Magnetohydrodynamics dynamical relaxation of coronal magnetic fields. II. 2D magnetic X-points

TL;DR

This study investigates the formation and nature of current sheets in 2D magnetic X-points under finite plasma pressure, revealing a slow, asymptotic singularity growth and a non-thin current sheet equilibrium.

Contribution

It demonstrates that finite plasma pressure prevents the formation of infinitesimally thin current sheets, leading to a quasi-static equilibrium with a finite thick current layer.

Findings

Current density concentrates at the null and separatrices.

The singularity growth rate is proportional to t^D, with 0 < D < 1.

Finite pressure influences the current sheet structure and growth rate.

Abstract

We provide a valid magnetohydrostatic equilibrium from the collapse of a 2D X-point in the presence of a finite plasma pressure, in which the current density is not simply concentrated in an infinitesimally thin, one-dimensional current sheet, as found in force-free solutions. In particular, we wish to determine if a finite pressure current sheet will still involve a singular current, and if so, what is the nature of the singularity. We use a full MHD code, with the resistivity set to zero, so that reconnection is not allowed, to run a series of experiments in which an X-point is perturbed and then is allowed to relax towards an equilibrium, via real, viscous damping forces. Changes to the magnitude of the perturbation and the initial plasma pressure are investigated systematically. The final state found in our experiments is a "quasi-static" equilibrium where the viscous relaxation has completely ended, but the peak current density at the null increases very slowly following an asymptotic regime towards an infinite time singularity. Using a high grid resolution allows us to resolve the current structures in this state both in width and length. In comparison with the well known pressureless studies, the system does not evolve towards a thin current sheet, but concentrates the current at the null and the separatrices. The growth rate of the singularity is found to be tD, with 0 < D < 1. This rate depends directly on the initial plasma pressure, and decreases as the pressure is increased. At the end of our study, we present an analytical description of the system in a quasi-static non-singular equilibrium at a given time, in which a finite thick current layer has formed at the null.