Implications of loop-top origin for microwave, hard X-ray, and low-energy gamma-ray emissions from behind the limb flares

TL;DR

This study analyzes multi-wavelength observations of behind-the-limb solar flares, focusing on the loop-top origin of emissions, and assesses the particle acceleration and energy requirements for different emission types.

Contribution

It provides a quantitative analysis of multi-wavelength data, emphasizing the importance of particle escape and the trans-relativistic nature of emissions from the loop-top region.

Findings

A simple power-law electron spectrum fits the observations.

A broken power-law spectrum may be needed in energy space.

Energy estimates suggest different sources for >100 MeV gamma-rays and HXRs.

Abstract

The Fermi gamma-ray Space Telescope (Fermi) has detected hard X-ray (HXR) and gamma-ray photons from three flares, which according to \stereo occurred in active regions behind the limb of the Sun as delineated by near Earth instruments. For two of these flares \r has provided HXR images with sources located just above the limb, presumably from the loop top (LT) region of a relatively large loop. Fermi-Gamma-ray Burst Monitor has detected HXRs and gamma-rays, and RSTN has detected microwaves emissions with similar light curves. This paper presents a quantitative analysis of these multi-wavelength observations assuming that HXRs and microwaves are produced by electrons accelerated at the LT source, with emphasize on the importance of the proper treatment of escape of the particles from the acceleration-source region and the trans-relativistic nature of the analysis. The observed spectra are used to determine the magnetic field and relativistic electron spectra. It is found that a simple power-law in momentum (with cut off above a few 100 MeV) agrees with all observations, but in energy space a broken power law spectrum (steepening at rest mass energy) may be required. It is also shown that the production of the $>100$ MeV photons detected by The Fermi-Large Area Telescope at the LT source would require more energy compared to photospheric emission. These energies are smaller than that required for electrons, so that the possibility that all the emissions originate in the LT cannot be ruled out on energetic grounds. However, the differences in the light curves and emission centroids of HXRs and $>100$ MeV gamma-rays favour a different source for the latter.