EDP Sciences
Free access
Volume 466, Number 2, May I 2007
Page(s) 705 - 712
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20066689

A&A 466, 705-712 (2007)
DOI: 10.1051/0004-6361:20066689

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

J. 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
    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)

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