The Thickness of Electric Current Sheets and Implications for Coronal Heating

TL;DR

This paper investigates the physical constraints on current sheet thickness in magnetic reconnection, emphasizing their role in solar coronal heating and the conditions leading to loss of equilibrium and explosive energy release.

Contribution

It provides a theoretical analysis of the equilibrium conditions of current sheets in different geometries and estimates when they become unstable, impacting understanding of solar corona heating.

Findings

Finite thickness sheets cannot be in equilibrium in 2D and 2.5D geometries.

Curved guide fields can support equilibrium in 3D finite sheets under certain conditions.

Loss of equilibrium in evolving sheets may trigger reconnection, explaining coronal heating phenomena.

Abstract

The thickness of current sheets is extremely important, especially as it relates to the onset of fast magnetic reconnection. Onset determines how much magnetic free energy can build up in a field before it is explosively released. This has implications for many phenomena on the Sun and throughout the universe, including the heating of the solar corona. Significant effort has been devoted to the question of whether equilibrium current sheets in realistic geometries have finite or zero thickness. Using a simple force balance analysis, we show why current sheets without a guide field (2D) and with a guide field that is invariant in the guide field direction (2.5D) cannot be in equilibrium if they have both finite thickness and finite length. We then estimate the conditions under which the tension of a curved line-tied guide field can facilitate equilibrium in 3D sheets that are finite in all dimensions. Finally, we argue that some quasi-statically evolving current sheets undergoing slow stressing (e.g., when the coronal magnetic field is subjected to photospheric boundary driving) may reach a critical shear, at which point they lose equilibrium, spontaneously collapse, and reconnect. The critical shear is generally consistent with the heating requirements of solar active regions.