Abstracts

Quick links to abstracts from WG1, WG2, WG3, WG4, WG5.

Link to bigger chart.


WG1: X-ray spectral issues and electron energetics (Massone/Battaglia) (17:6)
Silvia Allavena
Dipartimento di Matematica, Universita di Genova, Italy
(*)

Marina Battaglia
University of Glasgow, UK
(4)
Implications of hard X-ray footpoint sizes and positions on the thick target model
I will present RHESSI observations of solar flare footpoints in near-limb events. Using visibility forward fitting we found the positions/heights and the sizes of footpoints along and perpendicular to the magnetic field of the flaring loop at different energies in the HXR range. In half of the analyzed events a clear trend of decreasing height of the sources with energy is found. Assuming collisional thick-target transport, the HXR sources are located between 600 and 1200 km above the photosphere. The sizes along the path of electron propagation are up to a factor 4 larger than predicted by the thick-target model. I will discuss this finding and its implications on the thick target model, including processes such as magnetic mirroring, collisional pitch angle scattering and X-ray albedo that can affect the sizes of X-ray sources.
John C Brown
University of Glasgow, UK
(4)
The Thick Target Model - Past, Present and Future
Ewan Dickson
University of Glasgow, UK
(*)
Electron Anisotropy in Solar Flares
The angular variation of high energy electrons during a solar flare is key to understanding the acceleration mechanism. High resolution X-ray spectra observed by RHESSI can be used to estimate this anisotropy. The effect of photospheric albedo, Compton scattering of X-ray photons from the photosphere, should greatly influence the observed spectrum if the X-ray emitting electrons are highly beamed. The observed spectra will thus contain signatures of the anisotropy. The technique of regularised inversion is used to determine the proportion of the electron flux directed downwards towards the photosphere compared to the electron flux directed towards the observer. The RHESSI flare database has been searched and analysis performed on all flares found to have statistically significant counts above 300 keV. In total 9 flares suitable for analysis were found. The anisotropy of these flares both over the entire impulsive phase and for shorter time intervals was measured and the flares have all been found to exhibit angular distributions which is close to isotropic.
Solmaz Eradat Oskoui
St Andrews University, UK
(2)

Weiqun Gan
Purple Mountain Observatory, China
(4)
On the Calculations of Non-thermal Bremsstrahlung
We developed the integral numeration for the hard X-ray bremsstrahlung spectrum based on the version 2 in Solar SoftWare (SSW). Comparing the cost time for the four cases of the integral numeration, we showed that the best case is faster than the version 2 by 2 to 5 times. It is also showed that with the best case, the accuracy, the suppression of the spike structure, and insensitivity to the upper limit of the integration are improved a lot, in comparison with previous version 1 and 2. The test calculations suggested that the default upper limit of the electron spectrum had better be set as 10 to 8 keV instead of the present 32000 keV in the SSW. We conclude that the present best case can replace the version 2 in the SSW. In addition, we tested the integral numeration using the exact bremsstrahlung cross-section instead of the present approximative one, and proved that the cost time is tolerable for our improved case.
Natasha Jeffrey
University of Glasgow, UK
(4)
Compton backscattering and polarisation of hard X-ray sources
I wish to contribute ideas and results relating to the polarisation and directivity of hard X-ray (HXR) sources, modelled using MC simulations with the inclusion of the photospheric albedo component. I am currently modelling how the effect of Compton backscattering of photons (the albedo effect) in the photosphere from HXR flare sources changes the polarisation of the primary source. By creating electron distributions of varying directivity and hence polarisation, the anisotropy and polarisation of the resulting bremsstrahlung photon distribution can be entered into a Monte Carlo simulation code to produce an albedo component scattered from the photosphere with an altered polarisation dependent on viewing angle and energy. From our initial simulation results, I wish to discuss how the polarisation of an HXR source might be able to help predict the high energy cutoff in the electron distribution and discuss how the polarisation across a single HXR source at one location on the solar disk varies spatially, and could be measured by future missions such as GRIPS.
Jana Kasparova
Astronomical Institute AS CR, Czech Republic
(3)
Non-thermal distributions in RHESSI and RESIK spectra
The contribution will present results of joint analysis of RHESSI continuum and RESIK line flare spectra. We will show that RHESSI non-thermal component associated with the electron beam is well correlated with presence of non-thermal n-distribution of plasma electrons obtained from the RESIK spectra. In addition, such an n-distribution occurs during radio bursts observed in the 0.61-15.4 GHz range. Data also suggest that the n-distribution could also explain RHESSI emission below ~5 keV. Finally, on the basis of spectral line analysis we will argue that the n-distribution does not occupy the same location as the thermal component detected by RHESSI at ~10 keV.
Eduard Kontar
University of Glasgow, UK
(3)
Wave-particle interactions and their role in solar flares
Depending how it fits in the program of the group, I would like to present and discuss the recent observational evidence with RHESSI for magnetic fluctuation in a flaring loop where electrons are accelerated or modelling of the evolution of energetic electrons including Langmuir waves in the presence of non-uniform density/density fluctuations.
Robert Lin
University of California, Berkeley, USA
(4)

