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Spatially inhomogeneous acceleration of electrons in solar flares  

Eduard Kontar   Submitted: 2018-01-16 07:54

The imaging spectroscopy capabilities of the Reuven Ramaty high energy solar spectroscopic imager (RHESSI) enable the examination of the accelerated electron distribution throughout a solar flare region. In particular, it has been revealed that the energisation of these particles takes place over a region of finite size, sometimes resolved by RHESSI observations. In this paper, we present, for the first time, a spatially distributed acceleration model and investigate the role of inhomogeneous acceleration on the observed X-ray emission properties. We have modelled transport explicitly examining scatter-free and diffusive transport within the acceleration region and compare with the analytic leaky-box solution. The results show the importance of including this spatial variation when modelling electron acceleration in solar flares. The presence of an inhomogeneous, extended acceleration region produces a spectral index that is, in most cases, different from the simple leaky-box prediction. In particular, it results in a generally softer spectral index than predicted by the leaky-box solution, for both scatter-free and diffusive transport, and thus should be taken into account when modelling stochastic acceleration in solar flares.

Authors: Duncan J. Stackhouse and Eduard P. Kontar
Projects: RHESSI

Publication Status: Accepted tp Astronomy and Astrophysics journal
Last Modified: 2018-01-16 11:10
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Heating and cooling of coronal loops with turbulent suppression of parallel heat conduction  

Eduard Kontar   Submitted: 2017-12-01 06:07

Using the "enthalpy-based thermal evolution of loops" (EBTEL) model, we investigate the hydrodynamics of the plasma in a flaring coronal loop in which heat conduction is limited by turbulent scattering of the electrons that transport the thermal heat flux. The EBTEL equations are solved analytically in each of the two (conduction-dominated and radiation-dominated) cooling phases. Comparison of the results with typical observed cooling times in solar flares shows that the turbulent mean free-path λT lies in a range corresponding to a regime in which classical (collision-dominated) conduction plays at most a limited role. We also consider the magnitude and duration of the heat input that is necessary to account for the enhanced values of temperature and density at the beginning of the cooling phase and for the observed cooling times. We find through numerical modeling that in order to produce a peak temperature ≃ 1.5 x 107 K and a 200 s cooling time consistent with observations, the flare heating profile must extend over a significant period of time; in particular, its lingering role must be taken into consideration in any description of the cooling phase. Comparison with observationally-inferred values of post-flare loop temperatures, densities, and cooling times thus leads to useful constraints on both the magnitude and duration of the magnetic energy release in the loop, as well as on the value of the turbulent mean free-path λT.

Authors: Bian, Nicolas; Emslie, A. Gordon; Horne, Duncan; Kontar, Eduard P.
Projects: RHESSI

Publication Status: accepted for publication in ApJ
Last Modified: 2017-12-01 11:37
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Imaging Spectroscopy of Solar Radio Burst Fine Structures  

Eduard Kontar   Submitted: 2017-11-16 00:25

Solar radio observations provide a unique diagnostic of the outer solar atmosphere. However, the inhomogeneous turbulent corona strongly affects the propagation of the emitted radio waves, so decoupling the intrinsic properties of the emitting source from the effects of radio-wave propagation has long been a major challenge in solar physics. Here we report quantitative spatial and frequency characterization of solar radio burst fine structures observed with the LOw Frequency Array (LOFAR), an instrument with high time resolution that also permits imaging at scales much shorter than those corresponding to radio-wave propagation in the corona. The observations demonstrate that radio-wave propagation effects, and not the properties of the intrinsic emission source, dominate the observed spatial characteristics of radio burst images. These results permit more accurate estimates of source brightness temperatures, and open opportunities for quantitative study of the mechanisms that create the turbulent coronal medium through which the emitted radiation propagates.

