Low-energy cutoffs in electron spectra of solar flares: Statistical survey

Kontar, E.P., Dickson, E. and Kasparova, J. (2008) Low-energy cutoffs in electron spectra of solar flares: Statistical survey. Solar Physics, 252(1), pp. 139-147. (doi: 10.1007/s11207-008-9249-x)

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The Reuven Ramaty High Energy 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 regularized inversion technique, we determine the mean electron flux distribution from count spectra for a selection of events with flat pho- ton spectra in the 15 – 20 keV energy range. Such spectral behavior is expected for photon spectra either affected by photospheric albedo or produced by electron spectra with an ab- sence of electrons in a given energy range (e.g., a low-energy cutoff in the mean electron spectra of nonthemal particles). We have found 18 cases that exhibit a statistically signifi- cant 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 photo- spheric 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 ∼ 12 keV cannot be viewed as real features. If low-energy cutoffs exist in the mean electron spectra, their energies should be less than ∼ 12 keV.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Kontar, Professor Eduard and Dickson, Mr Ewan and Kasparova, Dr Jana
Authors: Kontar, E.P., Dickson, E., and Kasparova, J.
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Solar Physics
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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
398461Acceleration, propagation and emission of energetic particles from the sun to the earthEduard KontarParticle Physics & Astronomy Research Council (PPARC)PP/C001656/1Physics and Astronomy
465931Solar, stellar and cosmological plasmas: a synthesis of data, modelling and theory.Declan DiverScience & Technologies Facilities Council (STFC)ST/F002149/1Physics and Astronomy