Bipolar magnetic structures driven by stratified turbulence with a coronal envelope

TL;DR

This study demonstrates the spontaneous formation of bipolar magnetic structures in stratified turbulence with a coronal envelope, showing their development, decay, and association with magnetic pressure instability.

Contribution

It reveals how bipolar magnetic regions naturally form in stratified turbulent environments with a coronal layer, highlighting the role of negative effective magnetic pressure instability.

Findings

Bipolar magnetic regions form after 1-2 turbulent diffusion times.

Magnetic field strengths reach equipartition levels.

Formed structures resemble loop-like features in the corona.

Abstract

We report the spontaneous formation of bipolar magnetic structures in direct numerical simulations of stratified forced turbulence with an outer coronal envelope. The turbulence is forced with transverse random waves only in the lower (turbulent) part of the domain. Our initial magnetic field is either uniform in the entire domain or confined to the turbulent layer. After about 1--2 turbulent diffusion times, a bipolar magnetic region of vertical field develops with two coherent circular structures that live during one turbulent diffusion time, and then decay during 0.5 turbulent diffusion times. The resulting magnetic field strengths inside the bipolar region are comparable to the equipartition value with respect to the turbulent kinetic energy. The bipolar magnetic region forms a loop-like structure in the upper coronal layer. We associate the magnetic structure formation with the negative effective magnetic pressure instability in the two-layer model.