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SPRG Seminars - Archive
April 18, 2006
Ion Acceleration and Gamma Rays in RHESSI Flares
Albert Shih, UC Berkeley / Space Sciences Laboratory
The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) has observed both high-energy electron bremsstrahlung emission and nuclear gamma-ray line emission from solar flares. The gamma-ray lines are produced when energetic ions interact with the ambient medium through various processes including neutron capture and nuclear de-excitation. By comparing electron-related emission and ion-related emission in flares, one can put constraints on the acceleration mechanisms for electrons and ions. Of the gamma-ray lines, RHESSI is most sensitive to the neutron-capture line at 2.223 MeV, which is indirectly produced by protons accelerated above 30 MeV. RHESSI has observed eleven flares with significant neutron-capture line emission and can place upper limits on other flares. We compare the fluence in this line with the electron bremsstrahlung fluence above 300 keV for all RHESSI flares that extend to such energies and find a striking linear correlation between these two quantities across many orders of magnitude in size. This correlation implies that the mechanism that accelerates protons above 30 MeV is directly associated with the mechanism that accelerates electons above 300 keV. We also find that this correlation agrees with observations made by the Solar Maximum Mission (SMM) satellite. For the larger flares observed by RHESSI, we can also study the gamma-ray line flux ratios and line shapes as a way to probe the ambient and accelerated ion abundances, including looking for changes over the course of a flare.