Examining Flux Tube Interactions as a Cause of Sub-alfvénic Outflow

Abstract

Abstract In accepted models, magnetic tension drives reconnected magnetic flux away from the reconnection site at the local Alfvén speed. Numerous observational signatures of these outflows have been identified in solar flares, notable among them being supra-arcade downflows (SADs), almost none move at the Alfvén speed as predicted by models. Well-studied examples of SADs or SAD loops found in the flare of 2017 September 10 (SOL2017-09-10T15:35:00) move at a quarter or less of the expected Alfvén speed. Among those reasons posited to explain such discrepancies is the possibility that reconnected flux experiences a drag force during its outflow. Drag has not been included in previous reconnection models. Here, we develop the first such model in order to test the possibility that drag can explain sub-alfveńic reconnection outflows. Our model uses thin flux tube dynamics, previously shown to match features of flare observations other than outflow speed, including for the 2017 September 10 flare. We supplement the dynamics with a drag force representing the tube’s interaction with surrounding plasma through the formation of a wake. The wake’s width appears as a parameter in the force. We perform simulations, varying the drag parameter and synthesizing EUV observations, to test whether a drag force can produce a reasonable fit to observed features of the September 10 flare. We find that that slower retraction increases the brightness of emission and lowers the temperature of the synthetic plasma sheet. With proper choice of parameters the drag enables the simulation to agree reasonably with the observations.