Gottfried Mann
Astrohysikalisches Institut Potsdam, Germany
(3)
Hard X-ray spectra and the generation of energetic electrons during solar flares
During solar flares the Sun shows an enhanced hard X-ray emission as observed by RHESSI. That indicates the generation of energetic electrons during flares. RHESSI allows to measure hard X-ray spectra. From these spectra, the spectra of associated energetic electrons can be deduced by means of the forward-fitting method by Holman et al. 2003). Thus, 25 flares are evaluated by means of this method resulting in typical electron spectra for moderate and large flares. Adopting the standard flare model in terms of magnetic reconnection, magnetic energy is transfered into kinetic energy at slow-mode shocks. At these slow-mode shocks energetic electrons can be generated. The spectra of electrons energized at the slow-mode shocks are theoretically calculated and compared with the observed ones. That allows to deduce parameters, e.g. electron number density, magnetic field and temperature, in the flare and acceleration region.
Anna Maria Massone
CNR-SPIN, Italy
(4)
Analysis of performances and reliability of the different RHESSI imaging algorithms
During the last Annapolis RHESSI X workshop, the former Working Group 1 on RHESSI Imaging Techniques designed an imaging test to explore in a systematic fashion strengths and weaknesses of the variuos algorithms at disposal. In the special session devoted to this imaging test I will present results about the different reliability of the different methods for reconstructing different flaring morphologies and/or photometries.
Thomas Neukirch
University of St. Andrews, UK
(3)
Particle energisation in models of collapsing magnetic traps
Fast magnetic field relaxation during flares can lead to the energisation of particles in collapsing magnetic traps (e.g. Somov & Kosugi, 1997). We present some results of test particle calculations in a collapsing trap model first used by Giuliani et al. (2005) and a generalisation of that model to asymmetric magnetic fields.
Michele Piana
Dipartimento di Matematica, Universita di Genova, Italy
(4)
Electron maps can provide constraints on the solution of the electron continuity equation
Regularization techniques allow reconstruction of electron maps of flaring events from RHESSI visibilities. This contribution describes how these maps can be utilized to infer quantitative information on the energy loss term in the electron continuity equation and therefore to constrain assumptions on the energy loss mechanism and on the physical properties of the target.
Marco Prato
Università di Modena e Reggio Emilia, Italy
(3)
Imaging from visibilities: the Space-D algorithm
This talk deals a new approach for image reconstruction from visibilities. In particular, if one considers a priori information about the target image (e.g., the nonnegativity of the pixels), the image reconstruction problem can be reformulated as a constrained optimization problem, in which the stationary points of the objective function can be viewed as the solutions of a deconvolution problem with a suitable kernel. A fast and effective gradient-projection iterative algorithm is proposed in order to provide physically meaningful reconstructions.
Richard Schwartz
NASA/GSFC, USA
(*)

Yang Su
CUA/NASA-GSFC, USA
(2)
Nonuniform Ionization in the Thick Target and the Density Distribution in Flaring Loops
Nonuniform ionization in the thick target can cause a break-up at low energies and a break-down at high energies in X-ray spectra with size limits on the breaks. A model which includes relativistic effects on the energy loss rate and Haug cross-section will be presented. Evidence for the full spectral signature of nonuniform ionization was found in a solar flare. Also, the relation between emission and column density from the model mentioned above was used to determine the density distribution in the flaring loop and the position of neutral gas at the footpoints. We will also talk about related issues about RHESSI imaging and instrumental effects.

