RHESSI Observations of the Size Scales of Solar Hard X-ray Sources

Abstract

The Reuven Ramaty High-Energy Solar Spectroscopic Imager RHESSI telescope produces hard X-ray images by Fourier imaging techniques that are capable of determining the sizes and shapes of sources with spatial scales in the range ~2″–180″. Applying the method of Unpixelized Forward Fitting to RHESSI modulation profiles from simple flares, we have identified the presence of ‘halo’ sources whose size scale (~ 40″) greatly exceeds the ‘core’ sizes (≤ 6″–14″). Although such ‘core-halo’ structures have been observed at radio wavelengths using a similar technique, the radio and hard X-ray phenomena may be different. These observations raise questions about the nature of these ‘halos’. Among the possibilities are that they are albedo sources, thin-target loops, or unidentified diffuse structures.