Energetic proton back-precipitation onto the solar atmosphere in relation to long-duration gamma-ray flares

Abstract

Context. Gamma-ray emission during long-duration gamma-ray flare (LDGRF) events is thought to be caused mainly by > 300 MeV protons interacting with the ambient plasma at or near the photosphere. Prolonged periods of the gamma-ray emission have prompted the suggestion that the source of the energetic protons is acceleration at a coronal mass ejection (CME)-driven shock, followed by particle back-precipitation onto the solar atmosphere over extended times.
Aims: We study the latter hypothesis using test particle simulations, which allow us to investigate whether scattering associated with turbulence aids particles in overcoming the effect of magnetic mirroring, which impedes back-precipitation by reflecting particles as they travel sunwards.
Methods: The instantaneous precipitation fraction, P, the proportion of protons that successfully precipitate for injection at a fixed height, r_i, is studied as a function of scattering mean free path, λ and r_i. Upper limits to the total precipitation fraction, P̅, were calculated for eight LDGRF events for moderate scattering conditions (λ = 0.1 AU).
Results: We find that the presence of scattering helps back-precipitation compared to the scatter-free case, although at very low λ values outward convection with the solar wind ultimately dominates. For eight LDGRF events, due to strong mirroring, P̅ is very small, between 0.56 and 0.93% even in the presence of scattering.
Conclusions: Time-extended acceleration and large total precipitation fractions, as seen in the observations, cannot be reconciled for a moving shock source according to our simulations. Therefore, it is not possible to obtain both long duration γ ray emission and efficient precipitation within this scenario. These results challenge the CME shock source scenario as the main mechanism for γ ray production in LDGRFs.