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Diagnostics of non-thermal distributions in solar flare spectra observed by RESIK and RHESSI  

Jana Kasparova   Submitted: 2011-09-07 05:18

We focus on the non-thermal components of the electron distribution in thekeV range and analyse high-energy resolution X-ray spectra detected by RESIKand RHESSI for three solar flares.In the 2-4 keV range we assume that theelectron distribution can be modelled by an n-distribution. Using a method ofline-intensity ratios, we analyse allowed and satellite lines of Si observed byRESIK and estimate the parameters of this n-distribution. At higher energies weexplore RHESSI bremsstrahlung spectra. Adopting a forward-fitting approach andthick-target approximation, we determine the characteristics of injectedelectron beams. RHESSI non-thermal component associated with the electron beamis correlated well with presence of the non-thermal n-distribution obtainedfrom the RESIK spectra. In addition, such an n-distribution occurs during radiobursts observed in the 0.61-15.4 GHz range. Furthermore, we show that then-distribution could also explain RHESSI emission below ~5 keV. Therefore, twoindependent diagnostics methods indicate the flare plasma being affected by theelectron beam can have a non-thermal component in the ~2-5 keV range, which isdescribed by the n-distribution well. Finally, spectral line analysis revealsthat the n-distribution does not occupy the same location as the thermalcomponent detected by RHESSI at ~10 keV.

Authors: Kulinova,A., Kasparova, J., Dzifcakova, E., Sylwester, J., Sylwester, B., Karlický, M.
Projects: RHESSI

Publication Status: A&A (in pres)
Last Modified: 2011-09-07 05:42
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Beam-plasma system and its X-ray directivity  

Jana Kasparova   Submitted: 2009-09-02 05:22

An evolution of the electron distribution function in the beam-plasma system with the return current is computed numerically for different parameters. The X-ray bremsstrahlung corresponding to such an electron distribution is calculated and the directivity of the X-ray emission is studied. For computations of the electron distribution functions we used a 3-D particle-in-cell electromagnetic code. The directivity of the X-ray emission was calculated using the angle-dependent electron-ion bremsstrahlung cross-section. It was found that the resulting electron distribution function depends on the magnetic field assumed along the electron beam propagation direction. For small magnetic fields the electron distribution function becomes broad in the direction perpendicular to the beam propagation due to the Weibel instability and the return current is formed by the electrons in a broad and shifted bulk of the distribution. On the other hand, for stronger magnetic fields the distribution is more extended in the beam-propagation direction and the return current is formed by the electrons in the extended distribution tail. In all cases, the anisotropy of the electron distribution decreases rapidly due to fast collisionless processes. However, the magnetic field reduces this anisotropy decrease. The X-ray directivity shows the same trend and it is always closer to the isotropic case than that in a simple beaming model.

Authors: Karlický, M., Kasparova, J.
Projects: None

Publication Status: A&A (accepted)
Last Modified: 2009-09-02 08:10
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Response of optical hydrogen lines to beam heating: I. Electron beams  

Jana Kasparova   Submitted: 2009-04-15 02:12

We investigate the role of non-thermal electrons in the formation regions of Hα , Hbeta, and Hgamma lines in order to unfold their influence on the formation of these lines. We concentrate on pulse-beam heating varying on a subsecond timescale. Furthermore, we theoretically explore possibility that a new diagnostic tool exists indicating the presence of non-thermal electrons in the flaring chromosphere based on observations of optical hydrogen lines. To model the evolution of the flaring atmosphere and the time-dependent hydrogen excitation and ionisation, we used a 1-D radiative hydrodynamic code combined with a test-particle code that simulates the propagation, scattering, and thermalisation of a power-law electron beam in order to obtain the flare heating and the non-thermal collisional rates due to the interaction of the beam with the hydrogen atoms. All calculated models have shown a time-correlated response of the modelled Balmer line intensities on a subsecond timescale, with a subsecond timelag behind the beam flux. Depending on the beam parameters, both line centres and wings can show pronounced intensity variations. The non-thermal collisional rates generally result in an increased emission from a secondary region formed in the chromosphere.

Authors: Kasparova, J., Varady M., Heinzel, P., Karlický, M., Moravec Z.
Projects: None

Publication Status: A&A (accepted)
Last Modified: 2009-04-15 10:05
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Kappa distribution and hard X-ray emission of solar flares  

Jana Kasparova   Submitted: 2009-03-02 03:15

We investigate whether the so-called kappa distribution, often used to fit electron distributions detected in-situ in the solar wind, can describe electrons producing the hard X-ray emission in solar flares. Using Ramaty High Energy Solar Spectroscopic imager (RHESSI) flare data we fit spatially- and feature-integrated spectra, assuming kappa distribution for the mean electron flux spectrum. We show that a single kappa distribution generally cannot describe spatially integrated X-ray emission composed of both footpoint and coronal sources. In contrast, the kappa distribution is consistent with mean electron spectra producing hard X-ray emission in some coronal sources.

