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Low-Energy Cutoffs in Electron Spectra of Solar Flares: Statistical Survey
Authors:E P Kontar  E Dickson  J Ka?parová
Institution:(1) Department of Physics & Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK;(2) Astronomicky ústav AV ČR, v.v.i., Fričova 298, 251 65 Ondřejov, Czech Republic
Abstract: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 photon 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 absence 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 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 ∼ 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.
Keywords:Flares  X-ray bursts  Spectrum  Energetic particles  Electrons
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