Authors: E. P. Kontar, S. Yu, A. A. Kuznetsov, A. G. Emslie, B. Alcock, N. L. S. Jeffrey, V. N. Melnik, N. H. Bian, P. Subramanian
Projects: Other,SDO-AIA

Publication Status: https://www.nature.com/articles/s41467-017-01307-8
Last Modified: 2017-11-16 23:31
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Turbulent kinetic energy in the energy balance of a solar flare  

Eduard Kontar   Submitted: 2017-04-01 08:49

The energy released in solar flares derives from a reconfiguration of magnetic fields to a lower energy state, and is manifested in several forms, including bulk kinetic energy of the coronal mass ejection, acceleration of electrons and ions, and enhanced thermal energy that is ultimately radiated away across the electromagnetic spectrum from optical to X-rays. Using an unprecedented set of coordinated observations, from a suite of instruments, we here report on a hitherto largely overlooked energy component - the kinetic energy associated with small-scale turbulent mass motions. We show that the spatial location of, and timing of the peak in, turbulent kinetic energy together provide persuasive evidence that turbulent energy may play a key role in the transfer of energy in solar flares. Although the kinetic energy of turbulent motions accounts, at any given time, for only ~ (0.5-1)% of the energy released, its relatively rapid (~1-10 s) energization and dissipation causes the associated throughput of energy (i.e., power) to rival that of major components of the released energy in solar flares, and thus presumably in other astrophysical acceleration sites.

Authors: Kontar, E. P.; Perez, J. E.; Harra, L. K.; Kuznetsov, A. A.; Emslie, A. G.; Jeffrey, N. L. S.; Bian, N. H.; Dennis, B. R.
Projects: None,Hinode/EIS,RHESSI,SDO-AIA,SDO-HMI

Publication Status: to be published in Physical Review Letters
Last Modified: 2017-04-02 19:06
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The role of diffusion in the transport of energetic electrons during solar flares  

Eduard Kontar   Submitted: 2017-01-04 14:08

The transport of the energy contained in suprathermal electrons in solar flares plays a key role in our understanding of many aspects of flare physics, from the spatial distributions of hard X-ray emission and energy deposition in the ambient atmosphere to global energetics. Historically the transport of these particles has been largely treated through a deterministic approach, in which first-order secular energy loss to electrons in the ambient target is treated as the dominant effect, with second-order diffusive terms (in both energy and angle) being generally either treated as a small correction or even neglected. We here critically analyze this approach, and we show that spatial diffusion through pitch-angle scattering necessarily plays a very significant role in the transport of electrons. We further show that a satisfactory treatment of the diffusion process requires consideration of non-local effects, so that the electron flux depends not just on the local gradient of the electron distribution function but on the value of this gradient within an extended region encompassing a significant fraction of a mean free path. Our analysis applies generally to pitch-angle scattering by a variety of mechanisms, from Coulomb collisions to turbulent scattering. We further show that the spatial transport of electrons along the magnetic field of a flaring loop can be modeled rather effectively as a Continuous Time Random Walk with velocity-dependent probability distribution functions of jump sizes and occurrences, both of which can be expressed in terms of the scattering mean free path.

Authors: Bian, Nicolas H., Emslie, A. Gordon, and Kontar, Eduard P.
Projects: RHESSI

Publication Status: Accepted to ApJ
Last Modified: 2017-01-11 12:09
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Suppression of parallel transport in turbulent magnetized plasmas and its impact on non-thermal and thermal aspects of solar flares  

Eduard Kontar   Submitted: 2016-03-30 02:14

The transport of the energy contained in electrons, both thermal and suprathermal, in solar flares plays a key role in our understanding of many aspects of the flare phenomenon, from the spatial distribution of hard X-ray emission to global energetics. Motivated by recent RHESSI observations that point to the existence of a mechanism that confines electrons to the coronal parts of flare loops more effectively than Coulomb collisions, we here consider the impact of pitch-angle scattering off turbulent magnetic fluctuations on the parallel transport of electrons in flaring coronal loops. It is shown that the presence of such a scattering mechanism in addition to Coulomb collisional scattering can significantly reduce the parallel thermal and electrical conductivities relative to their collisional values. We provide illustrative expressions for the resulting thermoelectric coefficients that relate the thermal flux and electrical current density to the temperature gradient and the applied electric field. We then evaluate the effect of these modified transport coefficients on the flare coronal temperature that can be attained, on the post-impulsive-phase cooling of heated coronal plasma, and on the importance of the beam-neutralizing return current on both ambient heating and the energy loss rate of accelerated electrons. We also discuss the possible ways in which anomalous transport processes have an impact on the required overall energy associated with accelerated electrons in solar flares.

