Department of Astronomy, University of Maryland
Solar Gamma-Ray Observations of the Particle Release in Fourteen High-Energy Solar Energetic Particle Events
Solar flares and shocks formed by fast Coronal Mass Ejections (CMEs) can accelerate ions to energies >300 MeV/nucleon. These particles can escape the Sun where they are observed in space as Solar Energetic Particles (SEPs) and in neutron monitors as Ground Level Enhancements (GLEs). They also can interact at the Sun where they produce impulsive or late phase pion-decay gamma-ray emission (LPGRE). There is evidence that LPGRE is associated with CME shocks and represents a majority of the >100 MeV gamma-ray events observed. The solar particle release (SPR) time of the SEPs can be determined from their measured energy-dependent onset times. We have compared the SPR times in 14 events from 1989 to 2021, 10 of which were GLEs, with the time histories of hard X-rays and pion-decay gamma rays observed by instruments on 7 satellites. We find that the particle release typically occurs near the onset of LPGRE, suggesting that the same acceleration process is responsible for both SEPs and late phase gamma-rays. In contrast, most of the particle releases occur after the flares ended or during their decay phase. This suggests that any flare-accelerated > 300 MeV protons in SEP/LPGRE events were first trapped before release. But pion-decay gamma rays were only detected in 5 of the 10 flares where there were observations and in only 3 of these did the number of protons exceed even 10% of the number in the associated LPGRE. This indicates that a flare/trap origin is consistent with at most half of the SEP/LPGRE events while a CME shock origin is consistent with all 14.