsive phase of a C6.7 flare on 2019 May 9. The solar flare was simultaneously observed by multiple instruments, i.e., the New Vacuum Solar Telescope (NVST), the Interface Region Imaging Spectrograph, the Atmospheric Imaging Assembly (AIA), Fermi, the Mingantu Spectral Radioheliograph, and the Nobeyama Radio Polarimeters. Using the single Gaussian fitting and the moment analysis technique, redshifted velocities at slow speeds of 15-19 km s^-1 are found in the cool lines of C II and Si IV at one flare footpoint location. Redshifts are also seen in the H line-of-sight velocity image measured by NVST at double footpoints. Those redshifts with slow speeds can be regarded as the low-velocity downflows driven by the chromospheric condensation. Meanwhile, the converging motions from double footpoints to the loop top are found in the high-temperature EUV images, such as AIA 131, 94, and 335 . Their apparent speeds are estimated to be roughly 126-210 km s^-1, which could be regarded as the high-velocity upflows caused by the chromospheric evaporation. The nonthermal energy flux is estimated to be about 5.7 10¹⁰ erg s^-1 cm^-2. The characteristic timescale is roughly equal to 1 minute. All these observational results suggest an explosive chromospheric evaporation during the flare impulsive phase. While a hard X-ray/microwave pulse and a type III radio burst are found simultaneously, indicating that the explosive chromospheric evaporation is driven by the nonthermal electron.

Sizes and Shapes of Sources in Solar Metric Radio Bursts

Metric and decametric radio emissions from the Sun are the only direct source of information about the dynamics of nonthermal electrons in the upper corona. In addition, the combination of spectral and imaging (sizes, shapes, and positions) observations of low-frequency radio sources can be used as a unique diagnostic tool to probe plasma turbulence in the solar corona and inner heliosphere. The geometry of the low-frequency sources and its variation with frequency are still not understood, primarily due to the relatively low spatial resolution available for solar observations. Here we report the first detailed multifrequency analysis of the sizes of solar radio sources observed by the Low Frequency Array. Furthermore, we investigate the source shapes by approximating the derived intensity distributions using 2D Gaussian profiles with elliptical half-maximum contours. These measurements have been made possible by a novel empirical method for evaluating the instrumental and ionospheric effects on radio maps based on known source observations. The obtained deconvolved sizes of the sources are found to be smaller than previous estimations, and often show higher ellipticity. The sizes and ellipticities of the sources inferred using 2D Gaussian approximation, and their variation with frequency are consistent with models of anisotropic radio-wave scattering in the solar corona.

Properties and Space Weather Relevance

A comprehensive statistical analysis on the properties and accompanied phenomena of all M-class solar flares (as measured in soft X-rays) in the last two solar cycles (19962019) is presented here with a focus on their space weather potential. The information about the parent active region and the underlying sunspot (Hale) type is collected for each case, where possible, in order to identify photospheric precondition as precursors for the solar flare eruption or confinement. Associations with coronal mass ejections, solar energetic particles, and interplanetary radio emissions are also evaluated and discussed as possible proxies for flare eruption and subsequent space weather relevance. The results show that the majority (80%) of the analyzed M-class flares are of , -, and -- magnetic field configuration. The M-class population of flares is accompanied by CMEs in 41% of the cases and about half of the flare sample has been associated with radio emission from electron beams. A much lower association (10%) is obtained with shock wave radio signatures and energetic particles. Furthermore, a parametric scheme is proposed in terms of occurrence rates between M-class flares and a variety of accompanied solar phenomena as a function of flare sub-classes or magnetic type. This study confirms the well-known reduced but inevitable space weather importance of M-class flares.

Predicting the Daily 10.7-cm Solar Radio Flux Using the Long Short-Term Memory Method

As an important index of solar activity, the 10.7-cm solar radio flux (F_10.7) can indicate changes in the solar EUV radiation, which plays an important role in the relationship between the Sun and the Earth. Therefore, it is valuable to study and forecast F_10.7. In this study, the long short-term memory (LSTM) method in machine learning is used to predict the daily value of F_10.7. The F_10.7 series from 1947 to 2019 are used. Among them, the data during 19471995 are adopted as the training dataset, and the data during 19962019 (solar cycles 23 and 24) are adopted as the test dataset. The fourfold cross validation method is used to group the training set for multiple validations. We find that the root mean square error (RMSE) of the prediction results is only 6.20~6.35 sfu, and the correlation coefficient (R) is as high as 0.9883~0.9889. The overall prediction accuracy of the LSTM method is equivalent to those of the widely used autoregressive (AR) and backpropagation neural network (BP) models. Especially for 2-day and 3-day forecasts, the LSTM model is slightly better. All this demonstrates the potentiality of the LSTM method in the real-time forecasting of F_10.7 in future.

NenuFAR Performance for Solar Radio Observations

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First Solar Radio Burst Observations by the Mexican Array Radio Telescope (MEXART) at 140 MHz

The National Laboratory of Space Weather in Mexico (Laboratorio Nacional de Clima Espacial: LANCE) coordinates instrumentation for monitoring the space-weather impact over Mexico. Two of these instruments are the Mexican Array Radio Telescope (MEXART) and Compound Astronomical Low frequency Low cost Instrument for Spectroscopy and Transportable Observatory (CALLISTO) station of the e-CALLISTO network (CALLMEX). Both instruments are located at the same facility (Coeneo Michoacan, Mexico) and share a spectral band centered at 140 MHz. In this work, we show the capabilities of the e-CALLISTO network as support to identify a solar radio burst in the signal of the MEXART radiotelescope. We identified 75 solar radio bursts in the MEXART signal: five events of Type II and 70 of Type III between September 2015 and May 2019. The analysis of solar radio bursts in the MEXART signal provides us valuable information about the development of the radio event due to their high sensitivity, time resolution, and isotropic response. In the case of Type-III solar radio events, we identify four characteristic phases in the dynamical evolution of the signal at 140 MHz: a pre-phase, a main peak, a decay phase, and a post-event phase. A Morlet wave transform was done of MEXART signals in the Type-III solar radio busts; in their spectra, a pine tree structure was identified preceding the main event in the time series. These characteristics are not observable in the data from the e-CALLISTO network.

Detections of Multi-Periodic Oscillations During a Circular Ribbon Flare

We present the analysis of three kinds of oscillating behavior using multi-wavelength observations of the 10 November 2013 (SOL2013-11-10T05:14) circular-ribbon flare. This event is a typical circular-ribbon flare with an outer spine structure and homologous jets. We found three kinds of oscillations (or perturbations): i) flux oscillation (or QPP) with a dominant period of about 20 seconds in X-ray, EUV, and microwave emissions, ii) periodic jets with an intermittent cadence of around 72 seconds, iii) an outer loop perturbing half a cycle with a duration of about 168 seconds. Similar to the periodic jets that could be produced by a nonthermal process, like repeated magnetic reconnection, the flare QPP detected in the thermal emissions could have the same origin as the oscillation seen in the nonthermal emissions. The outer-loop perturbation is possibly triggered by a blast wave driven by the circular-ribbon flare, or it might be modulated by the sausage wave or the slow magnetoacoustic wave. The results obtained provide data for further numerical studies on the physical origin of the flare oscillations.

Solar Eclipse Observations with Small Radio Telescope in Hong Kong in the 21 CM Radio Frequency Band

Small radio telescope of 21 cm was used for studying the partial solar eclipse, with magnitude 0.89, in Hong Kong on 21st June, 2020. The radio telescope SPIDER 300A was designed and constructed by the Radio2Space Company, Italy. Radio flux density time curves (light curve) and a two- dimension mapping of the eclipse are presented in this paper. Standard radio data reduction methods were used to obtain the intensity time curve. We also adopted the semi-pipeline method for the reduction of data to obtain the same results as with the built-in software of the radio telescope SPIDER 300A. The total solar radio flux of the eclipse was found to reduce by maximum 555%, while the maximum eclipsed area of the same eclipse is 86.08%. Other radio observations of solar eclipses in Hong Kong are also discussed in this paper, including SPIDER 300A observation of partial solar eclipse on 26th December 2019 (APPENDIX A); and small radio telescope (SRT), developed by the Haystack Observatory, MIT, USA, observation of 2020 eclipse (APPENDIX B).

The Critical Frequency of the Ionospheric F2-LAYER as Obtained from Ionosonde Data and Observations of Solar Radio Bursts

Subject and Purpose. Studying the time variations shown by the critical frequencies of the ionospheric F2 layer through comparative analysis of ionosonde data and observations of type III solar radio bursts. Methods and Methodology. In this work, two independent methods have been used for identifying critical frequencies in the ionosphere, namely that of vertical sounding and observations of type III solar radio bursts near their cut-off frequency in the ionosphere. One of the ionosondes used for vertical sounding was located near Zmiiv (Kharkiv Region), rather close to the UTR-2 radio astronomy observatory where the solar bursts were observed. The radiation from such bursts represented probe signals for transmissive sounding. The solar radiation was received with an element of a low-frequency (1 to 40 MHz) antenna array. Results. On May 22, 2021 variations in the critical frequency f0F2 of the ionospheric F2-layer were followed between 07:00 and 17:00 UT. The value reached a maximum of 5.9 MHz at 07:45 to 08:00 UT and then decreased smoothlyto 4.9 MHz, staying there from 15:30 till 16:45 UT. At that time, a storm of type III solar bursts was recorded with the antenna for radio observations at 140 MHz, revealing a cut-off effect for the bursts. As has been found,their cut-off frequency can be used for estimating the critical frequency f0F2 in the ionosphere. Conclusions. The comparative analysis of solar burst observations and frequency-and-time measurements with an ionosonde has shown possibilities for evaluating the critical frequency f0F2 in the ionosphere from the data on the cut-off frequency for solar radio-frequ

Dispersion and damping rate of Langmuir wave in space plasma with regularized Kappa distributed electrons

The dispersion of Langmuir wave (LW) in an unmagnetized collisionless plasma with regularized Kappa distributed electrons is investigated from the kinetic theory. The frequency and damping rate of LW are analyzed for the parameters relating to the source region of a type III solar radio burst. It is found that the linear behavior of LW is greatly modified by the suprathermal index and the exponential cutoff parameter . In the region < 1.5, the damping rate of LW will be much larger than the one with Maxwellian distributed electrons. Hence, the nonlinear process of LW in low region may exhibit different properties in comparison with the one in large region.

Observations and Modeling of NOAA AR 12470

Waves and oscillations are important solar phenomena not only because they can propagate and dissipate energy in the chromosphere, but also because they carry information about the structure of the atmosphere in which they propagate. Among these phenomena, the one of the most interesting ones occurs in the sunspot umbra. In this area, continuously propagating magnetohydrodynamic (MHD) waves generated from below the photosphere create the famous 3-minute sunspot umbral oscillations that affect the line profile of spectral lines due to temperature, density, and velocity changes of the plasma in the region. In the past decades, numerous observations and models have been carried out about the nature of the 3-minute oscillation and its relation with the coronal heating problem, but the lack of direct observations of the temperature variation in the chromosphere has made it hard to answer these questions.The need for a better understanding of the fine structure of the 3-minute oscillation and its time evolution in sunspots has intensified with the development of better observing tools. Among modern observatories, the Atacama Large Millimeter/submillimeter Array (ALMA) opens up a new era of solar radio observation due to its high spatial and temporal resolution and image quality. When combined with other cutting-edge instruments, such as the Goode Solar Telescope (GST) at the Big Bear Solar Observatory (BBSO), the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, and Interface Region Imaging Spectrograph (IRIS), ALMA can provide unique electron temperature diagnostics that clarify the behavior of the solar chromosphere's response to propagating waves.In this dissertation, a study is carried out about sunspot oscillations and wave propagation in NOAA active region 12470 using an approximately 1 hr long dataset acquired on 2015 December 17 by the instruments listed above. The discovery of 3-minute sunspot oscillations seen in the mm-wave band is reported for the first time. The 2 s cadence of ALMA images makes it possible to well resolve the typical 3-minute period sunspot oscillation in the chromosphere. Fourier analysis is applied to the ALMA band 3 and GST H data sets to obtain the power spectra as well as phase information of the oscillations. The properties of the wave propagation are analyzed by combining multiple wavelengths that probe physical parameters of the solar atmosphere at different heights.The chromospheric radiation is synthesized in 1-D using a radiation transfer code which uses the Solar Irradiance Physical Modeling (SRPM) as an input. A good correlation of the phase relationship between the observed and modeled oscillations of H and temperature fluctuations has been found and it is consistent with the result from a possible physical model for impulse-driven acoustic waves propagating in the gravitationally stratified medium. An asymmetry in the time profile of the temperature fluctuations discovered in the ALMA data is found to require a nonlinear wave solution, which is applied to several atmospheric models in an attempt to match the asymmetry and the absolute brightness temperature of the simulations to the observations. The asymmetry is successfully reproduced using the nonlinear wave scenario, although the absolute brightness of the simulated atmosphere remains lower than observed. These results demonstrate the capability of ALMA mm observations to provide new insight into what is needed for improving such atmospheric models in the future.

A deep learning method for the recognition of solar radio burst spectrum

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A Transient-Motion-Powered Motion Detector

The emerging energy harvesting technology facilitates the development of ubiquitous and everlasting battery-free motion detectors. This article introduces a robust design of the transient-motion-powered motion detector, which is called ViPSN-pluck. "ViPSN" is the acronym for the vibration-powered sensing node while "pluck" stands for the plucking- motion energy harvester. By using a piezo-magneto-elastic structure, ViPSN-pluck can efficiently harvest energy from a transient motion. By properly making good use of this tiny harvested energy, ViPSN-pluck can effectively carry out motion detection and Bluetooth low-energy (BLE) wireless communication. Given the concurrency of mechanical potential energy precharging and motion detection, the transient-motion plucking energy harvester used in ViPSN-pluck has the merit of high energy reliability. This unique feature is unprecedented in the solar and radio-frequency (RF) energy harvesting cases, which might suffer from energy outages under fluctuating irradiance or RF signal strength, respectively. The working principle of ViPSN-pluck, in particular, the dynamic characteristics of the plucking energy harvester and the energy matching between generation and utilization, are discussed in detail to demonstrate the robustness in operation. The cyber-electromechanical synergy among the mechanical dynamics, power conditioning circuit, and low-power embedded system is highlighted. The design methodology of ViPSN-pluck provides a valuable reference for the developments of future motion-powered Internet of Things devices.

Demonstrating the capabilities of CubeSats to monitor essential climate variables of the water cycle [Instruments and Missions]

The Federated Satellite Systems/³Cat-5 (FSSCat) mission was the winner of the European Space Agency (ESA) Sentinel Small Satellite (S³) Challenge and overall winner of the 2017 Copernicus Masters competition. It consisted of two six-unit CubeSats. The Earth observation payloads were 1) the Flexible Microwave Payload 2 (FMPL-2) onboard ³Cat-5/A, an L-band microwave radiometer and GNSS reflectometer (GNSS-R) implemented using a software-defined radio (SDR), and 2) the HyperScout-2 onboard ³Cat-5/B, a hyperspectral camera, with the first experiment using artificial intelligence to discard cloudy images. FSSCat was launched on 3 September 2020 and injected into a 535-km synchronous orbit. ³Cat-5/A was operated for three months until the payload was probably damaged by a solar flare and coronal mass ejection. During this time, all scientific requirements were met, including the generation of coarse-resolution and downscaled soil moisture (SM) maps, sea ice extent (SIE) maps, concentration and thickness maps, and even wind speed (WS) and sea surface salinity (SSS) maps, which were not originally foreseen. ³Cat-5/B was operated a few more months until the number of images acquired met the requirements. This article briefly describes the FSSCat mission and the FMPL-2 payload and summarizes the main scientific results.

Impact of Solar Activity on Snow Cover Variation Over the Tibetan Plateau and Linkage to the Summer Precipitation in China

Solar activity is one of the main external forcing factors driving the Earth's climate system to change. The snow cover over the Tibetan Plateau is an important physical factor affecting the East Asian climate. At present, insufficient research on the connection between solar activity and snow cover over the Tibetan Plateau has been carried out. Using Solar Radio Flux (SRF), Solar Sunspot Number (SSN), and Total Solar Irradiance (TSI) data, this paper calculated the correlation coefficients with snow indices over the Tibetan Plateau, such as winter and spring snow depth (WSD/SSD) and snow day number (WSDN/SSDN). These snow indices are obtained from the daily gauge snow data in the Tibetan Plateau. Through correlation analyses, it is found that there are significant synchronous or lag correlations between snow indices and solar parameters on multi-time scales. In particular, the Spring Snow Day Number (SSDN) is of significant synchronous or lag correlation with SRF, SSN, and TSI on multi-time scales. It is further found that SSDN over the Tibetan Plateau has more stable positive correlations with SRF by using the 21-year running mean and cross spectrum analyses. Therefore, SSDN can be ascertained to be the most sensitive snow index to the solar activity compared with other snow indices. Moreover, its influence on summer precipitation of China is strongly regulated by solar activity. In high solar activity years (HSAY), the significant correlated area of summer precipitation in China to SSDN is located further north than that in low solar activity years (LSAY). Such impact by solar activity is also remarkable after excluding the impact of ENSO (i.e., El Nio-Southern Oscillation) events. These results provide support for the application of snow indices in summer rainfall prediction in China.

Simulations of the Antenna-shielding Effect of the Daocheng Solar Radio Telescope (DSRT)

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Measuring differential rotation from full disk ALMA images of the Sun

An automatic method for measuring the differential rotation profile of the Sun from short time span data, based on tracking features with an optical flow algorithm and robust iterative fitting, is presented. The method is first tested on the synthetic data and SDO/HMI magnetograms, and later applied to ALMA full disk band 6 solar images. The method successfully recovers the differential rotation profile, however, it is found to be very sensitive to input parameters and image quality, and further work is needed to devise methods for finding optimal parameters.

Fundamental Electromagnetic Emissions by a Weak Electron Beam in Solar Wind Plasmas with Density Fluctuations

The generation of Langmuir wave turbulence by a weak electron beam in a randomly inhomogeneous plasma and its subsequent electromagnetic radiation are studied owing to two-dimensional particle-in-cell simulations in conditions relevant to type III solar radio bursts. The essential impact of random density fluctuations of average levels of a few percents of the background plasma on the characteristics of the electromagnetic radiation at the fundamental plasma frequency _p is shown. Not only wave nonlinear interactions but also processes of Langmuir waves' transformations on the density fluctuations contribute to the generation of such emissions. During the beam relaxation, the amount of electromagnetic energy radiated at _p in a plasma with density fluctuations strongly exceeds that observed when the plasma is homogeneous. The fraction of Langmuir wave energy involved in the generation of electromagnetic emissions at _p saturates around 10^-4, i.e., one order of magnitude above that reached when the plasma is uniform. Moreover, whereas harmonic emission at 2 _p dominates over fundamental emission during the time evolution in a homogeneous plasma, fundamental emission is strongly dominant when the plasma contains density fluctuations, at least during several thousands of plasma periods before being overcome by harmonic emission when the total electromagnetic energy begins to saturate.

Observational Signatures of Tearing Instability in the Current Sheet of a Solar Flare

Magnetic reconnection is a fundamental physical process converting magnetic energy into not only plasma energy but also particle energy in various astrophysical phenomena. In this Letter, we show a unique data set of a solar flare where various plasmoids were formed by a continually stretched current sheet. Extreme ultraviolet images captured reconnection inflows, outflows, and particularly the recurring plasma blobs (plasmoids). X-ray images reveal nonthermal emission sources at the lower end of the current sheet, presumably as large plasmoids with a sufficiently amount of energetic electrons trapped in them. In the radio domain, an upward, slowly drifting pulsation structure, followed by a rare pair of oppositely drifting structures, was observed. These structures are supposed to map the evolution of the primary and the secondary plasmoids formed in the current sheet. Our results on plasmoids at different locations and scales shed important light on the dynamics, plasma heating, particle acceleration, and transport processes in the turbulent current sheet and provide observational evidence for the cascading magnetic reconnection process.

Spectral Analysis of Solar Radio Type III Bursts from 20 kHz to 410 MHz

We present the statistical analysis of the spectral response of solar radio type III bursts over the wide frequency range between 20 kHz and 410 MHz. For this purpose, we have used observations that were carried out using both spaced-based (Wind/Waves) and ground-based (Nanay Decameter Array and Nanay Radioheliograph) facilities. In order to compare the flux densities observed by the different instruments, we have carefully calibrated the data and displayed them in solar flux units. The main result of our study is that type III bursts, in the metric to hectometric wavelength range, statistically exhibit a clear maximum of their median radio flux density around 2 MHz. Although this result was already reported by inspecting the spectral profiles of type III bursts in the frequency range 20 kHz-20 MHz, our study extends such analysis for the first time to metric radio frequencies (i.e., from 20 kHz to 410 MHz) and confirms the maximum spectral response around 2 MHz. In addition, using a simple empirical model we show that the median radio flux S of the studied data set obeys the polynomial form Y = 0.04X ³ - 1.63X ² + 16.30X - 41.24, with $X=\mathrm{ln}({F}_{\mathrm{MHz}})$ and with $Y=\mathrm{ln}({S}_{\mathrm{SFU}})$ . Using the Sittler and Guhathakurtha model for coronal streamers, we have found that the maximum of radio power therefore falls in the range 4 to 10 R , depending on whether the type III emissions are assumed to be at the fundamental or the harmonic.

Short-Term Prediction of Solar F_{10.7} Radiation Flux Based on Deep Learning

Solar radiation flux $F_{10.7}$ is widely used in space environment models such as the empirical model of atmosphere and the ionospheric model as an input parameter, and its prediction accuracy directly affects the precision of spacecraft orbit prediction. The relationship between solar radiation flux ($F_{10.7}$) and sunspot number (SSN) is calculated by the time series method, and the linear relationship between them is given. On this basis, we proposed a Short-Term prediction method for $F_{10.7}$ in the next 7 days based on the 54-day solar radiation flux index combined with the sunspots number into the LSTM (Long and Short Term Memory) neural network, compared the prediction results with those of other forecasting methods. The results show: (1) The performance of the proposed 7-day method model is better than that of the Space Weather Prediction Center (SWPC), and the correlation coefficient (CC) between the predicted and observed values reaches 0.96. At the same time, the root mean square error is about 11.62 solar radiation flux units, and the RMSE of the forecast result is lower than that of SWPC, which decreases by about 11\%. (2) According to the statistical results of 23 and 24 solar activity cycle year, the optimal MAPE (Mean Absolute Percentage Error) of the average absolute percentage error of $F_{10.7}$ index on the seventh day of high solar activity year can reach 12.9\%, and the optimal MAPE of low solar activity year can reach 2.5\%. (3) The results of combined sunspot number LSTM and LSTM using only $F_{10.7}$ showed that the newly introduced sunspot number was effective in improving the prediction of LSTM, and the root mean square error (RMSE) of these two models were lower than that of SWPC by about 11\% and 5\%, respectively.

Digital Signal Processing in RadioSolariz Project Using SSE2

This paper aims at elaborating on the digital signal processing techniques used in data manipulation in the radioSolariz solar radio- telescope project. Focus is drawn on the implementation of different digital signal processing algorithms through the use of streaming single instruction multiple data extensions 2. This complementary instruction set to general purpose personal computer microprocessors offers increased computational power by realizing parallel processing. The benefit is a higher data throughput while lowering the electrical power consumption of the digital signal processing computer. Optimized code fragments are shown along with original code snippets and these are discussed and analyzed. Future work and implementation of other modern parallel processing technologies are envisaged.

Parker Solar Probe detects solar radio bursts related with a behind-the-limb active region

Context. The interpretation of solar radio bursts observed by Parker Solar Probe (PSP) in the encounter phase plays a key role in understanding intrinsic properties of the emission mechanism in the solar corona. Lower time-frequency resolution of the PSP receiver can be overcome by simultaneous ground-based observations using more advanced antennas and receivers.
Aims: In this paper we present such observations for which the active active region 12 765, begetter of type III, J, and U solar bursts, was within sight of ground-based instruments and behind the solar limb of the PSP spacecraft.
Methods: We used a subarray of the Giant Ukrainian Radio Telescope to get the spectral properties of radio bursts at the frequency range of 8-80 MHz, as well as the PSP radio instruments with a bandwidth of 10.5 kHz-19.2 MHz, during solar observations on June 5, 2020.
Results: We directly detected the radio events initiated by the active region behind the solar limb of the PSP spacecraft, using special conditions in the solar corona, due to the absence of active regions from the PSP side. Following the generation mechanism of solar radio emission, we refined the density model for the solar corona above the active region 12765 responsible for the radio bursts. Based on the PSP spacecraft position near the Sun and delays of radio waves between space- and ground-based records, we found the corresponding radio responses on the PSP spectrogram.
Conclusions: The absence of sunspots from the PSP side contributes to the propagation of radio waves from a dense loop of the Sun to quiet regions with low densities, through which PSP instruments can detect the radiation.

Multi-Instrument Investigation of the Impact of the Space Weather Events of 6-10 September 2017

We analyzed the space weather events of 6-10 September 2017 using the multi-instrument approach. We focused on the four X-class flares which emanated from the Active Region AR 12673 and the Ground Induced Currents hazard associated with the geomagnetic storm of 7-8 September 2017. The flare effect on the equatorial electrojet (EEJ) recorded on board the SWARM satellite and on the horizontal component of the geomagnetic field (H) records of ground-based magnetometers was further examined. During the X2.2/X1.3 flares of 6/7 September, the maximum percentage Global Navigation Satellite System (GNSS) vertical Total Electron Content (VTEC) increase was 6.9%/5.0% in Dakar/Porto Velho. During the X9.3/X8.2 flare of 6/10 September it was 7.9%/18.8% in Ascension Island/Kourou. The strongest Solar Flare Effect occurred in Mbour and Kourou during the respective flare. However, the highest EEJ increase was observed during the X2.2 and X9.3 flares. Interestingly, the X.9.3 flare resulted in a stronger ionospheric response than the X8.2 flare. Furthermore, global TEC map showed a higher response in the African and South American longitude during the respective event. The total radio fade-out lasted from 30 to 90 min at the Hermanus and Sao Luis ionosondes during the flares, while the risk level to critical ground infrastructures based on the geomagnetically induced currents hazard was very low risk. Our results highlight the potential GPS positioning errors induced by sudden increase in TEC and the loss of high-frequency communication and GNSS navigation signals associated with these solar events.

Quasi-Periodic Energy Release in a Three-Ribbon Solar Flare

Quasi-periodic pulsations (QPPs) are found in solar flares of various magnetic morphologies, e.g. in two-ribbon or circular-ribbon flares, and the mechanisms of their generation are not yet clear. Here we present the first detailed analysis of QPPs (with a period P =54 13 seconds) found in the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations of a relatively rare three-ribbon M1.1 class flare that occurred on 5 July 2012 (SOL2012-07-05T06:49). QPPs are manifested in the temporal profiles of temperature [T ] and emission measure [E M ] of "super-hot" (T_s30 - 50 MK) plasma but are almost invisible in the profiles of "hot" (T_h15 - 20 MK) plasma parameters when approximating X-ray spectra of the flare with the bremsstrahlung spectrum of a two-temperature thermal (Maxwellian) plasma. In addition, QPPs with a similar period are found in the temporal profiles of the flux and spectral index of nonthermal electrons if the observed X-ray spectra are approximated by a combination of the bremsstrahlung spectra of a single-temperature plasma and nonthermal electrons with a power-law energy distribution. QPPs are not well expressed in the X-ray flux according to RHESSI and GOES data, or in radio data. The QPPs are accompanied by apparent systematic movement of a single X-ray source at a low speed of 34 21 km s¹ along the central flare ribbon over a narrow (<5 Mm) "tongue" of negative magnetic polarity, elongated (20 Mm) between two areas of positive polarity. The results of magnetic extrapolation in the nonlinear force-free field (NLFFF) approximation show that the X-ray source could move along curved and twisted field lines between two sheared flare arcades. It is worth noting that in the homologous three-ribbon M6.1 flare (SOL2012-07-05T11:39), which occurred in the same region about five hours later, the X-ray sources moved much less systematically and did not produce similar QPPs. We interpret the observed QPPs as a result of successive episodes of energy release in different spatial locations. In each pulsation, (1 - 4)10²⁹ erg is released in the form of thermal energy of hot and super-hot plasmas (or accelerated electrons), which is comparable with the energy of a microflare. The total kinetic energy released during all QPPs is (0.7 - 3.5)10³⁰ erg, which is about an order of magnitude less than the free magnetic energy 1.56 10³¹ erg released in the flare region. We discuss possible propagating triggers of the quasi-periodic energy release (slow magnetoacoustic waves, asymmetric rise of curved/twisted field lines, flapping oscillations, and thermal instability in a reconnecting current sheet) and argue that the current state of available mechanisms and observations does not allow us to reach an unambiguous conclusion.

Common Origin of Quasi-Periodic Pulsations in Microwave and Decimetric Solar Radio Bursts

We analyse quasi-periodic pulsations (QPP) detected in the microwave and decimetre radio emission of the 5 September 2017 7:04 UT (SOL2017-09-05T07:04) solar flare, using simultaneous observations by the Siberian Radioheliograph 48 (SRH-48, 4 - 8 GHz) and Mingantu Spectral Radioheliograph (MUSER-I, 0.4 - 2 GHz). The microwave emission was broadband with a typical gyrosynchrotron spectrum, while a quasi- periodic enhancement of the decimetric emission appeared in a narrow spectral band (500 - 700 MHz), consistent with the coherent-plasma- emission mechanism. The periodicity that we found in microwaves is about 30 seconds, coming from a compact loop-like source with a typical height of about 31 Mm. The decimetric emission exhibited a periodicity of about 6 seconds. We suggest a qualitative scenario linking the QPPs observed in both incoherent and coherent spectral bands and their generation mechanisms. The properties of the QPPs found in the microwave signal are typical for perturbations of the flare loop by the standing sausage mode of a fast magnetohydrodynamic (MHD) wave. Our analysis indicated that this sausage-oscillating flare loop was the primary source of oscillations in the discussed event. The suggested scenario is that a fundamental sausage harmonic is the dominant cause for the observed QPPs in the microwave emission. The initiation of oscillations in the decimetric emission is caused by the third sausage harmonic via periodic and nonlinear triggering of the acceleration processes in the current sheets, formed at the interface between the sausage-oscillating flare loop and the external coronal loop that extended to higher altitudes. Our results demonstrate the possible role of MHD wave processes in the release and transport of energy during solar flares, linking coherent and incoherent radio emission mechanisms.