Authors: Bian, Nicolas H., Kontar, Eduard P., and Emslie, A. Gordon
Projects: RHESSI,SDO-AIA

Publication Status: accepted to ApJ
Last Modified: 2016-03-30 20:55
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Collisional relaxation of electrons in a warm plasma and accelerated nonthermal electron spectra in solar flares  

Eduard Kontar   Submitted: 2015-07-08 16:08

Extending previous studies of nonthermal electron transport in solar flares which include the effects of collisional energy diffusion and thermalization of fast electrons, we present an analytic method to infer more accurate estimates of the accelerated electron spectrum in solar flares from observations of the hard X-ray spectrum. Unlike for the standard cold-target model, the spatial characteristics of the flaring region, especially the necessity to consider a finite volume of hot plasma in the source, need to be taken into account in order to correctly obtain the injected electron spectrum from the source-integrated electron flux spectrum (a quantity straightforwardly obtained from hard X-ray observations). We show that the effect of electron thermalization can be significant enough to nullify the need to introduce an ad hoc low-energy cutoff to the injected electron spectrum in order to keep the injected power in non-thermal electrons at a reasonable value. Rather the suppression of the inferred low-energy end of the injected spectrum compared to that deduced from a cold-target analysis allows the inference from hard X-ray observations of a more realistic energy in injected non-thermal electrons in solar flares.

Authors: Kontar, E. P.; Jeffrey, N. L. S.; Emslie, A. G.; Bian, N. H.
Projects: RHESSI

Publication Status: accepted to ApJ
Last Modified: 2015-07-10 20:38
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The Formation of Kappa-Distribution Accelerated Electron Populations in Solar Flares  

Eduard Kontar   Submitted: 2014-10-07 02:29

Driven by recent RHESSI observations of confined loop-top hard X-ray sources in solar flares, we consider stochastic acceleration of electrons in the presence of Coulomb collisions. If electron escape from the acceleration region can be neglected, the electron distribution function is determined by a balance between diffusive acceleration and collisions. Such a scenario admits a stationary solution for the electron distribution function that takes the form of a kappa distribution. We show that the evolution toward this kappa distribution involves a "wave front" propagating forwards in velocity space, so that electrons of higher energy are accelerated later; the acceleration time scales with energy according to τ\rm acc ~ E3/2. At sufficiently high energies escape from the finite-length acceleration region will eventually dominate. For such energies, the electron velocity distribution function is obtained by solving a time-dependent Fokker-Planck equation in the "leaky-box" approximation. Solutions are obtained in the limit of a small escape rate from an acceleration region that can effectively be considered a thick target.

Authors: Bian, Nicolas H., Emslie, A. Gordon, Stackhouse, Duncan J., Kontar, Eduard P.
Projects: RHESSI

Publication Status: accepted in ApJ
Last Modified: 2014-10-07 10:58
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Turbulent pitch-angle scattering and diffusive transport of hard-X-ray producing electrons in flaring coronal loops  

Eduard Kontar   Submitted: 2013-12-11 23:26

Recent observations from {em RHESSI} have revealed that the number of non-thermal electrons in the coronal part of a flaring loop can exceed the number of electrons required to explain the hard X-ray-emitting footpoints of the same flaring loop. Such sources cannot, therefore, be interpreted on the basis of the standard collisional transport model, in which electrons stream along the loop while losing their energy through collisions with the ambient plasma; additional physical processes, to either trap or scatter the energetic electrons, are required. Motivated by this and other observations that suggest that high energy electrons are confined to the coronal region of the source, we consider turbulent pitch angle scattering of fast electrons off low frequency magnetic fluctuations as a confinement mechanism, modeled as a spatial diffusion parallel to the mean magnetic field. In general, turbulent scattering leads to a reduction of the collisional stopping distance of non-thermal electrons along the loop and hence to an enhancement of the coronal HXR source relative to the footpoints. The variation of source size L with electron energy E becomes weaker than the quadratic behavior pertinent to collisional transport, with the slope of L(E) depending directly on the mean free path lambda again pitch angle scattering. Comparing the predictions of the model with observations, we find that lambda sim(108-109) cm for sim30 keV, less than the length of a typical flaring loop and smaller than, or comparable to, the size of the electron acceleration region.