WG2: Flare morphology and interconnections between X-ray sources (Veronig/Saint-Hilaire) (10:4)
Markus Aschwanden
Solar and Astrophysics Dept, Lockheed Martin, USA
(*)
Flare statistics, fractal energy release, and self-organized criticality
We investigate flare morphology and flare statistics by measuring the fractal geometry or energy release regions, occurrence frequency distributions, and waiting time distributions. The hard X-ray observations from RHESSI are compared with the predictions of the new diffusive-fractal Self-Organized Criticality model proposed by Aschwanden and Charbonneau (2011). In addition we reconcile also the waiting time distributions of solar flare hard X-ray events from SMM, CGRO, ISEE-3, and RHESSI.
Brian Dennis
NASA Goddard Space Flight Center, USA
(4)
Extended HXR Sources – Albedo Patches or Coronal Sources
Extended HXR sources in the presence of compact footpoints have been reported based on visibility amplitudes from different detectors. Attempts have been made to determine the location and extent of these sources through direct imaging. Results of this work will be described for simulated sources and for specific flares at different solar longitudes, with a discussion of the possible nature of the extended sources as either albedo patches or coronal sources or a combination of the two.
Gordon Emslie
Western Kentucky University, USA
(5)

submitted by N Bian
Youping Li
Purple Mountain Observatory, China
(3)
Statistical Study on Thermal Energy of Solar Flares
We deduced the thermal energies of hot plasma for 1843 flare samples occurred during 2002-2009. The frequency distribution of this thermal energy presents a power law with the index -2.17 was found.
Hamish Reid
LESIA, Observatoire de Paris, France
(3)
Investigating the morphology of a flare using HXR, EUV and magnetic field data
Current observational evidence and standard models indicate that X-ray and EUV emission can be caused by the same population of energetic electrons streaming down into the chromosphere. Recent studies have also shown that UV emission late in a flare can be associated with lower energy X-ray photons than the EUV emission detected at the flare peak intensity. We analyse a confined flare whose magnetic connectivity has previously been investigated with MHD simulations and TRACE EUV data. We examine the flare using HXR observations from RHESSI and whether the HXR observations can be understood in the context of the previously studied magnetic topology.
Pascal Saint-Hilaire
UC Berkeley, USA
(*)
Caveats to fitting gaussians to visibilities in the case of thick-target modeling.
The spatial distribution of thick-target emission is strongly dependent on the atmosphere. A simple barometric atmosphere typically leads to very gaussian-like emission profiles. More complicated atmospheric models (as might occur in flares) lead to relatively strong differences in footpoint position and, particularly, sizes.
A decade of solar Type III radio bursts observed by the Nancay Radioheliograph
We present a statistical survey of radio Type III bursts observed by the Nancay Radioheliograph from 1998 to 2008, covering nearly a full solar cycle. In particular, sources sizes, positions, and fluxes were extracted and have been comprehensively cross-correlated. We find an east-west asymmetry in source positions, and that source FWHM sizes and fluxes follow power-law distributions. We derive a coronal density profile from these data, as well as discuss some implications for coronal heating.
Sabrina Savage
NASA/GSFC (ORAU), USA
(3)
Using Supra-Arcade Downflows as Probes of Electron Acceleration During Solar Flares
Extracting information from coronal features above flares has become more reliable with the availability of increasingly higher spatial- and temporal-resolution data in recent decades. We are now able to sufficiently probe the region high above long-duration flaring active regions where reconnection is expected to be continually occurring. Flows in the supra-arcade region, first observed with Yohkoh/SXT, have been theorized to be associated with newly-reconnected outflowing loops. High resolution data appears to confirm these assertions. Assuming that these flows are indeed reconnection outflows, then the detection of those directed toward the solar surface (i.e. downflowing) should be associated with particle acceleration between the current sheet and the loop footpoints rooted in the chromosphere. RHESSI observations of highly energetic particles with respect to downflow detections could potentially constrain electron acceleration models. I will discuss measurements of these supra-arcade downflows (SADs) in relation to reconnection model parameters and present preliminary findings comparing the downflow timings with high-energy RHESSI lightcurves.
Alexei Struminsky
Space Research Institute, Russia
(5)
Morphology of large impulsive flares and episodes of energy release
Time profiles of effective flare plasma temperature – T, heating function - (dT/dt)/T calculated from GOES data and hard X-ray intensity measured by RHESSI and INTEGRAL during large impulsive events on January 1 and September 13, 2005 are considered. The plasma was heated during about 10 min by accelerated electrons in several episodes, which differ by places and mechanisms of electron energy realese. In the beginning of events accelerated electrons interacted dominantly in the corona (thin target) and therefore a soft X-ray source was heated by downward fluxes only. Later electrons interacted both in the corona and chromosphere (thik target), so the plasma was heated by downwards from the region of coronal energy release and upwards fluxes from footpoints of magnetic loop. In some time moments a surplus of hard X-ray intensity in comparison with observed plasma heating is clear.
Astrid Veronig
University of Graz, Austria
(*)

