RHESSIIMAGING SURVEY OF γ-RAY BREMSSTRAHLUNG EMISSION IN SOLAR FLARES

Abstract

We present a high-energy (>150 keV) imaging survey of all solar γ-ray flares observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to study bremsstrahlung emission from relativistic electrons. Using RHESSI rear segment data, images in the energy range from 150 to 450 keV integrated over the total duration of the impulsive phase of the flare are derived. Out of the 29 γ-ray peaks in 26 RHESSI flares, we successfully obtained images for 21 γ-ray peaks in 20 flares. The remaining eight peaks have >150 keV fluences of less than a few hundred photons per cm2 and counting statistics are too poor for detailed imaging. The flux ratio of the footpoint sources is found to be similar at 50 keV and above 150 keV, indicating that relativistic electrons are present in both footpoints of the flare loop. No correlation between the footpoint separation and the fluence ratio of the 2.2 MeV line and the >300 keV photons is found. This indicates that the relative efficiency of proton to electron acceleration does not depend on loop length, as could have been expected from stochastic acceleration models. As previously reported, the three flares with the best counting statistics show not only footpoint emission, but also a coronal γ-ray bremsstrahlung source. For events with lower counting statistics, no coronal source could be identified. However, instrumental limitation could easily hide a coronal source for events with lower statistics, suggesting that coronal γ-ray bremsstrahlung sources are nevertheless a general feature of γ-ray flares.