Low-energy Cut-offs in Electron Spectra of Solar Flares: Statistical Survey

E.P. Kontar$^1$, J. Kasparova$^2$, E. Scullion$^3$, J.C. Brown$^1$

$^1$ Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, U.K.
$^2$Astronomical Institute of the Academy of Sciences of the Czech Republic, 25165 Ondrejov, Czech Republic
$^3$Armagh Observatory, UK

Ramaty High Energy Solar Spectroscopic Imager (RHESSI) X-ray data base has been searched to find solar flares with weak thermal component and flat photon spectra. Using a regularised inversion technique, we determine the mean electron flux distribution from count spectra of the events which had either flat photon spectra or a large change in local spectral index with energy. Such spectral behavior is expected for photon spectra either affected by photospheric albedo or produced by electron spectra with an absence of electrons in some energy range, e.g. low-energy cut-off in electron spectra. We have found a number of cases which exhibit a statistically significant dip in the range of 10-30 keV. The positions and spectral indices of events with low-energy cut-off indicate that such feature could be a result of photospheric albedo. Moreover, when the isotropic albedo correction was applied, all low-energy cut-offs in electron spectrum were removed.