Hard X-ray Spectra and Positions of Solar Flares observed by RHESSI: photospheric albedo, directivity and electron spectra

Abstract

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

8 pages, 6 figures, revised manuscript to Astronomy and Astrophysics

References in corpus (1)

• Stereoscopic electron spectroscopy of solar hard X-ray flares with a single spacecraft

Cited by in corpus (10)

• Combined Modeling of Acceleration, Transport, and Hydrodynamic Response in Solar Flares: I. The Numerical Model
• Low-energy cutoffs in electron spectra of solar flares: statistical survey
• On the variation of solar flare coronal x-ray source sizes with energy
• Energetic Electron Distribution of the Coronal Acceleration Region: First results from Joint Microwave and Hard X-ray Imaging Spectroscopy
• Understanding breaks in solar flares x-ray spectra: Evaluation of a co-spatial return-current model
• Measurements of electron anisotropy in solar flares using albedo with RHESSI X-ray data
• Non-thermal recombination - a neglected source of flare hard X-rays and fast electron diagnostic
• Detection of acceleration processes during the initial phase of the 12 June 2010 flare
• Electron plasma wake field acceleration in solar coronal and chromospheric plasmas
• Spectral component imaging of solar X-ray flares