Gas-dynamic shock heating of post-flare loops due to retraction following localized, impulsive reconnection

Abstract

We present a novel model in which field lines shortening after localized, three-dimensional reconnection heat the plasma as they compress it. The shortening progresses away from the reconnection site at the Alfvén speed, releasing magnetic energy and generating parallel, compressive flows. These flows, which are highly supersonic when β « 1, collide in a pair of strong gas-dynamic shocks at which both the mass density and temperature are raised. Reconnecting field lines initially differing by more that 100° can produce a concentrated knot of plasma hotter that 20 MK at the loop's apex, consistent with observations. In spite of these high temperatures, the shocks convert less than 10% of the liberated magnetic energy into heat-the rest remains as kinetic energy of bulk motion. These gas-dynamic shocks arise only when the reconnection is impulsive and localized in all three dimensions; they are distinct from the slow magnetosonic shocks of the Petschek steady-state reconnection model. © 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.