Authors: Kasparova, J., Karlický, M.
Projects: RHESSI

Publication Status: A&AL (accepted)
Last Modified: 2009-03-13 03:54
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Far-IR and radio thermal continua in solar flares  

Jana Kasparova   Submitted: 2009-01-26 05:17

With the invention of new far-infrared (FIR) and radio mm and sub-mm instruments (DESIR on SMESE satellite, ESO-ALMA), there is a growing interest in observations and analysis of solar flares in this so far unexplored wavelength region. Two principal radiation mechanisms play a role: the synchrotron emission due to accelerated particle beams moving in the magnetic field and the thermal emission due to the energy deposit in the lower atmospheric layers. In this contribution we explore the time-dependent effects of beams on thermal FIR and radio continua. We show how and where these continua are formed in the presence of time dependent beam heating and non-thermal excitation/ionisation of the chromospheric hydrogen plasma.

Authors: Kasparova, J., Heinzel, P., Karlický, M., Moravec Z., Varady M.
Projects: None

Publication Status: accepted to Central European Astrophysical Bulletin
Last Modified: 2009-01-26 08:33
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Subject will be restored when possible  

Jana Kasparova   Submitted: 2008-02-06 03:16

Hard X-Ray bremsstrahlung continuum spectra, such as from solar flares, are commonly described in terms of power-law fits, either to the photon spectra themselves or to the electron spectra responsible for them. In applications various approximate relations between electron and photon spectral indices are often used for energies both above and below electron low-energy cutoffs. We examine the form of the exact relationships in various situations, and for various cross-sections, showing that empirical relations sometimes used can be highly misleading and consider how to improve fitting procedures. We obtain expressions for photon spectra from single, double and truncated power-law electron spectra for a variety of cross-sections and for the thin and thick target models and simple analytic expressions for the Bethe-Heitler cases. We show that above a low-energy cutoff the Kramers and Bethe-Heitler results match reasonably well with results for exact cross-sections up to energies around 100 keV; that below the low-energy cutoff, Kramers and other constant spectral index forms commonly used are very poor approximations to accurate results; but that our analytical forms are a very good match. Analytical forms of the Bethe-Heitler photon spectra from general power-law electron spectra are an excellent match to exact results for both thin and thick targets and they enable much faster spectral fitting than evaluation of the full spectral integrations

Authors: Brown, J. C., Kasparova, J., Massone, A.M., Piana, M.
Projects: RHESSI

Publication Status: A&A (submitted)
Last Modified: 2008-02-06 05:32
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Hard X-ray Spectra and Positions of Solar Flares observed by RHESSI: photospheric albedo, directivity and electron spectra  

Jana Kasparova   Submitted: 2007-01-31 03:14

We investigate the signature of the photospheric albedo contribution in solar flare hard X-ray spectra, the effect of low energy cutoffs in electron spectra, and the directivity of hard X-ray emission. Using Ramaty High Energy Solar Spectroscopic Imager (RHESSI) flare data we perform a statistical analysis of spatially integrated spectra and positions of solar flares. We demonstrate clear centre-to-limb variation of photon spectral indices in the 15-20 keV energy range and a weaker dependency in the 20-50 keV range which is consistent with photospheric albedo as the cause. The results also suggest that low-energy cutoffs sometimes inferred in mean electron spectra are an artefact of albedo. We also derive the anisotropy (ratio of downward/observer directed photons) of hard X-ray emission in the 15-20 keV range for various heliocentric angles.

Authors: J. Kasparova, E. P. Kontar, J. C. Brown
Projects: RHESSI

Publication Status: revised manuscript to A&A
Last Modified: 2007-01-31 10:52
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Multi-wavelength analysis of high energy electrons in solar flares: a case study of August 20, 2002 flare  

Jana Kasparova   Submitted: 2005-09-05 13:42

A multi-wavelength spatial and temporal analysis of solar high energy electrons is conducted using the August 20, 2002 flare of an unusually flat (gamma=1.8) hard X-ray spectrum. The flare is studied using RHESSI, Hα , radio, TRACE, and MDI observations with advanced methods and techniques never previously applied in the solar flare context. A new method to account for X-ray Compton backscattering in the photosphere (photospheric albedo) has been used to deduce the primary X-ray flare spectra. The mean electron flux distribution has been analysed using both forward fitting and model independent inversion methods of spectral analysis. We show that the contribution of the photospheric albedo to the photon spectrum modifies the calculated mean electron flux distribution, mainly at energies below 100 keV. The positions of the Hα emission and hard X-ray sources with respect to the current-free extrapolation of the MDI photospheric magnetic field and the characteristics of the radio emission provide evidence of the closed geometry of the magnetic field structure and the flare process in low altitude magnetic loops. In agreement with the predictions of some solar flare models, the hard X-ray sources are located on the external edges of the Hα emission and show chromospheric plasma heated by the non-thermal electrons. The fast changes of Hα intensities are located not only inside the hard X-ray sources, as expected if they are the signatures of the chromospheric response to the electron bombardment, but also away from them.

Authors: Kasparova, J., Karlický, M., Kontar, E. P., Schwartz, R. A., Dennis, B. R.
Projects: RHESSI

Publication Status: Solar Physics (accepted)
Last Modified: 2005-09-05 13:42
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Abstracts by Author
Diagnostics of non-thermal distributions in solar flare spectra observed by RESIK and RHESSI
Beam-plasma system and its X-ray directivity
Response of optical hydrogen lines to beam heating: I. Electron beams
Kappa distribution and hard X-ray emission of solar flares
Far-IR and radio thermal continua in solar flares
Subject will be restored when possible
Hard X-ray Spectra and Positions of Solar Flares observed by RHESSI: photospheric albedo, directivity and electron spectra
Multi-wavelength analysis of high energy electrons in solar flares: a case study of August 20, 2002 flare

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