Kelvin-Helmholtz instability in a current-vortex sheet at a 3D magnetic null

TL;DR

This paper reports the first observation of a Kelvin-Helmholtz instability in a 3D current-vortex sheet at a magnetic null, revealing complex filamentation, reconnection, and plasma mixing in a magnetic topology relevant to space plasma phenomena.

Contribution

It demonstrates the occurrence of KH instability in a 3D current-vortex sheet at a magnetic null, highlighting its role in plasma mixing and magnetic reconnection.

Findings

KH instability causes filamentation and vortex branching.

Plasma mixing occurs across the fan separatrix surface.

Instability may enhance heating and magnetic connectivity.

Abstract

We report here, for the first time, an observed instability of a Kelvin-Helmholtz (KH) nature occurring in a fully three-dimensional (3D) current-vortex sheet at the fan plane of a 3D magnetic null point. The current-vortex layer forms self-consistently in response to foot point driving around the spine lines of the null. The layer first becomes unstable at an intermediate distance from the null point, with the instability being characterized by a rippling of the fan surface and a filamentation of the current density and vorticity in the shear layer. Owing to the 3D geometry of the shear layer, a branching of the current filaments and vortices is observed. The instability results in a mixing of plasma between the two topologically distinct regions of magnetic flux on either side of the fan separatrix surface, as flux is reconnected across this surface. We make a preliminary investigation of the scaling of the system with the dissipation parameters. Our results indicate that the fan plane separatrix surface is an ideal candidate for the formation of current-vortex sheets in complex magnetic fields and, therefore, the enhanced heating and connectivity change associated with the instabilities of such layers.