FLUKA Simulations of Pion Decay Gamma-radiation from Energetic Flare Ions

TL;DR

This paper models gamma-ray emission from solar flares caused by energetic ions using FLUKA simulations, analyzing how ion distributions and viewing angles influence observable spectra, with implications for interpreting Fermi LAT data.

Contribution

It introduces FLUKA-based simulations for pion decay gamma-ray production in solar flares, exploring the effects of ion distribution and viewing angle on the spectra.

Findings

Photon spectra depend strongly on viewing angle and ion distribution.

Flare spectra near disc center show limited >1 GeV emission under simple assumptions.

Simulation results agree with previous models for pion decay gamma-ray emissivity.

Abstract

Gamma-ray continuum at > 10 MeV photon energy yields information on > 0.2 - 0.3 GeV/nucleon ions at the Sun. We use the general-purpose Monte Carlo code FLUKA (FLUktuierende KAskade) to model the transport of ions injected into thick and thin target sources, the nuclear processes that give rise to pions and other secondaries and the escape of the resulting photons from the atmosphere. We give examples of photon spectra calculated with a range of different assumptions about the primary ion velocity distribution and the source region. We show that FLUKA gives results for pion decay photon emissivity in agreement with previous treatments. Through the directionality of secondary products, as well as Compton scattering and pair production of photons prior to escaping the Sun, the predicted spectrum depends significantly on the viewing angle. Details of the photon spectrum in the 100 MeV range may constrain the angular distribution of primary ions and the depths at which they interact. We display a set of thick-target spectra produced making various assumptions about the incident ion energy and angular distribution and the viewing angle. If ions are very strongly beamed downward, or ion energies do not extend much above 1 GeV/nucleon, the photon spectrum is highly insensitive to details of the ion distribution. Under the simplest assumptions, flares observed near disc centre should not display significant radiation above 1 GeV photon energy. We give an example application to Fermi Large Area Telescope data from the flare of 12 June 2010.