January 5, 2018, 1:00 pm - 2:00 pm

January 5, 1:00 pm - 2:00 pm

Characteristics of Sustained >100 MeV Gamma-Ray Emission Associated with Solar Flares

Gerald Share (University of Maryland, Department of Astronomy)

Proton and ion interactions in the solar atmosphere produce gamma radiation. The spectrum and temporal characteristics of the emission provides fundamental information on the accelerated ions interacting at the Sun. Most of the gamma-ray observations prior to 2008 were made in the nuclear energy range with radiation arising from interactions of one to tens of MeV ions accelerated in solar flares. There was also a class of what was called Long Duration Gamma-Ray Flares with higher-energy emission lasting several hours primarily associated with X-class flares. Since the launch of the Fermi satellite, with its sensitive Large Area Telescope (LAT), 32 such high-energy gamma ray events have been detected from significantly weaker X-ray flares. Because their temporal and spectral characteristics are clearly distinct from the associated solar flare, we prefer to call the radiation 'Sustained Gamma-Ray Emission (SGRE)'. We summarize the characteristics of these events. The gamma-ray spectra are consistent with the decay of pions produced by protons above 300 MeV and inconsistent with primary electron bremsstrahlung. The onset of the radiation often occurs minutes after the impulsive flare, but can also begin up to two hours later. The emission can last from a few minutes to tens of hours. The number of >500 MeV protons responsible for the SGRE is typically more than ten times the number of flare-accelerated protons. Because 30 of the 32 SGRE events are accompanied by fast CMEs, it is likely that these magnetic eruptions play a significant role in the acceleration of the protons to high energies. Using GOES/HEPAD data we also estimate that the number of protons in the accompanying SEPs is typically about 50-100 times the number of protons that produce the SGRE. We discuss the implications of these observations.