On the Shock Source of Sustained Gamma-Ray Emission from the Sun

TL;DR

This paper provides evidence that protons accelerated by CME-driven shocks propagate back to the Sun, causing sustained gamma-ray emission, with CME speed being a key factor in the occurrence of SGRE.

Contribution

It offers new statistical analysis linking CME properties, especially speed, to sustained gamma-ray emission, supporting the shock-precipitation model.

Findings

SGRE CMEs are similar to those in ground level enhancement events.

CME speed correlates with SGRE fluence.

High CME speed is necessary for SGRE occurrence.

Abstract

It has recently been shown that the spatially and temporally extended gamma-ray emission in solar eruptions are caused by greater than 300 MeV protons precipitating on the Sun from shocks driven by coronal mass ejections (CMEs). The gamma-rays result from the decay of neutral pions produced in the proton-proton interaction when the greater than 300 MeV protons collide with those in the chromosphere. The evidence comes from the close correlation between the durations of the sustained gamma-ray emission (SGRE) and the associated interplanetary (IP) type II radio bursts. In this paper, we provide further evidence that support the idea that protons accelerated in IP shocks driven by CMEs propagate toward the Sun, precipitate in the chromosphere to produce the observed SGRE. We present the statistical properties of the SGRE events and the associated CMEs, flares, and type II radio bursts. It is found that the SGRE CMEs are similar to those associated with ground level enhancement events. The CME speed is well correlated with the SGRE fluence. High CME speed is an important requirement for the occurrence of SGRE, while the flare size is not. Based on these results, we present a schematic model illustrating the spatially and temporally extended nature of SGRE related to the CME flux rope-shock structure.