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

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.