Spontaneous formation of flux concentrations in a stratified layer

TL;DR

This paper investigates the spontaneous formation of large-scale magnetic flux concentrations in a stratified layer due to the negative effective magnetic pressure instability, using advanced direct numerical simulations to extend previous findings.

Contribution

It extends previous DNS studies into larger scale separation regimes, demonstrating spontaneous magnetic structure formation without averaging, and characterizes the effective magnetic pressure's dependence on field strength.

Findings

Large-scale magnetic structures form spontaneously in simulations.

Effective magnetic pressure becomes negative below the equipartition field.

Dependence of magnetic pressure on field strength shows convergence at higher magnetic Reynolds numbers.

Abstract

The negative effective magnetic pressure instability discovered recently in direct numerical simulations (DNS) may play a crucial role in the formation of sunspots and active regions in the Sun and stars. This instability is caused by a negative contribution of turbulence to the effective mean Lorentz force (the sum of turbulent and non-turbulent contributions) and results in formation of large-scale inhomogeneous magnetic structures from initial uniform magnetic field. Earlier investigations of this instability in DNS of stably stratified, externally forced, isothermal hydromagnetic turbulence in the regime of large plasma beta are now extended into the regime of larger scale separation ratios where the number of turbulent eddies in the computational domain is about 30. Strong spontaneous formation of large-scale magnetic structures is seen even without performing any spatial averaging. These structures encompass many turbulent eddies. The characteristic time of the instability is comparable to the turbulent diffusion time, L^2/eta_t, where eta_t is the turbulent diffusivity and L is the scale of the domain. DNS are used to confirm that the effective magnetic pressure does indeed become negative for magnetic field strengths below the equipartition field. The dependence of the effective magnetic pressure on the field strength is characterized by fit parameters that seem to show convergence for larger values of the magnetic Reynolds number.