Ivan Zimovets
Space Research Institute of Russian Academy of Sciences, Russia
(3)
Quasi-Periodic Pulsations of Non-Thermal Hard X-ray Emission in Two-Ribbon Solar Flares
Some solar flares are accompanied by quasi-periodic pulsations of non-thermal hard X-ray emission with periods from several seconds up to several tens of seconds. It is shown that at least in two-ribbon flares there is a tendency for such pulsations to be emitted sequentially from different magnetic loops stacked into the flaring arcades. In this case, the triggering disturbances seem to propagate predominantly along the axis of the flaring arcades at the speeds of a few tens km/s, well below the Alfven and sound speeds. These observations strongly require incorporation of the third dimension into the standard two-dimensional models of eruptive solar flares. Here we discuss three possible mechanisms proposed for the interpretation of these observations: 1) asymmetric filament eruptions; 2) slow magnetoacoustic wave propagation along the flaring arcades; 3) unstable modes of oscillations of quasi-vertical current sheets having extension along the flaring arcade axis. The necessary observations to be performed to select the correct mechanism are also discussed.

WG3: Radio & X-ray flaring loops (Hurford/Fleishman) (18:13)
Hazel Bain
SSL, UC Berkeley, USA
(2)
Synchrotron emission in expanding CME loops
Hard X-ray and radio observations can be used to identify source of electron acceleration within the flare-CME system. Observations at meter wavelengths using the Nancay Radioheliograph are ideal for locating sources of synchrotron and plasma emission (Bastian & Gary 1997). Bastian et al. (2001) and Maia et al. (2007) imaged synchrotron emission present in the expanding loops of two independent CME's. It is unclear how these electrons were accelerated. Previous suggestions include acceleration at the reconnection site/current sheet between the flare and CME or in the foreshock generated as the ejected material propagates outward. We present observations of an event which occurred on the 14th of August which is a new candidate for studying these events.
Nicolas Bian
University of Glasgow, UK
(2)
Constraining turbulent acceleration models with imaging observations of thick-target coronal loops, N.Bian, E.Kontar, G.Emslie
RHESSI imaging observations of dense coronal sources allow an assessment of the density and volume of the acceleration region and, hence, of the specific acceleration rate of electrons to hard-x-ray-producing energies, during solar flares. The value of this key quantity is used to constrain various turbulent acceleration models, in particular the level of electromagnetic fluctuations required for acceleration to occur at this rate (electrons s-1 per electrons) in these models. This is combined with measurements of spectral indices and analysis of the variation of the source width with energy, resulting from turbulent transport of hard-x-ray-emitting electrons across the guiding magnetic field of the loop, which provide additional constraints on these models.
Christina Burge
University of Glasgow, UK
(*)
Stochastic Particle Acceleration At Reconnection Sites
The X-type neutral point has long been used as a simple model to describe the post-reconnection topology of acceleration regions in the corona. To make such a model more realistic, we introduce turbulence via a superposition of wave mode oscillations in the magnetic and electric fields, and conduct test particle simulations to integrate the orbits of particles in the presence of such fields. A method to incorporate pitch angle scattering by integrating these orbits stochastically is also being developed.
Steven Christe
NASA GSFC, USA
(*)
SDO and RHESSI Observations of Flares
We present statistics of flares with simultaneous observations in both hard X-ray (HXR) and ) and ultraviolet (UV/EUV) observations by RHESSI and SDO/AIA, respectively. The flares were chosen from the RHESSI flare list and range from GOES M class to GOES A class. Identifying the flare region using RHESSI imaging observations, we compare HXR (thermal) light curves and images with those observed by AIA in every available UV/EUV channels. Many events show complex morphologies with multiple flaring loops observed in AIA images though only simple morphology are revealed by RHESSI images. We compare the light curves in all wavelengths bands and present statistics of time lags and correlation between wavelengths.
Gregory Fleishman
NJIT, US
(1)
Cold, tenuous solar flare: acceleration without heating
