Current accumulation at an asymmetric 3D null point caused by generic shearing motions

TL;DR

This study explores how the orientation of shear motions affects current accumulation and magnetic reconnection at a 3D null point, revealing the importance of magnetic field structure in determining current localization.

Contribution

It demonstrates the dependence of current accumulation on the relative orientation between shear driving and the null's eigenvectors in a 3D magnetic null point.

Findings

Current accumulates along the fan eigenvector with the smallest eigenvalue.

Weak current layers form when driving is aligned with the weak eigenvector and impulsive.

Boundary effects dominate current accumulation under continuous stressing.

Abstract

Context. We investigate the dynamical evolution of the reconnection process at an initially linear 3D null point that is stressed by a localised shear motion across the spine axis. The fan plane is not rotationally symmetric and this allows for different behaviours depending on the alignment of the fan plane relative to the imposed driver direction. Aims. The aim is to show how the current accumulation and the associated reconnection process depends on the relative orientation between the driver imposed stress across the spine axis of the null and the main eigenvector direction in the fan plane. Methods. The time evolution of the 3D null point is investigated solving the 3D non-ideal MHD equations numerically in a Cartesian box. The magnetic field is frozen to the boundaries and the boundary velocity is only non-zero where the imposed driving is for stressing the system is applied. Results. The current accumulation is found to be along the direction of the fan eigenvector associated with the smallest eigenvalue until the direction of the driver is almost parallel to this eigenvector. When the driving velocity is parallel to the weak eigenvector and has an impulsive temporal profile the null only forms a weak current layer. However, when the null point is stressed continuously boundary effects dominates the current accumulation. Conclusions. There is a clear relation between the orientation of the current concentration and the direction of the fan eigenvector corresponding to the small eigenvalue. This shows that the structure of the magnetic field is the most important in determining where current is going to accumulate when a single 3D null point is perturbed by a simple shear motion across the spine axis. As the angle between the driving direction and the strong eigenvector direction increases, the current that accumulates at the null becomes progressively weaker.