Gamma-Ray Observations of the 2005 January 20 Solar Flare

The RHESSI spectrometer observed the entire impulsive phase of the flare. The measured spectrum was dominated by the positron-electron annihilation line at 511 keV and neutron-capture line at 2223 keV above a continuum formed by electron bremsstrahlung and nuclear continuum. There is only weak evidence for narrow nuclear de-excitation lines. We show that such a gamma-ray spectrum can be produced by a population of accelerated particles with a very hard spectrum. We use the 511-keV/2222-keV line flux ratio to infer that the power-law index of accelerated particles >30 MeV was ~2.2. This index is similar to the event-averaged index observed in particles at 1 AU. We show that our calculations also suggest an impulsive SEP composition of accelerated particles with an alpha/p ratio > 0.1. This work has been funded by NASA DPR13,777G and ONR.