Detection of negative effective magnetic pressure instability in turbulence simulations

TL;DR

This paper demonstrates the negative effective magnetic pressure instability in turbulence simulations, showing how it leads to large-scale magnetic flux concentrations, supporting earlier theoretical predictions and relevant to solar active region formation.

Contribution

First direct numerical simulation evidence of the negative effective magnetic pressure instability in stratified turbulence, confirming theoretical models and exploring its properties.

Findings

Flux concentrations form due to the instability

Magnetic energy weakly depends on Reynolds number at high values

Results support earlier mean-field and analytic work

Abstract

We present the first demonstration of the negative effective magnetic pressure instability in direct numerical simulations of stably stratified, externally forced, isothermal hydromagnetic turbulence in the regime of large plasma beta. By the action of this instability, an initially uniform horizontal magnetic field forms flux concentrations whose scale is large compared to the turbulent scale. We further show that the magnetic energy of these large-scale structures is only weakly dependent on the magnetic Reynolds number, provided its value is large enough for the instability to be excited. Our results support earlier mean-field calculations and analytic work which identified this instability. Applications to the formation of active regions in the Sun are discussed.