The negative magnetic pressure effect in stratified turbulence

TL;DR

This paper investigates how turbulence-induced negative magnetic pressure in stratified environments can lead to the formation of large-scale magnetic structures, challenging traditional flux tube models and providing numerical verification of this effect.

Contribution

It demonstrates through direct numerical simulations that turbulence can produce negative magnetic pressure effects, facilitating large-scale magnetic structure formation in stratified turbulence.

Findings

Negative magnetic pressure effect verified in simulations

Large-scale magnetic structures can form from uniform fields

Turbulence anisotropy influences magnetic pressure behavior

Abstract

While the rising flux tube paradigm is an elegant theory, its basic assumptions, thin flux tubes at the bottom of the convection zone with field strengths two orders of magnitude above equipartition, remain numerically unverified at best. As such, in recent years the idea of a formation of sunspots near the top of the convection zone has generated some interest. The presence of turbulence can strongly enhance diffusive transport mechanisms, leading to an effective transport coefficient formalism in the mean-field formulation. The question is what happens to these coefficients when the turbulence becomes anisotropic due to a strong large-scale mean magnetic field. It has been noted in the past that this anisotropy can also lead to highly non-diffusive behaviour. In the present work we investigate the formation of large-scale magnetic structures as a result of a negative contribution of turbulence to the large-scale effective magnetic pressure in the presence of stratification. In direct numerical simulations of forced turbulence in a stratified box, we verify the existence of this effect. This phenomenon can cause formation of large-scale magnetic structures even from initially uniform large-scale magnetic field.