# Fermi-LAT Observations of High-energy Behind-the-limb Solar Flares

## TL;DR

This paper reports Fermi-LAT observations of high-energy emissions from behind-the-limb solar flares, analyzing their spectra, localization, and acceleration mechanisms, revealing protons accelerated by CMEs impacting the solar surface.

## Contribution

It provides detailed multi-wavelength analysis and modeling of behind-the-limb flares, offering new insights into particle acceleration and emission processes in occulted solar events.

## Key findings

- High-energy gamma-ray emission associated with behind-the-limb flares.
- Protons accelerated in CME environments precipitate onto the solar surface.
- HXR emission from electron bremsstrahlung in coronal loop tops.

## Abstract

We report on the Fermi-LAT detection of high-energy emission from the behind-the-limb (BTL) solar flares that occurred on 2013 October 11, and 2014 January 6 and September 1. The Fermi-LAT observations are associated with flares from active regions originating behind both the eastern and western limbs, as determined by STEREO. All three flares are associated with very fast coronal mass ejections (CMEs) and strong solar energetic particle events. We present updated localizations of the >100 MeV photon emission, hard X-ray (HXR)and EUV images, and broadband spectra from 10 keV to 10 GeV, as well as microwave spectra. We also provide a comparison of the BTL flares detected by Fermi-LAT with three on-disk flares and present a study of some of the significant quantities of these flares as an attempt to better understand the acceleration mechanisms at work during these occulted flares. We interpret the HXR emission to be due to electron bremsstrahlung from a coronal thin-target loop top with the accelerated electron spectra steepening at semirelativistic energies. The >100 MeV gamma-rays are best described by a pion-decay model resulting from the interaction of protons (and other ions) in a thick-target photospheric source. The protons are believed to have been accelerated (to energies >10 GeV) in the CME environment and precipitate down to the photosphere from the downstream side of the CME shock and landed on the front side of the Sun, away from the original flare site and the HXR emission.