The Role of Inverse Compton Scattering in Solar Coronal Hard X-ray and Gamma-ray Sources

TL;DR

This paper investigates inverse Compton scattering as an alternative to bremsstrahlung for explaining coronal hard X-ray and gamma-ray emissions during solar flares, highlighting its potential significance especially for mildly relativistic electrons.

Contribution

It introduces inverse Compton scattering as a significant emission mechanism in solar flare coronal sources, considering anisotropic electron distributions and comparing it with bremsstrahlung.

Findings

ICS can significantly enhance coronal HXR and gamma-ray emission under certain viewing geometries.

Mildly relativistic electrons can up-scatter EUV/SXR photons to HXR energies.

In some observed events, ICS is a plausible alternative to bremsstrahlung as the dominant emission process.

Abstract

Coronal hard X-ray (HXR) and continuum gamma-ray sources associated with the impulsive phase of solar flares have been the subject of renewed interest in recent years. They have been interpreted in terms of thin-target, nonthermal bremsstrahlung emission. This interpretation has led to rather extreme physical requirements in some cases. For example, in one case, essentially all of the electrons in the source must be accelerated to nonthermal energies to account for the coronal HXR source. In other cases, the extremely hard photon spectra of the coronal continuum gamma-ray emission suggest that the low energy cutoff of the electron energy distribution lies in the MeV energy range. Here we consider the role of inverse Compton scattering (ICS) as an alternate emission mechanism in both the ultra- and mildly relativistic regimes. It is known that relativistic electrons are produced during powerful flares; these are capable of up-scattering soft photospheric photons to HXR and gamma-ray energies. Previously overlooked is the fact that mildly relativistic electrons, generally produced in much greater numbers in flares of all sizes, can up-scatter EUV/SXR photons to HXR energies. We also explore ICS on anisotropic electron distributions and show that the resulting emission can be significantly enhanced over an isotropic electron distribution for favorable viewing geometries. We briefly review results from bremsstrahlung emission and reconsider circumstances under which nonthermal bremsstrahlung or ICS would be favored. Finally, we consider a selection of coronal HXR and gamma-ray events and find that in some cases the ICS is a viable alternative emission mechanism.