RHESSI observations of particle acceleration in solar flares

The NASA Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) Small Explorer spacecraft was launched on 5 February 2002. Its primary objective is to investigate particle acceleration and energy release in solar flares through imaging spectroscopy of flare hard X-ray/gamma-ray continuum and gamma-ray lines emitted by energetic electrons and ions, respectively. Here we present preliminary results on the hard X-ray imaging spectroscopy of a flare, including the spatial variation with energy of the hard X-ray sources, the energy spectra and timing of the individual sources, and spectral features and total energy content. RHESSI observes continuous solar emission in the 3-10 keV energy range, with many microflares from many active regions. These microflares have a non-thermal power-law component similar to normal flares but with much steeper spectral slopes. We also present the first high-resolution gamma-ray line spectrum and the first imaging of gamma-ray lines, from the large X4.8 flare of 23 July 2002. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.