Measurement of Siberian Radioheliograph cable delays

To achieve the maximum dynamic range of solar radio images obtained using aperture synthesis in relatively wide frequency bands 0.10.5 % of the operating frequency, it is necessary to compensate the signal propagation delays in the antenna receive path before calculating visibility functions (hereinafter visibilities). When visibilities are corrected without delay compensation, the signal-to-noise ratio decreases due to residual phase slopes in the receiving system bandwidth. In addition to enhancing dynamic range, preliminary compensation for delays simplifies real-time imaging no antenna gain calibration is required to get a first approximation image. The requirements for the accuracy of antenna placement are also reduced in contrast to the measurement of the phase visibility error, the measurement of the delay is actually not so critical to the antenna position errors that are larger than the operating wavelength. The instantaneous frequency band of the Siberian Radioheliograph, which determines the minimum step for measuring the phase slope, and hence the accuracy of determining the delay, is 10 MHz. At the speed of light in an optical fiber of ~0.7c, a step of 10 MHz makes it possible to unambiguously measure the difference between electrical lengths of cables up to 20 m and to correct antenna positions by radio observations, even if the error in the position of the antennas exceeds the operating wavelength. Correction of the band phase slopes during the observation time adapts the radio telescope to the temperature drift of delays and decreases antenna gain phase spread. This, in turn, leads to more stable solutions to systems of equations containing antenna gains as unknowns.

DSC based Dual-Resunet for radio frequency interference identification

Radio frequency interference (RFI) will pollute the weak astronomical signals receivedby radio telescopes, which in return will seriously affect the time-domain astronomical observation and research. In this paper, we use a deep learning method to identify RFI in frequency spectrum data, and propose a neural network based on Unet that combines the principles of depthwise separable convolution and residual, named DSC Based Dual-Resunet. Compared with the existing Unet network, DSC Based Dual-Resunet performs better in terms ofaccuracy, F1 score, and MIoU, and is also better in terms of computation cost where the model size and parameter amount are 12.5% of Unet and the amount of computation is 38%ofUnet. The experimental results show that the proposed network is a high-performance and lightweight network, and it is hopeful to be applied to RFI identification of radio telescopes on a large scale.

Observational results of MUSER during 20142019

The solar radio signal that can be received by the ground-based telescopes covers a wide frequency range, allowing us to monitor the complex physical processes occurred from the solar surface to the vast interplanetary space. MingantU SpEctral Radioheliograph (MUSER), as the latest generation of solar dedicated radio spectral-imaging instrument in the centimeter-decimeter wavelengths, has accumulated a large number of observational data since its commissioning observation in 2014. This paper presents the main observational results identified by MUSER from 2014 to 2019, including the quiet Sun and 94 solar radio burst events. We find that there are 81 events accompanied with Geostationary Operational Environmental Satellites (GOES) soft X-ray (SXR) flares, among which the smallest flare class is B1.0. There are 13 events without accompanying any recorded flares, among which the smallest SXR intensity during the radio burst period is equivalent to level-A. The main characteristics of all radio burst events are presented, which shows the powerful ability of MUSER to capture the valuable information of the solar non-thermal processes and the importance for space weather. This work also provides a database for further in-depth research.

Energy and spectral analysis of confined solar flares from radio and X-ray observations

The energy and spectral shape of radio bursts may help us understand the generation mechanism of solar eruptions, including solar flares, coronal mass ejections, eruptive filaments, and various scales of jets. The different kinds of flares may have different characteristics of energy and spectral distribution. In this work, we selected 10 mostly confined flare events during October 2014 to investigate their overall spectral behaviour and the energy emitted in microwaves by using radio observations from microwaves to interplanetary radio waves, and X-ray observations of GOES, RHESSI, and Fermi/GBM. We found that: all the confined flare events were associated with a microwave continuum burst extending to frequencies of 9.4 15.4 GHz, and the peak frequencies of all confined flare events are higher than 4.995 GHz and lower than or equal to 17 GHz. The median value is around 9 GHz. The microwave burst energy (or fluence) and the peak frequency are found to provide useful criteria to estimate the power of solar flares. The observations imply that the magnetic field in confined flares tends to be stronger than that in 412 flares studied by Nita et al. (2004). All 10 events studied did not produce detectable hard X-rays with energies above 300 keV indicating the lack of efficient acceleration of electrons to high energies in the confined flares.

from solar patrols to pulsars and VLBI

The development of radio astronomy in Poland took some time to occur, in view of the destruction of the country during World War II. Also, the shift of the country's borders in favor of the Soviet Union led to the loss of two important astronomy observatories: the historic centers of astronomy in Wilno and Lww. The relocation of these important astronomical centers to Toru and Wrocaw in the West needed considerable effort. Hence radio astronomy was a second priority as the optical astronomers had to become operational first. Nevertheless, in the 1950s groups in Krakw and Toru started to erect radio telescopes. In Krakw solar radio astronomy became established. In Toru, there were experiments with low frequency parabolic antennas. First, a 15-m dish was built, and later a 32-m dish became operational. In Krakw the solar patrol continued and a LOFAR station was added. Contacts with radio observatories in other parts of the world were established. In 1988 a new regional university was established in Zielona Gra, and a group involved in pulsar research was built up there. Astrophysics departments existed at Warszawa (Warsaw) University and at the Nicolaus Copernicus Astronomical Center (CAMK) of the Polish Academy of Sciences, and the universities in Pozna, Gdask and Lublin had astronomical departments, but none of these was involved in radio astronomy.

the Form of Cycles

Analysis of the features of the form of low solar cycles 23 and 24 for the solar-activity indices (F is the solar radio flux at a wavelength of 10.7 cm, Rz and Ri are the relative numbers of sunspots, old and new versions) and the ionospheric index of this activity T. For this, the analyzed indices are reduced to the Rz scale and smoothed (with a 24-month Gaussian filter) values of these indices are considered. It was found that the cycle forms were preserved for cycles 23 and 24 and previous solar cycles by index Rz, i.e., a definite connection was made between the cycle amplitude and the time of the onset of the cycle maximum. The same relationship was observed for the Ri index, except for cycle 23, in which the observed cycle maximum occurred 7 months later than the expected time based on previous cycles. For indices F and T, the cycle shapes also persisted up to cycle 22, but the observed cycle maxima in cycles 23 and 24 occurred almost a year later than expected, which is one of the properties of the new regime of prolonged low solar activity. In this mode, the connection between the Ri and F indices is broken, which leads to different cycle forms for these indices.

Increased Microwave Radiation Brightness as a Sign of Flare-Producing Active Regions Based on Observations of NOAA Active Region 12371

Methods to predict solar flares based on observations in the microwave range with radio telescopes with high angular resolution continue to be developed. The results are presented for an analysis of observations of NOAA active region 12371, in a quasi-quiescent state characterized by increased brightness of emission in the microwave range, which causes multiple eruptive events. According to observations with the RATAN-600 radio telescope during the event of June 21, 2015, a sharp change in the structure of the image of the microwave radiation source above NOAA region 12 371 was recorded. It was presumably interpreted by short-term plasma heating over the region of the delta configuration of the magnetic field in the tail part of the active region. Due to the high sensitivity of RATAN-600 during polarization measurements, it is possible to localize the position of the cloud in which the emission or acceleration of fast particles occurs with known models of the magnetic field.

Spectral Diagnostics of Langmuir Coronal Plasma Turbulence Based on the Radio Emission at the Double Plasma Frequency

Analysis of the radiation of solar radio bursts with a fine structure generated at a double plasma frequency makes it possible to obtain information on the energy density of Langmuir waves in solar plasma and on the volume of the region of the generation of radio emission when Langmuir waves merge. As an example, we consider the spectrum of a radio burst on August 17, 1999. Assuming that this radio burst is the result of the merging of Langmuir waves, a possible spectrum of Langmuir waves has been found. This makes it possible to explain the form of the particular spectrum of the radio burst, and the relationship between the volume of the emitting region and the energy density of Langmuir waves has been obtained. It is shown that the linear size of the emitting region can be ~10⁸ cm or less. It is also shown that plasma inhomogeneity will lead to an increase in the width of the radio emission spectrum. Small (~10%) changes in the plasma concentration in discrete sources of radio emission may lead to the formation of local spectral maxima.

the Event of July 4, 2012

The temporal evolution of thermal bremsstrahlung sub-THz emission from a flare chromosphere and transition region is analyzed based on the F-CHROMA solar flare model database (https://www.fchroma.org) and solar flare observations with the RT-7.5 radio telescope at 93 and 140 GHz on July 4, 2012. The model is based on the RADYN numerical code, which describes the response of chromospheric background plasma to the action of a beam of accelerated electrons in the form of a triangular pulse. The positive slope of the spectrum in the sub-THz frequency range has been formed over the entire burst. A thin layer of low temperature and high plasma density (the so-called chromospheric condensation) appears. It moves to the region of higher altitudes and effectively absorbs the sub-THz emission. It is shown based on the RHESSI hard X-ray observations that the chromospheric thermal plasma cannot provide the observed sub-THz emission fluxes during the solar flare on July 4, 2012. The cause of the obtained discrepancy is discussed.

The Origin of Time Delays between Sub-Terahertz and Soft X-ray Emission from Solar Flares

The time delays between sub-terahertz (sub-THz) and soft X-ray (SXR) emission from solar flares with a positive spectrum slope at frequencies of >100 GHz are considered. Based on a cross-correlation analysis of the time profiles of sub-THz and SXR emission from 11 flare events obtained from the KOSMA (230 GHz), SST (212 GHz), and RT-7.5 (93 GHz) telescopes, as well as the GOES satellite in channel 18 , we identified two main types of sub-THz events. Type I includes four X-class events with a well-defined Neupert effect, i.e., the sub-THz flux outpaces the SXR flux by 29 min. Type II includes seven M-class flares, in which the time profiles behave similarly and the delays do not exceed 30 s. The different mechanisms of radio emission in different layers of the solar atmosphere may be responsible for the positive slope of the sub-THz spectrum of flares.

Decimetric Type-U Solar Radio Bursts and Associated EUV Phenomena on 2011 February 9

A GOES M1.9 flare took place in active region AR 11153 on 2011 February 9. With a resolution of 200 kHz and a time cadence of 80 ms, the reverse-drifting (RS) type-III bursts, intermittent sequence of type-U bursts, drifting pulsation structure (DPS), and fine structures were observed by the Yunnan Observatories Solar Radio Spectrometer (YNSRS). Combined information revealed by the multiwavelength data indicated that after the DPS was observed by YNSRS, the generation rate of type-U bursts suddenly increased to 5 times what it had been. In this event, the generation rate of type-U bursts may depend on the magnetic- reconnection rate. Our observations are consistent with previous numerical simulation results. After the first plasmoid produced (plasma instability occurred), the magnetic-reconnection rate suddenly increased by 5 to 8 times. Furthermore, after the DPS, the frequency range of the turnover frequency of type-U bursts was obviously broadened to thrice what it was before, which indicates a fluctuation amplitude of the density in the loop top. Our observations also support numerical simulations during the flare-impulsive phase. Turbulence occurs at the top of the flare loop and the plasmoids can trap nonthermal particles, causing density fluctuation at the loop top. The observations are generally consistent with the results of numerical simulations, helping us to better understand the characteristics of the whole physical process of eruption.

A DEFT Way to Forecast Solar Flares

Solar flares have been linked to some of the most significant space weather hazards at Earth. These hazards, including radio blackouts and energetic particle events, can start just minutes after the flare onset. Therefore, it is of great importance to identify and predict flare events. In this paper we introduce the Detection and EUV Flare Tracking (DEFT) tool, which allows us to identify flare signatures and their precursors using high spatial and temporal resolution extreme- ultraviolet (EUV) solar observations. The unique advantage of DEFT is its ability to identify small but significant EUV intensity changes that may lead to solar eruptions. Furthermore, the tool can identify the location of the disturbances and distinguish events occurring at the same time in multiple locations. The algorithm analyzes high temporal cadence observations obtained from the Solar Ultraviolet Imager instrument aboard the GOES-R satellite. In a study of 61 flares of various magnitudes observed in 2017, the "main" EUV flare signatures (those closest in time to the X-ray start time) were identified on average 6 minutes early. The "precursor" EUV signatures (second-closest EUV signatures to the X-ray start time) appeared on average 14 minutes early. Our next goal is to develop an operational version of DEFT and to simulate and test its real-time use. A fully operational DEFT has the potential to significantly improve space weather forecast times.

Multiple Electron Acceleration Instances during a Series of Solar Microflares Observed Simultaneously at X-Rays and Microwaves

Even small solar flares can display a surprising level of complexity regarding their morphology and temporal evolution. Many of their properties, such as energy release and electron acceleration can be studied using highly complementary observations at X-ray and radio wavelengths. We present X-ray observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager and radio observations from the Karl G. Jansky Very Large Array (VLA) of a series of GOES A3.4-B1.6 class flares observed on 2013 April 23. The flares, as seen in X-ray and extreme ultraviolet, originated from multiple locations within active region NOAA 11726. A veritable zoo of different radio emissions between 1 GHz and 2 GHz was observed cotemporally with the X-ray flares. In addition to broadband continuum emission, broadband short-lived bursts and narrowband spikes, indicative of accelerated electrons, were observed. However, these sources were located up to 150 away from the flaring X-ray sources but only some of these emissions could be explained as signatures of electrons that were accelerated near the main flare site. For other sources, no obvious magnetic connection to the main flare site could be found. These emissions likely originate from secondary acceleration sites triggered by the flare, but may be due to reconnection and acceleration completely unrelated to the cotemporally observed flare. Thanks to the extremely high sensitivity of the VLA, not achieved with current X-ray instrumentation, it is shown that particle acceleration happens frequently and at multiple locations within a flaring active region.

Modeling

There is a wide consensus that the ubiquitous presence of magnetic reconnection events and the associated impulsive heating (nanoflares) are strong candidates for solving the solar coronal heating problem. Whether nanoflares accelerate particles to high energies like full-sized flares is unknown. We investigate this question by studying the type III radio bursts that the nanoflares may produce on closed loops. The characteristic frequency drifts that type III bursts exhibit can be detected using a novel application of the time-lag technique developed by Viall & Klimchuk (2012) even when there are multiple overlapping events. We present a simple numerical model that simulates the expected radio emission from nanoflares in an active region, which we use to test and calibrate the technique. We find that in the case of closed loops the frequency spectrum of type III bursts is expected to be extremely steep such that significant emission is produced at a given frequency only for a rather narrow range of loop lengths. We also find that the signature of bursts in the time-lag signal diminishes as: (1) the variety of participating loops within that range increases; (2) the occurrence rate of bursts increases; (3) the duration of bursts increases; and (4) the brightness of bursts decreases relative to noise. In addition, our model suggests a possible origin of type I bursts as a natural consequence of type III emission in a closed-loop geometry.

The Sun at millimeter wavelengths. III. Impact of the spatial resolution on solar ALMA observations

Context. Interferometric observations of the Sun with the Atacama Large Millimeter/sub-millimeter Array (ALMA) provide valuable diagnostic tools for studying the small-scale dynamics of the solar atmosphere.
Aims: The aims are to perform estimations of the observability of the small-scale dynamics as a function of spatial resolution for regions with different characteristic magnetic field topology facilitate a more robust analysis of ALMA observations of the Sun.
Methods: A three-dimensional model of the solar atmosphere from the radiation- magnetohydrodynamic code Bifrost was used to produce high-cadence observables at millimeter and submillimeter wavelengths. The synthetic observables for receiver bands 3-10 were degraded to the angular resolution corresponding to ALMA observations with different configurations of the interferometric array from the most compact, C1, to the more extended, C7. The observability of the small-scale dynamics was analyzed in each case. The analysis was thus also performed for receiver bands and resolutions that are not commissioned so far for solar observations as a means for predicting the potential of future capabilities.
Results: The minimum resolution required to study the typical small spatial scales in the solar chromosphere depends on the characteristic properties of the target region. Here, a range from quiet Sun to enhanced network loops is considered. Limited spatial resolution affects the observable signatures of dynamic small-scale brightening events in the form of reduced brightness temperature amplitudes, potentially leaving them undetectable, and even shifts in the times at which the peaks occur of up to tens of seconds. Conversion factors between the observable brightness amplitude and the original amplitude in the fully resolved simulation are provided that can be applied to observational data in principle, but are subject to wavelength-dependent uncertainties. Predictions of the typical appearance at the different combinations of receiver band, array configuration, and properties of the target region are conducted.
Conclusions: The simulation results demonstrate the high scientific potential that ALMA already has with the currently offered capabilities for solar observations. For the study of small-scale dynamic events, however, the spatial resolution is still crucial, and wide array configurations are preferable. In any case, it is essential to take the effects due to limited spatial resolution into account in the analysis of observational data. Finally, the further development of observing capabilities including wider array configurations and advanced imaging procedures yields a high potential for future ALMA observations of the Sun.

First observations and performance of the RPW instrument on board the Solar Orbiter mission

The Radio and Plasma Waves (RPW) instrument on the ESA Solar Orbiter mission is designed to measure in situ magnetic and electric fields and waves from the continuum up to several hundred kHz. The RPW also observes solar and heliospheric radio emissions up to 16 MHz. It was switched on and its antennae were successfully deployed two days after the launch of Solar Orbiter on February 10, 2020. Since then, the instrument has acquired enough data to make it possible to assess its performance and the electromagnetic disturbances it experiences. In this article, we assess its scientific performance and present the first RPW observations. In particular, we focus on a statistical analysis of the first observations of interplanetary dust by the instrument's Thermal Noise Receiver. We also review the electro-magnetic disturbances that RPW suffers, especially those which potential users of the instrument data should be aware of before starting their research work.

Simulations of radio-wave anisotropic scattering to interpret type III radio burst data from Solar Orbiter, Parker Solar Probe, STEREO, and Wind

Aims: We use multi-spacecraft observations of individual type III radio bursts to calculate the directivity of the radio emission. We compare these data to the results of ray-tracing simulations of the radio-wave propagation and probe the plasma properties of the inner heliosphere.
Methods: We used ray-tracing simulations of radio- wave propagation with anisotropic scattering on density inhomogeneities to study the directivity of radio emissions. Simultaneous observations of type III radio bursts by four widely separated spacecraft were used to calculate the directivity and position of the radio sources. The shape of the directivity pattern deduced for individual events is compared to the directivity pattern resulting from the ray-tracing simulations.
Results: We show that simultaneous observations of type radio III bursts by four different probes provide an opportunity to estimate the radio source positions and the directivity of the radio emission. The shape of the directivity varies from one event to another and it is consistent with anisotropic scattering of the radio waves.

ARRAY(0x18a8790)

Solar Orbiter/RPW antenna calibration in the radio domain and its application to type III burst observations

Context. In order to allow for a comparison with the measurements from other antenna systems, the voltage power spectral density measured by the Radio and Plasma waves receiver (RPW) on board Solar Orbiter needs to be converted into physical quantities that depend on the intrinsic properties of the radiation itself (e.g., the brightness of the source).
Aims: The main goal of this study is to perform a calibration of the RPW dipole antenna system that allows for the conversion of the voltage power spectral density measured at the receiver's input into the incoming flux density.
Methods: We used space observations from the Thermal Noise Receiver (TNR) and the High Frequency Receiver (HFR) to perform the calibration of the RPW dipole antenna system. Observations of type III bursts by the Wind spacecraft are used to obtain a reference radio flux density for cross-calibrating the RPW dipole antennas. The analysis of a large sample of HFR observations (over about ten months), carried out jointly with an analysis of TNR-HFR data and prior to the antennas' deployment, allowed us to estimate the reference system noise of the TNR-HFR receivers.
Results: We obtained the effective length, l_eff, of the RPW dipoles and the reference system noise of TNR-HFR in space, where the antennas and pre-amplifiers are embedded in the solar wind plasma. The obtained l_eff values are in agreement with the simulation and measurements performed on the ground. By investigating the radio flux intensities of 35 type III bursts simultaneously observed by Wind and Solar Orbiter, we found that while the scaling of the decay time as a function of the frequency is the same for the Waves and RPW instruments, their median values are higher for the former. This provides the first observational evidence that Type III radio waves still undergo density scattering, even when they propagate from the source, in a medium with a plasma frequency that is well below their own emission frequency.

In-flight performance and first results

Context. The Radio and Plasma Waves (RPW) instrument on board Solar Orbiter has been operating nearly continuously since the launch in February 2020. The Time Domain Sampler (TDS) receiver of the RPW instrument is dedicated to waveform measurements of plasma waves and dust impact signatures in an intermediate frequency range from 0.2 to 200 kHz.
Aims: This article presents the first data from the RPW- TDS receiver and discusses the in-flight performance of the instrument and, in particular, the on-board wave and dust detection algorithm. We present the TDS data products and its scientific operation. We demonstrate the content of the dataset on several examples. In particular, we study the distribution of solar Langmuir waves in the first year of observations and one Type III burst event.
Methods: The on-board detection algorithm is described in detail in this article and classifies the observed waveform snapshots, identifying plasma waves and dust impacts based on the ratio of their maximum amplitude to their median and on the spectral bandwidth. The algorithm allows TDS to downlink the most scientifically relevant waveforms and to perform an on-board statistical characterization of the processed data.
Results: The detection algorithm of TDS is shown to perform very well in its detection of plasma waves and dust impacts with a high accuracy. The initial analysis of statistical data returned by TDS shows that sporadic Langmuir waves that are not associated with Type III events are routinely observed in the inner heliosphere, with a clear increase in occurrence rate closer to the Sun. We also present an example of RPW observations during an encounter of the source region of a Type III burst, which exploits the on-board calculated histograms data.

The first widespread solar energetic particle event observed by Solar Orbiter on 2020 November 29

Context. On 2020 November 29, the first widespread solar energetic particle (SEP) event of solar cycle 25 was observed at four widely separated locations in the inner (1 AU) heliosphere. Relativistic electrons as well as protons with energies > 50 MeV were observed by Solar Orbiter (SolO), Parker Solar Probe, the Solar Terrestrial Relations Observatory (STEREO)-A and multiple near-Earth spacecraft. The SEP event was associated with an M4.4 class X-ray flare and accompanied by a coronal mass ejection and an extreme ultraviolet (EUV) wave as well as a type II radio burst and multiple type III radio bursts.
Aims: We present multi-spacecraft particle observations and place them in context with source observations from remote sensing instruments and discuss how such observations may further our understanding of particle acceleration and transport in this widespread event.
Methods: Velocity dispersion analysis (VDA) and time shift analysis (TSA) were used to infer the particle release times at the Sun. Solar wind plasma and magnetic field measurements were examined to identify structures that influence the properties of the energetic particles such as their intensity. Pitch angle distributions and first-order anisotropies were analyzed in order to characterize the particle propagation in the interplanetary medium.
Results: We find that during the 2020 November 29 SEP event, particles spread over more than 230 in longitude close to 1 AU. The particle onset delays observed at the different spacecraft are larger as the flare-footpoint angle increases and are consistent with those from previous STEREO observations. Comparing the timing when the EUV wave intersects the estimated magnetic footpoints of each spacecraft with particle release times from TSA and VDA, we conclude that a simple scenario where the particle release is only determined by the EUV wave propagation is unlikely for this event. Observations of anisotropic particle distributions at SolO, Wind, and STEREO-A do not rule out that particles are injected over a wide longitudinal range close to the Sun. However, the low values of the first-order anisotropy observed by near-Earth spacecraft suggest that diffusive propagation processes are likely involved.

Multi-Wavelength Observations of Quasi-Periodic Pulsations in a Solar Flare

We report our analysis of multi-wavelength observations of quasi- periodic pulsations (QPPs) during the impulsive phase of the C6.7 flare on 9 May 2019. The flare was simultaneously observed by Fermi, the New Vacuum Solar Telescope, the Mingantu Spectral Radioheliograph, the Nobeyama Radio Polarimeters, and the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory. Three well-pronounced pulsations are detected in full-disk hard X-ray and microwave fluxes, as well as the local light curves at wavelengths of the H line core, AIA 304 , 171 , 211 , and 335 between 05:43:30 UT and 05:48:15 UT. The quasi- periods of about 90 - 110 seconds are determined from their Morlet wavelet power spectra. Meanwhile, a sequence of three groups of Type-III radio bursts is seen in the radio dynamic spectrum during the same time interval. Our observations suggest that the flare QPPs are possibly related to nonthermal electrons accelerated by the intermittent magnetic reconnection during the flare's impulsive phase.

Solar Flare Effects on the Earth's Lower Ionosphere

Solar flares significantly impact the conditions of the Earth's ionosphere. In particular, the sudden increase in X-ray flux during a flare penetrates down to the lowest-lying D-region and dominates ionization at these altitudes (60 - 100 km). Measurements of very low frequency (VLF: 3 - 30 kHz) radio waves that reflect at D-region altitudes provide a unique remote-sensing probe to investigate the D-region response to solar-flare emissions. Here, using a combination of VLF amplitude measurements at 24 kHz together with X-ray observations from the Geostationary Operational Environment Satellite (GOES) X-ray sensor, we present a large-scale statistical study of 334 solar-flare events and their impacts on the D-region over the past solar cycle. Focusing on both GOES broadband X-ray channels, we investigate how the flare peak fluxes and position on the solar disk dictate an ionospheric response and extend this to investigate the characteristic time delay between incident X-ray flux and the D-region response. We show that the VLF amplitude linearly correlates with both the 1 - 8 and 0.5 - 4 channels, with correlation coefficients of 0.80 and 0.79, respectively. For the two X-class flares in our sample, however, there appears to be a turnover in the linear relationship, similar to previous works. Unlike higher altitude ionospheric regions for which the location of the flare on the solar disk affects the ionospheric response, we find that the D-region response to solar flares does not depend on the flare location. By comparing the time delays between the peak X-ray fluxes in both GOES channels and VLF amplitudes, we find that there is an important difference between the D-region response and the X-ray spectral band. We also demonstrate for several flare events that show a negative time delay, the peak VLF amplitude matches with the impulsive 25 - 50 keV hard X-ray fluxes measured by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). These results highlight the importance of performing full spectral analysis when studying the ionospheric responses to solar flares.

Hysteresis, time lag, and relation between solar activity and cosmic rays during solar cycle 24

We study galactic cosmic ray (GCR) modulation during solar cycle 24. For this study we utilize neutron monitor data together with sunspot number (SSN) and 10.7 cm solar radio flux (SRF) data. We plot hysteresis curve between the GCR intensity and SSN, and GCR intensity and SRF. We performed time-lag correlation analysis to determine the time lag between GCR intensity and solar activity parameters. The time lag is determined not only for the whole solar cycle, but also during the two polarity states of the heliosphere (A<0 and A>0) in solar cycle 24. We notice differences in time lags during two polarity epochs of the solar cycle. We discuss these differences in the light of existing modulation models. We compare the results of this very weak solar activity cycle with the corresponding results reported for the previous comparatively more active solar cycles.

Statistical Analysis of the Orbital Motion of Selected Artificial Earth Satellites during Solar Cycle 24

A statistical analysis of selected parameters of solar activity and orbital motion of artificial Earth satellites (AES's) during solar cycle 24 is carried out. Inactive satellites, launch vehicle (LV) stages, and their debris moving mainly in low orbits are studied. Different analysis algorithms are applied to the time series of the solar radio flux F_10.7 and the calculated deceleration rate dP/dt of the investigated space objects (SOs): their annual statistical indices are estimated, these parameters are studied for periodicity (wavelet analysis), and a test additive decomposition into trend and seasonal components is performed. It is found that the satellite deceleration rate in the vicinity of the solar maximum (20122014) increases by a factor of ten. For the solar radio flux F_10.7 and the kinematic parameter dP/dt of SOs 06073 and 31117, seasonal changes, cyclicity with a period of 27 days, etc. are confirmed. A clear anticorrelation between the trends of the corresponding parameters within 0.730.95 for SO 31117 during 20112018 and 0.820.95 for SO 37794 during 20122018 is observed.

Relation of the Monthly Mean Ionospheric T Index to Solar and Geomagnetic Activity Indices

An analysis of the relation of the monthly mean ionospheric T index to the solar (F107) and geomagnetic (Ap) activity indices is presented based on a dataset of these indices for 19542020. F107 and Ap are the monthly mean flux of the solar radio emission at a wavelength of 10.7 cm and the planetary Ap-index of geomagnetic activity, respectively. It is found that the effective index F = (F107₀ + F107₁)/2 provides a high correlation between the ionospheric and solar indices; F107₀ and F107₁ are the F107 indices over the given and previous months. The dependence of T on F in the form of a second-degree polynomial makes it possible to reproduce 9598% of variations in T over the analyzed time interval. Thus, the additional contribution of Ap to T is insignificant. Nevertheless, the contribution of Ap to T depends on the time of year: it is insignificant in January and significant in July. This revealed property of the annual anomaly in ionospheric parameters is conserved also for the contribution of the aa index of geomagnetic activity to the ionospheric T index. In all considered cases, an increase in Ap or aa leads to a decrease in the ionospheric T index, i.e., a mean (global) decrease in the median of the concentration in the F2-layer maximum. Under the same other conditions, such decrease is more significant for July than for January.

On the Issue of the Origin of Type II Solar Radio Bursts

Type II solar radio bursts are among the most powerful events in the solar radio emission in the meter wavelength range. It is generally accepted that the agents generating type II radio bursts are magnetohydrodynamic shock waves. But the relationship between the shock waves and the other manifestations of the large-scale disturbances in the solar atmosphere (coronal mass ejections, Morton waves, EUW waves) remains unclear. To clarify a problem, it is important to determine the conditions of generation of type II radio bursts. Here, the model of the radio source is based on the generation of radio emission within the front of the collisionless shock wave where the Buneman instability of plasma waves is developed. In the frame of this model, the Alfvn magnetic Mach number must exceed the critical value, and there is a strict restriction on the perpendicularity of the front. The model allows us to obtain the information about the parameters of the shock waves and the parameters of the medium by the parameters of type II bursts. The estimates, obtained in this paper for several events with the band splitting of the fundamental and harmonic emission bands of the type II bursts, confirm the necessary conditions of the model. In this case the registration of type II radio bursts is an indication of the propagation of shock waves in the solar atmosphere, and the absence of type II radio bursts is not an indication of the absence of shock waves. Such a situation should be taken into account when investigating the relationship between type II radio bursts and other manifestations of solar activity.

Detection of Flare Multiperiodic Pulsations in Mid-ultraviolet Balmer Continuum, Ly, Hard X-Ray, and Radio Emissions Simultaneously

Quasi-periodic pulsations (QPPs), which usually appear as temporal pulsations of the total flux, are frequently detected in the light curves of solar/stellar flares. In this study, we present the investigation of nonstationary QPPs with multiple periods during the impulsive phase of a powerful flare on 2017 September 6, which were simultaneously measured by the Hard X-ray Modulation Telescope (Insight- HXMT), as well as the ground-based BLENSW. The multiple periods, detected by applying a wavelet transform and Lomb-Scargle periodogram to the detrended light curves, are found to be ~20-55 s in the Ly and mid- ultraviolet Balmer continuum emissions during the flare impulsive phase. Similar QPPs with multiple periods are also found in the hard X-ray emission and low-frequency radio emission. Our observations suggest that the flare QPPs could be related to nonthermal electrons accelerated by the repeated energy release process, i.e., triggering of repetitive magnetic reconnection, while the multiple periods might be modulated by the sausage oscillation of hot plasma loops. For the multiperiodic pulsations, other generation mechanisms could not be completely ruled out.

