On the Cause of Supra-Arcade Downflows in Solar Flares

TL;DR

This paper presents a model explaining supra-arcade downflows (SADs) as low-density regions carved by continuous, localized 3D magnetic reconnection outflow jets in solar flares, supported by simulations.

Contribution

It introduces a novel 3D reconnection model with localized outflows that explains SADs as collimated, low-density plasma regions, differing from previous models.

Findings

3D simulations confirm collimated outflow jets produce SAD-like regions.

Density stratification is essential for SAD formation.

Localized reconnection explains the persistent, thin SAD structures.

Abstract

A model of supra-arcade downflows (SADs), dark low density regions also known as tadpoles that propagate sunward during solar flares, is presented. It is argued that the regions of low density are flow channels carved by sunward-directed outflow jets from reconnection. The solar corona is stratified, so the flare site is populated by a lower density plasma than that in the underlying arcade. As the jets penetrate the arcade, they carve out regions of depleted plasma density which appear as SADs. The present interpretation differs from previous models in that reconnection is localized in space but not in time. Reconnection is continuous in time to explain why SADs are not filled in from behind as they would if they were caused by isolated descending flux tubes or the wakes behind them due to temporally bursty reconnection. Reconnection is localized in space because outflow jets in standard two-dimensional reconnection models expand in the normal (inflow) direction with distance from the reconnection site, which would not produce thin SADs as seen in observations. On the contrary, outflow jets in spatially localized three-dimensional reconnection with an out-of-plane (guide) magnetic field expand primarily in the out-of-plane direction and remain collimated in the normal direction, which is consistent with observed SADs being thin. Two-dimensional proof-of-principle simulations of reconnection with an out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion regions and the necessity of density stratification. Three-dimensional simulations confirm that localized reconnection remains collimated.