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On the Cause of Supra-Arcade Downflows in Solar Flares  

Paul Cassak   Submitted: 2013-09-03 09:29

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.

Authors: P. A. Cassak, J. F. Drake, J. T. Gosling, T.-D. Phan, M. A. Shay, L. S. Shepherd
Projects: None

Publication Status: Accepted to Ap. J. Lett
Last Modified: 2013-09-03 12:22
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Paul Cassak   Submitted: 2008-02-05 10:51

There is currently no explanation of why the corona has the temperature and density it has. We present a model which explains how the dynamics of magnetic reconnection regulates the conditions in the corona. A bifurcation in magnetic reconnection at a critical state enforces an upper bound on the coronal temperature for a given density. We present observational evidence from 107 flares in 37 sun-like stars that stellar coronae are near this critical state. The model may be important to self-organized criticality models of the solar corona.

Authors: P. A. Cassak, D. J. Mullan and M. A. Shay
Projects: None

Publication Status: Ap. J. Lett. (accepted)
Last Modified: 2008-02-05 12:02
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On the Cause of Supra-Arcade Downflows in Solar Flares
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