Coronal γ-ray bremsstrahlung from solar flare-accelerated electrons

The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) provides for the first time imaging spectroscopy of solar flares up to the gamma-ray range. The three RHESSI flares with best counting statistics are analyzed in the 200-800 keV range revealing gamma-ray emission produced by electron bremsstrahlung from footpoints of flare loops, but also from the corona. Footpoint emission dominates during the gamma-ray peak, but as the gamma-ray emission decreases the coronal source becomes more and more prominent. Furthermore, the coronal source shows a much harder spectrum ( with power-law indices gamma between 1.5 and 2) than the footpoints ( with gamma between 3 and 4). These observations suggest that flare-accelerated high-energy (similar to MeV) electrons stay long enough in the corona to lose their energy by collisions producing gamma-ray emission, while lower energetic electrons precipitate more rapidly to the footpoints.