Localized Acceleration of Energetic Particles by a Weak Shock in the Solar Corona

Globally propagating shocks in the solar corona have long been studied to quantify their involvement in the acceleration of energetic particles. However, this work has tended to focus on large events associated with strong solar flares and fast coronal mass ejections (CMEs), where the waves are sufficiently fast to easily accelerate particles to high energies. Here we present observations of particle acceleration associated with a global wave event which occurred on 2011 October 1. Using differential emission measure analysis, the global shock wave was found to be incredibly weak, with an Alfvn Mach number of ~1.008-1.013. Despite this, spatially resolved type III radio emission was observed by the Nanay RadioHeliograph at distinct locations near the shock front, suggesting localized acceleration of energetic electrons. Further investigation using magnetic field extrapolation identified a fan structure beneath a magnetic null located above the source active region, with the erupting CME contained within this topological feature. We propose that a reconfiguration of the coronal magnetic field driven by the erupting CME enabled the weak shock to accelerate particles along field lines initially contained within the fan and subsequently opening into the heliosphere, producing the observed type III emission. These results suggest that even weak global shocks in the solar corona can accelerate energetic particles via reconfiguration of the surrounding magnetic field.

Characterising coronal turbulence using snapshot imaging of radio bursts in 80-200 MHz

Context. Metrewave solar type-III radio bursts offer a unique means to study the properties of turbulence across coronal heights. Theoretical models have shown that the apparent intensity and size of the burst sources evolve at sub-second scales under the influence of local turbulence. The properties of the evolution vary with observation frequency. However, observational studies remained difficult due to the lack of high fidelity imaging capabilities at these fine temporal scales simultaneously across wide spectral bands.
Aims: I present a spectroscopic snapshot imaging (0.5 s, 160 kHz resolution) study of a type-III burst event across the 80-200 MHz band. By modelling the temporal variability of the source sizes and intensity at every observation frequency, the characteristics of coronal turbulence are studied across a heliocentric height range of 1.54-1.75 R.
Methods: To understand the morphological evolution of the type- III source, a 2D Gaussian fitting procedure is used. The observed trends in the source area and integrated flux density are analysed in the framework of theoretical and data-driven models.
Results: The strength of density fluctuations (N/N) in the corona is derived as a function of height (R). Combined with the archival low frequency data, N/N values across 1.5-2.2 R agree within a few factors. The burst decay time (_decay) and the full width at half maximum of the source showed a power-law dependence with frequency, roughly consistent with the results from data-driven models. However, the values of _decay across frequencies turned out higher than the expected trend. The intrinsic sizes of the burst source were derived, correcting for scatter broadening. This roughly matched the expected size of flux tubes at the coronal heights explored. I also report the observation of an intrinsic anti-phased pulsation in the area and flux density of the source.

A Machine Learning Perspective

Several techniques have been developed in the last two decades to forecast the occurrence of Solar Proton Events (SPEs), mainly based on the statistical association between the >10 MeV proton flux and precursor parameters. The Empirical model for Solar Proton Events Real Time Alert (ESPERTA; Laurenza et al., 2009, https://doi.org/10.1029/2007sw000379) provides a quite good and timely prediction of SPEs after the occurrence of M2 soft x-ray (SXR) bursts, by using as input parameters the flare heliolongitude, the SXR and the 1 MHz radio fluence. Here, we reinterpret the ESPERTA model in the framework of machine learning and perform a cross validation, leading to a comparable performance. Moreover, we find that, by applying a cut-off on the M2 flares heliolongitude, the False Alarm Rate (FAR) is reduced. The cut-off is set to E20 where the cumulative distribution of M2 flares associated with SPEs shows a break which reflects the poor magnetic connection between the Earth and eastern hemisphere flares. The best performance is obtained by using the SMOTE algorithm, leading to probability of detection of 0.83 and a FAR of 0.39. Nevertheless, we demonstrate that a relevant FAR on the predictions is a natural consequence of the sample base rates. From a Bayesian point of view, we find that the FAR explicitly contains the prior knowledge about the class distributions. This is a critical issue of any statistical approach, which requires to perform the model validation by preserving the class distributions within the training and test datasets.

Forecasting the Remaining Duration of an Ongoing Solar Flare

The solar X-ray irradiance is significantly heightened during the course of a solar flare, which can cause radio blackouts due to ionization of the atoms in the ionosphere. As the duration of a solar flare is not related to the size of that flare, it is not directly clear how long those blackouts can persist. Using a random forest regression model trained on data taken from X-ray light curves, we have developed a direct forecasting method that predicts how long the event will remain above background levels. We test this on a large collection of flares observed with GOES-15, and show that it generally outperforms simple linear regression, giving a median error of less than 2 min for the approximate end time of a flare. This random forest model is computationally light enough to be performed in real time, allowing for the prediction to be made during the course of a flare.

Particle-in-cell simulations of the plasma emission in type III solar radio bursts

Not Available

Detection and mitigation of RFI in SBRS observation data

In view of the inconsistency of channel gains and a large amount of interference noise in Solar Broadband Radio Spectrometer (SBRS) observation data, they will seriously affect the analysis of SBRS data. In this paper, a method of Radio Frequency Interference (RFI) detection and mitigation for SBRS observation data is reported. Firstly, the SBRS observation data are preprocessed, a part of the observation data was selected to calculate the mean and variance to achieve the normalization of the entire observation data, which can avoid the influence of strong noise on the normalization result. Furthermore, we proposed an adaptive threshold RFI detection method based on fusion wavelet transform reconstruction and an RFI elimination method based on neighborhood weighted filling. It is worth mentioning that to detect RFI interference signals of different magnitudes, we adopted an iterative approach to the RFI detection and mitigation process. Through qualitative analysis of real observation data and quantitative analysis of simulated data, it is shown that the method proposed in this paper can effectively eliminate RFI in SBRS observation data, and improve the quality of observation data for further scientific analysis.

The first detection of the solar U+III association with an antenna prototype for the future lunar observatory

We report about observations of solar U+III bursts on 2020 June 5 by means of a new active antenna designed to receive radiation in 4-70 MHz. This instrument can serve as a prototype of the ultra-long-wavelength radio telescope for observations on the farside of the Moon. Our analysis of experimental data is based on simultaneous records obtained with the antenna arrays GURT and NDA in high frequency and time resolution, e-Callisto network as well as by using the space-based observatories STEREO and WIND. The results from this observational study confirm the model of Reid and Kontar.

ORFEES - a radio spectrograph for the study of solar radio bursts and space weather applications

Radio bursts are sensitive tracers of non-thermal electron populations in the solar corona. They are produced by electron beams and shock waves propagating through the corona and the heliosphere, and by trapped electron populations in coronal mass ejections (CMEs) and in quiescent active regions. Combining space-borne and ground-based radio spectrographs allows one to track disturbances between the low corona, near or at the sites of particle acceleration, and the spacecraft. Radio observations are, therefore, a significant tool in probing the solar origin of heliospheric disturbances, which is a central research topic as witnessed by the Parker Solar Probe and Solar Orbiter missions. The full scientific return of these projects needs vigorous ground-based support, which at radio wavelengths covers altitudes up to about a solar radius above the photosphere. Besides research in solar and heliospheric physics, monitoring solar radio bursts also supports space weather services. On occasion, radio bursts can themselves be a space weather hazard. The Nanay radio astronomy station in central France has a long tradition of monitoring radio emission at decimetre-to-meter wavelengths. This article describes the radio spectrograph ORFEES (Observations Radiospectrographiques pour FEDOME et l'Etude des Eruptions Solaires). It observes the whole-Sun flux density between 144 and 1004 MHz, pertaining to regions between the low corona and about half a solar radius above the photosphere. ORFEES results from a partnership between Observatoire de Paris and the French Air Force, which operates the experimental space weather service FEDOME. The primary use of the instrument at the Paris Observatory is astrophysical observation. Low-resolution data with rapid availability are presently produced for the French Air Force. Similar information can be made available to a broader range of space weather service providers. This article gives an overview of the instrument design and access to the data and shows a few illustrative observations.

Impact of solar and geomagnetic activities on total column ozone in China

Dynamic impacts on sunspot number (SSN), solar radio flux at 10.7 cm (F10.7) and amplitude antipodal (aa) index influencing long-term trends of total column ozone (TCO) obtained from three stations; Xianghe (39.975N, 116.37E), Linan (30.3N, 119.73E), and Kunming (25.03N, 102.683E) over China have been studied. Descriptive, regression and neural network training techniques are used from 1980 to 2018. The annual mean TCO concentrations are 333.61 1.15 DU, 280.87 0.58 DU, and 260.33 0.49 DU at Xianghe, Linan, and Kunming stations, respectively. The mean monthly peak TCO values are ~417.13 DU (March), 316.79 DU (April), and 291.03 DU (April) at Xianghe, Linan, and Kunming, respectively. Irrespective of the decreasing seasonal trend during spring at Xianghe, the springtime TCO amount is greater than the TCO values in other seasons at all stations. Our analyses show direct forcing of SSN, F10.7, and aa on TCO changes throughout the different periods considered. At Xianghe, the value of r ranges between 0.09 and 0.48 and the p-value < 0.05 whereas the most contributing factor is aa index (about 40 %) to ozone enhancement. There is a dramatic improvement of the impact of the quasi-biennial oscillation (QBO)-modulated (70 hPa and 50 hPa) SSN, F10.7, and aa on TCO during winter in all the stations particularly the aa, which appreciated from ~40 % to 66 %. Ozone is affected negatively during descending and low solar cycle (SC) phases whereas it enhances during ascending and high SC phases with 30 hPa QBO as the most active force of enhancement. Our finding is a profitable tool for atmospheric model development, which could be used to predict future phenomena.

equatorial studies

Various solar flares and coronal mass ejections were associated with the intense solar activity located at RGN 2673 based on NASA's record. The effects of solar activity change the condition of the ionosphere leading to fading or loss of signal. The duration of signal loss may last for a few minutes or more than an hour depending on the scale of the solar flare. Having said that, such an event disturbed the HF (high-frequency) radio communication with high sunspot number (SSn) from 4th to 10th September 2017. The R1-R2 (minor moderate) and R3 (strong) radio blackouts occurred on 4th, 6th and 10th of September 2017. Therefore, this study aims to investigate and analyse the effects of intense solar activity towards HF radio communication based on the observations of Jicamarca (11.571 S, 76.525 W) and Fortaleza (3.7327 S, 38.527 W) ionosondes located at low latitudes and an amateur radio application executed at a ground station in UKM (2.92 N, 101.77 E). According to the data coverage from both the ionosondes on the specific dates, an increased value of critical frequency of F2 layer (foF2) during HF radio blackout was identified to be associated with high SSn. The X-class flares recorded on all three dates in September 2017 caused the foF2 to enhance with recovery times of tens of minutes to hours, based on the decay time of the flare. Moreover, HF selection for amateur radios experienced an increase of more than 8 MHz during the day compared to around 7 MHz on a normal day. This result is essential for frequency planning, especially for HF amateur radio users in and around Malaysia.

A Case Study

Combining in situ measurements of energetic electrons and remote sensing observations of hard X-rays and type III radio bursts, we examine the release times of energetic electrons in the July 23, 2016 event. We find that the releases of in situ energetic electrons from the Sun are delayed from those electrons that are responsible for the hard X-rays. We further find that the release time of in situ electrons is a function of electron energy. Under the assumption that the acceleration mechanism for the upward propagating electrons is of Fermi-type and is controlled by an energy-dependent diffusion coefficient, we fit these release times by a simple functional form, related to the turbulence spectral index. Implications of our study on the underlying electron acceleration mechanisms and the magnetic reconnection process in solar flares are discussed. Our results demonstrate the power of the recently developed fractional velocity dispersion analysis (FVDA) method in solar flare studies.

An analysis based on 19-year measurements in Boston, USA

Solar radiation plays a major role in atmospheric photochemistry, contributing to the formation and growth of ultrafine particles (PN). PN affect global Earth's radiation balance, climate system, and human health. However, the impact of solar activity on ambient PN remains unclear. In this study, we investigated the associations between daily ambient PN concentrations [particle number (PN)/cm³] and solar radio flux [solar activity index (F_10.7 in sfu)] as a solar activity parameter, shortwave solar radiation (SWR), daylight time (DL), cosmic ray-induced ionization (CRII), and air pollution [PM_2.5, black carbon (BC) and SO₂] over a 19-year period in Boston, MA. We used generalized additive models adjusted for local environmental conditions. We found that F_10.7 was the strongest predictor for daily PN concentrations over all time lags (0-28 days of lags) and seasons. The effects were higher in winter and fall. In winter, an interquartile (IQR) of 60 sfu F_10.7 corresponded to an increase of 5770 PN/cm³ in the day of PN collection. In fall, an IQR of 75.5 sfu F_10.7 was associated with an increase of 5429 PN/cm³. The effects of F_10.7 on PN concentrations were slightly greater when the models were adjusted for air pollution. In summer, ambient PN concentrations were statistically significantly associated with F_10.7, SWR, and BC, with the strongest association found for PN and BC in the day of PN collection. Unlike the effects of F_10.7, SWR and local pollutants on PN concentrations, DL and CRII were negatively associated with ambient PN in the analyses. These findings suggest that solar activity may have a significant impact on daily ambient PN concentrations that affect the Earth's climate system and human health.

Harmonic Maser Emissions from Electrons with Loss-cone Distribution in Solar Active Regions

Electron cyclotron maser emission (ECME) is regarded as a plausible source for coherent radio radiations from solar active regions (e.g., solar radio spikes). In this Letter, we present a 2D3V fully kinetic electromagnetic particle-in-cell simulation to investigate the wave excitations and subsequent nonlinear processes induced by the energetic electrons in the loss-cone distribution. The ratio of the plasma frequency to the electron gyrofrequency _pe/_ce is set to 0.25, adequate for solar active region conditions. As a main result, we obtain strong emissions at the second-harmonic X mode (X2). While the fundamental X mode (X1) and the Z mode are amplified directly via the electron cyclotron maser instability, the X2 emissions can be produced by nonlinear coalescence between two Z modes and between Z and X1 modes. This represents a novel generation mechanism for the harmonic emissions in plasmas with a low value of _pe/_ce, which may resolve the escaping difficulty of explaining solar radio emissions with the ECME mechanism.

Wave Excitation by Power-law-Distributed Energetic Electrons with Pitch-angle Anisotropy in the Solar Corona

Radio waves from the Sun are emitted, as a rule, due to energized electrons. Observations infer that the related energized electrons follow (negative) power-law velocity distributions above a break velocity U_b. They might also distribute anisotropically in the pitch-angle space. To understand radio wave generation better, we study the consequences of anisotropic power-law-distributed energetic electrons in current-free collisionless coronal plasmas utilizing 2.5-dimensional particle-in-cell simulations. We assume that the velocity distribution f_u of the energized electrons follows a plateau (f_u/u = 0) and a power-law distribution with spectral index for velocities below and above U_b, respectively. In the pitch-angle space, these energized electrons are spread around a center _c = 0.5. We found that the energetic plateau-power-law electrons can more efficiently generate coherent waves if the anisotropy of their pitch-angle distribution is sufficiently strong, i.e., a small pitch-angle spread _s. The break velocity U_b affects the excitation dominance between the electrostatic and electromagnetic waves: for larger U_b electrostatic waves are mainly excited, while intermediate values of U_b are required for an excitation dominated by electromagnetic waves. The spectral index controls the growth rate, efficiency, saturation, and anisotropy of the excited electromagnetic waves as well as the energy partition in different wave modes. These excited electromagnetic waves are predominantly right-handed polarized, in X- and Z-modes, as observed, e.g., in solar radio spikes. Additionally about 90% of the kinetic energy loss of the energetic electrons is dissipated, heating the ambient thermal electrons. This may contribute to the coronal heating.

Insights from Snapshot Spectroscopic Radio Observations of a Weak Type I Solar Noise Storm

We present a high-fidelity snapshot spectroscopic radio imaging study of a weak type I solar noise storm that took place during an otherwise exceptionally quiet time. Using high-fidelity images from the Murchison Widefield Array, we track the observed morphology of the burst source for 70 minutes and identify multiple instances where its integrated flux density and area are strongly anticorrelated with each other. The type I radio emission is believed to arise due to electron beams energized during magnetic reconnection activity. The observed anticorrelation is interpreted as evidence for presence of MHD sausage wave modes in the magnetic loops and strands along which these electron beams are propagating. Our observations suggest that the sites of these small scale reconnections are distributed along the magnetic flux tube. We hypothesize that small scale reconnections produces electron beams which quickly get collisionally damped. Hence, the plasma emission produced by them span only a narrow bandwidth and the features seen even a few mehahertz apart must arise from independent electron beams.

Space weather study through analysis of solar radio bursts detected by a single-station CALLISTO spectrometer

This article summarises the results of an analysis of solar radio bursts (SRBs) detected by the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (CALLISTO) spectrometer hosted by the University of Rwanda. The data analysed were detected during the first year (2014-2015) of the instrument operation. Using quick plots provided by the e-CALLISTO website, a total of 201 intense and well-separated solar radio bursts detected by the CALLISTO station located in Rwanda, are found consisting of 4 type II, 175 type III and 22 type IV radio bursts. It is found that all analysed type II and 37 % of type III bursts are associated with impulsive solar flares, while the minority ( 13 %) of type IV radio bursts are associated with solar flares. Furthermore, all type II radio bursts are associated with coronal mass ejections (CMEs), 44 % of type III bursts are associated with CMEs, and the majority ( 82 %) of type IV bursts were accompanied by CMEs. With aid of the atmospheric imaging assembly (AIA) images on board the Solar Dynamics Observatory (SDO), the location of open magnetic field lines of non-flare-associated type III radio bursts are shown. The same images are used to show the magnetic loops in the solar corona for type IV radio bursts observed in the absence of solar flares and/or CMEs. Findings from this study indicate that analysis of SRBs that are observed from the ground can provide a significant contribution to the early diagnosis of solar transients phenomena, such as solar flares and CMEs, which are major drivers of potential space weather hazards.

Trends and characteristics of high-frequency type II bursts detected by CALLISTO spectrometers

Solar radio type II bursts serve as early indicators of incoming geo- effective space weather events such as coronal mass ejections (CMEs). In order to investigate the origin of high-frequency type II bursts (HF type II bursts), we have identified 51 of them (among 180 type II bursts from SWPC reports) that are observed by ground-based Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (CALLISTO) spectrometers and whose upper- frequency cutoff (of either fundamental or harmonic emission) lies in between 150 MHz-450 MHz during 2010-2019. We found that 60% of HF type II bursts, whose upper-frequency cutoff 300 MHz originate from the western longitudes. Further, our study finds a good correlation (~ 0.73) between the average shock speed derived from the radio dynamic spectra and the corresponding speed from CME data. Also, we found that analyzed HF type II bursts are associated with wide and fast CMEs located near the solar disk. In addition, we have analyzed the spatio-temporal characteristics of two of these high-frequency type II bursts and compared the derived from radio observations with those derived from multi-spacecraft CME observations from SOHO/LASCO and STEREO coronagraphs.

Correlation analyses between solar activity parameters and cosmic ray muons between 2002 and 2012 at high cutoff rigidity

Solar activity modulates cosmic ray (CR) flux with different magnitudes on different time scales. Several investigations from locations with different, mainly low, cutoff rigidity utilizing different solar activity parameters have been carried out to characterize their influence on the observed CR flux.

In this study, the effect of seven solar, interplanetary, and geophysical parameters on the secondary CR muons has been investigated and established using correlation analyses. Daily mean values of the CR data for the period between 2002 and 2012 were obtained from the King Abdulaziz City for Science and Technology (KACST) muon detector, Riyadh, central Saudi Arabia (Rc = 14.4 GV). Considered parameters are sunspot number, radial component of the interplanetary magnetic field, Kp index, solar radio emission flux at 10.7 cm, Dst index, solar wind speed, and solar wind density. Same analyses have been carried out using data from the Oulu neutron monitor (Rc = 0.8 GV) for comparison purposes.

Obtained results showed that the intensities of the secondary CRs from both stations are significantly correlated with the Dst index and plasma density, and anti-correlated with the rest of the variables. These results are in agreement with those obtained by several investigators. Magnitude and strength of the correlations between the considered variables and CRs were higher and stronger for the CR neutrons than the CR muons. These findings may be due to the greater influence of seasonal variations on the CR muons than on the CR neutrons. Additionally, the muon detector is sensitive (due to its high cutoff rigidity) to higher-energy CR particles, which are less affected by solar disturbances.

Time-lag cross-correlation analyses between the monthly mean CR values from both stations and the monthly mean values of the considered variables were conducted. Comparable results with the previous investigations were found.

Multivariable models using the seven parameters were developed to predict the CR variations for both sites. For the data from the KACST detector, the model was able to predict the measured data with a correlation coefficient of 0.48 and a standard deviation of 0.60%. On the other hand, the developed model for the Oulu neutron monitor has a correlations coefficient of 0.77 and a standard deviation of 3.7%.

Multiwavelength observations of a metric type-II event

We have studied a complex metric radio event that originated in a compact flare, observed with the ARTEMIS-JLS radiospectrograph on February 12, 2010. The event was associated with a surge observed at 195 and 304 and with a coronal mass ejection observed by instruments on board STEREO A and B near the eastern and western limbs respectively. On the disk the event was observed at ten frequencies by the Nanay Radioheliograph (NRH), in H by the Catania observatory, in soft X-rays by GOES SXI and Hinode XRT, and in hard X-rays by RHESSI. We combined these data, together with MDI longitudinal magnetograms, to get as complete a picture of the event as possible. Our emphasis is on two type-II bursts that occurred near respective maxima in the GOES light curves. The first, associated with the main peak of the event, showed an impressive fundamental-harmonic structure, while the emission of the second consisted of three well-separated bands with superposed pulsations. Using positional information for the type-IIs from the NRH and triangulation from STEREO A and B, we found that the type-IIs were associated neither with the surge nor with the disruption of a nearby streamer, but rather with an extreme ultraviolet (EUV) wave probably initiated by the surge. The fundamental-harmonic structure of the first type-II showed a band split corresponding to a magnetic field strength of 18 G, a frequency ratio of 1.95 and a delay of 0.230.65 s of the fundamental with respect to the harmonic; moreover it became stationary shortly after its start and then drifted again. The pulsations superposed on the second type-II were broadband and had started before the burst. In addition, we detected another pulsating source, also before the second type-II, polarized in the opposite sense; the pulsations in the two sources were out of phase and hence hardly detectable in the dynamic spectrum. The pulsations had a measurable reverse frequency drift of about 2 s¹.

Generation of interplanetary type II radio emission

Context. Coronal mass ejections (CMEs) are eruptive phenomena that can accelerate energetic particles and drive shock waves. The CME-driven shocks propagate from the low corona to interplanetary space. The radio emission that results from fast electrons energised by shock waves are called type II bursts. This radio emission can provide information on the physical properties of the shock and its evolution as it travels through the corona and interplanetary space.
Aims: We present a comprehensive analysis of the shock wave associated with two type II radio bursts observed on 27 September 2012. The aim of the study is to isolate and understand the shock wave properties necessary for accelerating electrons, leading to the production of the radio emission.
Methods: First, we modelled the 3D expansion of the shock wave by exploiting multi-viewpoint reconstruction techniques based on extreme ultraviolet imaging. The physical properties of the shock front were then deduced by comparing the triangulated 3D expansion with properties of the background corona provided by a 3D magnetohydrodynamic model. The radio triangulation technique provided the location of radio source on the surface of the modelled wave in order to compare radio sources with the shock properties.
Results: This study is focused on the temporal evolution of the shock wave parameters and their role in the generation of radio emission. Results show a close relationship between the shock wave strength and its geometry. We deduce from this analysis that there may be several mechanisms at play that generally contribute to the generation of radio emission.
Conclusions: The comparison between the reconstructed sources of radio emission and the ambient shock wave characteristics reveals the complex relationship between shock parameters and show how they can influence the morphology of the observed type II radio emission.

On the Magnetoacoustic Waves and Physical Conditions in Zebra Radio Sources

Analysis of the solar radio zebra-pattern (ZP) spectrum for the burst on 21 June 2011 has shown that the frequencies corresponding to the stripes of this ZP experience quasiperiodic oscillations relative to some average values. The period of such oscillations, expressed in the number of the ZP stripes, is 2.41 0.21 , and expressed in frequencies, it is (5.00 0.68 ) MHz. The change in the period of oscillations with time anticorrelates with the amplitude of the oscillations. The values of the harmonic numbers for the corresponding bands are given, and thus the magnetic-field strength is also estimated on the basis of the theory of double plasma resonance (DPR). In addition, a possible change in the L_bh/L_nh parameter in the ZP-generation region is taken into account (L_bh and L_nh respectively are the magnetic-field and density scales). Calculations of the frequency- drift rate, carried out using an improved method for its determination, have shown that the drift values (3 -8 MHzs¹) are in accordance with Kaneda et al. (Astrophys. J. Lett.855, L29, 2018). By using two density models of the solar atmosphere, the wavelength of these oscillations has also been determined. For the model presented by Aschwanden (Space Sci. Rev.101, 1, 2002), the wavelength is about 1370 km while for the barometric density model, the wavelength is about 4650 km. The wavelength increases with time; for example, in the first model, the wavelength increases with time from 1200 to 1490 km. The calculated kink and sausage wave velocities turned out to be significantly lower than the observed ones. The reason for this discrepancy requires additional analysis.

Kodaikanal Solar Observatory Radio Spectrograph

The Indian Institute of Astrophysics (IIA) has commissioned a low frequency (<100 MHz) spectrograph at its Kodaikanal Solar Observatory (KSO) for coordinated observations of transients in the solar atmosphere with other existing optical observing facilities there. The hardware set-up and initial observations are presented. The availability of different instruments in the same observatory helps to quickly plan and jointly observe the energetic phenomena and their signatures in all the three major atmospheric regions of the Sun, i.e. photosphere, chromosphere and corona.

Long-term trends of the F2-region at mid-latitudes in the Southern Hemisphere

The long-term variations in f_oF2 at Hobart (52.88S, 147.32E), Canberra (35.28S, 149.13 E) and Christchurch (43.53S, 172.64E) stations, located in the mid-latitude zone in the Southern Hemisphere were analyzed using 1947-2006 years of the data. The saturation, solar and geomagnetic activity and seasonal effects were removed mainly by using 12-month running mean, linear and multiple regression (twofold regression) methods to find possible signatures of climate change in long-term trends in the f_oF2. The solar activity proxies, sunspot number, R_Z, and F10.7 solar radio flux were used in regression to find the f_oF2 residuals at midday (12 LT) and midnight (00 LT) of the stations. The long-term trends obtained at 12 LT are more significant and consistent with the model results. The trends estimated with F10.7 solar flux are negative and the trends estimated with R_Z are positive (small and not significant). The f_oF2 decreased by 0.1-0.4 MHz for the 5 solar cycles period which could be mainly due to enhanced CO₂ in the troposphere that is cooling the upper atmosphere. Further research is needed to see if the f_oF2 trends are also affected by other factors such as thermospheric winds, neutral constituents, the secular variation of Earth's magnetic field, long-term changes in stratospheric ozone, solar and geomagnetic activities.

Model of the E-Layer Critical Frequency for the Auroral Region

A new foE model for the auroral region is constructed; the model is based on an analysis of the models of auroral electron precipitations, the boundaries of the discrete and diffusive aurora, the main ionospheric trough, and measurements of the E-layer critical frequency foE. The model is an analytical model. It consists of solar (foE_sol) and auroral (foE_avr) components. The solar component of the model does not depend of geomagnetic activity. It depends on solar activity via the F index, which is determined by the solar radio emission flux at a wavelength of 10.7 cm over the previous day and three solar rotations. The auroral component of the model does not depend of solar activity. It depends on geomagnetic activity via the effective Kp* index, which takes into account the prehistory of changes in this activity. The model indirectly takes into account the dependence of the relative contribution of foE_sol and foE_avr to the total foE value on the difference in the heights of the maxima of these model components via the addition of a coefficient. The model qualitatively takes into account the effect of the winter anomaly in foE_avr via the addition of a function. It is found that the errors of the new foE model in the auroral region at the nighttime hours are much lower than those in the international IRI model (with the STORM-E option) for both moderate and high geomagnetic activity. For example, the comparison with data from ionospheric stations shows that the IRI model underestimates foE in these conditions by approximately a factor of 2 on average. The average shift in foE relative to the experimental data in the new model does not exceed 20%.

Auto Recognition of Solar Radio Bursts Using the C-DCGAN Method

Solar radio bursts can be used to study the properties of solar activities and the underlying coronal conditions on the basis of the present understanding of their emission mechanisms. With the construction of observational instruments, around the world, a vast volume of solar radio observational data has been obtained. Manual classifications of these data require significant efforts and human labor in addition to necessary expertise in the field. Misclassifications are unavoidable due to subjective judgments of various types of radio bursts and strong radio interference in some events. It is therefore timely and demanding to develop techniques of auto-classification or recognition of solar radio bursts. The latest advances in deep learning technology provide an opportunity along this line of research. In this study, we develop a deep convolutional generative adversarial network model with conditional information (C-DCGAN) to auto-classify various types of solar radio bursts, using the solar radio spectral data from the Culgoora Observatory (1995, 2015) and the Learmonth Observatory (2001, 2019), in the metric decametric wavelengths. The technique generates pseudo images based on available data inputs, by modifying the layers of the generator and discriminator of the deep convolutional generative adversarial network. It is demonstrated that the C-DCGAN method can reach a high-level accuracy of auto-recognition of various types of solar radio bursts. And the issue caused by inadequate numbers of data samples and the consequent over- fitting issue has been partly resolved.

