Hard X-Ray Observations of Coronal Sources: Implications for Particle Acceleration

The properties of hard X-ray emission from solar flares can provide insight into particle acceleration and transport processes. Commonly, at higher energies the bright footpoint emission from the flare loop prevents a detailed analysis of the weaker loop-top source due to the limited dynamic range. Thus, flares close to the solar limb, where the footpoints are occulted, are interesting events to study because they can reveal the coronal loop-top emission and thus the electron properties at their acceleration site. We present results of a survey study of partially occulted flares observed with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). We found that most of the flare spectra allow a fit to a thermal plus non-thermal component, either with a broken power-law or a kappa function. The spatial separation between the thermal and non-thermal component, as derived from imaging, is usually small. The light curve analysis shows for many flares a very good correlation between the derivative of the soft X-ray flux and the hard X-rays. We discuss implications for particle acceleration models that result from our study.