Authors: Kontar, E. P.; Bian, N. H.; Emslie, A. G.; Vilmer, N.
Projects: RHESSI

Publication Status: accepted in ApJ
Last Modified: 2013-12-12 11:38
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Electron acceleration during three-dimensional relaxation of an electron beam-return current plasma system in a magnetic field  

Eduard Kontar   Submitted: 2012-07-29 12:13

We investigate the effects of acceleration during non-linear electron-beam relaxation in magnetized plasma in the case of electron transport in solar flares. The evolution of electron distribution functions is computed using a three-dimensional particle-in-cell electromagnetic code. Analytical estimations under simplified assumptions are made to provide comparisons. We show that, during the non-linear evolution of the beam-plasma system, the accelerated electron population appears. We found that, although the electron beam loses its energy efficiently to the thermal plasma, a noticeable part of the electron population is accelerated. For model cases with initially monoenergetic beams in uniform plasma, we found that the amount of energy in the accelerated electrons above the injected beam-electron energy varies depending the plasma conditions and could be around 10-30% of the initial beam energy. This type of acceleration could be important for the interpretation of non-thermal electron populations in solar flares. Its neglect could lead to the over-estimation of accelerated electron numbers. The results emphasize that collective plasma effects should not be treated simply as an additional energy-loss mechanism, when hard X-ray emission in solar flares is interpreted, notably in the case of RHESSI data.

Authors: M. Karlický and E.P. Kontar
Projects: RHESSI

Publication Status: accepted for publication in A&A
Last Modified: 2012-07-30 16:11
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A Classification Scheme For Turbulent Acceleration Processes In Solar Flares  

Eduard Kontar   Submitted: 2012-06-07 13:02

We establish a classification scheme for stochastic acceleration models involving low-frequency plasma turbulence in a strongly magnetized plasma. This classification takes into account both the properties of the accelerating electromagnetic field, and the nature of the transport of charged particles in the acceleration region. We group the acceleration processes as either resonant, non-resonant or resonant-broadened, depending on whether the particle motion is free-streaming along the magnetic field, diffusive or a combination of the two. Stochastic acceleration by moving magnetic mirrors and adiabatic compressions are addressed as illustrative examples. We obtain expressions for the momentum-dependent diffusion coefficient D(p), both for general forms of the accelerating force and for the situation when the electromagnetic force is wave-like, with a specified dispersion relation omega=omega(k). Finally, for models considered, we calculate the energy-dependent acceleration time, a quantity that can be directly compared with observations of the time profile of the radiation field produced by the accelerated particles, such as during solar flares.

Authors: N. H. Bian, A. G. Emslie, and E. P. Kontar
Projects: RHESSI

Publication Status: submitted to ApJ
Last Modified: 2012-06-14 13:55
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Turbulent cross-field transport of non-thermal electrons in coronal loops: theory and observations  

Eduard Kontar   Submitted: 2012-02-01 15:17

Context. A fundamental problem in astrophysics is the interactionbetween magnetic turbulence and charged particles. It is now possibleto use Ramaty High Energy Solar Spectroscopic Imager (RHESSI)observations of hard X-rays (HXR) emitted by electrons to identify thepresence of turbulence and to estimate the magnitude of the magneticfield line diffusion coefficient at least in dense coronal flaringloops.
Aims: We discuss the various possible regimes of cross-field transportof non-thermal electrons resulting from broadband magnetic turbulencein coronal loops. The importance of the Kubo number K as a governingparameter is emphasized and results applicable in both the large andsmall Kubo number limits are collected.
Methods: Generic models, based on concepts and insights developed inthe statistical theory of transport, are applied to the coronal loopsand to the interpretation of hard X-ray imaging data in solar flares.The role of trapping effects, which become important in the non-linearregime of transport, is taken into account in the interpretation ofthe data.
Results: For this flaring solar loop, we constrain the ranges ofparallel and perpendicular correlation lengths of turbulent magneticfields and possible Kubo numbers. We show that a substantial amount ofmagnetic fluctuations with energy ~1% (or more) of the backgroundfield can be inferred from the measurements of the magnetic diffusioncoefficient inside thick-target coronal loops.

