Statistical Study of Coronal Hard X-ray Source Heights and Fluxes

Hard X-ray observation of partly occulted flares has proven useful for studying flare-accelerated electrons in the solar corona. These nonthermal electrons emit bremsstrahlung hard X-rays (HXRs), but are difficult to observe for on-disk flares because the much brighter chromospheric footpoints tend to dominate HXR images. Previous research using the RHESSI spacecraft has performed individual and statistical study of HXR sources in partly occulted flares to investigate, for example, spectral characteristics and the relationship between nonthermal coronal sources and thermal loops.

Source heights are not usually measured in these cases because of the difficulty in determining heliographic locations of flares beyond the limb. Occasionally, multi-spacecraft observation will identify a source location and thus calculate an absolute HXR source height. Microflare source heights have also been studied statistically by fitting distribution functions to the observed projected locations. But so far, a statistical study of coronal HXR sources in which source heights are determined individually for each flare has not been performed.

In this work, we study flares jointly observed by RHESSI and STEREO/EUVI over a 2+ year time range. From RHESSI data, we obtain coronal HXR source positions and fluxes. The alternate viewing angle offered by STEREO provides flare locations, enabling RHESSI source positions to be translated into absolute heights above the photosphere. We will present the distribution of coronal HXR source heights and will discuss their possible corresponding features in the standard flare model, i.e. thermal loops, nonthermal looptop sources, above-the-looptop sources, and ejecta.