Characteristics of Late-Phase 100 MeV Gamma-Ray Emission in Solar Eruptive Events
Journal Article - Open Access
NAVAL RESEARCH LAB WASHINGTON DC WASHINGTON United States
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We characterize and catalog 30 solar eruptive events observed by the Fermi Large Area Telescope LAT having late-phase 100 MeV -ray emission LPGRE, identified 30 yr ago in what were called long-duration gamma-ray flares. We show that LPGRE is temporally and spectrally distinct from impulsive phase emission in these events. The spectra are consistent with the decay of pions produced by 300MeV protons and are not consistent with primary electron bremsstrahlung. Impulsive 100 keV X-ray emission was observed in all 27 LPGRE events where observations were made. All but two of the LPGRE events were accompanied by a fast and broad coronal mass ejection CME. TheLPGRE start times range from CME onset to 2 hr later. Their durations range from 0.1 to 20 hr and appear to be correlated with durations of 100 MeV solar energetic particle SEP proton events. The power-law spectral indices of the 300MeV protons producing LPGRE range from 2.5 to 6.5 and vary during some events. Combined -ray line and LAT measurements indicate that LPGRE proton spectra are steeper above 300 MeV than they are below 300 MeV. The number of LPGRE protons 500MeV is typically about 10 the number in the impulsive phase of the solar eruptive event and ranges in nine events from 0.01 to 0.5 the number in the accompanying SEP event, with large systematic uncertainty. What appears to be late-phase electron bremsstrahlung with energies up to 10 MeV was observed in one LPGRE event. We discuss how current models of LPGRE may explain these characteristics.