Authors: Bian, N. H.; Kontar, E. P.; MacKinnon, A. L.
Projects: RHESSI

Publication Status: published in Astronomy and Astrophysics
Last Modified: 2012-02-03 12:03
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Wave-particle interactions in non-uniform plasma and the interpretation of Hard X-ray spectra in solar flares  

Eduard Kontar   Submitted: 2012-01-04 04:26

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Kontar, E. P.; Ratcliffe, H.; Bian, N. H.
Projects: RHESSI

Publication Status: accepted to Astronomy and Astrophysics
Last Modified: 2012-01-04 12:15
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Acceleration, magnetic fluctuations and cross-field transport of energetic electrons in a solar flare loop  

Eduard Kontar   Submitted: 2011-02-23 04:14

Plasma turbulence is thought to be associated with various physical processes involved in solar flares, including magnetic reconnection, particle acceleration and transport. Using Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations and the X-ray visibility analysis, we determine the spatial and spectral distributions of energetic electrons for a flare (GOES M3.7 class, April 14, 2002 23:55 UT), which was previously found to be consistent with a reconnection scenario. It is demonstrated that because of the high density plasma in the loop, electrons have to be continuously accelerated about the loop apex of length ∼2x109cm and width ∼7x108cm. Energy dependent transport of tens of keV electrons is observed to occur both along and across the guiding magnetic field of the loop. We show that the cross-field transport is consistent with the presence of magnetic turbulence in the loop, where electrons are accelerated, and estimate the magnitude of the field line diffusion coefficient for different phases of the flare. The energy density of magnetic fluctuations is calculated for given magnetic field correlation lengths and is larger than the energy density of the non-thermal electrons. The level of magnetic fluctuations peaks when the largest number of electrons is accelerated and is below detectability or absent at the decay phase. These hard X-ray observations provide the first observational evidence that magnetic turbulence governs the evolution of energetic electrons in a dense flaring loop and is suggestive of their turbulent acceleration.

Authors: Kontar, E. P., Hannah, I. G., Bian, N. H.
Projects: RHESSI

Publication Status: ApJ Letters, in press
Last Modified: 2011-02-23 09:39
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The sub-arcsecond hard X-ray structure of loop footpoints in a solar flare  

Eduard Kontar   Submitted: 2010-05-22 10:41

The newly developed X-ray visibility forward fitting technique is applied to Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data of a limb flare to investigate the energy and height dependence on sizes, shapes, and position of hard X-ray chromospheric footpoint sources. This provides information about the electron transport and chromospheric density structure. The spatial distribution of two footpoint X-ray sources is analyzed using PIXON, Maximum Entropy Method, CLEAN and visibility forward fit algorithms at nonthermal energies from sim 20 to sim 200 keV. We report, for the first time, the vertical extents and widths of hard X-ray chromospheric sources measured as a function of energy for a limb event. Our observations suggest that both the vertical and horizontal sizes of footpoints are decreasing with energy. Higher energy emission originates progressively deeper in the chromosphere consistent with downward flare accelerated streaming electrons. The ellipticity of the footpoints grows with energy from sim 0.5 at sim 20 keV to sim 0.9 at sim 150 keV. The positions of X-ray emission are in agreement with an exponential density profile of scale height sim 150 km. The characteristic size of the hard X-ray footpoint source along the limb is decreasing with energy suggesting a converging magnetic field in the footpoint. The vertical sizes of X-ray sources are inconsistent with simple collisional transport in a single density scale height but can be explained using a multi-threaded density structure in the chromosphere.

Authors: Kontar, E. P., Hannah, I. G., Jeffrey, N. L. S., Battaglia, M.
Projects: RHESSI

Publication Status: Submitted to ApJ
Last Modified: 2010-05-23 14:14
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Local re-acceleration and a modified thick target model of solar flare electrons  

Eduard Kontar   Submitted: 2009-10-13 08:14

The collisional thick target model (CTTM) of solar hard X-ray (HXR) bursts has become an almost 'Standard Model' of flare impulsive phase energy transport and radiation. However, it faces various problems in the light of recent data, particularly the high electron beam density and anisotropy it involves.} {We consider how photon yield per electron can be increased, and hence fast electron beam intensity requirements reduced, by local re-acceleration of fast electrons throughout the HXR source itself, after injection.} {We show parametrically that, if net re-acceleration rates due to e.g. waves or local current sheet electric ({cal E}) fields are a significant fraction of collisional loss rates, electron lifetimes, and hence the net radiative HXR output per electron can be substantially increased over the CTTM values. In this local re-acceleration thick target model (LRTTM) fast electron number requirements and anisotropy are thus reduced. One specific possible scenario involving such re-acceleration is discussed, viz, a current sheet cascade (CSC) in a randomly stressed magnetic loop.} {Combined MHD and test particle simulations show that local {cal E} fields in CSCs can efficiently accelerate electrons in the corona and and re-accelerate them after injection into the chromosphere. In this HXR source scenario, rapid synchronisation and variability of impulsive footpoint emissions can still occur since primary electron acceleration is in the high Alfvén speed corona with fast re-acceleration in chromospheric CSCs. It is also consistent with the energy-dependent time-of-flight delays in HXR features.

