GEANT4 Simulations of Gamma-Ray Emission from Accelerated Particles in Solar Flares

TL;DR

This study uses GEANT4 simulations to analyze how particle acceleration and atmospheric effects influence gamma-ray spectra from solar flares, aiding interpretation of observational data.

Contribution

The paper introduces detailed Monte Carlo simulations of gamma-ray production in solar flares, accounting for angular distributions and atmospheric reprocessing effects.

Findings

The ratio of 511 keV line to continuum serves as a diagnostic of particle acceleration.

Pion decay produces positron annihilation signatures at specific atmospheric depths.

Compton scattering of annihilation photons can mimic orthopositronium decay spectra.

Abstract

Gamma-ray spectroscopy provides diagnostics of particle acceleration in solar flares, but care must be taken when interpreting the spectra due to effects of the angular distribution of the accelerated particles (such as relativistic beaming) and Compton reprocessing of the radiation in the solar atmosphere. In this paper, we use the GEANT4 Monte Carlo package to simulate the interactions of accelerated electrons and protons and study these effects on the gamma-rays resulting from electron bremsstrahlung and pion decay. We consider the ratio of the 511~keV annihilation-line flux to the continuum at 200~keV and in the energy band just above the nuclear de-excitation lines (8--15~MeV) as a diagnostic of the accelerated particles and a point of comparison with data from the X17 flare of 2003 October 28. We also find that pion secondaries from accelerated protons produce a positron annihilation line component at a depth of $\sim$ 10 g cm$^{-2}$, and that the subsequent Compton scattering of the 511~keV photons produces a continuum that can mimic the spectrum expected from the 3$\gamma$ decay of orthopositronium.