We report the observation of an unusual cold, tenuous solar flare, which reveals itself via numerous and prominent non-thermal manifestations, while lacking any noticeable thermal emission signature. RHESSI hard X-rays and 0.1-18 GHz radio data from OVSA and Phoenix-2 show copious electron acceleration (10^35 electrons per second above 10 keV) typical for GOES M-class flares with electrons energies up to 100 keV, but GOES temperatures not exceeding 6,1 MK. The imaging, temporal, and spectral characteristics of the flare have led us to a firm conclusion that the bulk of the microwave continuum emission from this flare was produced directly in the acceleration region. The implications of this finding for the flaring energy release and particle acceleration are discussed.
Dale Gary
New Jersey Institute of Technology, USA
(4)
The Expanded Owens Valley Solar Array (EOVSA)
The Owens Valley Solar Array, located near Big Pine, California, has operated over nearly 30 years with periodic upgrades to go from a single pair of antennas to most recently 7 antennas. Its operation has relied on time-sharing of a three-baseline analog correlator and switching between R and L polarizations, and its all-analog approach to frequency-agility, although originally ahead of its time, is now thoroughly outdated. However, all of that is about to change, with the EOVSA project. This generously funded project will increase the number of antennas to 13, enlarge the footprint of the array to 1.5 km baselines, and use technology developed for FASR to completely modernize the analog and digital signal processing to provide fully-correlated, dual-polarization, full-Sun, broadband observations covering 1-18 GHz in under 1 s. We give a brief description of the design of the instrument, and then discuss the science and operational goals of the project. The work is already underway, with expected completion in late 2013. To be ready to fully interpret these unique microwave imaging data, the radio group at NJIT has initiated several efforts to improve modeling of radio and X-ray emission from flaring loops (see presentations by Gregory Fleishman and Gelu Nita in this workshop). We invite others at the workshop to start thinking about how they might use the completely open and community accessible EOVSA database, and perhaps get involved by developing software tools for the instrument.
Iain Hannah
University of Glasgow, UK
(4)
Measuring the shape of HXR coronal loops
(This is for WG1/3) I want to discuss visibility forward fitting a loop shape to dense coronal loops and using the resulting energy dependent lengths and widths to constrain the acceleration and transport of the energetic electrons. In particular the widths gives a measure of cross-field diffusion which can be interpreted as magnetic fluctuations and turbulent acceleration. This is related to the work presented by Kontar and Bian.
Gordon Holman
NASA Goddard Space Flight Center, USA
(1)
The Impact of Return-Current Losses on Solar Flare Radiation Signatures
Energy losses associated with a cospatial return current can significantly change the distribution of accelerated electrons in solar flares. This in turn affects the observed X-ray and radio emission from flares and, because of increased heating in the corona, the thermal evolution of the flare plasma. I will discuss the classical evolution of the steady-state electron beam-return-current system and the consequences for observable emissions. As part of this discussion, I will examine Gordon Emslie's 1980 conjucture that return-current losses result in an upper limit to the brightness of flare hard X-ray sources.
Gordon Hurford
University of California, Berkeley, USA
(4)
Update on the Solar Orbiter/STIX X-Ray Telescope
Energy losses associated with a cospatial return current can significantly change the distribution of accelerated electrons in solar flares. This in turn affects the observed X-ray and radio emission from flares and, because of increased heating in the corona, the thermal evolution of the flare plasma. I will discuss the classical evolution of the steady-state electron beam-return-current system and the consequences for observable emissions. As part of this discussion, I will examine Gordon Emslie's 1980 conjucture that return-current losses result in an upper limit to the brightness of flare hard X-ray sources.
Andrew Inglis
NASA Goddard Space Flight Center, USA
(*)
Instrumental oscillations in RHESSI count rates during solar flares