Catalog of Long-term Transient Sources in the First 10 yr of Fermi-LAT Data

We present the first Fermi Large Area Telescope (LAT) catalog of long- term -ray transient sources (1FLT). This comprises sources that were detected on monthly time intervals during the first decade of Fermi-LAT operations. The monthly timescale allows us to identify transient and variable sources that were not yet reported in other Fermi-LAT catalogs. The monthly data sets were analyzed using a wavelet-based source detection algorithm that provided the candidate new transient sources. The search was limited to the extragalactic regions of the sky to avoid the dominance of the Galactic diffuse emission at low Galactic latitudes. The transient candidates were then analyzed using the standard Fermi-LAT maximum likelihood analysis method. All sources detected with a statistical significance above 4 in at least one monthly bin were listed in the final catalog. The 1FLT catalog contains 142 transient -ray sources that are not included in the 4FGL-DR2 catalog. Many of these sources (102) have been confidently associated with active galactic nuclei (AGNs): 24 are associated with flat-spectrum radio quasars, 1 with a BL Lac object, 70 with blazars of uncertain type, 3 with radio galaxies, 1 with a compact steep-spectrum radio source, 1 with a steep-spectrum radio quasar, and 2 with AGNs of other types. The remaining 40 sources have no candidate counterparts at other wavelengths. The median -ray spectral index of the 1FLT-AGN sources is softer than that reported in the latest Fermi-LAT AGN general catalog. This result is consistent with the hypothesis that detection of the softest -ray emitters is less efficient when the data are integrated over year-long intervals.

Implications of Flat Optically Thick Microwave Spectra in Solar Flares for Source Size and Morphology

The study aims to examine the spectral dynamics of the low-frequency, optically thick gyrosynchrotron microwave emission in solar flares to determine the characteristics of the emitting source. We present the high-resolution spectra of a set of microwave bursts observed by the Expanded Owens Valley Solar Array (EOVSA) during its commissioning phase in the 2.5-18 GHz frequency range with 1 second time resolution. Out of the 12 events analyzed in this study, nine bursts exhibit a direct decrease with time in the optically thick spectral index _l, an indicator of source morphology. Particularly, five bursts display a "flat" spectrum (_l 1.0) compared to that expected for a homogeneous/uniform source (_l 2.9). These flat spectra at low frequencies (<10 GHz) can be defined as the emission from a spatially inhomogeneous source with a large area and/or with multiple emission components. In a subset of six events with partial cross- correlation data, both the events with flat spectra show a source size of ~120 at 2.6-3 GHz. Modeling based on inhomogeneity supports the conclusion that multiple discrete sources can only reproduce a flat spectrum. We report that these flat spectra appear predominantly in the decay phase and typically grow flatter over the duration in most of the bursts, which indicates an increasing inhomogeneity and complexity of the emitting volume as the flare progresses. This large volume of flare emission filled with the trapped energetic particles is often invisible in other wavelengths, like hard X-rays, presumably due to the collisionless conditions in these regions of low ambient density and magnetic field strength.

Radio, X-Ray, and Extreme-ultraviolet Observations of Weak Energy Releases in the "Quiet" Sun

We analyzed ground-based low frequency (<100 MHz) radio spectral and imaging data of the solar corona obtained with the facilities in the Gauribidanur observatory during the same time as the very weak soft X-ray flares (sub-A-class, flux <10^-7Wm^-2 in the 1-8 wavelength range) from the quiet Sun observed with the X-ray Solar Monitor (XSM) on board Chandrayaan-2 during the recent solar minimum. Nonthermal type I radio burst activity was noticed in close temporal association with the X-ray events. The estimated brightness temperature (T_b) of the bursts at a typical frequency like 80 MHz is 3 10⁵ K. Extreme-ultraviolet (EUV) observations at 94 with the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) revealed a brightening close to the same location and time as the type I radio bursts. As far as we know reports of simultaneous observations of X-ray and/or EUV counterpart to weak transient radio emission at low frequencies from the quiet Sun in particular are rare. Considering this and the fact that low frequency radio observations are sensitive to weak energy releases in the solar atmosphere, the results indicate that coordinated observations of similar events would be useful to understand transient activities in the quiet Sun.

Non-thermal Electron Energization During the Impulsive Phase of an X9.3 Flare Revealed by Insight-HXMT

The X9.3 flare SOL20170906T11:55 was observed by the CsI detector aboard the first Chinese X-ray observatory Hard X-ray Modulation telescope (Insight-HXMT). Using the wavelets method, we report 22 s quasiperiodic pulsations during the impulsive phase. The spectra from 100 keV to 800 keV show the evolution with the gamma-ray flux of a power-law photon index from ~1.8 before the peak, ~2.0 around the flare peak, to ~1.8 again. The gyrosynchrotron microwave spectral analysis reveals a 36"6 0"6 radius gyrosynchrotron source with mean transverse magnetic field around 608.2 Gauss. The penetrated 10 keV non-thermal electron density is about 10^6.7 cm^-3 at peak time. The magnetic field strength followed the evolution of high-frequency radio flux. Further gyrosynchrotron source modeling analysis implies that there exists a quite steady gyrosynchrotron source, and the non-thermal electron density and transverse magnetic field evolution are similar to higher-frequency light curves. The temporal spectral analysis reveals that those non-thermal electrons are accelerated by repeated magnetic reconnection, likely from a lower corona source.

Analysis of Type II and Type III Radio Bursts Associated with SEPs from Non-Interacting/Interacting Radio-Loud CMEs

We analyze radio bursts observed in events with interacting/non- interacting CMEs that produced major SEPs (I_p > 10 MeV) from April 1997 to December 2014. We compare properties of meter (m), decahectometer (DH) type II and DH type III bursts, and time lags for interacting-CME-associated (IC) events and non-interacting-CME- associated (NIC) events. About 70% of radio emissions were observed in events of both types from meters to kilometers. We found high correlations between the drift rates and mid-frequencies of type II radio bursts calculated as the mean geometric between their starting and ending frequencies for both NIC and IC-associated events (correlation coefficient R² = 0.98, power-law index = 1.68 0.16 and R² = 0.93, = 1.64 0.19 respectively). We also found a correlation between the frequency drift rates of DH type II bursts and space speeds of CMEs in NIC-associated events. The absence of such correlation for IC-associated events confirms that the shock speeds changed in CME-CME interactions. For the events with western source locations, the mean peak intensity of SEPs in IC-associated events is four times larger than that in NIC-associated SEP events. From the mean time lags between the start times of SEP events and the start of m, DH type II, and DH type III radio bursts, we inferred that particle enhancements in NIC-associated SEP events occurred earlier than in IC- associated SEP events. The difference between NIC events and IC events in the mean values of parameters of type II and type III bursts is statistically insignificant.

Response and periodic variation of total atmospheric ozone to solar activity over Mountain Waliguan

Modern-Era Retrospective Analysis for Research and Applications, Version-2 (MERRA-2) total column ozone (TCO) data from 1980 to 2018 and ground based TCO data were used to investigate its responses to periodic variations concerning stratospheric dynamics such as tropopause heights and Quasi-Biennial Oscillations (QBO), and solar activity signatures of Magnesium II core-to-wing ratio (Mg II) and F10.7 cm solar radio flux (F10.7) observed over Mountain (Mt.) Waliguan (36.17N, 100.53E), China. Cross-correlation and wavelet analyses were used for carrying out the investigation. The MgII and F10.7 show a periodicity of around an 11-year solar cycle (SC) while the TCO trend is though periodic and it does not exhibit one to one correspondence with the 11-year SC. The highest value in TCO is 355.7 DU noted in February (1982) whereas, the lowest value of TCO is 256.02 DU in October (1988) during the entire study period. The average value in TCO is 291.17 0.94 DU with an amplitude of variation of 99.68 DU (~28%). The trend of TCO depicts a persistent distinct seasonal pattern with maxima in winter/spring (December through April) and minima in summer/autumn (August through November). The long-term variability shows a notable decreasing trend in TCO by ~30%, 13%, and 5% per decade for SCs 21-23, where SC 24 represents an increasing trend of ~1% per decade. Hence, this illustrates the significant recovery of TCO over Mt. Waliguan, China which is as a result of the reduction of the ozone depletion agents arising as a positive outcome of the implementation of the Montreal regulatory policy. From wavelet analysis, the F10.7, MgII, tropopause height, QBO and TCO show a significant dominant periodicity of 10-12 months. It is concluded that the variations in mean TCO are strongly correlated with variations of the F10.7 cm and Mg-II solar indices, and the mean TCO responds rapidly to solar activity variations.

Growth of Radio Astronomy in India

In this autobiographical account, I first describe my family, then childhood and education in India. During 1953-55, I worked in the new field of radio astronomy at the Division of Radiophysics of the Commonwealth Scientific and Industrial Research Organisation in Australia. During 1956-57, I worked at the Radio Astronomy Station of Harvard University at Fort Davis, Texas, where I made observations of solar radio bursts at decimeter wavelengths. I then joined Stanford University as a graduate student in 1957. I contributed to the successful operation of the Stanford Cross Antenna and then used it for studying microwave radio emission from the Sun. I was awarded the Ph.D. degree by Stanford University in 1960 and was then appointed as an Assistant Professor for three years. With an urge to contribute to evolving scientific endeavors in India, I joined the Tata Institute of Fundamental Research (TIFR) at Mumbai, India, in April 1963. In my stay of more than three decades at TIFR, I conceived of, and guided, construction of two of the world's largest radio telescopes, namely the Ooty Radio Telescope and the Giant Metrewave Radio Telescope. These instruments have led to several outstanding contributions and discoveries in the areas of radio galaxies, quasars, pulsars, and cosmology.

Particle-in-cell simulation of plasma emission in solar radio bursts

Aims: The present study aims to shed light on our understanding of the radiation processes of solar radio bursts associated with nonthermal electron propagation in the corona and interplanetary space.
Methods: We performed 2.5-dimensional particle-in-cell (PIC) simulations to investigate the plasma emission excited by a relativistic electron beam using different pitch angles in the magnetized plasma.
Results: Langmuir waves at the fundamental and harmonic frequencies were excited via the energy dissipation of the electron beam. For the first time, the backward Langmuir waves up to the third harmonic frequencies were reproduced in the cases of large pitch angles, likely arising from the relecting and scattering of density fluctuations to the Langmuir waves during electrom beam-plasma interaction. Electromagnetic (EM) waves were generated via the mode conversion of electrostatic (ES) waves and the nonlinear wave coupling. Specifically, the harmonic EM emission comes from the coupling of forward and backward Langmuir waves, namely, L + L' 2H, while the higher harmonic EM emissions generally come from the coupling of the Langmuir wave and lower-order harmonic EM wave, namely, L + (n 1)H nH. When the electron beam exhibits a large pitch angle, another possible mechanism for the third harmonic EM emission might be the coalescence of three ES waves, namely, L + L' + L 3H.

Statistical analysis of solar radio fiber bursts and relations with flares

Fiber bursts are a type of fine structure that frequently occurs in solar flares. Although observations and theory of fiber bursts have been studied for decades, their microphysical process, emission mechanism, and especially the physical links with the flaring process still remain unclear. We performed a detailed statistical study of fiber bursts observed by the Chinese Solar Broadband Radio Spectrometers in Huairou with high spectral-temporal resolutions in the frequency ranges of 1.102.06 GHz and 2.603.80 GHz during 20002006. We identify more than 900 individual fiber bursts in 82 fiber events associated with 48 solar flares. From the soft X-ray observations of the Geostationary Operational Environmental Satellite, we found that more than 40% of fiber events occurred in the preflare and rising phases of the associated solar flares. Most fiber events are temporally associated with hard X-ray bursts observed by RHESSI or microwave bursts observed by the Nobeyama Radio Polarimaters, which implies that they are closely related to the nonthermal energetic electrons. The results indicate that most fiber bursts have a close temporal relation with energetic electrons. Most fiber bursts are strongly polarized, and their average duration, relative bandwidth, and relative frequency-drift rate are about 1.22 s, 6.31%, and 0.069 s¹. The average duration and relative bandwidth of fiber bursts increase with solar flare class. The fiber bursts associated with X-class flares have a significantly lower mean relative frequency-drift rate. The average durations in the postflare phase are clearly longer than the duration in the preflare and rising phases. The relative drift rate in the rising phase is clearly higher than that in preflare and postflare phases. The hyperbola correlation of the average duration and the relative drift rate of the fiber bursts is very interesting. These characteristics are very important for understanding the formation of solar radio fiber bursts and for revealing the nonthermal processes of the related solar flares.

A Search for the Counterparts of Quiet-Sun Radio Transients in Extreme Ultraviolet Data

Nonthermal radio transients from the quiet Sun have been recently discovered and it has been hypothesized using rough calculations that they might be important for coronal heating. It is well realized that energy calculations using coherent emissions are often subject to poorly constrained parameters and hence have large uncertainties. However, energy estimates using observations in the extreme ultraviolet (EUV) and soft X-ray bands are routinely done and the techniques are pretty well established. This work presents the first attempt to identify the EUV counterparts of these radio transients and then use them to estimate the energy deposited into the corona during the event. I show that the group of radio transients studied here is associated with a brightening observed in the EUV waveband and is produced by an energy release of 10²⁵ ergs. The fact that the flux density of the radio transient is only 2 mSFU suggests that it might be possible to do large statistical studies in the future for understanding the relationship between these radio transients and other EUV and X-ray counterparts, as well as for understanding their importance in coronal heating.

Quasi-Periodic Pulsations Detected in Ly and Nonthermal Emissions During Solar Flares

We report quasi-periodic pulsations (QPPs) with double periods during three solar flares (viz. SOL2011-Feb-15T01:44, SOL2011-Sep-25T04:31, SOL2012-May-17T01:25). The flare QPPs were observed from light curves in Ly- , hard X-ray (HXR), and microwave emissions, with the Ly- emission recorded by the Geostationary Operational Environmental Satellite, the HXR emission recorded by the Reuven Ramaty High-Energy Solar Spectroscopic Imager and the Fermi Gamma-ray Burst Monitor, and the microwave emission recorded by the Nobeyama Radio Polarimeters and Radioheliograph. By using the Markov chain Monte Carlo (MCMC) method, QPPs with double periods of about two minutes and one minute were first found in the Ly- emission. Then using the same method, a QPP with nearly the same period of about two minutes was also found in HXR and microwave emissions. Considering the possible common origin (nonthermal electrons) between Ly- and HXR/microwave emission, we suggest that the two-minute QPP results from the periodic acceleration of nonthermal electrons during magnetic reconnections. The ratio between the double periods in the Ly- emission was found to be close to two, which is consistent with the theoretical expectation between the fundamental and harmonic modes. However, we cannot rule out other possible driving mechanisms for the one-minute QPPs in HXR/microwave emissions due to their relatively large deviations.

Radio Interferometric Observations of the Sun Using Commercial Dish TV Antennas

The radio astronomy group in the Indian Institute of Astrophysics (IIA) has been carrying out routine observations of radio emission from the solar corona at low frequencies (40 - 440 MHz) at the Gauribidanur observatory, about 100 km north of Bangalore. Since IIA has been performing regular observations of the solar photosphere and chromosphere using different optical telescopes in its Kodaikanal Solar Observatory (KSO) also (see https://www.iiap.res.in/kodai.htm), the possibilities of obtaining two-dimensional radio images of the solar chromosphere using low-cost instrumentation to supplement the optical observations are being explored. As a part of the exercise, recently the group had developed prototype instrumentation for interferometric observations of radio emission from the solar chromosphere at high frequencies (11.2 GHz) using two commercial dish TV antennas. The hardware set-up and initial observations are presented.

Evolvement of microwave spike bursts in a solar flare on 2006 December 13

Solar radio spikes are one of the most intriguing spectral types of radio bursts. Their very short lifetimes, small source size and super- high brightness temperature indicate that they should be involved in some strong energy release, particle acceleration and coherent emission processes closely related to solar flares. In particular, for the microwave spike bursts, their source regions are much close to the related flaring source region which may provide the fundamental information of the flaring process. In this work, we identify more than 600 millisecond microwave spikes which recorded by the Solar Broadband Radio Spectrometer in Huairou (SBRS/Huairou) during an X3.4 solar flare on 2006 December 13 and present a statistical analysis about their parametric evolution characteristic. We find that the spikes have nearly the same probability of positive and negative frequency drifting rates not only in the flare rising phase, but also in the peak and decay phases. So we suppose that the microwave spike bursts should be generated by shock-accelerated energetic electrons, just like the terminational shock (TS) wave produced by the reconnection outflows near the loop top. The spike bursts occurred around the peak phase have the highest central frequency and obviously weak emission intensity, which imply that their source region should have the lowest position with higher plasma density due to the weakened magnetic reconnection and the relaxation of TS during the peak phase. The right-handed polarization of the most spike bursts may be due to the TS lying on the top region of some very asymmetrical flare loops.

Correction of the temperature effect in calibration of a solar radio telescope

This work analyzes the annual fluctuation of the observation data of the Mingantu Solar radio Telescope (MST) in S, C and X bands. It is found that the data vary with local air temperature as the logarithmic attenuation of equipment increases with temperature and frequency. A simplified and effective calibration method is proposed, which is used to calibrate the MST data in 2018-2020, while the correction coefficients are calculated from data in 2018-2019. For S, C and X bands, the root mean square errors of one polarization are 2.7, 5.7 and 20 sfu, and the relative errors are 4%, 6% and 8% respectively. The calibration of MUSER and SBRS spectra is also performed. The relative errors of MUSER at 1700 MHz, SBRS at 2800 MHz, 3050 MHz and 3350 MHz are 8%, 8%, 11% and 10% respectively. We found that several factors may affect the calibration accuracy, especially at X-band. The method is expected to work for other radio telescopes with similar design.

spectrometer commissioning and observations of type III solar radio bursts

The Astrophysics Directorate of CONIDA has installed two radio spectrometer stations belonging to the e-CALLISTO network in Lima, Peru. Given their strategic location near the Equator, it is possible to observe the Sun evenly throughout the whole year. The receiver located at Pucusana, nearby the capital city of Lima, took data from October 2014 until August 2016 in the metric and decimetric bands looking for radio bursts. During this period, this e-CALLISTO detector was unique in its time-zone coverage. To asses the suitability of the sites and the performance of the antennas, we analyzed the radio ambient background and measured their radiation pattern and beamwidth. To demonstrate the capabilities of the facilities for studying solar dynamics in these radio frequencies, we have selected and analyzed type III Solar Radio Bursts. The study of this kind of burst helps to understand the electron beams traversing the solar corona and the solar atmospheric density. We have characterized the most common radio bursts with the following mean values: a negative drift rate of -25.8 3.7 MHz s^-1, a duration of 2.6 0.3 s and 35 MHz bandwidth in the frequency range of 114 to 174 MHz. In addition, for some events, it was possible to calculate a global frequency drift which on average was 0.4 0.1 MHz s^-1.

A long-term multifrequency study of solar rotation using the solar radio flux and its relationship with solar cycles

This paper examines long-term (more than four solar cycles) temporal and spatial fluctuations in the solar rotation by investigating radio- emission escapes from various layers of the solar atmosphere during the years 1967-2010. The flux modulation approach can also be used to investigate variations in solar rotation, which is a contentious topic in solar physics. This study makes use of a time-series of radio flux data at various frequencies (245-15 400 MHz) obtained at Sagamore Hill Solar Radio Observatory in Massachusetts, USA, and at other observatories from 1967 to 2010. The periodicity present in the temporal variation of the time-series is estimated through a Lomb-Scargle periodogram. The rotation period estimated for five radio emissions (606, 1415, and 2695 MHz from the corona, and 4995 and 8800 MHz from the transition region) through a statistical approach shows continuous temporal and spatial variations throughout the years. The smoothed rotation period shows the presence of periodic ~22-yr and ~11-yr components. The 22-yr component could be linked to the reversal of the solar magnetic field (Hale) cycle, while the 11-yr component is most likely related to the sunspot (Schwabe) cycle. In addition to these two components, random components are also prominently present in the analysed data. The cross-correlation between the sunspot number and the rotation period obtained shows a strong correlation with the 11-yr Schwabe and 22-yr Hale cycle. The corona rotates faster or slower than the transition region in different epochs. The alternation of the faster rotation speed between the corona and transition region also follows the 22-yr cycle.

The Role of Flare-Driven Ionospheric Electron Density Changes on the Doppler Flash Observed by SuperDARN HF Radars

Trans-ionospheric high frequency (HF: 3-30 MHz) signals experience strong attenuation following a solar flare-driven sudden ionospheric disturbance (SID). Solar flare-driven HF absorption, referred to as short-wave fadeout, is a well-known impact of SIDs, but the initial Doppler frequency shift phenomena, also known as "Doppler flash" in the traveling radio wave is not well understood. This study seeks to advance our understanding of the initial impacts of solar flare-driven SID using a physics-based whole atmosphere model for a specific solar flare event. First, we demonstrate that the Doppler flash phenomenon observed by Super Dual Auroral Radar Network (SuperDARN) radars can be successfully reproduced using first-principles based modeling. The output from the simulation is validated against SuperDARN line-of-sight Doppler velocity measurements. We then examine which region of the ionosphere, D, E, or F, makes the largest contribution to the Doppler flash. We also consider the relative contribution of change in refractive index through the ionospheric layers versus lowered reflection height. We find: (a) the model is able to reproduce radar observations with an root-median- squared-error and a mean percentage error () of 3.72 m/s and 0.67%, respectively; (b) the F-region is the most significant contributor to the total Doppler flash (48%), 30% of which is contributed by the change in F-region's refractive index, while the other 18% is due to change in ray reflection height. Our analysis shows lowering of the F-region's ray reflection point is a secondary driver compared to the change in refractive index.

Data set for solar flare prediction using helioseismic and magnetic imager vector magnetic field data

It is known that solar flares can affect the near-Earth space, incurring in consequences for radio communications. Therefore, there is a need to research systems for monitoring solar events. This article presents a data set which can be used in the analysis of such events. This data set originated from a set of records from magnetic attributes and solar flare data. In order to create this data set, authors used the SunPy library which provided access to data from the Joint Science Operations Center (JSOC) and Space Weather Prediction Center (SWPC). By integrating data from those two sources, 8,874 samples were obtained comprehending the period between May, 2010 and December, 2019. The collected data were stored as a CSV data set. This data set can be used to support the research of solar flare forecasting, as well as to be compared to other data sets or expanded with new attributes.

A study based on the dominance of hemispheric sunspot activity during the solar cycles 18-24

Solar activity indices differ over the solar disk, and different activity parameters are not considered to be symmetric between the northern and southern solar hemispheres. In the present paper, the daily data of a set of solar parameters (solar radio flux F10.7, total solar irradiance TSI, plage area PA, coronal index CI, solar flare index SFI, and solar mean magnetic field B) as well as the daily hemispheric sunspot areas (SSAs) and sunspot numbers (SSNs) during a timeframe 1945-2017 (covering almost the last seven solar cycles, 18 24) have been employed to investigate the north-south (N-S) asymmetry of the considered solar parameters based on the dominance of hemispheric distributions of SSAs and SSNs. The N-S asymmetry for each solar parameter has been investigated and the results revealed that it is a significant aspect through different years in the entire period. The grand average of each solar parameter for the northern and southern groups over each solar activity cycle has been calculated to investigate the statistical significance of N-S asymmetry of each solar parameter in each solar activity cycle. The solar cycles 19 and 24 displayed the dominance of the southern F10.7 and PA over the northern one. However, the solar cycle 23 showed the reverse. The grand average of CI displayed the southern preference in the solar cycle 19 while, the northern dominance of CI is revealed for the solar cycle 23. The grand averages of CI demonstrated nearly symmetric distribution in solar cycles 18, and 20-22. The N-S asymmetry of the grand averages of SFI exhibited a southern dominance during solar cycles 21 and 24. On the other hand, the northern preference is observed for the grand average of SFI through the solar cycle 23. The asymmetry of the grand averages of B obviously has the same dominance (sign) of hemispheric sunspot activity indices for the solar cycles 21-24. The periodic behavior of the N-S asymmetry of SSAs, SSNs, F10.7, and B has been investigated using Fast Fourier Transformation. Many mid- and long term periodicities have been detected. We present our results and discuss their possible explanations.

First Frequency-time-resolved Imaging Spectroscopy Observations of Solar Radio Spikes

Solar radio spikes are short duration and narrow bandwidth fine structures in dynamic spectra observed from the GHz to tens of MHz range. Their very short duration and narrow frequency bandwidth are indicative of subsecond small-scale energy release in the solar corona, yet their origin is not understood. Using the LOw Frequency ARray, we present spatially, frequency, and time resolved observations of individual radio spikes associated with a coronal mass ejection. Individual radio spike imaging demonstrates that the observed area is increasing in time and the centroid positions of the individual spikes move superluminally parallel to the solar limb. Comparison of spike characteristics with that of individual Type IIIb striae observed in the same event show similarities in duration, bandwidth, drift rate, polarization, and observed area, as well the spike and striae motion in the image plane suggesting fundamental plasma emission with the spike emission region on the order of ~10⁸ cm, with brightness temperature as high as 10¹³ K. The observed spatial, spectral, and temporal properties of the individual spike bursts are also suggestive of the radiation responsible for spikes escaping through anisotropic density turbulence in closed loop structures with scattering preferentially along the guiding magnetic field oriented parallel to the limb in the scattering region. The dominance of scattering on the observed time profile suggests the energy release time is likely to be shorter than what is often assumed. The observations also imply that the density turbulence anisotropy along closed magnetic field lines is higher than along open field lines.

Second Harmonic Electromagnetic Emissions by an Electron Beam in Solar Wind Plasmas with Density Fluctuations

Two-dimensional particle-in-cell simulations are performed to study the electromagnetic radiation emitted at the second harmonic 2_p of the plasma frequency by a weak electron beam propagating in a background plasma with random density fluctuations, in solar wind conditions relevant to Type III solar radio bursts. The dynamics of the waves, the beam, and the plasma are calculated over several thousands of plasma periods. For relevant comparisons, simulations with and without applied density fluctuations are performed for the same parameters. This Letter evidences for the first time the impact of density fluctuations on the physical mechanisms driving the generation of electromagnetic waves emitted at 2_p. Results obtained show that (i) the beam radiates electromagnetic waves at 2_p as a result of nonlinear processes of Langmuir waves' coalescence, despite wave scattering on the density fluctuations that strongly affect the Langmuir spectra; (ii) the fraction of initial beam energy transferred asymptotically to the electromagnetic waves at 2_p is by one order of magnitude smaller when the plasma involves density fluctuations of average level around 5%; (iii) compared to the homogeneous case, the ratio of electromagnetic energy radiated at 2_p to the energy carried by the Langmuir wave turbulence is significantly larger during all the nonlinear stage; (iv) asymptotically, when the plasma is inhomogeneous, electromagnetic emissions at 2_p present isotropized spectra whereas quadrupolar radiation occurs for the homogeneous plasma case.

The Possibility of Observing Several Hydrogen Radio Lines in Solar Activity Features over Sunspots

The first solar radio line attributed to atomic hydrogen was theoretically grounded by J.P. Wild in 1952. Its Zeeman profile was calculated and reliably established in the radiation from quiet Sun regions and activity features over sunspots in 2018 using observations with the RATAN-600 radio telescope, leading to the determination of several parameters of the line emitting regions and the estimation of magnetic fields in the solar atmosphere. In this paper, we calculate the Zeeman profile for two more hydrogen radio lines and show that they can be observed in activity features over sunspots.

2019 International Women's Day event. Two-step solar flare with multiple eruptive signatures and low Earth impact

Context. We present a detailed analysis of an eruptive event that occurred on 2019 March 8 in the active region AR 12734, which we refer as the International Women's Day event. The event under study is intriguing based on several aspects: (1) low-coronal eruptive signatures come in `pairs', namely, there is a double-peaked flare, two coronal dimmings, and two extreme ultraviolet (EUV) waves; (2) although the event is characterized by a complete chain of eruptive signatures, the corresponding coronagraphic signatures are weak; and (3) although the source region of the eruption is located close to the center of the solar disc and the eruption is thus presumably Earth-directed, heliospheric signatures are very weak with very weak Earth impact.
Aims: In order to understand the initiation and evolution of this particular event, we performed a comprehensive analysis using a combined observational-modeling approach.
Methods: We analyzed a number of multi-spacecraft and multi-instrument (both remote-sensing and in situ) observations, including soft X-ray, EUV, radio and white-light emission, as well as plasma, magnetic field, and particle measurements. We employed 3D nonlinear force-free modeling to investigate the coronal magnetic field configuration in and around the active region, the graduated cylindrical shell model to make a 3D reconstruction of the CME geometry, and the 3D magnetohydrodynamical numerical model EUropean Heliospheric FORecasting Information Asset to model the background state of the heliosphere.
Results: Our results reveal a two-stage C1.3 flare, associated with two EUV waves that occur in close succession and two-stage coronal dimmings that evolve co-temporally with the flare and type II and III radio bursts. Despite its small GOES class, a clear drop in magnetic free energy and helicity is observed during the flare. White light observations do not unambiguously indicate two separate CMEs, but rather a single entity most likely composed of two sheared and twisted structures corresponding to the two eruptions observed in the low corona. The corresponding interplanetary signatures are that of a small flux rope swith indications of strong interactions with the ambient plasma, which result in a negligible geomagnetic impact.
Conclusions: Our results indicate two subsequent eruptions of two systems of sheared and twisted magnetic fields, which already begin to merge in the upper corona and start to evolve further out as a single entity. The large-scale magnetic field significantly influences both the early and the interplanetary evolution of the structure. During the first eruption, the stability of the overlying field was disrupted, enabling the second eruption. We find that during the propagation in the interplanetary space the large-scale magnetic field, that is, the location of heliospheric current sheet between the AR and the Earth, is likely to influence propagation, along with the evolution of the erupted structure(s).

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ALMA observations of the variability of the quiet Sun at millimeter wavelengths

Aims: We address the variability of the quiet solar chromosphere at 1.26 mm and 3 mm with a focus on the study of spatially resolved oscillations and transient brightenings, which are small, weak events of energy release. Both phenomena may have a bearing on the heating of the chromosphere.
Methods: We used Atacama Large Millimeter/submillimeter Array (ALMA) observations of the quiet Sun at 1.26 mm and 3 mm. The spatial and temporal resolution of the data were 1 2 and 1 s, respectively. The concatenation of light curves from different scans yielded a frequency resolution in spectral power of 0.50.6 mHz. At 1.26 mm, in addition to power spectra of the original data, we degraded the images to the spatial resolution of the 3 mm images and used fields of view that were equal in area for both data sets. The detection of transient brightenings was made after the effect of oscillations was removed.
Results: At both frequencies, we detected p-mode oscillations in the range 3.64.4 mHz. The corrections for spatial resolution and field of view at 1.26 mm decreased the root mean square (rms) of the oscillations by a factor of 1.6 and 1.1, respectively. In the corrected data sets, the oscillations at 1.26 mm and 3 mm showed brightness temperature fluctuations of 1.7 1.8% with respect to the average quiet Sun, corresponding to 137 and 107 K, respectively. We detected 77 transient brightenings at 1.26 mm and 115 at 3 mm. Although their majority occurred in the cell interior, the occurrence rate per unit area of the 1.26 mm events was higher than that of the 3 mm events; this conclusion does not change if we take into account differences in spatial resolution and noise levels. The energy associated with the transient brightenings ranged from 1.8 10²³ to 1.1 10²⁶ erg and from 7.2 10²³ to 1.7 10²⁶ erg for the 1.26 mm and 3 mm events, respectively. The corresponding power-law indices of the energy distribution were 1.64 and 1.73. We also found that ALMA bright network structures corresponded to dark mottles or spicules that can be seen in broadband H images from the GONG network.
Conclusions: The fluctuations associated with the p-mode oscillations represent a fraction of 0.550.68 of the full power spectrum. Their energy density at 1.26 mm is 3 10² erg cm³. The computed low- end energy of the 1.26 mm transient brightenings is among the smallest ever reported, irrespective of the wavelength of the observation. Although the occurrence rate per unit area of the 1.26 mm transient brightenings was higher than that of the 3 mm events, their power per unit area is smaller likely due to the detection of many weak 1.26 mm events.

