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.
Publication Status: submitted to Solar Physics
Last Modified: 2008-09-23 20:57