Long Duration Gamma-ray Flares and High Energy Solar Energetic Particles: Is there a Connection?

Abstract

Long Duration Gamma-Ray Flares (LDGRFs) are characterized by delayed and long-duration gamma-ray emission above $\sim$50 MeV. Despite dozens of observations in the last decade with $\it{Fermi}$/LAT, the nature of this emission has been a challenge to explain. The highest energy emission has generally been attributed to the decay of pions produced by the interaction of high-energy protons with ambient solar material. The fact that the $\gamma$-ray emission is delayed from the onset of the initial eruption and that the emission is, in some cases, unusually long in duration suggests that particle acceleration occurs within large volumes extending to high altitudes, either by stochastic acceleration within large coronal loops or by back-precipitation from CME-driven shocks. We have tested these models by a making direct comparisons between the properties of the accelerated ion population at the flare derived from the observations of $\it{Fermi}$/LAT and those of solar energetic particles detected at Earth by PAMELA at comparable high energies. We investigated 27 high-energy gamma ray events (from \cite{ref:SHARE2018}), and for 14 events we compare the two populations (SEPs in space and the interacting population at the Sun) and discuss the implications in terms of potential sources of the LDGRFs.