Multi-objective Optimization of Interferometric Array u-v Coverage

A new principle is introduced to optimize the configuration of an interferometric array, based on the trade-off between the uniform and Gaussian u-v distributions. The multi-objective optimization method, nondominated sorting genetic algorithm II (NSGA-II), is applied to achieve the optimal trade-off. The resulting array having a single configuration can meet the observation requirements of both compact and extended sources. This method has been successfully applied to design a 16-element array as the initial stage of the Daocheng Solar Radio Telescope to illustrate its feasibility. NSGA-II is improved by introducing artificial intervention into the genetic operator to solve the equality constraints. The improved NSGA-II is applied to obtain the Pareto optimal set, and a 16-element array configuration is retrieved with the best u-v trade-off between the snapshot mode and Earth rotation synthesis mode.

Development of a local empirical model of ionospheric total electron content (TEC) and its application for studying solar-ionospheric effects

Regular and irregular variations in total electron content (TEC) are one of the most significant observables in ionospheric studies. During the solar cycle 24, the variability of ionosphere is studied using global positioning system derived TEC at a mid-latitude station, Tehran (35.70N, 51.33E). Based on solar radio flux and seasonal and local time- dependent features of TEC values, a semi-empirical model is developed to represent its monthly/hourly mean values. Observed values of TEC and the results of our semi-empirical model then are compared with estimated values of a standard plasmasphere-ionosphere model. The outcome of this model is an expected mean TEC value considering the monthly/hourly regular effects of solar origin. Thus, it is possible to use it for monitoring irregular effects induced by solar events. As a result, the connection of TEC variations with solar activities are studied for the case of coronal mass ejections accompanying extreme solar flares. TEC response to solar flares of class X is well reproduced by this model. Our resulting values show that the most powerful flares (i.e. class X) induce a variation of more than 20 percent in daily TEC extent.

Radial differential rotation of solar corona using radio emissions

The present work is an effort to investigate possible radial variations in the solar coronal rotation by analysing the solar radio emission data at 15 different frequencies (275-1755 MHz) for the period starting from 1994 July to 1999 May. We used a time series of disc-integrated radio flux recorded daily at these frequencies through radio telescopes situated at the Astronomical Observatory of the Jagellonian University in Cracow. The different frequency radiation originates from different heights in the solar corona. Existing models indicate its origin at the height range from nearly ~12 000 km (for emission at 275 MHz), below up to ~2400 km (for emission at 1755 MHz). There are some data gaps in the time series used for the study, so we used statistical analysis using the Lomb-Scargle periodogram (LSP). This method has successfully estimated the periodicity present in time series even with such data gaps. The rotation period estimated through LSP shows variation in the rotation period, which is compared with the earlier reported estimate using auto correlation technique. This study indicates some similarity as well as a contradiction with studies reported earlier. The radial and temporal variation in the solar rotation period are presented and discussed for the whole period analysed.

A geometrical description for interplanetary propagation of Earth-directed CMEs based on radiative proxies

We present a 3D geometrical model to describe the propagation and expansion of coronal mass ejections (CMEs) in the interplanetary space based on radiative proxies to be implemented in previous procedures that use SXR and microwave emissions to estimate the Earth-directed CME propagation speed. We carefully selected a sample of 45 well-defined CME-ICME events to evaluate our model. We computed this 3D geometrical model for each event as a tool to improve the arrival time predictions based on radiative proxies. We conducted a different analysis for each radiative proxy: SXR emission and microwave emission at 9 GHz, and we compared the results separately with the observations by the Wind spacecraft. In general, the results showed that the implementation of our 3D geometrical model improves the predictions and provides an important complement to the arrival time prediction method based on radiative proxies, especially for CME events whose source origin were located at helio longitudes far from the central meridian at least 10. Improvements for this tool based on observations by Parker Solar Probe and Solar Orbiter must be developed in the future work.

Glaciological Monitoring Using the Sun as a Radio Source for Echo Detection

Ice-penetrating radar observations are critical for projecting ice-sheet contribution to sea-level rise; however, these prognostic models have significant uncertainties due to an incomplete understanding of glacial subsurface processes. Existing radars that can characterize subsurface conditions are too resource-intensive to simultaneously monitor ice sheets at both the necessary temporaldaily to multiannualand spatialtributary to continentalscales. Here, we investigate using an ambient radio source, instead of transmitting a signal, for glaciological monitoring. We demonstrate, for the first time, passive radio sounding using the Sun to accurately measure ice thickness on Store Glacier, Greenland. Passive radar sounding could provide low- resource time-series measurements of the cryosphere, enabling us to observe and understand evolving englacial and subglacial conditions across Greenland and Antarctica with unprecedented coverage and resolution.

Eruptive versus confined X-class flares in solar cycles 23 and 24

A systematic analysis on the properties of all GOES X-class solar flares (SFs) in solar cycles 23 and 24 (1996-2019) is performed. The occurrence rates and parameters of the eruptive and confined SFs are presented. The aspect of eruptivity versus confinement is investigated with respect to the co-occurrence of coronal mass ejections (CMEs), radio emissions, energetic protons and geomagnetic storms. The absence of interplanetary type III radio bursts, in addition to the lack of CMEs, is found to be a very good proxy for confinement, in contrast to the sunspot type of the parent active region, as both eruptive and confined SFs are predominantly of -- magnetic type. The remaining parameters, protons and geomagnetic storms, imply observing from a specific location in the heliophere and thus are biased for Earth-reaching phenomena. Finally, the relationships between the two types of SFs and the considered here solar activity phenomena are discussed in view of previous studies.

An Interplanetary Type IIIb Radio Burst Observed by Parker Solar Probe and Its Emission Mechanism

Type IIIb radio bursts were identified as a chain of quasi-periodic striae in dynamic spectra, drifting from high to low frequencies in a manner similar to type III bursts, which fine structures may provide a clue to a better understanding of emission mechanisms. The approaching observation of the Parker Solar Probe (PSP) spacecraft provides a new chance of probing type IIIb bursts in the vicinity of the Sun. In this Letter, combining the in situ measurement of PSP and the empirical model of solar atmospheres in open magnetic field regions, we analyze in detail a typical event of interplanetary (IP) type IIIb bursts observed by PSP, which was first reported by Pulupa et al. Our results show that the electron cyclotron maser (ECM) emission can probably play an important role in the excitation mechanism of the IP type IIIb burst and the formation of the fine striae structure may be attributed to the modulation of Alfvn waves on the growth rate of the ECM instability.

Broken Power-law Energy Spectra of the Accelerated Electrons Detected in Radio and Hard X-Rays during the SOL2013-05-13 Event

Solar flares, resulting from magnetic activity of the Sun, are among the most energetic events in the solar system and in extreme cases directly affect our highly technological society. In this work, we analyze a solar flare detected at millimeter and centimeter wavelengths, as well as X-rays above 1 MeV. Observations of solar flares at these energy bands provide diagnostics of the energetic accelerated electrons and the magnetic fields where the emission is produced. During the SOL2013-05-13 solar flare, radio data were obtained by the telescope system POlarisation Emission of Millimeter Activity at the Sun, which observes the Sun at 45 and 90 GHz with polarization measurements, and at microwaves (1-15 GHz) by the Radio Solar Telescope Network. For the same event, X-ray emission was detected by the RHESSI and Fermi satellites. Spectra at both wavelengths were constructed and fit separately to yield the accelerated electron energy distribution that produced the emission. The optically thin radio spectral index was calculated by fitting the Ramaty model of gyrosynchrotron emission to the observed radio spectrum, whereas the hard X-ray spectral index was obtained from the spectral fit assuming a thermal emission model plus a nonthermal broken power-law distribution. Finally, both spectral indexes were compared and confirmed that the index obtained from the radio spectrum agrees with the index of the X-ray spectrum for energies above the break energy of ~600 keV. Thus, the hard X-rays more energetic than 600 keV and high radio frequencies of solar flares are emitted by the same population of high- energy accelerated electrons. This result indicates that the accelerated electrons have an energy distribution best represented by a broken power law, with a breakup above energies around 1 MeV.

Radio evidence for a shock wave reflected by a coronal hole

We report the first unambiguous observational evidence in the radio range of the reflection of a coronal shock wave at the boundary of a coronal hole. The event occurred above an active region located at the northwest limb of the Sun and was characterized by an eruptive prominence and an extreme-ultraviolet wave steepening into a shock. The EUV observations acquired by the Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory and the Extreme Ultraviolet Imager instrument on board the Solar TErrestrial RElations Observatory were used to track the development of the EUV front in the inner corona. Metric type II radio emission, a distinguishing feature of shock waves propagating in the inner corona, was simultaneously recorded by ground- based radio spectrometers. The radio dynamic spectra displayed an unusual reversal of the type II emission lanes, together with type III- like herringbone emission, indicating shock-accelerated electron beams. Combined analysis of imaging data from the two space-based EUV instruments and the Nanay Radioheliograph evidences that the reverse- drifting type II emission was produced at the intersection of the shock front, reflected at a coronal hole boundary, with an intervening low- Alfvn-speed region characterized by an open field configuration. We also provide an outstanding data-driven reconstruction of the spatiotemporal evolution in the inner corona of the shock-accelerated electron beams produced by the reflected shock.

Harmonic electron-cyclotron maser emissions driven by energetic electrons of the horseshoe distribution with application to solar radio spikes

Context. Electron-cyclotron maser emission (ECME) is the favored mechanism for solar radio spikes and has been investigated extensively since the 1980s. Most studies relevant to solar spikes employ a loss- cone-type distribution of energetic electrons, generating waves mainly in the fundamental X/O mode (X1/O1), with a ratio of plasma oscillation frequency to electron gyrofrequency (_pe/_ce) lower than 1. Despite the great progress made in this theory, one major problem is how the fundamental emissions pass through the second- harmonic absorption layer in the corona and escape. This is generally known as the escaping difficulty of the theory.
Aims: We study the harmonic emissions generated by ECME driven by energetic electrons with the horseshoe distribution to solve the escaping difficulty of ECME for solar spikes.
Methods: We performed a fully kinetic electromagnetic particle-in-cell simulation with _pe/_ce = 0.1, corresponding to the strongly magnetized plasma conditions in the flare region, with energetic electrons characterized by the horseshoe distribution. We also varied the density ratio of energetic electrons to total electrons (n_e/n₀) in the simulation. To analyze the simulation result, we performed a fast Fourier transform analysis on the fields data.
Results: We obtain efficient amplification of waves in Z and X2 modes, with a relatively weak growth of O1 and X3. With a higher-density ratio, the X2 emission becomes more intense, and the rate of energy conversion from energetic electrons into X2 modes can reach 0.06% and 0.17%, with n_e/n₀ = 5% and 10%, respectively.
Conclusions: We find that the horseshoe-driven ECME can lead to an efficient excitation of X2 and X3 with a low value of _pe/_ce, providing novel means for resolving the escaping difficulty of ECME when applied to solar radio spikes. The simultaneous growth of X2 and X3 can be used to explain some harmonic structures observed in solar spikes.

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ALMA small-scale features in the quiet Sun and active regions

Aims: The main aim of the present analysis is to decipher (i) the small-scale bright features in solar images of the quiet Sun and active regions obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) and (ii) the ALMA correspondence of various known chromospheric structures visible in the H images of the Sun.
Methods: Small- scale ALMA bright features in the quiet Sun region were analyzed using single-dish ALMA observations (1.21 mm, 248 GHz) and in an active region using interferometric ALMA measurements (3 mm, 100 GHz). With the single-dish observations, a full-disk solar image is produced, while interferometric measurements enable the high-resolution reconstruction of part of the solar disk, including the active region. The selected quiet Sun and active regions are compared with the H (core and wing sum), EUV, and soft X-ray images and with the magnetograms.
Results: In the quiet Sun region, enhanced emission seen in the ALMA is almost always associated with a strong line-of-sight magnetic field. Four coronal bright points were identified, while other small-scale ALMA bright features are most likely associated with magnetic network elements and plages. In the active region, in 14 small-scale ALMA bright features randomly selected and compared with other images, we found five good candidates for coronal bright points, two for plages, and five for fibrils. Two unclear cases remain: a fibril or a jet, and a coronal bright point or a plage. A comparison of the H core image and the 3 mm ALMA image of the analyzed active region showed that the sunspot appears dark in both images (with a local ALMA radiation enhancement in sunspot umbra), the four plage areas are bright in both images and dark small H filaments are clearly recognized as dark structures of the same shape also in ALMA.

A Bayesian Inference-Based Empirical Model for Scintillation Indices for High-Latitude

Solar wind parameters, the solar radio flux index (F10.7), the Sun's declination and the SuperMAG Electrojet index are used to construct a Bayesian inference-based empirical model for scintillation indices (S₄ and ) at high latitudes. For the present study, measurements from three Global Positioning System (GPS) L1 receivers located in the auroral zone, the cusp and in the polar cap are selected, respectively. The solar wind characteristics include the solar wind speed (V_SW) and ram pressure (_SW) as well as the Geocentric Solar Magnetospheric (GSM) B_y and the B_z components of the interplanetary magnetic field (IMF). Following a brief assessment on the independence of the variables (predictors), prior probabilities of occurrence in the case of a multinomial classification are constructed. Posterior-probabilities are then deduced for any arbitrary set of predictors. We show that the model captures most variations seen in the measured indices whether they are associated or not with transient interplanetary events. Although the model tends to underestimate the actual phase index measurements, 95% of the validated events are predicted with an error less than 0.034 rad in . For the amplitude scintillation index, 5% of validated events have an error larger than 0.019.

SDO and Udaipur-CALLISTO Observations

In this article, we present a multi-wavelength investigation of a C-class flaring activity that occurred in the active region NOAA 12734 on 8 March 2019. The investigation utilizes data from the Atmospheric Imaging Assembly (AIA) and the Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) and the Udaipur-CALLISTO solar radio spectrograph of the Physical Research Laboratory. This low intensity C1.3 event is characterized by typical features of a long- duration event (LDE), viz. extended flare arcade, large-scale two-ribbon structures and twin coronal dimmings. The eruptive event occurred in a coronal sigmoid and displayed two distinct stages of energy release, manifested in terms of temporal and spatial evolution. The formation of twin-dimming regions are consistent with the eruption of a large flux rope with footpoints lying in the western and eastern edges of the coronal sigmoid. The metric radio observations obtained from Udaipur- CALLISTO reveals a broad-band (50 -180 MHz), stationary plasma emission for 7 min during the second stage of the flaring activity that resemble a type IV radio burst. A type III decametre-hectometre radio bursts with starting frequency of 2.5 MHz precedes the stationary type IV burst observed by Udaipur-CALLISTO by 5 min. The synthesis of multi- wavelength observations and non-linear force-free field (NLFFF) coronal modeling together with magnetic decay index analysis suggest that the sigmoid flux rope underwent a zipping-like uprooting from its western to eastern footpoints in response to the overlying asymmetric magnetic field confinement. The asymmetrical eruption of the flux rope also accounts for the observed large-scale structures viz. apparent eastward shift of flare ribbons and post-flare loops along the polarity inversion line (PIL), and provides evidence for lateral progression of magnetic reconnection site as the eruption proceeds.

risk factors for Global Navigation Satellite Systems

Extreme space weather events affect the stability and quality of the global navigation satellite systems (GNSS) of the second generation (GPS, GLONASS, Galileo, BeiDou/Compass) and GNSS augmentation. We review the theory about mechanisms behind the impact of geomagnetic storms, ionospheric irregularities, and powerful solar radio bursts on the GNSS user segment. We also summarize experimental observations of the space weather effects on GNSS performance in 20002020 to confirm the theory. We analyze the probability of failures in measurements of radio navigation parameters, decrease in positioning accuracy of GNSS users in dual-frequency mode and differential navigation mode (RTK), and in precise point positioning (PPP). Additionally, the review includes data on the occurrence of dangerous and extreme space weather phenomena and the possibility for predicting their impact on the GNSS user segment. The main conclusions of the review are as follows: 1) the positioning error in GNSS users may increase up to 10 times in various modes during extreme space weather events, as compared to the background level; 2) GNSS space and ground segments have been significantly modernized over the past decade, thus allowing a substantial increase in noise resistance of GNSS under powerful solar radio burst impacts; 3) there is a great possibility for increasing the tracking stability and accuracy of radio navigation parameters by introducing algorithms for adaptive lock loop tuning, taking into account the influence of space weather events; 4) at present, the urgent scientific and technical problem of modernizing GNSS by improving the scientific methodology, hardware and software for monitoring the system integrity and monitoring the availability of required navigation parameters, taking into account the impact of extreme space weather events, is still unresolved.

Recent Observational Studies on the Fine Structures of Solar Type II Radio Bursts

In the solar radio dynamic spectrum, type II radio bursts appear as narrow-band features drifting slowly from higher to lower frequency. These signals are due to plasma radiation excited by energetic electrons. For fundamental plasma radiation, the emission is close to the local plasma frequency. Type II bursts provide important insights in the studies of shock waves driven by solar eruptions, shock-accelerated energetic electrons and space weather forecasting. In addition, type II radio bursts have a variety of fine structures in the dynamic spectrum. According to their spectrotemporal features, they can be roughly categorized into type II radio bursts with band-splitting, multi-branch, herringbone, and sudden-spectral change structures. These fine structures can be used to diagnose coronal parameters such as electron density and magnetic field strength, to determine the speed and configuration of the associated shock waves, and to further understand the solar eruption processes. However, there are still many unresolved problems in the cause of these fine structures, which require further research. In particular, it is of great importance to use data with high angular resolution available from newly-built radio heliographs in China for research. This article reviews recent progresses on observational studies of the fine structures of type II radio bursts and outlines outstanding issues for future studies.

probing high-energy structures around the central molecular zone

Recent observations have revealed interstellar features that apparently connect energetic activity in the central region of our Galaxy to its halo. The nature of these features, however, remains largely uncertain. We present a Chandra mapping of the central 2 4 field of the Galaxy, revealing a complex of X-ray-emitting threads plus plume-like structures emerging from the Galactic Centre (GC). This mapping shows that the northern plume or fountain is offset from a well-known radio lobe (or the GCL), which however may represent a foreground H II region, and that the southern plume is well wrapped by a corresponding radio lobe recently discovered by MeerKAT. In particular, we find that a distinct X-ray thread, G0.17-0.41, is embedded well within a non-thermal radio filament, which is locally inflated. This thread with a width of ~1.6 arcsec (FWHM) is ~2.6 arcmin or 6 pc long at the distance of the GC and has a spectrum that can be characterized by a power law or an optically- thin thermal plasma with temperature 3 keV. The X-ray-emitting material is likely confined within a strand of magnetic field with its strength 1 mG, not unusual in such radio filaments. These morphological and spectral properties of the radio/X-ray association suggest that magnetic field re-connection is the energy source. Such re- connection events are probably common when flux tubes of antiparallel magnetic fields collide and/or become twisted in and around the diffuse X-ray plumes, representing blowout superbubbles driven by young massive stellar clusters in the GC. The understanding of the process, theoretically predicted in analog to solar flares, can have strong implications for the study of interstellar hot plasma heating, cosmic ray acceleration and turbulence.

On the influence of Langmuir wave spectra on the spectra of electromagnetic waves generated in solar plasma with double plasma frequency

In this paper, we consider the spectral dependences of transverse electromagnetic waves generated in solar plasma at the coalescence of Langmuir waves. It is shown that different spectra of Langmuir waves lead to characteristic types of transversal electromagnetic wave spectra, what makes it possible to diagnose the features of the spectra of Langmuir waves generated in solar plasma.

Plasmaspheric scale height modeling based on COSMIC radio occultation data

The scale height in the plasmasphere (H_P) is an important factor to present the dynamic nature and variations of the plasmasphere. It is challenging to build reliable plasmaspheric scale height model because of poor coverage and limited continuity of plasmaspheric sounder data. In this paper, an empirical orthogonal function (EOF) analysis method is applied to organize the H_P obtained from total electron content (TEC) measurements of precise orbit determination (POD) antennas and electron density profiles of Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission, from the year 2007-2014, to reconstruct a global plasmaspheric scale height model. The model depends on parameters including the month of the year, the local time, the geographic latitude and the solar radio flux index F10.7. Comparison of this model and the scale height deduced from the International Satellites for Ionospheric Studies (ISIS) data shows remarkable consistency in H_P values and variations.

Scaling features and IMF dependence

We investigated the average polar patterns of ionospheric electron density and the corresponding patterns of scaling features as a function of interplanetary magnetic field orientation. The focus is on the Northern Hemisphere using electron density data recorded on-board ESA Swarm A satellite. The first- and second-order scaling exponents have been evaluated by means of the q^th -order structure functions. We used electron density measurements over a period of 15 months from April 1, 2014 to June 30, 2015, which corresponds to the maximum of solar cycle 24 and which is characterized by an average value of the solar radio flux (F10.7) index equal to (140 30) sfu. Electron density, first- and second-order scaling exponents have been mapped and discussed for four main IMF orientations provided by B_y and B_z components under conditions of high solar activity. Large spatial changes of the second-order scaling exponent pattern are observed with a steepening of the associated spectral exponent in correspondence with the nightside polar cap trailing edge. Intermittency, defined as the departure from linearity of the dependence of scaling exponents on moment order q, is also evaluated finding that it is generally higher near the equatorward boundary of the auroral oval than elsewhere. On the whole, the found patterns of the electron density first- and second-order scaling exponents suggest the occurrence of turbulence at the high latitudes.

the Gauribidanur and Greenland sites

The instability of the Sun's magnetic field can ignite many eruptive events on the solar surface, including flares, coronal mass ejections, and prominence eruptions. The inner heliosphere environment is affected, and consequently, these events are said to contribute to the celestial weather change. As one of the many eruptive events, a solar flare is of the most frequent due to the magnetic reconnection process in which the accelerated electrons from the reconnection sites escape into the interplanetary space and cause solar radio bursts type III (SRBT III). When it is observed near the Earth, this SRBT III is in the form of radio dynamics spectrum; thus, monitoring this spectrum is vital to the further analysis of the said SRBT III. In this paper, we investigate the background levels: short and long periods of the CALLISTO instruments from two different stations where for each site, a 10-day background- level observation is randomly selected. For the purpose of this study, the mean differences and coefficient of variation (CV) distributions for every frequency channel are determined where most of the frequency channels have displayed small mean differences between these two background levels: short and long periods and the CV distributions as well. These short-period observations, within 15 min of the background levels, are found significant enough to warrant further analysis of the solar radio bursts detected by the CALLISTO instruments.

Gyroresonance and Free-Free Radio Emissions from Multithermal Multicomponent Plasma

The solar atmosphere contains thermal plasma at a wide range of temperatures. This plasma is often quantified, in both observations and models, by a differential emission measure (DEM). The DEM is a distribution of the thermal electron density squared over temperature. In observations, the DEM is computed along a line of sight, while in the modeling it is over an elementary volume element (voxel). This description of the multithermal plasma is convenient and widely used in the analysis and modeling of extreme ultraviolet emission, which has an optically thin character. However, there is no corresponding treatment in the radio domain, where the optical depth of emission can be large, more than one emission mechanism is involved, and plasma effects are important. Here, we extend the theory of thermal gyroresonance and free- free radio emissions in the classical single-temperature Maxwellian plasma to the case of a multitemperature plasma. The free-free component is computed using the DEM and temperature-dependent ionization states of coronal ions, contributions from collisions of electrons with neutral atoms, the exact Gaunt factor, and the magnetic field effect. For the gyroresonant component, another measure of the multitemperature plasma is used, which describes the distribution of the thermal electron density over temperature. We give representative examples demonstrating important changes in the emission intensity and polarization due to the effects considered. The theory is implemented in available computer code.

Coronal Conditions for the Occurrence of Type II Radio Bursts

Type II radio bursts are generally observed in association with flare- generated or coronal-mass-ejection-driven shock waves. The exact shock and coronal conditions necessary for the production of type II radio emission are still under debate. Shock waves are important for the acceleration of electrons necessary for the generation of the radio emission. Additionally, the shock geometry and closed field line topology, e.g., quasi-perpendicular shock regions or shocks interacting with streamers, play an important role for the production of the emission. In this study we perform a 3D reconstruction and modeling of a shock wave observed during the 2014 November 5 solar event. We determine the spatial and temporal evolution of the shock properties and examine the conditions responsible for the generation and evolution of type II radio emission. Our results suggest that the formation and evolution of a strong, supercritical, quasi-perpendicular shock wave interacting with a coronal streamer were responsible for producing type II radio emission. We find that the shock wave is subcritical before and supercritical after the start of the type II emission. The shock geometry is mostly quasi-perpendicular throughout the event. Our analysis shows that the radio emission is produced in regions where the supercritical shock develops with an oblique to quasi-perpendicular geometry.

Energy Budget of Plasma Motions, Heating, and Electron Acceleration in a Three-loop Solar Flare

Nonpotential magnetic energy promptly released in solar flares is converted to other forms of energy. This may include nonthermal energy of flare-accelerated particles, thermal energy of heated flaring plasma, and kinetic energy of eruptions, jets, upflows/downflows, and stochastic (turbulent) plasma motions. The processes or parameters governing partitioning of the released energy between these components are an open question. How these components are distributed between distinct flaring loops and what controls these spatial distributions are also unclear. Here, based on multiwavelength data and 3D modeling, we quantify the energy partitioning and spatial distribution in the well-observed SOL2014-02-16T064620 solar flare of class C1.5. Nonthermal emission of this flare displayed a simple impulsive single-spike light curve lasting about 20 s. In contrast, the thermal emission demonstrated at least three distinct heating episodes, only one of which was associated with the nonthermal component. The flare was accompanied by upflows and downflows and substantial turbulent velocities. The results of our analysis suggest that (i) the flare occurs in a multiloop system that included at least three distinct flux tubes; (ii) the released magnetic energy is divided unevenly between the thermal and nonthermal components in these loops; (iii) only one of these three flaring loops contains an energetically important amount of nonthermal electrons, while two other loops remain thermal; (iv) the amounts of direct plasma heating and that due to nonthermal electron loss are comparable; and (v) the kinetic energy in the flare footpoints constitutes only a minor fraction compared with the thermal and nonthermal energies.

The active region source of a type III radio storm observed by Parker Solar Probe during encounter 2

Context. We investigated the source of a type III radio burst storm during encounter 2 of NASA's Parker Solar Probe (PSP) mission.
Aims: It was observed that in encounter 2 of NASA's PSP mission there was a large amount of radio activity and, in particular, a noise storm of frequent, small type III bursts from 31 March to 6 April 2019. Our aim is to investigate the source of these small and frequent bursts.
Methods: In order to do this, we analysed data from the Hinode EUV Imaging Spectrometer, PSP FIELDS, and the Solar Dynamics Observatory Atmospheric Imaging Assembly. We studied the behaviour of active region 12737, whose emergence and evolution coincides with the timing of the radio noise storm and determined the possible origins of the electron beams within the active region. To do this, we probed the dynamics, Doppler velocity, non-thermal velocity, FIP bias, and densities, and carried out magnetic modelling.
Results: We demonstrate that although the active region on the disc produces no significant flares, its evolution indicates it is a source of the electron beams causing the radio storm. They most likely originate from the area at the edge of the active region that shows strong blue-shifted plasma. We demonstrate that as the active region grows and expands, the area of the blue-shifted region at the edge increases, which is also consistent with the increasing area where large-scale or expanding magnetic field lines from our modelling are anchored. This expansion is most significant between 1 and 4 April 2019, coinciding with the onset of the type III storm and the decrease of the individual burst's peak frequency, indicating that the height at which the peak radiation is emitted increases as the active region evolves.

Periodicities in an active region correlated with Type III radio bursts observed by Parker Solar Probe

Context. Periodicities have frequently been reported across many wavelengths in the solar corona. Correlated periods of ~5 min, comparable to solar p-modes, are suggestive of coupling between the photosphere and the corona.
Aims: Our study investigates whether there are correlations in the periodic behavior of Type III radio bursts which are indicative of nonthermal electron acceleration processes, and coronal extreme ultraviolet (EUV) emission used to assess heating and cooling in an active region when there are no large flares.
Methods: We used coordinated observations of Type III radio bursts from the FIELDS instrument on Parker Solar Probe (PSP), of EUV emissions by the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) and white light observations by SDO Helioseismic and Magnetic Image (HMI), and of solar flare X-rays by Nuclear Spectroscopic Telescope Array (NuSTAR) on April 12, 2019. Several methods for assessing periodicities are utilized and compared to validate periods obtained.
Results: Periodicities of ~5 min in the EUV in several areas of an active region are well correlated with the repetition rate of the Type III radio bursts observed on both PSP and Wind. Detrended 211 and 171 light curves show periodic profiles in multiple locations, with 171 peaks sometimes lagging those seen in 211 . This is suggestive of impulsive events that result in heating and then cooling in the lower corona. NuSTAR X-rays provide evidence for at least one microflare during the interval of Type III bursts, but there is not a one-to-one correspondence between the X-rays and the Type III bursts. Our study provides evidence for periodic acceleration of nonthermal electrons (required to generate Type III radio bursts) when there were no observable flares either in the X-ray data or the EUV. The acceleration process, therefore, must be associated with small impulsive events, perhaps nanoflares.

Temporal and Spatial Association Between a Solar Flare, CME, and Radio Burst on 19 November 2013

We present multi-wavelength and multi-instrument observations and analysis of a major X 1.0 class flare, radio burst, halo coronal mass ejection (CME), and loop eruption from the solar active region NOAA 11893 on 19 November 2013 (SOL2013-11-19T10:26). The aim of this work is twofold: The first aim to study the evolution of the loop eruption and the second is to find the link between this eruption and radio emissions. Initial signatures of eruption from the solar source region are confirmed using observations from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory in the hot channel at 94 wavelength. These observations confirm that the source of the CME was associated with a magnetic-loop eruption, which was visible before the flare initiation. The photospheric magnetic configuration displayed a complex network of sunspots. After the eruption, a CME was observed by the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph with linear speed and acceleration of 740 kms¹ and 2 ms², respectively. Dynamic radio-spectrum observation from the Learmonth Observatory in the metric frequency range shows Type- III and Type-II radio emissions that reveal the field-line opening and coronal-shock formation closely associated with the CME. From the metric Type-II radio observation and assuming Newkirk's density model, we estimate the shock formation height range of 1.14 -1.54 _R with the corresponding shock speed (650 kms¹). With the heliographic observations from the Nanay heliograph at different frequencies we could disentangle Type-II bursts from Type-III bursts. Likely, the Type-III burst would correspond to the loop eruption while the Type-II comes from the northern flank of the CME.

