Solar Radio Spikes and Type IIIb Striae Manifestations Triggered by a Coronal Mass Ejection by Clarkson et al

Radio bursts are frequently emitted in the outer solar corona due to the acceleration of energetic electrons in solar flares and coronal mass ejections (CME). The shortest observed bursts are radio spikes, which have narrow spectral widths and are emitted over decades in frequency. Despite being studied for many years, imaging observations are sparse and restricted to localisation of the emission at decimetre frequencies (e.g. Benz, 2002). Recently, LOw Frequency ARray (LOFAR) observations allowed for tracking of the individual spike source evolution at fixed frequencies showing that radio-wave scattering governs the time duration and fixed frequency motion, as well as a morphological similarity to Type IIIb striae in the same event (Clarkson et al., 2021).

Clarkson et al 2023 conducted an analysis of over 1000 spikes using LOFAR to provide a statistical determination of their characteristics from imaging and dynamic spectra, as well as comparison with 250 individual Type IIIb striae from the same event.


Figure 1 provides an overview of the event associated with a C-class flare from an active region towards the western limb, were extended magnetic structures bridge the solar hemispheres. The Type IIIb bursts are a useful diagnostic of the coronal loop that their sources partially trace. Figure 1e shows a bi-directional Type IIIb that exhibits normal and reverse-drifting components, each characterized by a drift rate that corresponds to a different beam velocity, suggesting possible asymmetry in the injection of energy in each direction. The centroids of each component propagate in opposite directions across the sky-plane, indicating that the likely region of beam acceleration is where the two directions emanate. However, individual Type IIIb striae imaging reveals strong signs of radio-wave propagation effects in the form of centroid displacement over time at fixed frequencies in a direction tangential to the beam motion, consistent with anisotropic density fluctuations in a non-radial magnetic field. Correspondingly, the striae peak centroids do not correlate with the direction of the magnetic field structure that is assumed from the scatter-induced motion, suggesting that the location of beam acceleration was lower in the loop structure in a region near the CME flank.

Figure 1: An overview of the event on 15 July 2017. (a) SDO/AIA 171 Å image at 11:20:57 UT, superimposed with a PFSS extrapolation at noon, and a LASCO C2 image showing the streamer-puff (above) and narrow (below) CME fronts. (b-d) Dynamic spectra showing samples of the spike emission. (e) Dynamic spectra of a Type IIIb J burst.

Figure 1(b-d) show examples of spike clusters. Each individual spike presents comparative centroid motion to the Type IIIb striae. The ensemble of spike centroids drift along the curvature of the unobserved magnetic structure. The centroid motion is often superluminal, yet there is a spread in the velocity at a given frequency, possibly caused by a varying anisotropy and turbulence level over time, as well as a spread of emission angles within the loop. The spike intensity contours overlap in position and shape with striae that are temporally near, with both bursts evolving similarly throughout the decay phase. Moreover, the altitude of the spike and Type IIIb striae sources at a given frequency increases over time, interpreted as a projection effect from a restoring field that was perturbed by the passing CME shock. The majority of spikes occur post-CME, suggesting that the perturbation and reconfiguration of the magnetic field may have incited frequent magnetic reconnection in numerous sites.

Figure 2: Ensemble of spike centroid motions split into three frequency groups 5 MHz wide. The direction of motion is from dark to light. The centroids are overlaid onto an AIA 171 \r{A} image with a PFSS extrapolation showing possible closed magnetic geometry within the region. The red box bounds the active region.

The spike linear sizes and decay times (Figure 3, left) reveal a $1/f$ dependence similar to that for Type III bursts, indicating that scattering could be important up to at least 1 GHz. We find that the inhomogeneity time, with density fluctuations between 0.3-1% (red curves in Figure 3, left), can provide a closer approximation to observations than the plasma collision time that spikes have previously been compared to in order to explain their short time profiles (e.g. Melnik et al., 2014). Our analysis also showed that spike bandwidth ratios at tens and hundreds of MHz can differ by an order of magnitude. At decametre frequencies, the bandwidth ratios are contained within a narrow range and suggest intrinsic source sizes <1 arcsec. Above 200-400 MHz there is an abrupt jump in bandwidth ratio and a wider spread. When assuming plasma emission, we found that this variation could be replicated using the Langmuir wave dispersion relation of the form
\[ \omega=\omega_\mathrm{pe}+\frac{3k^2v_\mathrm{Te}^2}{2\omega_\mathrm{pe}}+ \frac{\omega_\mathrm{ce}^2}{2\omega_\mathrm{pe}}\sin^2{\psi}, \]
where $\psi$ is the angle between the plasma wave and the magnetic field direction. The broad spread above 400 MHz can be accounted for by different magnetic field strengths, which may vary from event to event. However, we do not rule out the ECM mechanism for spikes at GHz frequencies. For decametre spikes, the best match to observations suggested a magnetic field strength that was twice the average, with $\psi\simeq23$ degrees.

Figure 3: Decay times (left) and bandwidth ratios (right) of solar radio spikes. The blue data points show the median and interquartile range of the spikes measured in this study with LOFAR. The left panel includes the plasma collision time for various coronal temperatures (grey lines) and inhomogeneity time for select values of $\delta{n}/n$. The right panel includes the bandwidth ratio using equation 2 for various magnetic field strengths, with $\psi=23$ degrees.


We statistically analysed solar radio spikes and Type IIIb striae associated with a closed loop system, where the emission was likely produced via frequent magnetic reconnection triggered by a CME. The emitting region for these bursts is smaller than 1 arcsec and evolves over several tens of minutes due to perturbed magnetic geometry caused by the CME shock. The emitting location will be determined by an interplay of the electron beam acceleration site, the beam characteristics, and the turbulent conditions. The radio sources display characteristics of strong anisotropic scattering which produces non-radial evolution of their spatial location over time. Consequently, the observed source locations do not correspond to the intrinsic emitter, with the trajectory of the Type IIIb burst inferring a location closer to the Sun along the ascending leg near the CME flank. Radio-wave scattering is important in both the decameter and decimeter wavelength domains, governing the observed decay time and sizes. Assuming plasma emission, the observed spike bandwidth ratios can be replicated via the Langmuir-wave dispersion relation, with an increase above 400 MHz caused by the strong variation in magnetic field strength between events. In the decametre range, the spike observations suggest a magnetic field strength stronger than average by a factor of ~2.

Based on the recent paper by Clarkson, D. L., Kontar, E. P., Vilmer, N., Gordovskyy, M., Chen, X., Chrysaphi, N., Solar Radio Spikes and Type IIIb Striae Manifestations of Subsecond Electron Acceleration Triggered by a Coronal Mass Ejection, ApJ, 946, 33. DOI: 10.3847/1538-4357/acbd3f


Benz, A. O., Saint-Hilaire, P., Vilmer, N.: 2002, A&A, 383

Clarkson, D. L., Kontar, E. P., Gordovskyy, M. et al.: 2021, ApJL, 917, 2

Melnik, V. N., Shevchuk, N. V., Konovalenko, A. A. et al.: 2014, Sol. Phys., 289, 5