Tuesday, July 16, 2013

1307.3946 (P. A. Cassak et al.)

On the Cause of Supra-Arcade Downflows    [PDF]

P. A. Cassak, J. F. Drake, J. T. Gosling, T. -D. Phan, M. A. Shay, L. S. Shepherd
A model of supra-arcade downflows (SADs), low-density plasma voids also known as tadpoles that propagate sunward during solar flares, is presented. It is argued that regions of density depletion 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. The jets penetrate the arcade, carving out plasma depletion regions which appear as SADs. The present interpretation differs from previous models in that reconnection is localized in space but not in time. The model explains why SADs are not filled in from behind as they would if they were isolated descending flux ropes due to temporally localized reconnection or the wakes behind them. Proof-of-principle simulations of reconnection with an out-of-plane (guide) magnetic field confirm the creation of depletion regions and the necessity of density stratification. In the standard two-dimensional picture of guide field reconnection, outflow jets expand in the normal (inflow) direction with distance from the reconnection site, so they will not carve thin SADs as seen in observations. However, three-dimensional simulations of reconnection with an X-line of limited length reveal that outflow jets expand primarily in the out-of-plane direction and remain collimated in the normal direction. Therefore, the density depletions are collimated, consistent with observed SADs.
View original: http://arxiv.org/abs/1307.3946

No comments:

Post a Comment