A Quarter Century of Wind Spacecraft Discoveries

The Wind spacecraft, launched on November 1, 1994, is a critical element in NASA's Heliophysics System Observatory (HSO) - a fleet of spacecraft created to understand the dynamics of the Sun-Earth system. The combination of its longevity (>25 years in service), its diverse complement of instrumentation, and high resolution and accurate measurements has led to it becoming the "standard candle" of solar wind measurements. Wind has over 55 selectable public data products with over 1,100 total data variables (including OMNI data products) on SPDF/CDAWeb alone. These data have led to paradigm shifting results in studies of statistical solar wind trends, magnetic reconnection, large- scale solar wind structures, kinetic physics, electromagnetic turbulence, the Van Allen radiation belts, coronal mass ejection topology, interplanetary and interstellar dust, the lunar wake, solar radio bursts, solar energetic particles, and extreme astrophysical phenomena such as gamma-ray bursts. This review introduces the mission and instrument suites then discusses examples of the contributions by Wind to these scientific topics that emphasize its importance to both the fields of heliophysics and astrophysics.

BoxCox multi-output linear regression for 10.7 cm solar radio flux prediction

We consider the problem of predicting the mid-term daily 10.7cm solar radio flux (F10.7), a widely-used solar activity index. A novel approach is proposed for this task, in which BoxCox transformation with a proper parameter is first applied to make the data satisfy the property of homoscedasticity that is a basic assumption of regression models, and then a multi-output linear regression model is used to predict future F10.7 values. The experiment shows that the BoxCox transformation significantly improves the predictive performance and our new approach works substantially better than the prediction from the US Airforce and other alternative methods like Auto-regressiveModel, Multi-layer Perceptron, and Support Vector Regression.

Solar radio filtering algorithm based on improved long short-term memory

The effective observation of burst events in solar radio research has been impeded by various interference signals, especially interference signals with a wide frequency range and high intensity, as they can partially or completely obscure the observation of burst events. Image processing methods that directly remove the interference signal channels and subtract the average of the interference signal channel are not suitable for processing all types of interference signals. This paper proposes the use of a specific kind of recurrent neural networks, called long short-term memory networks, to predict the value of the radio frequency interference signals with high intensity of the burst event in the solar radio spectrum. The predicted interference can then be removed in accordance with the principle that signals can be linearly added. Therefore, predicted value is subtracted from the data containing the burst event signals and the RFI signals (The radio frequency interference signals to be processed in this article refer to the signal of the broadcast signal that can be received in the frequency range, the signal transmitted by the mobile phone, and the signal transmitted by the sea vessel, and the like) to remove the interference. Then, in order to reduce the error caused by the stepwise prediction in the network and further improve the prediction accuracy, this paper analyzes the characteristics of the value of the radio interference and applies the digital mapping method to convert the prediction problem into the classification problem in the time series. The experimental results show that the proposed method can effectively remove the radio interference in the solar spectrum and clearly show the burst events.

Survey of electron density changes in the daytime ionosphere over the Arecibo Observatory due to lightning and solar flares

Optical observations of transient luminous events and remote-sensing of the lower ionosphere with low-frequency radio waves have demonstrated that thunderstorms and lightning can have substantial impacts in the nighttime ionospheric D region. However, it remains a challenge to quantify such effects in the daytime lower ionosphere. The wealth of electron density data acquired over the years by the Arecibo Observatory incoherent scatter radar (ISR) with high vertical spatial resolution (300-m in the present study), combined with its tropical location in a region of high lightning activity, indicate a potentially transformative pathway to address this issue. Through a systematic survey, we show that daytime sudden electron density changes registered by Arecibo's ISR during thunderstorm times are on average different than the ones happening during fair weather conditions (driven by other external factors). These changes typically correspond to electron density depletions in the D and E region. The survey also shows that these disturbances are different than the ones associated with solar flares, which tend to have longer duration and most often correspond to an increase in the local electron density content.

Fine structure of type III solar radio bursts from Langmuir wave motion in turbulent plasma

The Sun frequently accelerates near-relativistic electron beams that travel out through the solar corona and interplanetary space. Interacting with their plasma environment, these beams produce type III radio burststhe brightest astrophysical radio sources seen from Earth. The formation and motion of type III fine frequency structures is a puzzle, but is commonly believed to be related to plasma turbulence in the solar corona and solar wind. Combining a theoretical framework with kinetic simulations and high-resolution radio type III observations using the Low-Frequency Array, we quantitatively show that the fine structures are caused by the moving intense clumps of Langmuir waves in a turbulent medium. Our results show how type III fine structure can be used to remotely analyse the intensity and spectrum of compressive density fluctuations, and can infer ambient temperatures in astrophysical plasma, substantially expanding the current diagnostic potential of solar radio emission.

The lower dayside ionosphere of Mars from 14 years of MaRS radio science observations

This work uses a subset of "quiet" MaRS ionospheric dayside observations (MaRS_quiet, 2004-2017) and a 1-D photochemical model (IonA-2) to investigate the potential formation processes of the excess electron densities merged with the base of the main ionosphere (Mm). 42% of the investigated MaRS observations contain identified Mm, which occur in a large variety of shapes ranging from smoothly decreasing electron densities to peak structures below the base of M1. The Mm appear over the full range of accessible solar zenith angles (50 - 90) and are found between approximately 70 and 110 km altitude. Their base is found on average deeper in the atmosphere than the base of the averaged undisturbed MaRS electron density profiles. This indicates a dependence of the Mm formation on energy sources that penetrate deep into the atmosphere. This is supported by a strong positive correlation with increasing solar activity when solar flares, coronal mass ejections, and enhanced short solar X-ray and Ly- intensities are more common. No relationship is found between the Mm occurrence rate and the magnitude/inclination of the weak crustal crustal magnetic field in MaRS_quiet.

Investigations with the IonA-2 photochemical model for undisturbed and flare conditions show that the ionization of the local neutral atmosphere by solar X-ray radiation <2 nm provides a satisfying explanation for detected Mm features with smoothly decreasing electron densities below the M1 base in combination with moderate slopes of the lower Mm region _Mm and altitudes of the lower boundary h_L,S. While sufficient ionization energy reaches the region of interest during flares, no Mm features with peaks below the M1 base occur in any of the model electron density profiles. This supports the conclusion that the subgroup of merged excess electron densities with peaks or intermediate features (Mi) below the M1 base must have an origin different from the sole variability of solar X-ray radiation during undisturbed and solar flare conditions. The size of the identified Mm makes an exclusive meteoric origin of the Mm peak structures unlikely.

It is indicative from the IonA-2 model results that the general increase/decrease of solar X-ray <2 nm leads to a correlated response of the Mm region. The sporadic occurrence of the merged excess electron densities in the MaRS observations is therefore assumed to be a combination of observational (increased observation noise level compared to the available amount of X-ray radiation <2 nm, shift of the lower baseline by ionospheric deviations from radial symmetry) and environmental (e.g. variations in solar X-ray) factors.

Medium-term Predictions of F10.7 and F30 cm Solar Radio Flux with the Adaptive Kalman Filter

The solar radio flux at F10.7 and F30 cm is required by most models characterizing the state of the Earth's upper atmosphere, such as the thermosphere and ionosphere, to specify satellite orbits, re-entry services, collision avoidance maneuvers, and modeling of the evolution of space debris. We develop a method called RESONANCE (Radio Emissions from the Sun: ONline ANalytical Computer-aided Estimator) for the prediction of the 13-month smoothed monthly mean F10.7 and F30 indices 1-24 months ahead. The prediction algorithm has three steps. First, we apply a 13-month optimized running mean technique to effectively reduce the noise in the radio flux data. Second, we provide initial predictions of the F10.7 and F30 indices using the McNish-Lincoln method. Finally, we improve these initial predictions by developing an adaptive Kalman filter with identification of the error statistics. The rms error of predictions with lead times from 1 to 24 months is 5-27 solar flux units (sfu) for the F10.7 index and 3-16 sfu for F30, which statistically outperforms current algorithms in use. The proposed approach based on the Kalman filter is universal and can be applied to improve the initial predictions of a process under study provided by any other forecasting method. Furthermore, we present a systematic evaluation of re-entry forecast as an application to test the performance of F10.7 predictions on past ESA re-entry campaigns for payloads, rocket bodies, and space debris that re-entered from 2006 to 2019 June. The test results demonstrate that the predictions obtained by RESONANCE in general also lead to improvements in the forecasts of re-entry epochs.

Statistics of Low Frequency Cutoffs for Type III Radio Bursts Observed by Parker Solar Probe during Its Encounters 1-5

The low frequency cutoffs f_lo and the observed plasma frequency f_p of 176 type III radio bursts are investigated in this paper. These events are observed by the Parker Solar Probe when it is in the encounter phase from the first to the fifth orbit. The result shows that the distribution of cutoffs f_lo is widely spread between 200 kHz and 1.6 MHz. While the plasma frequency f_p at the spacecraft is between 50 and 250 kHz, which is almost all smaller than f_lo. The result also shows that the maximum probability distribution of f_lo (680 kHz) is remarkably higher than that observed by Ulysses and Wind (100 kHz) in previous research. Three possible reasons, i.e., solar activity intensity, event electing criteria, and radiation attenuation effect, are also preliminarily discussed.

Observational Evidence for Beat Phenomenon in Complex Solar Type III Radio Bursts

We present new observational evidence for one of the most important three wave interactions, called the electrostatic decay instability (ESD) $L\to {L}^{{\prime} }+S$ in the source regions of complex solar type III radio bursts (L is the electron beam-excited Langmuir wave, and ${L}^{{\prime} }$ and S are the ESD excited daughter Langmuir and ion sound waves, respectively). The STEREO in situ wave observations in the source regions of complex type III bursts show that Langmuir waves often occur as one-dimensional magnetic field aligned beat-type wave packets, with peak intensities well in excess of the threshold for excitation of ESD, and with spectra containing (a) two closely spaced narrow peaks (L and ${L}^{{\prime} }$ ) corresponding probably to the beating modes responsible for the beat patterns at frequencies very close to the local electron plasma frequency, f_pe, and (b) narrow peaks at ion sound frequencies, f_S, which are very close to beat frequencies. Using the FFT and higher order spectral techniques, we show that the frequency, wavevector and phase resonance conditions required for excitation of ESD are well satisfied for these wave packets, and the speeds of electron beams derived from the resonance conditions agree reasonably well with those derived from the drift rates of the associated type III events. We also show that the merging of (L) and ( ${L}^{{\prime} }$ ) most probably is the excitation mechanism of the second harmonic radio emission ${T}_{2{f}_{{pe}}}$ of these type III bursts.

comparison with IRI-2016 and IRI-2012 models

In this study the statistics of ionospheric total electron content (TEC), derived from a GSV4004B dual-frequency Global Positioning System (GPS) receiver at Agartala station (23.450N, 91.150E) located in northern equatorial ionization anomaly (EIA) crest region of the Indian subcontinent, is reported with a performance analysis of IRI-2016 and IRI-2012 models during the ascending, maxima, declining and minima phases (2013-2018) of the solar cycle 24. Variations of model total electron content, as obtained from the IRI-2016 and IRI-2012 for the three options of topside electron density namely NeQuick, IRI 2001 and IRI 01-corr, are compared with the observed total electron content during different periods of interest viz. monthly, seasonal, annual and the correlations with solar activity parameters viz. sunspot number (SSN), 10.7 cm solar radio flux (F10.7), solar EUV flux, are also investigated. All the three options of IRI-2016 and IRI-2012 models show an earlier occurrence of diurnal maximum total electron content, as compared to the observed diurnal maximum GPS total electron content, throughout all the months during the complete period of observation. As the solar activity decreases (from 2015 to 2018), the model starts underestimating GPS total electron content, which becomes significantly high during the very low solar activity period of 2017-18 for all the months. IRI-2016 model underestimates the GPS total electron content before the hours of diurnal maximum and overestimates after the hours of diurnal maximum in the years from 2013-2018. IRI-2012 model underestimates the GPS total electron content before the hours of diurnal maximum and overestimates after the hours of diurnal maximum in the years from 2013-17 but overestimate during the whole day in the year of 2018. Overestimation by IRI-2012 is much more than that by IRI-2016 in the year of 2018. Predictions given by IRI-2016 are better than that given by IRI-2012 for our region. The seasonal mean maximum total electron content values are highest during the spring equinox months and lowest during the winter months except the year of 2014 and 2013. The correlation analysis, between the GPS total electron content and solar indices, show that the correlation coefficient is higher for the solar EUV flux, as compared to the sunspot number (SSN) and 10.7 cm solar radio flux (F10.7).

First Type III Solar Radio Bursts of Solar Cycle 25

The minimum of the previous solar cycle, Solar Cycle 24, occurred in December 2019, which also marked the start of the new Solar Cycle 25. The first radio bursts of the new solar cycle were observed in the spring season 2020. In this work we will present three type III solar bursts which were observed in May and June 2020 at radio frequencies between 18 - 90 MHz. There are two radio observatories in Finland that are capable of doing low-frequency solar radio observations: Aalto University Metshovi Radio Observatory (MRO) and Kilpisjrvi Atmospheric Imaging Receiver Array (KAIRA) of the Sodankyl Geophysical Observatory, University of Oulu. The instruments of the two institutes have different design and characteristics, and they operate in rather different radio interference environments. We will compare simultaneous observations from these two instruments and we will also discuss the properties of these type III solar bursts.

An investigation of flare emissions at multiple wavelengths

We report multi-wavelength observations of four solar flares on 2014 July 07. We firstly select these flares according to the soft X-ray (SXR) and extreme ultraviolet (EUV) emissions recorded by the Extreme Ultraviolet Variability Experiment and Geostationary Orbiting Environmental Satellites. Then their locations and geometries are identified from the full-disk images measured by the Atmospheric Imaging Assembly (AIA), and the time delays among the light curves in different channels are identified. The electron number densities are estimated using the differential emission measure method. We find that three of four flares show strong emissions in SXR channels and high temperature (>6 MK) EUV wavelengths during the impulsive phase, i.e., AIA 131 and 94 , and then they emit peak radiation subsequently in the middle temperature (0.6-3 MK) EUV channels. Moreover, they last for a long time and have smaller electron densities, which are probably driven by the interaction of hot diffuse flare loops. Only one flare emits radiation at almost the same time in all the observed wavelengths, lasts for a relatively short time, and has a larger electron density. It is also accompanied by a type III radio burst. The bright emission at the EUV channel could be corresponding to the associated erupting filament.

Temporal variation of solar flare index during solar cycles 21 - 24

The present investigation attempts to quantify the temporal variation of Solar Flare Index (SFI) with other activity indices during solar cycles 21 - 24 by using different techniques such as linear regression, correlation, cross-correlation with phase lag-lead, etc. Different Solar Activity Indices (SAI) considered in this present study are Sunspot Number (SSN), 10.7 cm Solar Radio Flux (F10.7), Coronal Index (CI) and MgII Core-to-Wing Ratio (MgII). The maximum cycle amplitude of SFI and considered SAI has a decreasing trend from solar cycle 22, and cycle 24 is the weakest solar cycle among all other cycles. The SFI with SSN, F10.7, CI and MgII shows hysteresis during all cycles except for solar cycle 22 where both paths for ascending and descending phases are intercepting each other, thereby representing a phase reversal. A positive hysteresis circulation exists between SFI and considered SAI during solar cycles 22 and 23, whereas a negative circulation exists in cycles 21 and 24. SFI has a high positive correlation with coefficient values of 0.92, 0.94, 0.84 and 0.81 for SSN, F10.7, CI and MgII respectively. According to cross-correlation analysis, SFI has a phase lag with considered SAI during an odd-number solar cycle (solar cycles 21 and 23) but no phase lag/lead during an even-numbered solar cycle (solar cycles 22 and 24). However, the entire smoothed monthly average SFI data indicate an in-phase relationship with SSN, F10.7 and MgII, and a one-month phase lag with CI. The presence of those above characteristics strongly confirms the outcomes of different research work with various solar indices and the highest correlation exists between SFI and SSN as well as F10.7 which establishes that SFI may be considered as one of the prime activity indices to interpret the characteristics of the Sun's active region as well as for more accurate short-range or long-range forecasting of solar events.

Electron Acceleration during Macroscale Magnetic Reconnection

The first self-consistent simulations of electron acceleration during magnetic reconnection in a macroscale system are presented. Consistent with solar flare observations, the spectra of energetic electrons take the form of power laws that extend more than two decades in energy. The drive mechanism for these nonthermal electrons is Fermi reflection in growing and merging magnetic flux ropes. A strong guide field suppresses the production of nonthermal electrons by weakening the Fermi drive mechanism. For a weak guide field the total energy content of nonthermal electrons dominates that of the hot thermal electrons even though their number density remains small. Our results are benchmarked with the hard x-ray, radio, and extreme ultraviolet observations of the X8.2-class solar flare on September 10, 2017.

Low Radio Frequency Observations from the Moon Enabled by NASA Landed Payload Missions

A new era of exploration of the low radio frequency universe from the Moon will soon be underway with landed payload missions facilitated by NASA's Commercial Lunar Payload Services (CLPS) program. CLPS landers are scheduled to deliver two radio science experiments, Radio wave Observations at the Lunar Surface of the photoElectron Sheath (ROLSES) to the nearside and Lunar Surface Electromagnetics Experiment (LuSEE) to the farside, beginning in 2021. These instruments will be pathfinders for a 10 km diameter interferometric array, Farside Array for Radio Science Investigations of the Dark ages and Exoplanets (FARSIDE), composed of 128 pairs of dipole antennas proposed to be delivered to the lunar surface later in the decade. ROLSES and LuSEE, operating at frequencies from 100 kHz to a few tens of megahertz, will investigate the plasma environment above the lunar surface and measure the fidelity of radio spectra on the surface. Both use electrically short, spiral- tube deployable antennas and radio spectrometers based upon previous flight models. ROLSES will measure the photoelectron sheath density to better understand the charging of the lunar surface via photoionization and impacts from the solar wind, charged dust, and current anthropogenic radio frequency interference. LuSEE will measure the local magnetic field and exo-ionospheric density, interplanetary radio bursts, Jovian and terrestrial natural radio emission, and the galactic synchrotron spectrum. FARSIDE, and its precursor risk-reduction six antenna-node array PRIME, would be the first radio interferometers on the Moon. FARSIDE would break new ground by imaging radio emission from coronal mass ejections (CME) beyond 2R, monitor auroral radiation from the B-fields of Uranus and Neptune (not observed since Voyager), and detect radio emission from stellar CMEs and the magnetic fields of nearby potentially habitable exoplanets.

Study of the truncation strategy in the FPGA of a solar radio digital receiver

Computation resource is the limiting factor in higher operational accuracy of field-programmable gate arrays (FPGAs) in solar radio digital receivers. The data truncation strategy which determines the accuracy of data is then the essential technology in the design of a receiving system. Based on the solar radio spectrometer (dual channel, 14 bit, 1.25 gigasamples per second) at the Chashan Solar Radio Observatory (CSO), this paper presents a data truncation strategy which can realize real-time solar radio observation (35-40 GHz) with high time and frequency resolution as well as a large dynamic range, and at the same time saves the computation resource to a large extent. Simulations of truncations during signal processing are carried out in MATLAB, and the best truncation mechanisms are deduced for windowing and fast Fourier transform (FFT). Using the simulation results, the best truncation strategies have been implemented in the solar radio receiver at CSO with the result that the best truncation bits for the windowing operation are [27 : 14], with an error of 2.5 10^-4, and the best truncation bits for the FFT output are [20 : 5] with an error of 1.5 10^-3. Compared with the processing of full-precision data, occupation of the computation resources in the FPGA can be reduced significantly. For instance, the lookup table, lookup table RAM, flip flop, and digital signal processing slices are reduced by 7.36%, 14.65%, 8.38%, and 24.94%, respectively, which guarantees broad-band real-time solar radio observations (35-40 GHz).

How modernized and strengthened GPS signals enhance the system performance during solar radio bursts

A Solar Radio Burst (SRB) is one of the most severe natural hazards affecting the performance of the global navigation satellite systems (GNSS). Considering the influence of different threat factors, the GNSS developers upgrade the systems to amend the accuracy and noise-proof features of the systems. In particular, GPS gradually replaces "old" satellites (GPS IIA, GPS IIR-A, GPS IIR-B) with new-generation equipment (GPS IIR-M, GPS IIF, GPS III) featured by an increase in the emitted signal power at L2 frequency and by new civilian codes. In this work, based on examples of the extreme SRB of September 24, 2011, and the severe SRB of September 6, 2017, we study how such modernization can improve the GPS system performance during solar flares accompanied by intense SRB. We recorded SRB-related drops in signal strength (S), which were 7.5/0 dB-Hz for the S1C, 10/7 dB-Hz for the S2X, 17/8 dB-Hz for the S2W and 9/7.5 for the S5/S5X in 2011/2017 correspondingly. The drop in the S2W signal strength for the modernized blocks was comparable in amplitude to those of the "old" blocks. However, the modernized IIR-M/IIF blocks were featured by about 5 dB-Hz higher signal strength. This resulted in a double and triple decrease in loss-of-lock density for the IIR-M/IIF satellites in 2011 and 2017, respectively, as compared to IIA/IIR-A during SRBs. Therefore, the increase in the emitted signal power and new civilian codes potentially enhance the stability of the GPS operation.

Solar Radio Burst Effects on Radio- and Radar-Based Systems

Radio emission from solar flares can attain such high flux density that the Sun becomes the dominant source of broadband radio noise in the terrestrial environment. The effects of this radio noise on wireless communication and navigation systems can take many forms, depending on the design and operation of the affected system. These effects can be of special concern for regional or global systems, since the effects can occur simultaneously over the entire sunlit hemisphere of Earth. This chapter reviews the origin of solar radio bursts, the threat they pose based on statistics of the flux-density distribution of such events vs. frequency, and some of the effects that have been documented in the literature. The chapter concludes with a discussion of the potential impacts on current and future technology and how these impacts can be mitigated (1) through improved radio monitoring of the Sun in both circular polarizations to supply meaningful real-time warnings, (2) through improved scientific understanding of the solar phenomena underlying the radio bursts, and (3) through improved system design that takes account of solar radio noise.

Possible Evidence of a Termination Shock

Solar flare termination shocks have been suggested as one of the viable mechanisms for accelerating electrons and ions to high energies. Observational evidence of such shocks, however, remains rare. Using radio dynamic spectroscopic imaging of a long-duration C1.9 flare obtained by the Karl G. Jansky Very Large Array (VLA), Chen et al. suggested that a type of coherent radio bursts, referred to as "stochastic spike bursts," were radio signatures of nonthermal electrons interacting with myriad density fluctuations at the front of a flare termination shock. Here we report another stochastic spike burst event recorded during the extended energy release phase of a long-duration M8.4-class eruptive flare on 2012 March 10. VLA radio spectroscopic imaging of the spikes in 1.0-1.6 GHz shows that, similar to the case of Chen et al., the burst centroids form an extended, 10-long structure in the corona. By combining extreme-ultraviolet imaging observations of the flare from two vantage points with hard X-ray and ultraviolet observations of the flare ribbon brightenings, we reconstruct the flare arcade in three dimensions. The results show that the spike source is located at 60 Mm above the flare arcade, where a diffuse supra-arcade fan and multitudes of plasma downflows are present. Although the flare arcade and ribbons seen during the impulsive phase do not allow us to clearly understand how the observed spike source location is connected to the flare geometry, the cooling flare arcade observed 2 hr later suggests that the spikes are located in the above-the-loop-top region, where a termination shock presumably forms.

Quasi-periodic Particle Acceleration in a Solar Flare

A common feature of electromagnetic emission from solar flares is the presence of intensity pulsations that vary as a function of time. Known as quasi-periodic pulsations (QPPs), these variations in flux appear to include periodic components and characteristic timescales. Here, we analyze a GOES M3.7 class flare exhibiting pronounced QPPs across a broad band of wavelengths using imaging and time series analysis. We identify QPPs in the time series of X-ray, low-frequency radio, and extreme ultraviolet (EUV) wavelengths using wavelet analysis, and localize the region of the flare site from which the QPPs originate via X-ray and EUV imaging. It was found that the pulsations within the 171 , 1600 , soft X-ray, and hard X-ray light curves yielded similar periods of ${122}_{-22}^{+26}$ s, ${131}_{-27}^{+36}$ s, ${123}_{-26}^{+11}$ s, and ${137}_{-56}^{+49}$ s, respectively, indicating a common progenitor. The low-frequency radio emission at 2.5 MHz contained a longer period of 231 s. Imaging analysis indicates that the location of the X-ray and EUV pulsations originates from a hard X-ray footpoint linked to a system of nearby open magnetic field lines. Our results suggest that intermittent particle acceleration, likely due to "bursty" magnetic reconnection, is responsible for the QPPs. The precipitating electrons accelerated toward the chromosphere produce the X-ray and EUV pulsations, while the escaping electrons result in low-frequency radio pulses in the form of type III radio bursts. The modulation of the reconnection process, resulting in episodic particle acceleration, explains the presence of these QPPs across the entire spatial range of flaring emission.

Narrowband Spikes Observed during the 2013 November 7 Flare

Narrowband spikes have been observed in solar flares for several decades. However, their exact origin is still discussed. To contribute to understanding of these spikes, we analyze the narrowband spikes observed in the 800-2000 MHz range during the impulsive phase of the 2013 November 7 flare. In the radio spectrum, the spikes started with typical broadband clouds of spikes, and then their distribution in frequencies changed into unique, very narrow bands having noninteger frequency ratios. We successfully fitted frequencies of these narrow spike bands by those, calculating dispersion branches and growth rates of the Bernstein modes. For comparison, we also analyzed the model where the narrow bands of spikes are generated at the upper-hybrid frequencies. Using both models, we estimated the plasma density and magnetic field in spike sources. Then, the models are discussed, and arguments in favor of the model with the Bernstein modes are presented. Analyzing frequency profiles of this spike event by the Fourier method, we found the power-law spectra with the power-law indices varying in the -0.8 to -2.75 interval. Because at some times this power-law index was close to the Kolmogorov spectral index (-5/3), we propose that the spikes are generated through the Bernstein modes in turbulent plasma reconnection outflows or directly in the turbulent magnetic reconnection of solar flares.

comparison based on CTIPe model simulations and satellite measurements

The ionospheric total electron content (TEC) provided by the International GNSS Service (IGS) and the TEC simulated by the Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) model have been used to investigate the delayed ionospheric response against solar flux and its trend during the years 2011 to 2013. The analysis of the distinct low-latitude and midlatitude TEC response over 15 E shows a better correlation of observed TEC and the solar radio flux index F10.7 in the Southern Hemisphere compared to the Northern Hemisphere. Thus, a significant hemispheric asymmetry is observed.

The ionospheric delay estimated using model-simulated TEC is in good agreement with the delay estimated for observed TEC against the flux measured by the Solar Dynamics Observatory (SDO) extreme ultraviolet (EUV) Variability Experiment (EVE). The average delay for the observed (modeled) TEC is 17(16) h. The average delay calculated for observed and modeled TEC is 1 and 2 h longer in the Southern Hemisphere compared to the Northern Hemisphere.

Furthermore, the observed TEC is compared with the modeled TEC simulated using the SOLAR2000 and EUVAC flux models within CTIPe over northern and southern hemispheric grid points. The analysis suggests that TEC simulated using the SOLAR2000 flux model overestimates the observed TEC, which is not the case when using the EUVAC flux model.

LOFAR imaging of the solar corona during the 2015 March 20 solar eclipse

The solar corona is a highly-structured plasma which can reach temperatures of more than 2 MK. At low frequencies (decimetric and metric wavelengths), scattering and refraction of electromagnetic waves are thought to considerably increase the imaged radio source sizes (up to a few arcminutes). However, exactly how source size relates to scattering due to turbulence is still subject to investigation. The theoretical predictions relating source broadening to propagation effects have not been fully confirmed by observations due to the rarity of high spatial resolution observations of the solar corona at low frequencies. Here, the LOw Frequency ARray (LOFAR) was used to observe the solar corona at 120180 MHz using baselines of up to 3.5 km (corresponding to a resolution of 12') during the partial solar eclipse of 2015 March 20. A lunar de-occultation technique was used to achieve higher spatial resolution (0.6') than that attainable via standard interferometric imaging (2.4'). This provides a means of studying the contribution of scattering to apparent source size broadening. It was found that the de-occultation technique reveals a more structured quiet corona that is not resolved from standard imaging, implying scattering may be overestimated in this region when using standard imaging techniques. However, an active region source was measured to be 4' using both de-occultation and standard imaging. This may be explained by the increased scattering of radio waves by turbulent density fluctuations in active regions, which is more severe than in the quiet Sun.

The Potential of Near Real Time Solar Observation at 1.4 GHz

Soil Moisture and Ocean Salinity (SMOS) is an ESA mission observing Earth at 1.4 GHz with full polarimetry. SMOS images are affected by a noise of solar origin produced by the Sun appearing in the antenna's field of view. In this paper, we study whether this solar signal is of any use for scientific and space weather observations. We analyze the response of the SMOS Sun brightness temperature (B_T) to thermal and nonthermal solar emissions, and compare them with observations from ground radio telescopes, GOES X ray flares, and CMEs from SOHO/LASCO. We find that the SMOS Sun B_T can detect weak variations in the solar emission such as the progress of the 11 year activity cycle, the solar rotation, and the thermal emission from flares. Solar radio bursts detected by the SMOS Sun B_T are generally observed during flares from the visible hemisphere of the Sun that are associated with a CME. We also find a correlation between the amount of solar flux released at 1.4 GHz and the speed, angular width, and kinetic energy of the CMEs. We conclude that the unique capability of the SMOS mission to perform 24 h near real time observation of the Sun with full polarimetry makes it a promising instrument for monitoring solar interferences affecting GNSS, radar, and L band wireless communications, as well as for early assessment of flares geoeffectiveness. Nevertheless, the current limitations of the solar data as byproduct of the SMOS data reduction pipeline make it necessary to create a dedicated product for solar observations.