We seek to illustrate the analysis problems posed by RHESSI spacecraft motion by studying persistent instrumental oscillations found in the lightcurves measured by RHESSI's X-ray detectors in the 6-12 keV and 12-25 keV energy range during the decay phase of the flares of 2004 November 4 and 6. The various motions of the RHESSI spacecraft which may contribute to the manifestation of oscillations are studied. The response of each detector in turn is also investigated. We find that on 2004 November 6 the observed oscillations correspond to the nutation period of the RHESSI instrument. These oscillations are also of greatest amplitude for detector 5, while in the lightcurves of many other detectors the oscillations are absent. We also find that the variation in detector pointing is much larger during this flare than the counterexample of 2004 November 4. Sufficiently large nutation motions of the RHESSI spacecraft lead to clearly observable oscillations in count rates, posing a significant hazard for data analysis. Dynamic correction of the RHESSI counts, accounting for the livetime, data gaps, and the transmission of the bi-grid collimator of each detector, is required to overcome this issue and is presented here. These corrections should be applied to all future oscillation studies.
Marian Karlicky
Astronomical Institute, Academy of Sciences, Ondrejov, Czech Republic
(1)
Successive merging of plasmoids and fragmentation in flare current sheet
A concept of the successive merging of plasmoids and fragmentation in the current sheet in the standard flare model is presented. Then, using a 2.5-D electromagnetic particle-in-cell model with free boundary conditions, these processes were modeled. We recognized the plasmoids which mutually interacted and finally merged into one large plasmoid. Between interacting plasmoids further plasmoids and current sheets on smaller and smaller spatial scales were formed. During interactions of the plasmoids the electrons were very efficiently accelerated and heated. Formulas for schematic fractal reconnection structures were derived. Relevant X-ray and radio signatures of these processes are discussed.
Larisa Kashapova
Institute of Solar-Terrestrial Physics SB RAS, Russia
(4)
Simulation of gyrosynchrotron emission of the 2 May 2007 solar flare with using of force-free magnetic field reconstruction and HXR data
Our contribution is devoted to effect of magnetic field topology on gyrosynchrotron solar radio emission of the flare observed on 2 May 2007. We used reconstructed 3D magnetic structure of AR 10953 in force-free approach at the beginning of the flare for information about distribution of magnetic field along the loop. Reconstructed force lines demonstrate a good correspondence with real loop structures seen on UV and X-ray images. HXR data was used for estimation of plasma parameters and identification of the flare loop. Gyrosynchrotron emission was calculated with using GS simulator code by (Nita, Fleishman and Gary, 2009). The results are compared with NoRH observations.
Alexey Kuznetsov
Armagh Observatory, UK
(1)
3D simulations of gyrosynchrotron emission from anisotropic electron distributions in coronal magnetic loops
Microwave emission of solar flares is formed primarily by incoherent gyrosynchrotron radiation generated by the accelerated electrons in coronal magnetic loops. The observed emission depends on many factors, including pitch-angle distribution of the emitting electrons and the source geometry. In this work, we perform systematic simulations of solar microwave emission using recently developed tools (GS Simulator and fast gyrosynchrotron codes) capable of simulating maps of radio brightness and polarization as well as spatially resolved emission spectra. A 3D model of a symmetric potential magnetic loop is used. We compare the emission from isotropic and anisotropic (of loss-cone type) electron distributions. We also investigate effects caused by inhomogeneous distribution of the emitting particles along the loop. It is found that the effects of the electron anisotropy are the most pronounced near the footpoints and they also depend strongly on the loop orientation. Concentration of the emitting particles at the loop top results in the corresponding shift of the visible intensity maximum, thus decreasing the effects of the anisotropy. The high-frequency (>50 GHz) emission spectral index is determined mainly by the energy spectrum of the emitting electrons; however, at the intermediate frequencies (around 10 GHz), the spectrum shape is strongly dependent on the electron anisotropy and spatial distribution. The implications of the obtained results for the diagnostics of the energetic electrons in solar flares are discussed.
Tereza Satiko Nishida Pinto
INPE, Brazil
(*)
3-D modeling of solar flare microwave emission using force free field extrapolation
We present the first results of the application of force free extrapolated magnetic field to modeling energetic electrons and microwave emission spatial distribution in a flaring region. The Nobeyama brightness maps constrain the extrapolated lines and the extreme ultraviolet imaging helps to infere the alpha parameter of the force free field. The spatial distribution of mildly relativistic electrons along extrapolated field lines is obtained through numerical solution of the nonstationary Fokker Planck equation. The model of the flaring loops is assumed to be stationary during the time scale of the flare.
