The Origin of Underdense Plasma Downflows Associated with Magnetic Reconnection in Solar Flares

TL;DR

This study uses 3D magnetohydrodynamics modeling to explain dark plasma downflows in solar flares as turbulent structures formed below the flare termination shock, providing new insights into flare energy release.

Contribution

It introduces a 3D MHD model that explains dark downflows as self-organized turbulent structures, challenging previous interpretations and linking them to the flare energy release process.

Findings

Dark downflows are formed in a turbulent interface region.

Downflows are self-organized structures, not direct reconnection outflows.

Turbulence and shocks in the interface are crucial for energy release.

Abstract

Magnetic reconnection is a universal process that powers explosive energy release events such as solar flares, geomagnetic substorms, and some astrophysical jets. A characteristic feature of magnetic reconnection is the production of fast reconnection outflow jets near the plasma Alfv\'{e}n speeds. In eruptive solar flares, dark, finger-shaped plasma downflows moving toward the flare arcade have been commonly regarded as the principal observational evidence for such reconnection-driven outflows. However, they often show a speed much slower than that expected in reconnection theories, challenging the reconnection-driven energy release scenario in standard flare models. Here, we present a three-dimensional magnetohydrodynamics model of solar flares. By comparing the model-predictions with the observed plasma downflow features, we conclude that these dark downflows are self-organized structures formed in a turbulent interface region below the flare termination shock where the outflows meet the flare arcade, a phenomenon analogous to the formation of similar structures in supernova remnants. This interface region hosts a myriad of turbulent flows, electron currents, and shocks, crucial for flare energy release and particle acceleration.