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

TL;DR

This paper introduces a new model where impulsive, localized magnetic reconnection causes flux tube shortening, generating gas-dynamic shocks that heat plasma to over 20 MK, with most energy remaining as bulk motion.

Contribution

It presents a novel mechanism for shock heating in post-flare loops due to flux tube retraction after 3D reconnection, differing from traditional steady-state models.

Findings

Reconnection-driven flux tube shortening generates strong gas-dynamic shocks.

Shocks heat plasma to temperatures exceeding 20 MK.

Less than 10% of magnetic energy converts into heat, most remains as kinetic energy.

Abstract

We present a novel model in which shortening of a magnetic flux tube following localized, three-dimensional reconnection generates strong gas-dynamic shocks around its apex. The shortening releases magnetic energy by progressing away from the reconnection site at the Alfven speed. This launches inward flows along the field lines whose collision creates a pair of gas-dynamic shocks. The shocks raise both the mass density and temperature inside the newly shortened flux tube. Reconnecting field lines whose initial directions differ by more that 100 degrees can produce a concentrated knot of plasma hotter that 20 MK, 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.