Spontaneous Formation of Surface Magnetic Structure from Large-scale Dynamo in Strongly-stratified Convection

TL;DR

This study demonstrates the spontaneous emergence of surface magnetic structures from a large-scale dynamo driven by strongly-stratified thermal convection, revealing new self-organization phenomena in stellar magnetic fields.

Contribution

First simulation showing surface magnetic structure formation from a large-scale dynamo in strongly-stratified convection, suggesting an unknown self-organization mechanism.

Findings

Surface magnetic structures form spontaneously in strongly-stratified convection.

Large-scale vertical magnetic fields organize into band-like bipolar structures.

An unknown mechanism likely drives the self-organization of magnetic structures.

Abstract

We report the first successful simulation of spontaneous formation of surface magnetic structures from a large-scale dynamo by strongly-stratified thermal convection in Cartesian geometry. The large-scale dynamo observed in our strongly-stratified model has physical properties similar to those in earlier weakly-stratified convective dynamo simulations, indicating that the $\alpha^2$-type mechanism is responsible for it. Additionally to the large-scale dynamo, we find that large-scale structures of the vertical magnetic field are spontaneously formed in the convection zone surface only for the case of strongly-stratified atmosphere. The organization of the vertical magnetic field proceeds in the upper convection zone within tens of convective turn-over time and band-like bipolar structures are recurrently-appeared in the dynamo-saturated stage. We examine possibilities of several candidates as the origin of the surface magnetic structure formation, and then suggest the existence of an as-yet-unknown mechanism for the self-organization of the large-scale magnetic structure, which should be inherent in the strongly-stratified convective atmosphere.