Mid-term Periodicities in Solar Radio Emission Corresponding to Sunspot Number During Solar Cycle 23

We present a systematic time-series analysis of solar radio emission in nine different frequencies to compare with that of daily sunspot number (SSN) during Solar Cycle 23 (1996-2009). Owing to the contribution from quiet-sun emission, the total solar fluxes in microwaves do not decrease as significantly as the sunspot number does during 2006 to 2009. Lomb- Scargle (LS) and wavelet analysis techniques are employed to infer the various periodicities present in the time-series data. False alarm probability (FAP) levels are estimated by the use of background mean power spectrum in the global wavelet spectrum. The LS periodogram contains resolved period peaks, some of which are below FAP levels, for example a well-known rotational period. These peaks are assessed with global significance levels of the wavelet analysis. In all the data sets, the period for solar rotational modulation (26-31 days) is present. The periodogram for the SSN presents Riger type periods (130-180 days), mid-term periods (300-400 days) and long-term periods (430-850 days). These periods in north and south are not similar, especially long term periods are missing in SSN data of the southern hemisphere. Corresponding to the SSN periodicities, Riger and near Riger type of oscillations (130-180 days), quasi-biennial periodicities in the range of 1.2 to 3 years were detected in the time-series data of radio frequencies. Several of these detected periods fall in the range of the periods that are suggested to be connected with magneto-Rossby wave spherical harmonics. Our analysis found reduced power levels in the LS periodograms of low frequencies because of the fact that these low frequency emissions originate higher up in the corona with diminishing contrast to small scale structures.

AIM-E auroral ionosphere model adjustment for the regular E layer

The E-Region Auroral Ionosphere Model (AIM-E) was developed to determine the chemical composition and electron density in the auroral zone at E-layer heights (90150 km). Solar and magnetic activity input parameters for AIM-E are the three-hour Ap index and the daily solar radio flux at a wavelength of 10.7 cm (index F10.7). In this paper, we compare AIM-E calculations of the electron density for the daytime with EUV radiation spectrum specified in two different ways: 1) the EUV spectrum theoretically calculated using the F10.7 index as an input parameter; 2) using TIMED satellite direct measurements of the EUV spectrum. We have corrected the EUVAC EUV radiation model to specify a photoionization source in AIM-E. Calculations of regular E-region critical frequencies show good agreement with the vertical sounding data from Russian high-latitude stations. Results we obtained make it possible to do a quick on-line assessment of the regular E layer, using the daily index F10.7 as an input parameter.

History of Space Weather Research and Operation in Japan and New Emphasis in Southeast Asia

Japanese space weather research and operation began in 1896 with research on radio propagation. LF, MF, and HF radio communication used to be important before the era of communication satellites; however, they are affected by ionospheric conditions and solar activity. A center for the research and development of radio systems was established at Hiraiso, which is now also a space weather observatory for measuring solar activity, geomagnetic fields, and ionospheric conditions. The need for space weather information initially decreased in the communication satellite age, but since the beginning of the 21st century it has been increasing again with the growing use of positioning satellite systems. The role of Southeast Asian countries in space weather research has become important and the range of activities is also growing. The contribution of these countries to space weather research and operation is expected to grow in future.

Galactic and cosmological fast radio bursts as scaled-up solar radio bursts

Fast radio bursts (FRBs) are bright milliseconds radio transients with large dispersion measures. Recently, FRB 200428 was detected in temporal coincidence with a hard X-ray flare from the Galactic magnetar SGR 1935+2154, which supports that at least some FRBs are from magnetar activity. Interestingly, a portion of X-ray flares from magnetar XTE J1810-197 and the Sun are also accompanied by radio bursts. Many features of Galactic FRB 200428 and cosmological FRBs resemble solar radio bursts. However, a common physical origin among FRBs, magnetar radio pulses, and solar radio bursts has not yet been established. Here, we report a universal correlation between X-ray luminosity and radio luminosity over 20 orders of magnitude among solar type III radio bursts, XTE J1810-197 and Galactic FRB 200428. This universal correlation reveals that the energetic electrons that produce the X-ray flares and those that cause radio emissions have a common origin, which can give stringent limits on the generation process of radio bursts. Moreover, we find similar occurrence frequency distributions of energy, duration, and waiting time for solar radio bursts, SGR 1935+2154 and repeating FRB 121102, which also support the tight correlation and the X-ray flares temporally associated with radio bursts. All of these distributions can be understood by avalanche models of self-organized criticality systems. The universal correlation and statistical similarities indicate that the Galactic FRB 200428 and FRBs seen at cosmological distances can be treated as scaled-up solar radio bursts.

The effects of density inhomogeneities on the radio wave emission in electron beam plasmas

Type III radio bursts are radio emissions associated with solar flares. They are considered to be caused by electron beams travelling from the solar corona to the solar wind. Magnetic reconnection is a possible accelerator of electron beams in the course of solar flares since it causes unstable distribution functions and density inhomogeneities (cavities). The properties of radio emission by electron beams in an inhomogeneous environment are still poorly understood. We capture the nonlinear kinetic plasma processes of the generation of beam-related radio emissions in inhomogeneous plasmas by utilizing fully kinetic particle-in-cell code numerical simulations. Our model takes into account initial electron velocity distribution functions (EVDFs) as they are supposed to be created by magnetic reconnection. We focus our analysis on low-density regions with strong magnetic fields. The assumed EVDFs allow two distinct mechanisms of radio wave emissions: plasma emission due to wave-wave interactions and so-called electron cyclotron maser emission (ECME) due to direct wave-particle interactions. We investigate the effects of density inhomogeneities on the conversion of free energy from the electron beams into the energy of electrostatic and electromagnetic waves via plasma emission and ECME, as well as the frequency shift of electron resonances caused by perpendicular gradients in the beam EVDFs. Our most important finding is that the number of harmonics of Langmuir waves increases due to the presence of density inhomogeneities. The additional harmonics of Langmuir waves are generated by a coalescence of beam-generated Langmuir waves and their harmonics.

Response of Mars's Topside Ionosphere to Changing Solar Activity and Comparisons to Venus

Studying how planetary ionospheres respond to changes in solar irradiance can inform our understanding of how planetary atmospheres have reacted to our changing Sun over the course of solar system history. The response of Mars's main ionospheric peak to changing solar irradiance has been well studied. Less well understood are the altitudes above the main peak. We use radio occultation observations by Mars Global Surveyor and ground based solar radio flux measurements to quantify the response of Mars's electron densities to changing solar irradiance at these altitudes. This analysis shows that the electron densities respond modestly at and just above the main peak, with the response increasing above or below the peak. We use a one dimensional photochemical equilibrium model to reproduce the observed electron densities and investigate the influence of minor ions, changing electron temperatures, and changing neutral densities and composition on the resultant electron density. We find that the neutral atmosphere is primarily responsible for the observed behavior. We also make comparisons to Venus, which shows a more dramatic increase in its ionospheric response as a function of altitude. We show that while the ionospheric responses of Mars and Venus as a function of altitude are markedly different, recasting the analysis in terms of number of scale heights above the main peak brings the two planets into closer agreement, especially at the main peak. The agreement is worse at higher altitudes and this is attributed to Venus's larger proportion of atomic oxygen.

On the Relationship of the O(¹D) 630.0 nm Dayglow Emission to the F10.7 cm Solar Flux and the Solar Zenith Angle

The Wind Imaging Interferometer (WINDII) Empirical Model, which provides the characteristics of the O(¹D) 630.0 nm atomic oxygen dayglow emission from the upper atmosphere has been reviewed and updated. It now includes the Integrated Emission Rate, the peak Volume Emission Rate, the Altitude of that peak and the Full Width at Half Maximum as functions of the F10.7 cm Solar Radio Flux and the solar zenith angle (SZA). The model employs 98,617 WINDII observations obtained between the years 1992 and 1996, and the model and observations of the Integrated Emission Rate agree well with one another within 2 standard deviations of 588.7 Rayleigh (R) (10⁶ photons cm² sec¹). It is also demonstrated that the impact of latitude, longitude and day of year, independently of their contribution to the SZA, is very small. The WINDII Empirical Model is also shown to agree with results from the TRANSCAR photochemical model. The dayglow is challenging to measure with ground based instruments, as the solar scattered light from the daytime sky must be accurately subtracted from the data. Ground based measurements of the integrated emission rate have been made by others, with good agreement for observations from Hyderabad during the 2015 summer and winter, but mixed agreement with measurements made over Boston in 2003. The latter results are reviewed and assessed.

Mingantu Spectral Radioheliograph for Solar and Space Weather Studies

The Chinese Spectral Radioheliograph (CSRH) covering 400 MHz-15 GHz frequency range was constructed during 2009-2016 in Mingantu Observing Station, National Astronomical Observatories, Chinese Academy of Sciences at Zhengxiangbaiqi, Inner Mongolia of China. CSRH is renamed as {\it M}ingant{\it U} {\it S}p{\it E}ctral {\it R}adioheliograph (MUSER) after its accomplishment. Currently, MUSER consists of two arrays spreading over 3 spiral-shaped arms of $\sim$3 km maximum baseline in both east-west and north-south directions. MUSER array configuration is optimized to meet the needs of observing the full-disk Sun over ultrawide wavebands with high temporal, spatial and spectral resolution and a high dynamic range of images. MUSER-I covers 400 MHz-2.0 GHz with 40 4.5-m-diameter antennas and MUSER-II covers 2-15 GHz with 60 2-m-diameter antennas. MUSER-I can obtain solar full-disk radio images in 64 frequency channels with 25 ms cadence and 51.6$^{\prime\prime}$ to 10.3$^{\prime\prime}$ spatial resolution corresponding to the frequency range from 400 MHz to 2 GHz, whereas MUSER-II can obtain full-disk images in 520 channels with 206.25 ms cadence and 10.3$^{\prime\prime}$ to 1.3$^{\prime\prime}$ resolution from 2 to 15 GHz. Snapshot image quality with 25 dB dynamic range can be realized. The extension of MUSER to lower frequency band covering 30 MHz - 400 MHz with an array of 224 logarithm-periodic dipole antennas (LPDAs) has been approved and will be constructed during the next 4 years. {MUSER} has the following merits as a solar dedicated instrument: unique high temporal-spatial-spectral resolutions simultaneously over a wide frequency range; innovative high- performance ultrawide-band, dual-polarization feeds for wideband signal collection; advanced high data-rate, large-scale digital correlation receiver for multiple-frequency and fast snapshot observations; and applications of new technologies such as using optical fiber to realize remote antenna and wide-band analog signal transmission. MUSER opens a new window to measure the solar magnetic fields, trace the dynamic evolution of energetic electrons in radio frequencies, which will help us to better understand the origin of solar activities and the basic drivers of space weather.

Radio astronomical tools for the study of solar energetic particles II.Time-extended acceleration at subrelativistic and relativistic energies

Solar energetic particle (SEP) events are commonly separated in two categories: numerous 'impulsive' events of relatively short duration, and a few 'gradual' events, where SEP-intensities may stay enhanced over several days at energies up to several tens of MeV. In some gradual events the SEP spectrum extends to relativistic energies ($> 1$ GeV), over shorter durations. The two categories are strongly related to an idea developed in the 1960s based on radio observations: Type III bursts, which were addressed in a companion chapter, outline impulsive acceleration of electrons to subrelativistic energies, while the large and the relativistic SEP events were ascribed to a second acceleration process. At radio wavelengths, typical counterparts were bursts emitted by electrons accelerated at coronal shock waves (type II bursts) and by electron populations in large-scale closed coronal structures (type IV bursts). Both burst types are related to coronal mass ejections (CMEs). Type II bursts from metric to kilometric wavelengths tend to accompany large SEP events, which is widely considered as a confirmation that CME- driven shocks accelerate the SEPs. But type II bursts, especially those related to SEP events, are most often accompanied by type IV bursts, where the electrons are rather accelerated in the wake of the CME. Individual event studies suggest that although the CME shock is the most plausible accelerator of SEPs up to some yet unknown limiting energy, the relativistic SEP events show time structure that rather points to coronal acceleration related to type IV bursts. This chapter addresses the question what type II bursts tell us about coronal shock waves and how type II and type IV radio bursts are related with relativistic proton signatures as seen by particle detectors on the Earth and by their gamma-ray emission in the solar atmosphere, focussing on two relativistic SEP events, on 2005 Jan 20 and 2017 Sep 10. The importance of radio emissions as a complement to the upcoming SEP observations from close to the Sun is underlined.

Polar and Equatorial Radii Derived from SST and ALMA

At subterahertz frequenciesi.e., millimeter and submillimeter wavelengthsthere is a gap in measurements of the solar radius, as well as other parameters of the solar atmosphere. As the observational wavelength changes, the radius varies because the altitude of the dominant electromagnetic radiation is produced at different heights in the solar atmosphere. Moreover, radius variations throughout long time series are indicative of changes in the solar atmosphere that may be related to the solar cycle. Therefore, the solar radius is an important parameter for the calibration of solar atmospheric models enabling a better understanding of the atmospheric structure. In this work, we use data from the Solar Submillimeter-wave Telescope (SST) and the Atacama Large Millimeter/submillimeter Array (ALMA) at frequencies of 100, 212, 230, and 405 GHz to measure the equatorial and polar radii of the Sun. The radii measured with extensive data from the SST agree with the radius-versus-frequency trend present in the literature. The radii derived from ALMA maps at 230 GHz also agree with the radius-versus- frequency trend, whereas the 100 GHz radii are slightly above the values reported by other authors. In addition, we analyze the equatorial and polar radius behavior over the years by determining the correlation coefficient between solar activity and subterahertz radius time series at 212 and 405 GHz (SST). The variations of the SST-derived radii over 13 yr are correlated to the solar activity when considering equatorial regions of the solar atmosphere and anticorrelated when considering polar regions. The ALMA-derived radius time series for 100 and 230 GHz show very similar behaviors with those of SST.

Multiwavelength Observations of the Formation and Eruption of a Complex Filament

We present an analysis of the formation and eruption of a filament and fast coronal mass ejection associated with a flare that occurred in active region 11429 using observations in the ultraviolet, extreme ultraviolet, X-ray, and radio wavelength bands. Precursor activity began as an interaction between two filaments, F1 and F2, that are identified as having twisted magnetic flux ropes (MFRs). Transient brightenings in all wavelengths are observed as a result of this interaction, likely the result of magnetic reconnection between the two filaments. This interaction results in a reconfiguration of the two filaments into a long overlying filament and a shorter low-lying filament. The upper filament subsequently undergoes a partial confined eruption. Plasma flows originating near the east footpoint of F1 lead to an extension of the upper filament into the filament channel to the west, resulting in a new active region filament (ARF). This new filament begins a slow rise and expansion. During its slowly rising phase, the MFR in which the filament is embedded becomes visible, with both the filament and flux rope rising and expanding simultaneously. The twist of the magnetic rope is determined as four turns. The erupting configuration changes from a twisted arch shape to a reversed shape within 75 s at the beginning of the fast-rise phase, representing a transformation from twist to writhe. The observations provide a clear example of filament formation via the tether-cutting reconnection of two nearby filaments. A helical kink instability may be the trigger of the ARF eruption.

PIC Simulation of Double Plasma Resonance and Zebra Pattern of Solar Radio Bursts

The latest study has reported that plasma emission can be generated by energetic electrons of Dory-Guest-Harris distribution via the electron cyclotron maser instability (ECMI) in plasmas characterized by a large ratio of plasma oscillation frequency to electron gyro-frequency (_pe/_ce). In our study, on the basis of the ECMI- plasma emission mechanism, we examine the double plasma resonance (DPR) effect and the corresponding plasma emission at both harmonic (H) and fundamental (F) bands using particle-in-cell simulations with various _pe/_ce. This allows us to directly simulate the feature of the zebra pattern (ZP) observed in solar radio bursts for the first time. We find that (1) the simulations reproduce the DPR effect nicely for the upper hybrid and Z modes, as seen from their variation of intensity and linear growth rate with _pe/_ce, (2) the intensity of the H emission is stronger than that of the F emission by 2 orders of magnitude and varies periodically with increasing _pe/_ce, while the F emission is too weak to be significant (therefore, we suggest that it is the H emission accounting for solar ZPs), (3) the peak-valley contrast of the total intensity of H is 4, and the peak lies around integer values of _pe/_ce (=10 and 11) for the present parameter setup. We also evaluate the effect of energy of energetic electrons on the characteristics of ECMI-excited waves and plasma radiation. The study provides novel insight on the physical origin of ZPs of solar radio bursts.

Insights on Magnetic Field Dynamics during a Microflare

A solar type-I noise storm is produced by accelerated particle beams generated at active regions undergoing magnetic field restructuring. Their intensity varies by orders of magnitude within subsecond and sub- MHz scales. But the morphological evolution of these sources is not studied at these scales due to the lack of required imaging cadence and fidelity in meterwave bands. Using data from the Murchison Widefield Array, this work explores the coevolution of size, sky-orientation, and intensity of a noise storm source associated with a weak microflare. This work presents the discovery of two correlated modes of evolution in the source parameters: a sausage like "S" mode where the source intensity and size show an anticorrelated evolution; and a torsional like "T" mode where the source size and sky-orientation show a correlated evolution. A flare mediated mode conversion is observed from "T" to "S" for the first time in these sources. These results support the idea of build up of magnetic stress energy in braided active region loops, which later become unstable causing flares and particle acceleration until they relax to a minimally braided state. The discovered mode conversion can be a future diagnostic for such events.

The Source Size, Duration, and Position

The observed features of the radio source indicate that the waves of solar radio bursts are convoluted with complex propagation effects. In this work, we perform ray-tracing simulations on radio wave transport in the corona and interplanetary region with anisotropic electron density fluctuations. For the first time, the variation of the apparent source size, burst duration, and source position for the fundamental emission and harmonic emission at the frequency of 35 MHz are simulated as a function of the anisotropic parameter and the angular scattering rate coefficient = ²/h₀, where ² = n²/n² is the density fluctuation level and h₀ is its correlation length near the wave excitation site. It is found that isotropic fluctuations produce a much larger decay time than a highly anisotropic fluctuation for fundamental emission. By comparing the observed duration and source size with the simulation results in the parameter space, we can estimate the scattering coefficient and the anisotropic parameter = 8.9 10^-5 km^-1 and = 0.719 with a point pulse source assumption. Position offsets due to wave scattering and refraction can produce the co-spatial of the fundamental and harmonic waves in the observation of some type III radio bursts. The visual speed due to the wave propagation effect can reach 1.5c for = 2.4 10^-4 km^-1 and = 0.2 for the fundamental emission in the sky plane, accompanied with large expansion rate of the source size. The direction of the visual speed is mostly identical to the direction of the offset, thus, for the observation aimed at obtaining the source position, the source centroid at the starting time is closer to the wave excitation site.

Harmonic Electron Cyclotron Maser Emission Excited by Energetic Electrons Traveling inside a Coronal Loop

A complete understanding of solar radio bursts requires developing numerical techniques that can connect large-scale activities with kinetic plasma processes. As a starting point, this study presents a numerical scheme combining three different techniques: (1) extrapolation of the magnetic field overlying a specific active region in order to derive the background field, (2) guiding-center simulation of the dynamics of millions of particles within a selected loop to reveal the integral velocity distribution function (VDF) around certain sections of the loop, and (3) particle-in-cell simulation of kinetic instabilities driven by energetic electrons initiated by the obtained distributions. Scattering effects at various levels (weak, moderate, and strong) due to wave turbulence-particle interaction are considered using prescribed timescales of scattering. It was found that the obtained VDFs contain strip-like and loss-cone features with positive gradient, and both features are capable of driving electron cyclotron maser emission, which is a viable radiation mechanism for some solar radio bursts, in particular, solar radio spikes. The strip-like feature is important in driving the harmonic X mode, while the loss-cone feature can be important in driving the fundamental X mode. In the weak-scattering case, the rate of energy conversion from energetic electrons to X2 can reach up to $\sim 2.9\times {10}^{-3}\,{E}_{{k}_{0}}$ , where ${E}_{{k}_{0}}$ is the initial kinetic energy of energetic electrons. The study demonstrates a novel way of exciting the X2 mode in the corona during solar flares and provides new sight into how escaping radiation can be generated within a coronal loop.

LOFAR Observations of a Jet-driven Piston Shock in the Low Solar Corona

The Sun produces highly dynamic and eruptive events that can drive shocks through the corona. These shocks can accelerate electrons, which result in plasma emission in the form of a type II radio burst. Despite the large number of type II radio burst observations, the precise origin of coronal shocks is still subject to investigation. Here, we present a well-observed solar eruptive event that occurred on 2015 October 16, focusing on a jet observed in the extreme ultraviolet by the Atmospheric Imaging Assembly (SDO/AIA), a streamer observed in white light by the Large Angle and Spectrometric Coronagraph (SOHO/LASCO), and a metric type II radio burst observed by the LOw Frequency Array (LOFAR). LOFAR interferometrically imaged the fundamental and harmonic sources of a type II radio burst and revealed that the sources did not appear to be cospatial, as would be expected from the plasma emission mechanism. We correct for the separation between the fundamental and harmonic using a model that accounts for scattering of radio waves by electron density fluctuations in a turbulent plasma. This allows us to show the type II radio sources were located 0.5R above the jet and propagated at a speed of 1000 km s^-1, which was significantly faster than the jet speed of 200 km s^-1. This suggests that the type II burst was generated by a piston shock driven by the jet in the low corona.

Multi-wavelength analysis of CME-driven shock and Type II solar radio burst band-splitting

It is now well established that Coronal Mass Ejections (CMEs) may produce shocks in near Sun and interplanetary medium. A Type-II radio burst is characterized by shock and associated emission with very slow frequency drift rate (0.1 MHz/sec). A CME driven shock and their velocity, acceleration/deceleration signature can be observed by a Type- II solar radio burst which drifts with the shock speed and is split in bands of plasma radio emission. These emissions can be seen in radio spectrographs as split bands both in fundamental and harmonic frequencies close to a ratio of 1:2. In this paper, we present an analysis of a CME associated with a band splitting Type-II radio burst observed using multi-instruments in multi-wavelengths. We observed the CME event that occurred in 02 May 2013 (05:24 UT) and is associated with an M1.1 class solar flare from the active region NOAA 11731 located at N10W23 on the solar disk. We use the widely accepted Newkirk coronal density model to estimate the height in the solar atmosphere to compare our results. We conclude that the speed of CME is high enough to produce a Type-II solar radio burst. The analysis of this paper also involved an estimation of the coronal ambient magnetic field and its comparison with the empirical active region magnetic field model (Dulk and McLean in Sol. Phys. 57:279, 1978). This shows the good results. Observations provide sufficient evidence that the unusual patch signature in Type-II solar radio burst is due to the CMECME interaction.

Short-term periodicities in the downward longwave radiation and their associations with cosmic ray and solar interplanetary data

In this study downward longwave (LW) atmospheric radiation data for the period of 2014-2020 were used to search for short-term periodicities using fast Fourier transform (FFT). Several local peaks in the power spectrum density were found and established. The time series exhibits a series of significant peaks (exceeding the 95% confidence limit), such as at 273 days, 227 days, 200 days, 178 days, 157 days, 110 days, 120 days, 87 days, 73 days, 53-56 days, 35-30 days, 25-27 days, 21 days, 13 days, and 9-10 days.

Moreover, cosmic ray data from KACST muon detector and the Oulu neutron monitor, as well as the data for the solar radio flux at 10.7 cm (F10.7 cm), Dst index, and solar wind speed for the same period as the LW data, were used to look for common cyclic variations and periodicities matching those found in the LW radiation. This was done to investigate the possible effect of the solar activity parameters on LW radiation. Several common periodicities were observed in the spectra of all the variables considered, such as 227 days, 154-157 days, 25-27 days, and 21 days. Some of the periodicities found in the LW radiation spectrum can be attributed to the modulation of the cosmic ray intensity by solar activity. Others are attributed to the disturbances in the interplanetary magnetic field. Based on the spectral results, we suggest that the solar signals may directly or indirectly affect the variations of the downward longwave radiation, which in turn may affect climate change.

Moving solar radio bursts and their association with coronal mass ejections

Context. Solar eruptions, such as coronal mass ejections (CMEs), are often accompanied by accelerated electrons that can in turn emit radiation at radio wavelengths. This radiation is observed as solar radio bursts. The main types of bursts associated with CMEs are type II and type IV bursts that can sometimes show movement in the direction of the CME expansion, either radially or laterally. However, the propagation of radio bursts with respect to CMEs has only been studied for individual events.
Aims: Here, we perform a statistical study of 64 moving bursts with the aim to determine how often CMEs are accompanied by moving radio bursts. This is done in order to ascertain the usefulness of using radio images in estimating the early CME expansion.
Methods: Using radio imaging from the Nanay Radioheliograph (NRH), we constructed a list of moving radio bursts, defined as bursts that move across the plane of sky at a single frequency. We define their association with CMEs and the properties of associated CMEs using white-light coronagraph observations. We also determine their connection to classical type II and type IV radio burst categorisation.
Results: We find that just over a quarter of type II and half of type IV bursts that occurred during the NRH observing windows in Solar Cycle 24 are accompanied by moving radio emission. All but one of the moving radio bursts are associated with white-light CMEs and the majority of moving bursts (90%) are associated with wide CMEs (> 60 in width). In particular, all but one of the moving bursts corresponding to type IIs are associated with wide CMEs; however, and unexpectedly, the majority of type II moving bursts are associated with slow white-light CMEs (< 500 km s¹). On the other hand, the majority of moving type IV bursts are associated with fast CMEs (> 500 km s¹).
Conclusions: The observations presented here show that moving radio sources are almost exclusively associated with CMEs. The majority of events are also associated with wide CMEs, indicating that strong lateral expansion during the early stages of the eruption may play a key role in the occurrence of the radio emission observed.

Spectroscopic observations of a flare-related coronal jet

Context. Coronal jets are ubiquitous in active regions and coronal holes.
Aims: In this paper, we study a coronal jet related to a C3.4 circular-ribbon flare in the active region 12434 on 2015 October 16.
Methods: The flare and jet were observed in ultraviolet and extreme ultraviolet wavelengths by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO). The line-of-sight magnetograms of the photosphere were observed by the Helioseismic and Magnetic Imager on board SDO. The whole event was covered by the Interface Region Imaging Spectrograph during its imaging and spectroscopic observations. Soft X-ray fluxes of the flare were recorded by the GOES spacecraft. Hard X-ray (HXR) fluxes at 450 keV were obtained from observations of RHESSI and Fermi. Radio dynamic spectra of the flare were recorded by the ground-based stations belonging to the e-Callisto network.
Results: Two minifilaments were located under a 3D fan-spine structure before flare. The flare was generated by the eruption of one filament. The kinetic evolution of the jet was divided into two phases: a slow rise phase at a speed of 131 km s¹ and a fast rise phase at a speed of 363 km s¹ in the plane- of-sky. The slow rise phase may correspond to the impulsive reconnection at the breakout current sheet. The fast rise phase may correspond to magnetic reconnection at the flare current sheet. The transition between the two phases occurred at 09:00:40 UT. The blueshifted Doppler velocities of the jet in the Si IV 1402.80 line range from 34 to 120 km s¹. The accelerated high-energy electrons are composed of three groups. Those propagating upward along the open field generate type III radio bursts, while those propagating downward produce HXR emissions and drive chromospheric condensation observed in the Si IV line. The electrons trapped in the rising filament generate a microwave burst lasting for 40 s. Bidirectional outflows at the base of jet are manifested by significant line broadenings of the Si IV line. The blueshifted Doppler velocities of outflows range from 13 to 101 km s¹. The redshifted Doppler velocities of outflows range from 17 to 170 km s¹.
Conclusions: Our multiwavelength observations of the flare-related jet are in favor of the breakout jet model and are important for understanding the acceleration and transport of nonthermal electrons.

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3D Quasilinear Effects

A conventional model for the generation of Langmuir waves in Type-III radio bursts is based on a one-dimensional (1D) version of the quasilinear equations. In this model a wave with phase velocity v resonates with an electron with velocity v =v, causing the waves to grow at a rate d F (v )/d v >0 , where F (v ) is the 1D-distribution function. The backreaction on the electrons drives the electrons towards a plateau distribution: d F (v )/d v 0 . In the 3D-generalization, none of these features apply: waves with phase speed v can resonate with electrons with speed v , depending on the angle between the wave normal and the electron velocity, wave growth occurs only if the distribution function is both an increasing function of v and also has an anisotropic pitch-angle distribution, and the backreaction involves diffusion in both speed v and in pitch-angle . In this article we discuss implications of the generalization from 1D to 3D on models for Type-III bursts. An effect that is absent in 1D, but may be important in 3D, is scattering of Langmuir waves by turbulence in the ambient plasma. Pitch-angle scattering by the scattered Langmuir waves may play an important role in the evolution of the Type-III beam.

An Observational Revisit of Stationary Type IV Solar Radio Bursts

Stationary type IV solar radio bursts (IVSs) are broadband continuum emission observed at decimetric-decametric wavelength without apparent source motions. They are closely associated with solar flares and/or coronal mass ejections. Earlier studies on IVSs suffered from limited number of events, frequency coverage and available channels, and spatial resolution. Here we present an analysis on 34 IVSs using two-dimensional imaging data provided by Nanay Radioheliograh (NRH) at 10 frequencies from 150 to 445 MHz. The events are recorded from 2010 to 2014. We focus on general properties including the spatial dispersion of sources with frequency, brightness temperature (T_B) and corresponding spectra, and polarization. Main findings are: (i) In the majority of events (23/34) regular and systematic source dispersion with frequency can be clearly recognized. (ii) In most (31/34) events the maximum brightness temperature (T_BM^E) exceeds 10⁸K, and exceeds 10⁹K in 23 events. The histogram distribution of T_BM^f, i.e. the maximum brightness temperature of a source at certain frequency (f ) of a specific event (referred to as event-f source, there are 247 such sources in total) exhibits a clear declining trend with increasing frequencies. The dominant type of T_B spectra is power-law like with a negative index. (iii) In most events (30/34) the sense of polarization remains unchanged and the number of events with right and left-handed polarization are comparable. In 57% of all 247 event-f sources the level of polarization does not change considerably, in about 39% sources the level of polarization exhibits significant variation yet with a fixed sense, and in only 4% the sense of polarization changes. These results provide strong constraints on radiation mechanism of IVSs.