Authors: Brown, J. C.; Turkmani, R.; Kontar, E. P.; MacKinnon, A. L.; Vlahos, L.
Projects: RHESSI

Publication Status: accepted for publication in Astronomy and Astrophysics
Last Modified: 2009-10-13 08:25
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Onsets and spectra of impulsive solar energetic electron events observed near the Earth  

Eduard Kontar   Submitted: 2009-03-19 03:20

Impulsive solar energetic electrons are often observed in the interplanetary space near the Earth and have an attractive diagnostic potential for poorly understood solar flare acceleration processes. We investigate the transport of solar flare energetic electrons in the heliospheric plasma to understand the role of transport to the observed onset and spectral properties of the impulsive solar electron events. The propagation of energetic electrons in solar wind plasma is simulated from the acceleration region at the Sun to the Earth, taking into account self-consistent generation and absorption of electrostatic electron plasma (Langmuir) waves, effects of non-uniform plasma, collisions and Landau damping. The simulations suggest that the beam-driven plasma turbulence and the effects of solar wind density inhomogeneity play a crucial role and lead to the appearance of a) spectral break for a single power-law injected electron spectrum, with the spectrum flatter below the break, b) apparent early onset of low-energy electron injection, c) the apparent late maximum of low-energy electron injection. We show that the observed onsets, spectral flattening at low energies, and formation of a break energy at tens of keV is the direct manifestation of wave-particle interactions in non-uniform plasma of a single accelerated electron population with an initial power-law spectrum.

Authors: Kontar, E. P. and Reid, H. A. S.
Projects: RHESSI

Publication Status: accepted to ApJ Letters
Last Modified: 2009-03-19 10:39
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Chromospheric magnetic field and density structure measurements using hard X-rays in a flaring coronal loop  

Eduard Kontar   Submitted: 2008-08-28 13:27

A novel method of using hard X-rays as a diagnostic for chromospheric density and magnetic structures is developed to infer sub-arcsecond vertical variation of magnetic flux tube size and neutral gas density.Using Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) X-ray data and the newly developed X-ray visibilities forward fitting technique we find the FWHM and centroid positions of hard X-ray sources with sub-arcsecond resolution (sim 0.2'') for a solar limb flare. We show that the height variations of the chromospheric density and the magnetic flux densities can be found with unprecedented vertical resolution of sim 150 km by mapping 18-250 keV X-ray emission of energetic electrons propagating in the loop at chromospheric heights of 400-1500 km. Our observations suggest that the density of the neutral gas is in good agreement with hydrostatic models with a scale height of around 140pm 30 km. FWHM sizes of the X-ray sources decrease with energy suggesting the expansion (fanning out) of magnetic flux tube in the chromosphere with height. The magnetic scale height B(z)(dB/dz)-1 is found to be of the order of 300 km and strong horizontal magnetic field is associated with noticeable flux tube expansion at a height of sim 900 km.

Authors: Kontar, E. P., Hannah, I. G., and MacKinnon, A. L.
Projects: RHESSI

Publication Status: Astronomy and Astrophysics in press
Last Modified: 2008-08-28 21:18
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Low-Energy Cutoffs in Electron Spectra of Solar Flares: Statistical Survey  

Eduard Kontar   Submitted: 2008-07-02 03:22

The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) X-ray data base (February 2002 - May 2006) has been searched to find solar flares with weak thermal components and flat photon spectra. Using a regularised inversion technique, we determine the mean electron flux distribution from count spectra for a selection of events with flat photon spectra in the 15-20 keV energy range. Such spectral behaviour is expected for photon spectra either affected by photospheric albedo or produced by electron spectra with an absence of electrons in a given energy range, e.g. a low-energy cutoff in the mean electron spectra of non-themal particles. We have found 18 cases which exhibit a statistically significant local minimum (a dip) in the range of 13-19 keV. The positions and spectral indices of events with low-energy cutoff indicate that such features are likely to be the result of photospheric albedo. It is shown that if the isotropic albedo correction is applied, all low-energy cutoffs in the mean electron spectrum are removed and hence the low energy cutoffs in the mean electron spectrum of solar flares above sim12 keV cannot be viewed as real features. If low-energy cutoffs exist in the mean electron spectra, their energies should be less than sim12 keV.

