Energetic proton precipitation onto the solar atmosphere in relation to Long Duration Gamma Ray Flares

Abstract:

Gamma rays produced during Long Duration Gamma Ray Flares (LDGRFs) are thought to be caused by > 300 MeV protons interacting with the ambient plasma at or near the photosphere. Gamma ray emissions lasting up to a full day have prompted the suggestion that a CME driven shock acts as a source of these protons, in which back-precipitation onto the solar atmosphere occurs over extended times. We study the influence of the magnetic mirror effect on back-precipitation by initially deriving an analytical expression for the height above the solar surface where mirroring takes place. We then use test particle simulations to investigate the role of injection height and turbulence-associated scattering on the fraction P of the injected population that precipitate. We find that increasing the injection height causes a dramatic decrease in P and that a decreased scattering mean free path leads to an increase in its value. We conclude that although scattering aids energetic protons in their propagation toward the solar surface, the precipitation fraction remains very small, being typically between 0.24 and 2.00%. We find that the rapid decrease in back-precipitation due to magnetic mirroring does not appear to match the slow decay of observed gamma ray profiles for scattering mean free paths λ ≥ 0.1 AU.