Intense L-Band Solar Radio Bursts Detection Based on GNSS Carrier-To-Noise Ratio Decrease over Multi-Satellite and Multi-Station

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origins and radio-wave propagation effects

Solar eruptive events are associated with radio emissions that appear as impulsive increases in intensity, known as solar radio bursts. Turbulence in the solar corona impacts the propagation of radio waves, obscuring the intrinsic emission properties. Here, anisotropic scattering on small-scale density fluctuations is investigated using novel 3D radio-wave propagation simulations. Several observed radio properties are simultaneously reproduced for the first time, verifying the necessity to consider anisotropic scattering. The sub-second evolution of fine radio burst properties at a single frequency is also investigated, enabled by conducting observations that utilise the unprecedented imaging capabilities of the LOw-Frequency ARray (LOFAR). The fundamental and harmonic sources of a Type IIIb burst are quantitatively compared, demonstrating that harmonic emissions arise from an intrinsic source with a finite size and finite emission duration. Drift-pair burst observations are successfully described by the radio echo hypothesis. It is shown that the radio echo, which produces the second Drift-pair component, is detected only when the anisotropy is strong. A dependence of the observed properties on the source's intrinsic location and on the assumed emission-to-plasma frequency ratio is inferred. Moreover, the subbands of a split-band Type II burst are simultaneously imaged for the first time. Despite the large separations observed between subband sources, it is shown that once scattering is quantitatively accounted for, the sources become co- spatial. Corrections on the observed source locations also allude to lower coronal densities. Additionally, the first observation of a Type II burst that transitions between a stationary and drifting statetermed as a transitioning Type II burstis reported. The radio emissions are related to a jet eruption that drives a streamer-puff CME. Overall, state-of-the-art simulations and radio observations are combined and compared. The importance of accounting for radio-wave propagation effectsprimarily anisotropic scatteringand the consequence of neglecting to do so on any subsequent interpretations is illustrated.

The Delayed Ionospheric Response to the 27 day Solar Rotation Period Analyzed With GOLD and IGS TEC Data

The delayed ionospheric response is analyzed for two well defined 27 day solar rotation periods in the year 2019 with solar radio flux index F10.7 and Global scale Observations of the Limb and Disk (GOLD) data, like solar extreme ultraviolet (EUV) flux proxy, O/N₂ column density ratio and peak electron density, as well as International Global Navigation Satellite System Service rapid high rate total electron content (TEC) map data. Although the correlation between GOLD solar EUV flux proxy and TEC is similar to the correlation between F10.7 and TEC, it is shown that the estimated delays based on GOLD data are in much better agreement with recent studies using EUV measurements compared to the delays based on F10.7 data. The GOLD peak electron density correlates well with TEC and allows insight to a local time interval when the ionosphere is not controlled by solar activity changes (17:00 LT to 21:00 LT). The present study investigates the impact of the solar activity (F10.7, GOLD EUV flux proxy) and O/N₂ column density ratio on the ionospheric delay for two representative solar rotation periods. The capabilities of GOLD data for future research on the ionospheric response to the 27 day solar rotation period are demonstrated and discussed. These results are crucial information for precise ionospheric models and forecasts.

The Impact of Solar Flux Proxies and Indices

We present a new high resolution empirical model for the ionospheric total electron content (TEC). TEC data are obtained from the global navigation satellite system (GNSS) receivers with a 1 1 spatial resolution and 5 min temporal resolution. The linear regression model is developed at 45N, 0E for the years 2000-2019 with 30 min temporal resolution, unprecedented for typical empirical ionospheric models. The model describes dependency of TEC on solar flux, season, geomagnetic activity, and local time. Parameters describing solar and geomagnetic activity are evaluated. In particular, several options for solar flux input to the model are compared, including the 10.7 cm solar radio flux (F_10.7), the Mg II core to wing ratio, and formulations of the solar extreme ultraviolet flux (EUV). Ultimately, the extreme ultraviolet flux presented by the Flare Irradiance Spectral Model, integrated from 0.05 to 105.05 nm, best represents the solar flux input to the model. TEC time delays to this solar parameter on the order of several days as well as seasonal modulation of the solar flux terms are included. The Ap₃ index and its history are used to reflect the influence of geomagnetic activity. The root mean squared error of the model (relative to the mean TEC observed in the 30 min window) is 1.9539 TECu. A validation of this model for the first 3 months of 2020 shows excellent agreement with data. The new model shows significant improvement over the International Reference Ionosphere 2016 (IRI 2016) when the two are compared during 2008 and 2012.

Radio astronomical tools for the study of solar energetic particles I. Correlations and diagnostics of impulsive acceleration and particle propagation

Solar energetic particles (SEPs) are sporadically ejected from the Sun during flares and coronal mass ejections. They are of major astrophysical interest, because the proximity of the Sun allows for detailed multi-messenger studies. They affect space weather due to interactions with electronics, with the Earth's atmosphere, and with humans if they leave the protective shield of the magnetosphere of the Earth. Since early studies in the 1950s, starting with particle detectors on the ground, SEP events have been related to radio bursts.Two subjects are addressed in this chapter: attempts to establish quantitative correlations between SEPs and microwave bursts produced by gyro synchrotron radiation of mildly relativistic electrons, and the information derived from type III radio bursts on impulsive processes of particle acceleration and the coronal and interplanetary propagation. Type III radio bursts produced by electron beams on open magnetic field lines have a wide range of applications, including the identification of acceleration regions, the identification of confined particle acceleration with coronal signatures, but no SEPs, and the paths that the electrons, and energetic charged particles in general, take to travel from the low corona to the Heliosphere in case they escape. Simple scenarios of coronal particle acceleration are confirmed in relatively simple and short events. But the comparison with particle transport models shows that longer and delayed acceleration episodes exist especially in large SEP events. They will be discussed in a companion chapter.

First Results from Joint Microwave and Hard X-Ray Imaging Spectroscopy

Nonthermal sources located above bright flare arcades, referred to as the "above-the-loop-top" sources, have been often suggested as the primary electron acceleration site in major solar flares. The X8.2 limb flare on 2017 September 10 features such an above-the-loop-top source, which was observed in both microwaves and hard X-rays (HXRs) by the Expanded Owens Valley Solar Array and the Reuven Ramaty High Energy Solar Spectroscopic Imager, respectively. By combining the microwave and HXR imaging spectroscopy observations with multifilter extreme ultraviolet and soft X-ray imaging data, we derive the coronal magnetic field and energetic electron distribution of the source over a broad energy range from <10 keV up to MeV during the early impulsive phase of the flare. The source has a strong magnetic field of over 800 G. The best-fit electron distribution consists of a thermal "core" from 25 MK plasma. A nonthermal power-law "tail" joins the thermal core at 16 keV with a spectral index of 3.6, which breaks down at above 160 keV to >6.0. Temporally resolved analysis suggests that the electron distribution above the break energy rapidly hardens with the spectral index decreasing from >20 to 6.0 within 20 s, or less than 10 Alfvn crossing times in the source. These results provide strong support for the above-the-loop-top source as the primary site where an ongoing bulk acceleration of energetic electrons is taking place very early in the flare energy release.

Diagnosing a Solar Flaring Core with Bidirectional Quasi-periodic Fast Propagating Magnetoacoustic Waves

Quasi-periodic fast propagating (QFP) waves are often excited by solar flares, and could be trapped in the coronal structure with low Alfvn speed, so they could be used as a tool for diagnosing both the flaring core and magnetic waveguide. As the periodicity of a QFP wave could originate from a periodic source or be dispersively waveguided, it is a key parameter for diagnosing the flaring core and waveguide. In this paper, we study two QFP waves excited by a Geostationary Operational Environmental Satellite-class C1.3 solar flare occurring at active region NOAA 12734 on 2019 March 8. Two QFP waves were guided by two oppositely oriented coronal funnels. The periods of two QFP waves were identical and were roughly equal to the period of the oscillatory signal in the X-ray and 17 GHz radio emission released by the flaring core. It is very likely that the two QFP waves could be periodically excited by the flaring core. Many features of this QFP wave event are consistent with the magnetic tuning fork model. We also investigated the seismological application with QFP waves, and found that the magnetic field inferred with magnetohydrodynamic seismology was consistent with that obtained in the magnetic extrapolation model. Our study suggests that the QFP wave is a good tool for diagnosing both the flaring core and the magnetic waveguide.

Harmonic Radio Emission in Randomly Inhomogeneous Plasma

In the present paper, we describe a theoretical model of the generation of harmonic emissions of type III solar radio bursts. The goal of our study is to fully take into account the most efficient physical processes involved in the generation of harmonic electromagnetic emission via nonlinear coupling of Langmuir waves in randomly inhomogeneous plasma of solar wind ( $l+{l}^{{\prime} }\to t$ ). We revisit the conventional mechanism of coalescence of primarily generated and back-scattered Langmuir waves in quasihomogeneous plasma. Additionally, we propose and investigate another mechanism that generates harmonic emission only in a strongly inhomogeneous plasma: the nonlinear coupling of incident and reflected Langmuir waves inside localized regions with enhanced plasma density (clumps), in the close vicinity of the reflection point. Both mechanisms imply the presence of strong density fluctuations in plasma. We use the results of a probabilistic model of beam-plasma interaction and evaluate the efficiency of energy transfer from Langmuir waves to harmonic emission. We infer that harmonic emissions from a quasihomogeneous plasma are significantly more intense than found in previous studies. The efficiency of Langmuir wave conversion into electromagnetic harmonic emission is expected to be higher at large heliospheric distances for the mechanism operating in quasihomogeneous plasma and at small heliocentric distances for the one operating in inhomogeneous plasma. The evaluation of emission intensity in quasihomogeneous plasma may also be applied for type II solar radio bursts. The radiation pattern in both cases is quadrupolar, and we show that emission from density clumps may efficiently contribute to the visibility of harmonic radio emission.

A statistical study of solar radio Type III bursts and space weather implication

Solar radio bursts (SRBs) are the signatures of various phenomenon that happen in the solar corona and interplanetary medium (IPM). In this article, we have studied occurrence of Type III bursts and their association with the Sunspot number. This study confirms that occurrence of Type III bursts correlate well with Sunspot number. Further, using the data obtained using e-CALLISTO network, we have investigated drift rates of isolated Type III bursts and duration of the group of Type III bursts. Since Type II, Type III and Type IV bursts are signatures of solar flares and/or CMEs, we can use the radio observations to predict space weather hazards. In this article, we have discussed two events that have caused near Earth radio blackouts. Since e-CALLISTO comprises more than 152 stations at different longitudes, we can use it to monitor the radio emissions from the solar corona 24 h a day. Such observations play a crucial role in monitoring and predicting space weather hazards within few minutes to hours of time.

Spatial quasi-periodic variations of the plasma density and magnetic field in zebra radio sources

Context. Radio bursts and their fine structures are an integral part of solar flares. Fine structures in particular are used for diagnostics of solar flare processes. The so-called zebras belong to the most important of such fine structures.
Aims: We analyze seven zebra events in order to search for spatial variations in the plasma density and magnetic field in zebra-stripe sources.
Methods: We used an improved method for estimating the gyroharmonic numbers of zebra-stripe frequencies. We compared observed zebra-stripe frequencies with those calculated in the zebra model. The differences in these frequencies vary and thus show spatial variations in the plasma density and magnetic field.
Results: In six out of seven analyzed zebras, we found a rather high correlation coefficient (about 0.7 and higher) between spatial variations in the density and magnetic field and a strictly periodic function. These density and magnetic field variations are explained by the torsional or sausage magnetoacoustic waves in the loop in which zebra-stripe sources are located. We present the wavelengths of these waves in dependence on the zebra frequency and estimate their periods.

Properties of High-Frequency Type II Radio Bursts and Their Relation to the Associated Coronal Mass Ejections

Solar radio bursts are often early indicators of space weather events such as coronal mass ejections (CMEs). In this study, we determined the properties of a sample of 40 high-starting-frequency ( 150 MHz) type II radio bursts and the characteristics of the associated CMEs such as width, location and speed during 2010-2016. The high starting frequency implies shock formation closer to the solar surface, which has important ramifications for the analysis of particle acceleration near the Sun. We found the CME heliocentric distances at the onset time of metric type II bursts range from 1.16 to 1.90 solar radii (Rs). The study was also extended to 128 metric type II bursts to include lower-starting- frequency events for further analysis. The projected CME heights range from 1.15 to 2.85 Rs. The lower starting frequency correspond to shocks forming at larger heights. A weak correlation was found between the type-II starting frequency and CME heights, which is consistent with the density decline in the inner corona. The analysis confirmed a good correlation between the drift rate and the starting frequency of type II bursts (correlation coefficient 0.8). Taking into account the radial variation of CMEs speeds from the inner corona to the interplanetary medium, we observed the deviations from the universal drift-rate spectrum of type II bursts and confirmed the previous results relating type II bursts to CMEs.

A New Universality Class Pertaining to Quantum Critical Systems

Radio Diagnostics of Particles and Plasma in the Solar Corona

Radio diagnostics, in addition to their capabilities in exploring intense, impulsive bursts, also provide a high sensitivity to much weaker events, which may not show any substantial signature in other wavelengths.

The initial case study examines a complex event consisting of multiple radio sources/bursts associated with a fast coronal mass ejection (CME) and an M 2.1 class solar flare (SOL2015-09-20). 'First-light' data from the Owens Valley Radio Observatory-Long Wavelength Array is put in context with observations from Large Angle and Spectrometric Coronagraph onboard the Solar and Heliospheric Observatory, along with the WAVES radio spectrograph onboard WIND, the Expanded Owens Valley Solar Array, and the Air Force Radio Solar Telescope Network. One burst source exhibiting an outward motion is focused upon indicating movement associated with the core of the CME and is classified as type IVm burst. The source height, smoothness of the emission in frequency and time, along with a lower density in the region, indicate the likelihood of gyrosynchrotron as the underlying mechanism over plasma emission. Spectral fitting techniques are used to estimate the physical conditions during the outward movement of the source.

The second study investigates whether energy bursts from small breaks in stressed magnetic fields (nanoflares) can accelerate particles like full-sized flares, and if so, how efficiently? Since nanoflares may produce numerous 'mildly energetic' particles, at those energies, the emission in X-ray will be dominated by the thermal component. Type III radio bursts generated by propagating energetic electrons are best suited for the purpose. A model is created to simulate type III emission that may be produced by thousands of nanoflares occurring per second and the novel time-lag technique used to detect the motion of particles. The technique indeed detects the signature of type IIIs despite the numerous overlapping bursts and added noise that is expected in a radio instrument. Based on the findings of the model and associated testing, data from the Very Large Array, Low Frequency Array, and Long Wavelength Array are currently being looked at for signatures of such bursts in the corona. A similar test is performed on data from the FIELDS instrument onboard Parker Solar Probe to look for signatures of particle acceleration in the solar wind from small- scale reconnection events.

POEM AHAA PODHOA XAPAKTEPCTK EPBX VA 25-DO KA COHEHO AKTBHOCT, OPEEX COCTOH CCTEM OCVEPPOEM AHAA PODHOA XAPAKTEPCTK EPBX VA 25-DO KA COHEHO AKTBHOCT, OPEEX COCTOH CCTEM OCVEPProblems of Analysis and Forecast of Characteristics of the First Phase 25 Cycle of Solar Activity, Determining States of Biosphere Systems

The paper considers the results of the analysis and prediction of the dynamics of so-lar activity from the series of the solar radio flux density at a frequency of 2.8 GHz and the relative Wolf numbers, proposed by the leading analytical groups, as well as by the authors of this publication. The typological characteristics of the upcoming situations of socio-economic development of Russia are indicated.

Analysis of Solar Research Activities Published in North Korean Journals

We have analyzed 42 research papers regarding on the solar astronomy written by North Korea scientists to investigate the current status of astronomical activities in North Korea. The papers are surveyed from the 'Bulletin of Astronomy', the 'Physics', the 'Bulletin of Academy of Science', and the 'Natural Science' in North Korea, and SCI journals. In addition, we refer to the presentation material announced in the 2015 IAU by director of Pyongyang Astronomical Observatory (PAO) and the 2013 OAD/IAU reports. We have analyzed the papers statistically according to three criteria such as research subject, research field, and research members. The main research subjects are the sunspot (28%), observation system (21%), and space environments (19%). The research fields are distributed with data analysis (50%), numerical method (29%), and instrument development (21%). There have been 25 and 9 researchers in the solar astronomy and space environment, respectively since 1995. North Korea's solar research activities were also investigated in three area: instrument, solar physics, and international research linkage. PAO has operated two of sunspot telescope and solar horizontal telescope for spectroscopy and polarimetry, but there is no specific information on solar radio telescopes. North Korea has cooperated in solar research with Europe and China. We expect that the results of this study will be used as useful resource in supporting astronomical cooperation between South and North Korea in the future..

External triggering of O-star formation by a cloud-cloud collision

We have performed a multi-wavelength study of the mid-infrared bubble S44 to investigate the origin of isolated high-mass star(s) and the star-formation process around the bubble formed by the H II region. We report on the results of new CO observations (¹²CO, ¹³CO J = 1-0, and ¹²CO J = 3-2) toward the isolated bubble S44 using the NANTEN2, Mopra, and ASTE radio telescopes. We found two velocity components at -84 km s^-1 and -79 km s^-1 in the direction of the bubble. These two clouds are likely to be physically associated with the bubble, because of the enhanced ¹²CO J = 3-2/1-0 intensity ratio from a ring-like structure affected by ultraviolet radiation from embedded high-mass star(s) and of the morphological correspondence between the 8 m emission and the CO distribution. Assuming a single object, we estimate a spectral type of the embedded star inside the bubble to be O8.5-9 $({\sim}20\,M_{\odot})$ from the radio-continuum free-free emission. We hypothesize that the two clouds collided with each other 3 Myr ago, triggering the formation of the isolated high-mass star in S44, as also occurred in RCW 120 and RCW 79. We argue that this scenario can explain the origin of the isolated O-star inside the bubble.

Connection of the Intensity of the Flux of SCR Protons with the Velocity of the CME and with the Fading of the Radio Emission of the Sun in the Decameter Range

The relationship between SCR and CME and with fading of the continuum of noise storms and type IV radio bursts in the decameter range is investigated. I was shown earlier that about 60% of CMEs associated with solar proton events are accompanied by deep fading of the solar radio emission in the decameter range, which coincides in time with CME registration. It has also been shown that fading is characterized by fading depth, the frequency bandwidth in which the fading occurs, as well as the duration of the fading and the frequency at which the maximum fading depth is observed. Further detailed studies have shown that for proton events accompanied by fading of the solar radio emission in the decameter range, the relationship between the intensity of the SCR proton flux and the CME velocity is much worse than for events without fading of the solar radio emission in the decameter range. However, it was found that for such events, the relationship between the flux of SCR protons and the CME velocity significantly increases if we take into account the fading depth of the solar radio emission in the decameter range. Earlier in (Isaeva, 2019), the results of a study of the relationship between the intensity of fading of the continuum of noise storms with the parameters of X-ray bursts, with the CME velocity and the velocity of coronal shock waves, as well as with the intensity of the SCR proton flux were presented. This paper presents the results of studying the relationship between the intensity of the SCR proton flux with the parameters of type II and IV radio bursts, as well as with the CME velocity and with the velocity of coronal shock waves, depending on the intensity of fading of the solar radio emission in the decameter range at a frequency of 27 MHz. The frequency of 27 MHz was chosen because in the region of this frequency the maximum fading depth of the solar radio emission in the decameter range is observed.

Spectral signature of solar active region in millimetre and submillimetre wavelengths

Active regions were observed with different instruments covering the spectral band from 17 to 405 GHz. The observations were made with the Nobeyama Radioheliograph (17 GHz), the Atacama Large Millimetre Array (107 and 238 GHz), and the Solar Submillimeter Telescope (212 and 405 GHz). A procedure was developed that allows the comparison between observations taken with telescopes of different operational characteristics and mainly of different spatial resolution. The brightness temperature and density flux spectra of several active regions corresponding to a different phase of its lifetime were obtained. The flux density invariably increases in all cases from 107 to 405 GHz and the mean spectral index is 2 showing that the dominant emission mechanism at submillimeter frequencies is still thermal. We show that Solar Submillimeter Telescope (SST) and Atacama Large Millimeter/submillimeter Array (ALMA) observations are compatible within the uncertainties, a result of great interest for future joint observations.

Performance Comparison of Power Divider and Fiber Splitter in the Fiber-Based Frequency Transmission System of Solar Radio Observation

Time-frequency synchronization plays an important role in the construction of solar radio telescopes (such as heliograph and interferometry). In the development of a synchronization system, time- frequency signal is divided and transmitted to each antenna in an array, therefore, the performance of the signal splitting devices determines the quantity of the data. To address this situation, we designed a frequency transmitting system, and conducted a test to compare the phase difference among outputs (10 MHz - 1.4 GHz), and the deterioration of the frequency stability (10 MHz 1GHz) brought by different splitting devices (fiber splitter and power divider). The following results had been obtained: 1) Phase difference introduced by both fiber splitter and power divider can be restricted in a range of 4, and the fiber splitter is rather stable, which indicate that a better imaging effect of heliograph can be achieved when using fiber splitters. 2) Frequency stability deterioration get better (worse) with increasing temperature for fiber splitter (power divider) in the testing frequency range for a short-time sampling (100 ms), and for a long-time sampling (10 min), the frequency stability of different devices is determined by both temperature and signal frequency. By estimating the SNR, the performance of optical splitter is found to be slightly better than power splitter. This article provides a basis for the selection and compensation of frequency transfer system components for integrated aperture heliograph, and provide a feasible solution of the construction of low-cost radioheliograph.

Radio Measurements of the Magnetic field in the Solar Chromosphere and the Corona

The structure of the upper solar atmosphere, on all observable scales, is intimately governed by the magnetic field. The same holds for a variety of solar phenomena that constitute solar activity, from tiny transient brightening to huge Coronal Mass Ejections. Due to inherent difficulties in measuring magnetic field effects on atoms (Zeeman and Hanle effects) in the corona, radio methods sensitive to electrons are of primary importance in obtaining quantitative information about its magnetic field. In this review we explore these methods and point out their advantages and limitations. After a brief presentation of the magneto-ionic theory of wave propagation in cold, collisionless plasmas, we discuss how the magnetic field affects the radio emission produced by incoherent emission mechanisms (free-free, gyroresonance and gyrosynchrotron processes) and give examples of measurements of magnetic filed parameters in the quiet sun, active regions and radio CMEs. We proceed by discussing how the inversion of the sense of circular polarization can be used to measure the field above active regions. Subsequently we pass to coherent emission mechanisms and present results of measurements from fiber bursts, zebra patterns and type II burst emission. We close this review with a discussion of the variation of the magnetic field, deduced by radio measurement, from the low corona up to 10 solar radii and with some thoughts about future work.

Imaging Spectroscopy of CME-associated Solar Radio Bursts using OVRO-LWA

We present the first results of a solar radio event observed with the Owens Valley Radio Observatory Long Wavelength Array at metric wavelengths. We examine a complex event consisting of multiple radio sources/bursts associated with a fast coronal mass ejection (CME) and an M2.1 GOES soft X-ray flare from 2015 September 20. Images of 9 s cadence are used to analyze the event over a 120 minute period, and solar emission is observed out to a distance of 3.5 R, with an instantaneous bandwidth covering 22 MHz within the frequency range of 40-70 MHz. We present our results from the investigation of the radio event, focusing particularly on one burst source that exhibits outward motion, which we classify as a moving type IV burst. We image the event at multiple frequencies and use the source centroids to obtain the velocity for the outward motion. Spatial and temporal comparison with observations of the CME in white light from the C2 coronagraph of the Large Angle and Spectrometric COronagraph, indicates an association of the outward motion with the core of the CME. By performing graduated- cylindrical-shell reconstruction of the CME, we constrain the density in the volume. The electron plasma frequency obtained from the density estimates do not allow us to completely dismiss plasma emission as the underlying mechanism. However, based on source height and smoothness of the emission in frequency and time, we argue that gyrosynchrotron is the more plausible mechanism. We use gyrosynchrotron spectral-fitting techniques to estimate the evolving physical conditions during the outward motion of this burst source.

ALMA and IRIS Observations of the Solar Chromosphere. II. Structure and Dynamics of Chromospheric Plages

We propose and employ a novel empirical method for determining chromospheric plage regions, which seems to better isolate a plage from its surrounding regions than other methods commonly used. We caution that isolating a plage from its immediate surroundings must be done with care in order to successfully mitigate statistical biases that, for instance, can impact quantitative comparisons between different chromospheric observables. Using this methodology, our analysis suggests that = 1.25 mm free-free emission in plage regions observed with the Atacama Large Millimeter/submillimeter Array (ALMA)/Band6 may not form in the low chromosphere as previously thought, but rather in the upper chromospheric parts of dynamic plage features (such as spicules and other bright structures), i.e., near geometric heights of transition- region temperatures. We investigate the high degree of similarity between chromospheric plage features observed in ALMA/Band6 (at 1.25 mm wavelengths) and the Interface Region Imaging Spectrograph (IRIS)/Si IV at 1393 . We also show that IRIS/Mg II h and k are not as well correlated with ALMA/Band6 as was previously thought, and we discuss discrepancies with previous works. Lastly, we report indications of chromospheric heating due to propagating shocks supported by the ALMA/Band6 observations.

ALMA and IRIS Observations of the Solar Chromosphere. I. An On-disk Type II Spicule

We present observations of the solar chromosphere obtained simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph. The observatories targeted a chromospheric plage region of which the spatial distribution (split between strongly and weakly magnetized regions) allowed the study of linear-like structures in isolation, free of contamination from background emission. Using these observations in conjunction with a radiative magnetohydrodynamic 2.5D model covering the upper convection zone all the way to the corona that considers nonequilibrium ionization effects, we report the detection of an on-disk chromospheric spicule with ALMA and confirm its multithermal nature.

On the Occurrence of Type IV Solar Radio Bursts in Solar Cycle 24 and Their Association with Coronal Mass Ejections

Solar activities, in particular coronal mass ejections (CMEs), are often accompanied by bursts of radiation at meter wavelengths. Some of these bursts have a long duration and extend over a wide frequency band, namely, type IV radio bursts. However, the association of type IV bursts with CMEs is still not well understood. In this article, we perform the first statistical study of type IV solar radio bursts in solar cycle 24. Our study includes a total of 446 type IV radio bursts that occurred during this cycle. Our results show that a clear majority, 81% of type IV bursts, were accompanied by CMEs, based on a temporal association with white-light CME observations. However, we found that only 2.2% of the CMEs are accompanied by type IV radio bursts. We categorized the type IV bursts as moving or stationary based on their spectral characteristics and found that only 18% of the total type IV bursts in this study were moving type IV bursts. Our study suggests that type IV bursts can occur with both "Fast" (500 km s^-1) and "Slow" (<500 km s^-1), and also both "Wide" (60) and "Narrow" (<60), CMEs. However, the moving type IV bursts in our study were mostly associated with "Fast" and "Wide" CMEs (52%), similar to type II radio bursts. Contrary to type II bursts, stationary type IV bursts have a more uniform association with all CME types.

Ionospheric TEC prediction using Long Short-Term Memory deep learning network

In this paper, the prediction model for ionospheric total electron content (TEC) based on Long Short-Term Memory (LSTM) deep learning network and its performance are discussed. The input parameters of the model are previous values of daily TEC, solar radio flux at 10.7 cm parameter of 81 day moving average (F107_81\oline), sunspot number (SSN), geomagnetic Kp index, and disturbance storm time (Dst) index, and the outputs are TEC values for the target day. TEC data from January 1, 2001 to December 31, 2016 were used in this study. The dataset almost covers most of the years of the last two solar cycles (23, 24), and it is separated as 81.3% for training, 6.2% for validation, and 12.5% for testing. At BJFS IGS station (39.61 N, 115.89 E), LSTM yielded good TEC estimates with an RMSE of 4.07 TECU in 2001, it was 33% and 48% lower than the RMSE observed in TEC prediction using BP and IRI-2016 models, respectively. In the year of low solar activity (2016), the RMSE predicted by LSTM was 1.78 TECU, it provided 30% and 54% lower RMSE for TEC prediction than for BP and IRI-2016 models. Under the condition of magnetic storm, the LSTM TEC predictions are more consistent with the corresponding IGS Global Ionospheric Maps (GIMs) TEC than TEC predictions by BP and IRI-2016 models. LSTM can better grasp the influence of different external conditions on TEC. Seventeen grid points along 120 E meridian in latitude range from 80 S to 80 N were selected to further study the performance of LSTM model in different latitude. Results show that the prediction accuracy of LSTM is better than that of BP at different latitudes, especially at low latitudes. The performances of the two models are highly correlated with latitude and solar activity, and are both better than that of IRI-2016.

Study of ionospheric D region changes during solar flares using MF radar measurements

During solar flares, the X-ray radiation suddenly increases, resulting in an increase in the electron density of the atmospheric D region and a strong absorption of short-wave radio waves. Based on Langfang medium frequency (MF) radar, this paper analyzed the variation characteristics of D region in the lower ionosphere from 62 km to 82 km. The analysis focused on multiple C-level and M-level solar flare events before and after the large-scale flare event at 11:53 (UT) on September 6, 2017. The results show that it is difficult to detect the electron density over 70 km in Langfang during solar flares, but the electron density value can be obtained as low as 62 km, and the stronger the flare intensity, the lower the detectable electron density height. Besides, the equal electron density height, the received power of X and O waves will also be significantly reduced during the flares, and the reduction of equal electron density height has a weak linear relationship with flare intensity.

Radio observations of solar active regions at 7.36 and 37 GHz

We present solar radio observations that were made semi-simultaneously with two Nordic-Baltic radio telescopes observing at two different frequency bands, 7.36 GHz and 37 GHz. The two radio telescopes are the Aalto University Metahovi radio telescope located in Kylmala, Finland, and the Ventspils International Radio-Astronomy Center (VIRAC) radio telescope located in Irbene, Latvia. The observations presented here were made between autumn 2017 and autumn 2018. We use them to study the active solar regions that are seen in the radio intensity maps at both frequencies. We find that the location of the maximum intensity is different for the two frequency bands, and that their peak intensities do not correlate. Based on these findings we propose a simplified scenario for how the radio sources can form at centimetre and millimetre wavelengths.

LOFAR observations of radio burst source sizes and scattering in the solar corona

Low frequency radio wave scattering and refraction can have a dramatic effect on the observed size and position of radio sources in the solar corona. The scattering and refraction is thought to be due to fluctuations in electron density caused by turbulence. Hence, determining the true radio source size can provide information on the turbulence in coronal plasma. However, the lack of high spatial resolution radio interferometric observations at low frequencies, such as with the LOw Frequency ARray (LOFAR), has made it difficult to determine the true radio source size and level of radio wave scattering. Here we directly fit the visibilities of a LOFAR observation of a Type IIIb radio burst with an elliptical Gaussian to determine its source size and position. This circumvents the need to image the source and then de-convolve LOFAR's point spread function, which can introduce spurious effects to the source size and shape. For a burst at 34.76 MHz, we find full width at half maximum (FWHM) heights along the major and minor axes to be 18.8' 0.1' and 10.2' 0.1', respectively, at a plane of sky heliocentric distance of 1.75 R. Our results suggest that the level of density fluctuations in the solar corona is the main cause of the scattering of radio waves, resulting in large source sizes. However, the magnitude of may be smaller than what has been previously derived in observations of radio wave scattering in tied-array images.