Authors: Kontar, E.P., Dickson, E., Kasparova, J.
Projects: RHESSI

Publication Status: submitted to Solar Physics
Last Modified: 2008-09-23 20:57
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Stereoscopic electron spectroscopy of solar hard X-ray flares with a single spacecraft  

Eduard Kontar   Submitted: 2006-11-06 05:36

Hard X-ray (HXR) spectroscopy is the most direct method of diagnosing energetic electrons in solar flares. Here we present a technique which allows us to use a single HXR spectrum to determine an effectively stereoscopic electron energy distribution. Considering the Sun's surface to act as a 'Compton mirror' allows us to look at emitting electrons also from behind the source, providing vital information on downward-propagating particles. Using this technique we determine simultaneously the electron spectra of downward and upward directed electrons for two solar flares observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The results reveal surprisingly near-isotropic electron distributions, which contrast strongly with the expectations from the standard model which invokes strong downward beaming, including collisional thick-target model.

Authors: Eduard P. Kontar and John C. Brown
Projects: RHESSI

Publication Status: Accepted to ApJ Letters
Last Modified: 2006-11-09 02:58
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Abstracts by Author
Spatially inhomogeneous acceleration of electrons in solar flares
Heating and cooling of coronal loops with turbulent suppression of parallel heat conduction
Imaging Spectroscopy of Solar Radio Burst Fine Structures
Turbulent kinetic energy in the energy balance of a solar flare
The role of diffusion in the transport of energetic electrons during solar flares
Suppression of parallel transport in turbulent magnetized plasmas and its impact on non-thermal and thermal aspects of solar flares
Collisional relaxation of electrons in a warm plasma and accelerated nonthermal electron spectra in solar flares
The Formation of Kappa-Distribution Accelerated Electron Populations in Solar Flares
Turbulent pitch-angle scattering and diffusive transport of hard-X-ray producing electrons in flaring coronal loops
Electron acceleration during three-dimensional relaxation of an electron beam-return current plasma system in a magnetic field
A Classification Scheme For Turbulent Acceleration Processes In Solar Flares
Turbulent cross-field transport of non-thermal electrons in coronal loops: theory and observations
Wave-particle interactions in non-uniform plasma and the interpretation of Hard X-ray spectra in solar flares
Acceleration, magnetic fluctuations and cross-field transport of energetic electrons in a solar flare loop
The sub-arcsecond hard X-ray structure of loop footpoints in a solar flare
Local re-acceleration and a modified thick target model of solar flare electrons
Onsets and spectra of impulsive solar energetic electron events observed near the Earth
Chromospheric magnetic field and density structure measurements using hard X-rays in a flaring coronal loop
Low-Energy Cutoffs in Electron Spectra of Solar Flares: Statistical Survey
Stereoscopic electron spectroscopy of solar hard X-ray flares with a single spacecraft
Compton backscattered and primary X-rays from solar flares: angle dependent Green's function correction for photospheric albedo
Solar Flare Hard X-ray Spectra Possibly Inconsistent with the Collisional Thick Target Model
Regularized Energy-Dependent Solar Flare Hard X-Ray Spectral Index
Determination Of Electron Flux Spectra In A Solar Flare With An Augmented Regularization Method: Application To RHESSI Data
Generalized Regularization Techniques With Constraints For The Analysis Of Solar Bremsstrahlung X-Ray Spectra
Nonuniform Target Ionisation and Fitting Thick Target Electron Injection Spectra to RHESSI Data
Nonuniform Target Ionisation and Fitting Thick Target Electron Injection Spectra to RHESSI Data
Nonuniform Target Ionisation and Fitting Thick Target Electron Injection Spectra to RHESSI Data
An Explanation for Non-Power-law Behavior in the Hard X-ray Spectrum of the July 23, 2002 Solar

Related Pages
MSU Solar Physics.
Max Millennium Science Mail Archive.
Max Millennium Message of the Day Mail Archive.
Max Millennium Flare Catalog

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