A model of double coronal hard X-ray sources in solar flares

TL;DR

This paper presents a novel model explaining double coronal hard X-ray sources in solar flares, showing that termination shocks driven by reconnection outflows efficiently accelerate electrons, producing synthetic images and spectra consistent with observations.

Contribution

The first model demonstrating electron acceleration by termination shocks in double coronal X-ray sources during solar flares.

Findings

Electrons are efficiently accelerated at termination shocks.

Synthetic HXR images show two distinct sources matching observations.

Spectral slopes of sources are similar, consistent with real data.

Abstract

A number of double coronal X-ray sources have been observed during solar flares by RHESSI, where the two sources reside at different sides of the inferred reconnection site. However, where and how are these X-ray-emitting electrons accelerated remains unclear. Here we present the first model of the double coronal hard X-ray (HXR) sources, where electrons are accelerated by a pair of termination shocks driven by bi-directional fast reconnection outflows. We model the acceleration and transport of electrons in the flare region by numerically solving the Parker transport equation using velocity and magnetic fields from the macroscopic magnetohydrodynamic simulation of a flux rope eruption. We show that electrons can be efficiently accelerated by the termination shocks and high-energy electrons mainly concentrate around the two shocks. The synthetic HXR emission images display two distinct sources extending to $>$100 keV below and above the reconnection region, with the upper source much fainter than the lower one. The HXR energy spectra of the two coronal sources show similar spectral slopes, consistent with the observations. Our simulation results suggest that the flare termination shock can be a promising particle acceleration mechanism in explaining the double-source nonthermal emissions in solar flares.