---
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. \\textbackslash\
  \ 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. \\textbackslash 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, \\ensuremath\\lambda and r$_i$. Upper limits\
  \ to the total precipitation fraction, P\u0305, were calculated for eight LDGRF\
  \ events for moderate scattering conditions (\\ensuremath\\lambda = 0.1 AU).\
  \ \\textbackslash Results: We find that the presence of scattering helps back-precipitation\
  \ compared to the scatter-free case, although at very low \\ensuremath\\lambda\
  \ values outward convection with the solar wind ultimately dominates. For eight\
  \ LDGRF events, due to strong mirroring, P\u0305 is very small, between 0.56 and\
  \ 0.93\\% even in the presence of scattering. \\textbackslash 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 \\ensuremath\\\
  gamma ray emission and efficient precipitation within this scenario. These results\
  \ challenge the CME shock source scenario as the main mechanism for \\ensuremath\\\
  gamma ray production in LDGRFs."
adsnote: Provided by the SAO/NASA Astrophysics Data System
adsurl: https://ui.adsabs.harvard.edu/abs/2022A&A...658A..23H
archivePrefix: arXiv
authors:
- A. Hutchinson
- S. Dalla
- T. Laitinen
- G. A. de Nolfo
- A. Bruno
- J. M. Ryan
- C. O. G. Waterfall
category: publications
date: '2022-02-01'
doi: 10.1051/0004-6361/202142002
draft: false
eid: A23
eprint: '2012.05146'
featured: false
journal: A&A
month: February
pages: A23
primaryClass: astro-ph.SR
projects: []
publication: A&A
publication_types:
- '2'
tags:
- Astroparticle physics
- 'Sun: coronal mass ejections (CMEs)'
- 'Sun: particle emission'
- 'Sun: x-rays'
- Gamma rays
- Turbulence
- Astrophysics - solar and stellar astrophysics
- Astrophysics - high energy astrophysical phenomena
title: 'Energetic proton back-precipitation onto the solar atmosphere in relation
  to long-duration gamma-ray flares'
type: ARTICLE
volume: '658'
year: '2022'
---