Hard X-ray spectra and positions of solar flares observed by RHESSI: photospheric albedo, directivity and electron spectra

¹ Astronomical Institute, Academy of Sciences of the Czech Republic, Fričova 298, 251 65 Ondřejov, Czech Republic e-mail: kasparov@asu.cas.cz
² Department of Physics and Astronomy, University of Glasgow, G12 8QQ, UK e-mail: eduard,john@astro.gla.ac.uk

Received: 3 November 2006
Accepted: 31 January 2007

Abstract

Aims.We investigate the signature of the photospheric albedo contribution in solar flare hard X-ray spectra, the effect of low energy cutoffs in electron spectra, and the directivity of hard X-ray emission.

Methods.Using Ramaty High Energy Solar Spectroscopic Imager (RHESSI) flare data we perform a statistical analysis of spatially integrated spectra and positions of solar flares.

Results.We demonstrate clear centre-to-limb variation of photon spectral indices in the 15–20 keV energy range and a weaker dependency in the 20–50 keV range which is consistent with photospheric albedo as the cause. The results also suggest that low-energy cutoffs sometimes inferred in mean electron spectra are an artefact of albedo. We also derive the anisotropy (ratio of downward/observer directed photons) of hard X-ray emission in the 15–20 keV range for various heliocentric angles.