Kelvin-Helmholtz instability and collapse of a twisted magnetic null point with anisotropic viscosity

TL;DR

This study compares isotropic and anisotropic viscosity effects on Kelvin-Helmholtz instability and null point collapse in magnetic reconnection, revealing that anisotropic viscosity accelerates instability growth and null collapse, with distinct heating characteristics.

Contribution

It introduces a detailed comparison of isotropic versus anisotropic viscosity in 3D MHD simulations of magnetic null point dynamics, highlighting the impact on instability growth and energy dissipation.

Findings

Anisotropic viscosity leads to faster instability growth.

Null point collapse occurs sooner with anisotropic viscosity.

Anisotropic viscosity results in greater overall heating.

Abstract

Context: Magnetic null points are associated with high-energy coronal phenomena such as solar flares and are often sites of reconnection and particle acceleration. Dynamic twisting of a magnetic null point can generate a Kelvin-Helmholtz instability (KHI) within its fan plane and, under continued twisting, can instigate spine-fan reconnection and an associated collapse of the null point. Aim: This article aims to compare the effects of isotropic and anisotropic viscosity in simulations of the KHI and collapse in a dynamically twisted magnetic null point. Methods: Simulations were performed using the 3D magnetohydrodynamics code Lare3d with a custom anisotropic viscosity module. A pair of high resolution simulations was performed, one using isotropic viscosity and another using anisotropic viscosity, keeping all other factors identical, and the results analysed in detail. A further parameter study was performed over a range of values for viscosity and resistivity. Results: Both viscosity models permit the growth of the Kelvin-Helmholtz instability and the eventual collapse of the null point. Over all studied parameters, anisotropic viscosity allows a faster growing instability, while isotropic viscosity damps the instability to the extent of stabilisation in some cases. Although the viscous heating associated with anisotropic viscosity is generally smaller, the ohmic heating dominates and is enhanced by the current sheets generated by the instability, leading to a greater overall heating rate when using anisotropic viscosity. The collapse of the null point occurs significantly sooner when anisotropic viscosity is employed.