A&A 466, 705-712 (2007)
Hard X-ray spectra and positions of solar flares observed by RHESSI: photospheric albedo, directivity and electron spectraJ. Kasparová1, E. P. Kontar2, and J. C. Brown2
1 Astronomical Institute, Academy of Sciences of the Czech Republic, Fricova 298, 251 65 Ondrejov, Czech Republic
2 Department of Physics and Astronomy, University of Glasgow, G12 8QQ, UK
(Received 3 November 2006 / Accepted 31 January 2007)
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.
Key words: Sun: flares -- Sun: X-rays, gamma rays -- Sun: particle emission -- scattering -- techniques: spectroscopic
© ESO 2007