Gelu Nita
New Jersey Institute of Technology, USA
(*)
GX_Simulator: An Interactive IDL Widget Tool for Visualization and Simulation of Imaging Spectroscopy Models and Data
An interactive IDL widget application intended to provide a flexible tool that allows the user to generate spatially resolved radio and/or X-ray spectra is presented. The object-based architecture of this application provides full interaction with local 3D magnetic field extrapolation models that may be embedded in a global coronal model. By the use of various mouse tools provided, the user is allowed to explore the magnetic connectivity of the model by generating magnetic field lines originating in user-specified volume voxels. Such lines may be selected to create magnetic flux tubes, which are further populated with user-defined analytical thermal/non thermal particle distribution models. By default, the application integrates IDL callable DLL and Shared libraries containing fast GS emission codes developed in FORTRAN and C++ based on the newly developed Fleishman –Kuznetsov approximation, and IDL X-ray codes developed by Eduard Kontar. However, the interactive interface allows interchanging these default libraries with any user-defined IDL or external callable codes designed to solve the radiation transfer equation in the same or other wavelength ranges of interest.
Panagiota Petkaki
DAMTP, University of Cambridge, UK
(1)
Acceleration of charged particles by fluctuating and steady electric fields in X-type magnetic fields
Magnetic reconnection is one of the mechanisms for releasing energy from magnetized plasmas. In astrophysical plasmas the released energy is associated with the production of energetic particles. Magnetic reconnection is often collisionless and could provide both the energy release mechanism and the particle accelerator in solar/space plasmas. I will discuss particle acceleration when fluctuating (in-time) electric fields are superposed on an static X-type magnetic field in collisionless hot solar plasma. This system is chosen to mimic the reconnective dissipation of a linear MHD disturbanceis also compared to particle acceleration from constant electric fields. Time evolution of ion and electron distributions are obtained by numerically integrating particle trajectories. The frequencies of the electric field represent a turbulent range of waves. Depending on the frequency and amplitude of the electric field, electrons and ions are accelerated to different degrees and have energy distributions of bimodal form consisting of a lower energy part and a high energy tail. For some frequencies a substantial fraction (20-30%) of the proton distribution is accelerated to gamma-ray producing energie and the bulk of the electron distribution is accelerated to hard X-ray producing energies. The acceleration mechanism is important for solar flares and solar noise storms but it could be applicable to all collisionless astrophysical plasmas.
Heather Ratcliffe
University of Glasgow, UK
(4)
The effect of Langmuir wave diffusion on the electron distribution
We consider the coupling of electrons and Langmuir waves in a non-uniform plasma using weak turbulence theory. The temporal evolution of the particle spectrum in a plasma with density perturbations is analysed. Assuming a Gaussian correlation function for density fluctuations and including the effects of collisions on the electrons and waves, we numerically solve the system of nonlinear kinetic equations for particles and Langmuir waves. We find a level of Langmuir waves, and hence estimate the possible radio emission.
Valentina Zharkova
Department of Mathematics, University of Bradford, UK
(4)
The effects of self-induced electric field on HXR and MW emission and polarisation in flaring loops with magnetic field convergence
We discuss practical aspects for interpretation of observed HXR and MW intensities, directivity and polarisation from flares by using the recent transport models for beam electron precipitation into flaring atmospheres considering energy losses and anisotropic scattering in self-induced electric and converging magnetic fields. We describe proposed additions to RHESSI interpretation tools allowing to account for these effects and to derive the parameters of beam electrons with higher accuracy. We also consider the role of electric field in generation of Langmuir turbulence and the consequences for observations of MW bursts in flares

