Sharp magnetic structures from dynamos with density stratification

TL;DR

This paper demonstrates that sharp bipolar magnetic structures naturally form in density-stratified turbulent dynamos, confirmed by both direct numerical simulations and mean-field models, driven by converging surface flows and Lorentz forces.

Contribution

It shows that sharp magnetic structures are a generic feature of dynamos in stratified layers, supported by both DNS and MFS, expanding understanding of magnetic pattern formation.

Findings

Sharp bipolar structures form in stratified dynamo simulations.

Mean-field models qualitatively reproduce DNS results.

Surface converging flows and Lorentz forces drive structure formation.

Abstract

Recent direct numerical simulations (DNS) of large-scale turbulent dynamos in strongly stratified layers have resulted in surprisingly sharp bipolar structures at the surface. Here we present new DNS of helically and non-helically forced turbulence with and without rotation and compare with corresponding mean-field simulations (MFS) to show that these structures are a generic outcome of a broader class of dynamos in density-stratified layers. The MFS agree qualitatively with the DNS, but the period of oscillations tends to be longer in the DNS. In both DNS and MFS, the sharp structures are produced by converging flows at the surface and might be driven in nonlinear stage of evolution by the Lorentz force associated with the large-scale dynamo-driven magnetic field if the dynamo number is at least 2.5 times supercritical.