Radio, Hard X-Ray, and Gamma-Ray Emissions Associated with a Far-Side Solar Event

TL;DR

This study analyzes a far-side solar eruptive event, revealing that high-energy particles observed from Earth originated from flare-accelerated particles trapped in coronal loops, reaccelerated by a shock wave, challenging previous CME-shock models.

Contribution

It provides new insights into particle acceleration mechanisms in far-side solar events, emphasizing the role of static coronal loops and shock reacceleration, distinct from CME-driven shock scenarios.

Findings

High-energy gamma-ray burst linked to flare-accelerated protons.

Particles were trapped and reaccelerated in static coronal loops.

Event involved shock wave not driven by CME.

Abstract

The SOL2014-09-01 far-side solar eruptive event produced hard electromagnetic and radio emissions observed with detectors at near-Earth vantage points. Especially challenging was a long-duration >100 MeV $\gamma$-ray burst probably produced by accelerated protons exceeding 300 MeV. This observation raised a question of how high-energy protons could reach the Earth-facing solar surface. Some preceding studies discussed a scenario in which protons accelerated by a CME-driven shock high in the corona return to the solar surface. We continue with the analysis of this challenging event, involving radio images from the Nan\c{c}ay Radioheliograph and hard X-ray data from the High Energy Neutron Detector (HEND) of the Gamma-Ray Spectrometer onboard the Mars Odyssey space observatory located near Mars. HEND recorded unocculted flare emission. The results indicate that the emissions observed from the Earth's direction were generated by flare-accelerated electrons and protons trapped in static long coronal loops. Their reacceleration is possible in these loops by a shock wave, which was excited by the eruption, being initially not CME-driven. The results highlight the ways to address remaining questions.