Photospheric albedo and directivity as observed from hard X-ray RHESSI spectra

J. Kasparova$^1$, E. Kontar$^2$

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

Spatially integrated hard X-ray photon spectra contain photospheric albedo component which is formed by photons backscattered in the photosphere. In our contribution we will present results of statistical analysis of a large set of solar flares observed by RHESSI. We will demonstrate centre-to-limb variation of photon spectral indices in several energy ranges which is consistent with photospheric albedo as the cause.We will also determine the directivity of downward directed hard X-ray emission and discuss implications for directivity of electron distribution.