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

TL;DR

This study analyzes RHESSI solar flare data to investigate low-energy cutoffs in electron spectra, finding that observed features are likely due to photospheric albedo effects rather than real electron energy cutoffs.

Contribution

It demonstrates that apparent low-energy cutoffs in electron spectra are artifacts of photospheric albedo, challenging previous assumptions about electron energy distributions in solar flares.

Findings

Low-energy cutoffs above ~12 keV are not real, caused by albedo effects.

Photospheric albedo correction removes low-energy cutoffs from electron spectra.

Most observed cutoffs are due to spectral effects, not actual electron energy limits.

Abstract

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 of 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 10--20 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 was applied, all low-energy cutoffs in the mean electron spectrum were removed and hence the low energy cutoffs in the mean electron spectrum of solar flares above $\sim$12 keV cannot be viewed as real features in the electron spectrum. If low-energy cutoffs exist in the mean electron spectra, the energy of low energy cutoffs should be less than $\sim$12 keV.