WG4: Footpoint signatures of energetic electrons (Hannah) (3:16)
Lyndsay Fletcher
School of Physics and Astronomy, University of Glasgow, UK
(5)
tbd
tbd
Iain Hannah
University of Glasgow, UK
(3)

Aidan O'Flannagain
Trinity College Dublin, Ireland
(5)
Episodic Hard X-ray Bursts in Solar Flares
During solar flares, hard X-rays (HXR) are emitted by accelerated electrons interacting with the dense underlying chromosphere. HXR spectra observed during these peaks can be used to calculate the properties of the flaring plasma, such as the density distributions at the time of the peak (e.g., Aschwanden et al. 2002). However, the impulsive nature of HXR bursts and the difficulties associated with HXR imaging-spectroscopy makes this a challenging task. Here, a flare that exhibited multiple separated HXR peaks in described, and used to determine the evolution of the flaring plasma with time. Our results provide new information on the structure of plasma in flaring loops and on the fundamental physics of chromospheric evaporation.

WG5: Low atmosphere response to electrons and ions (MacKinnon/Milligan) (8:4)
Joel Allred
NASA/GSFC, USA
(3)
Modeling White Light Emission from Flares
Using models for how energetic electrons heat the atmosphere during flares, we simulate the radiative-hydrodynamic response of the lower solar atmosphere to flare heating. The simulations account for much of the non-LTE, optically thick radiative transfer that occurs in the chromosphere. Our models predict an increase in white light continuum during the flare on the order of 20%, but this is highly sensitive to the electron beam flux used in the simulation. We find that a majority of the white light continuum originates in the chromosphere as a result of Balmer and Paschen recombinations, but a significant portion also forms in the photosphere which has been heated by radiative backwarming.
David Graham
University of Glasgow, UK
(*)

Hugh Hudson
UC Berkeley/University of Glasgow, US/UK
(4)
EVE and RHESSI
The RHESSI gamma-ray event SOL2010-06-12T00:57 was well-observed by SDO (HMI, AIA, EVE). The HMI data are particularly interesting, showing a white-light flare and a line-of-sight magnetic-field variation (but no seismic wave). I discuss the EVE data here. They provide the first XUV spectra with good temporal coverage and moderate spectral resolution of solar flares. The data, though Sun-as-a-star, show us some interesting things about this M2 event, including a redshift in the He II 304A line that is consistent with the observations reported by Milligan and Dennis (2009).
Matthieu Kretzschmar
LPC2E CNRS/University of Orleans, France
(3)
Sun as a star observation of white light flares
We will present Sun as a star observations of white light (WL) continuum emission in solar flares and provide statistical evidence that this emission is present in basically all flares from X-ray class C to X. We find that the WL continuum is consistent with a black body spectrum at T~9000K and with the flaring areas observed by imagers. We estimate the spectral energy distribution of flare emission and find that the WL continuum, which mostly occurs during the impulsive phase, represents about 70% of the total radiated energy. Additionally, we will also present and discuss the new high cadence (up to 100Hz) flare observations by the PROBA2/LYRA radiometer.
Alec MacKinnon
University of Glasgow, UK
(3)

Sarah Matthews
UCL-MSSL, UK
(*)
White-light flares, sun-quakes and magnetic transients - kith or kin?
Some of the most challenging observations to explain in the context of existing flare models are those related to the lower atmosphere and below the solar surface. Such observations, including WLF emission, changes in the photospheric magnetic field and seismic emission, indicate the poorly understood connections between energy release in the corona and its impact in the photosphere and the solar interior. Using data from Hinode, TRACE, RHESSI, GONG and Yohkoh we investigate the the characteristics of, and connections between, signatures observed at various atmospheric depths and how they relate to existing models.
Ryan Milligan
QUB, UK
(4)
Investigating the Influence of Nonthermal Electrons On Increased EUV Irradiance During Solar Flares Using RHESSI and SDO/EVE
This work aims to investigate the the physical processes that drive increases in EUV irradiance during solar flares. Previous studies have shown that this increase is greatest during a flare's impulsive phase, suggesting that they are driven by nonthermal electrons, although HXR observations have never been utilized to confirm this. The main focus of this talk therefore is to expand upon previous works by combining flare observations from RHESSI, which can determine the characteristics of the nonthermal electrons, with those from the EUV Variability Experiment (EVE) onboard SDO, which can measure the associated changes in the EUV irradiance.
Albert Shih
NASA/GSFC, USA
(1)
Temporal variability of ion acceleration and abundances in RHESSI flares
We use the latest RHESSI gamma-ray analysis techniques to study the temporal behavior of the RHESSI flares, and determine what changes can be attributed to an evolving acceleration mechanism or to evolving abundances.