The Acceleration and Confinement of Energetic Electrons by a Termination Shock in a Magnetic Trap: An Explanation for Nonthermal Loop-top Sources during Solar Flares

TL;DR

This paper presents a model combining magnetohydrodynamic simulations and particle transport to explain how a termination shock in solar flares accelerates electrons, producing nonthermal sources observed at the loop-top.

Contribution

It introduces a novel combined simulation approach demonstrating how a termination shock can accelerate and confine electrons, explaining nonthermal loop-top sources in solar flares.

Findings

Electrons are significantly energized at the shock front.

A magnetic trap below the shock confines electrons for extended periods.

The model produces a power-law energy spectrum consistent with observations.

Abstract

Nonthermal loop-top sources in solar flares are the most prominent observational signature that suggests energy release and particle acceleration in the solar corona. Although several scenarios for particle acceleration have been proposed, the origin of the loop-top sources remains unclear. Here we present a model that combines a large-scale magnetohydrodynamic simulation of a two-ribbon flare with a particle acceleration and transport model for investigating electron acceleration by a fast-mode termination shock at the looptop. Our model provides spatially resolved electron distribution that evolves in response to the dynamic flare geometry. We find a concave-downward magnetic structure located below the flare termination shock, induced by the fast reconnection downflows. It acts as a magnetic trap to confine the electrons at the looptop for an extended period of time. The electrons are energized significantly as they cross the shock front, and eventually build up a power-law energy spectrum extending to hundreds of keV. We suggest that this particle acceleration and transport scenario driven by a flare termination shock is a viable interpretation for the observed nonthermal loop-top sources.