Shocks and Thermal Conduction Fronts in Retracting Reconnected Flux Tubes

TL;DR

This paper develops a model for plasma heating in solar flares caused by reconnected flux tubes retracting at Alfvénic speeds, emphasizing the role of shocks and thermal conduction in energy dissipation.

Contribution

It introduces generalized thin flux tube equations including anisotropic viscosity and thermal conductivity, and analyzes shock structures in reconnected flux tubes under solar coronal conditions.

Findings

Gas-dynamic shocks generate large velocity and temperature gradients.

Shock thickness can reach the entire flux tube length in solar conditions.

Thermal fronts and isothermal sub-shocks are key features of the shock structure.

Abstract

We present a model for plasma heating produced by time-dependent, spatially localized reconnection within a flare current sheet separating skewed magnetic fields. The reconnection creates flux tubes of new connectivity which subsequently retract at Alfv\'enic speeds from the reconnection site. Heating occurs in gas-dynamic shocks which develop inside these tubes. Here we present generalized thin flux tube equations for the dynamics of reconnected flux tubes, including pressure-driven parallel dynamics as well as temperature dependent, anisotropic viscosity and thermal conductivity. The evolution of tubes embedded in a uniform, skewed magnetic field, following reconnection in a patch, is studied through numerical solutions of these equations, for solar coronal conditions. Even though viscosity and thermal conductivity are negligible in the quiet solar corona, the strong gas-dynamic shocks generated by compressing plasma inside reconnected flux tubes generate large velocity and temperature gradients along the tube, rendering the diffusive processes dominant. They determine the thickness of the shock that evolves up to a steady-state value, although this condition may not be reached in the short times involved in a flare. For realistic solar coronal parameters, this steady-state shock thickness might be as long as the entire flux tube. For strong shocks at low Prandtl numbers, typical of the solar corona, the gas-dynamic shock consists of an isothermal sub-shock where all the compression and cooling occur, preceded by a thermal front where the temperature increases and most of the heating occurs. We estimate the length of each of these sub-regions and the speed of their propagation.