Nonuniformity effects in the negative effective magnetic pressure instability

TL;DR

This paper investigates how non-uniform magnetic fields influence turbulent pressure suppression and the resulting large-scale instability, using simulations to reveal how turbulence contributes negatively to magnetic pressure and promotes flux concentration.

Contribution

It demonstrates the impact of magnetic field non-uniformity on turbulent pressure suppression and the development of magnetic flux concentrations through combined DNS and MFS approaches.

Findings

Non-uniform magnetic fields enhance turbulent pressure suppression.

Large-scale instability causes magnetic flux concentrations.

Effective mean-field pressure is uniformly reduced within structures.

Abstract

Using direct numerical simulations (DNS) and mean-field simulations (MFS), the effects of non-uniformity of the magnetic field on the suppression of the turbulent pressure is investigated. This suppression of turbulent pressure can lead to an instability which, in turn, makes the mean magnetic field even more non-uniform. This large-scale instability is caused by a resulting negative contribution of small-scale turbulence to the effective (mean-field) magnetic pressure. We show that enhanced mean current density increases the suppression of the turbulent pressure. The instability leads to magnetic flux concentrations in which the normalized effective mean-field pressure is reduced to a certain value at all depths within a structure.