Particle Acceleration and Transport at the Sun Inferred from Fermi/LAT Observations of >100 MeV Gamma-rays

TL;DR

This paper investigates the origin of sustained gamma-ray emission (SGRE) from the Sun, linking it to shock-accelerated protons and their transport, using Fermi/LAT observations and solar event data.

Contribution

It demonstrates that SGRE is caused by protons accelerated in shocks and propagating sunward, with detailed analysis of magnetic connectivity affecting observed spectra.

Findings

SGRE duration correlates linearly with type II burst duration.

High-energy protons are accelerated near the shock nose but may not reach Earth-based detectors.

Magnetic connectivity influences the observed proton spectra and gamma-ray emissions.

Abstract

The sustained gamma-ray emission (SGRE) events from the Sun are associated with an ultrafast (2000 km/s or greater) halo coronal mass ejection (CME) and a type II radio burst in the decameter-hectometric (DH) wavelengths. The SGRE duration is linearly related to the type II burst duration indicating that >300 MeV protons required for SGREs are accelerated by the same shock that accelerates tens of keV electrons that produce type II bursts. When magnetically well connected, the associated solar energetic particle (SEP) event has a hard spectrum, indicating copious acceleration of high-energy protons. In one of the SGRE events observed on 2014 January 7 by Fermi/LAT, the SEP event detected by GOES has a very soft spectrum with not many particles beyond 100 MeV. This contradicts the presence of the SGRE, implying the presence of significant number of >300 MeV protons. Furthermore, the durations of the type II burst and the SGRE agree with the known linear relationship between them (Gopalswamy et al. 2018, ApJ 868, L19). We show that the soft spectrum is due to poor magnetic connectivity of the shock nose to an Earth observer. Even though the location of the eruption (S15W11) is close to the disk center, the CME propagated non-radially making the CME flank crossing the ecliptic rather than the nose. High-energy particles are accelerated near the nose, so they do not reach GOES but they do precipitate to the vicinity of the eruption region to produce SGRE. This study provides further evidence that SGRE is caused by protons accelerated in shocks and propagating sunward to interact with the atmospheric ions.