The nature of separator current layers in MHS equilibria I. Current parallel to the separator

TL;DR

This study investigates the formation and characteristics of current layers along magnetic separators in 3D null configurations, revealing that they become twisted and evolve into structures resembling infinite-time singularities.

Contribution

It provides new insights into the nature of separator current layers with parallel currents, including their formation, twisting, and growth dynamics in non-resistive MHD relaxations.

Findings

Current layers form along separators during relaxation.

Separator current layers become twisted with increasing initial current.

Layer dimensions and strength grow with initial current, resembling infinite-time singularities.

Abstract

Separators, which are in many ways the three-dimensional equivalent to two-dimensional nulls, are important sites for magnetic reconnection. Magnetic reconnection occurs in strong current layers which have very short length scales. The aim of this work is to explore the nature of current layers around separators. A separator is a special field line which lies along the intersection of two separatrix surfaces and forms the boundary between four topologically distinct flux domains. In particular, here the current layer about a separator that joins two 3D nulls and lies along the intersection of their separatrix surfaces is investigated. A magnetic configuration containing a single separator embedded in a uniform plasma with a uniform electric current parallel to the separator is considered. This initial magnetic setup, which is not in equilibrium, relaxes in a non-resistive manner to form an equilibrium. The relaxation is achieved using the 3D MHD code, Lare3d, with resistivity set to zero. A series of experiments with varying initial current are run to investigate the characteristics of the resulting current layers present in the final (quasi-) equilibrium states. In each experiment, the separator collapses and a current layer forms along it. The dimensions and strength of the current layer increase with initial current. It is found that separator current layers formed from current parallel to the separator are twisted. Also the collapse of the separator is a process that evolves like an infinite-time singularity where the length, width and peak current in the layer grow slowly whilst the depth of the current layer decreases.