Nonlinear regimes in mean-field full-sphere dynamo

TL;DR

This paper uses a mean-field dynamo model to explore nonlinear magnetic field regimes in a fully convective star, revealing oscillating dynamo periods, regime transitions, and magnetic field configurations influenced by nonlinear effects and feedback mechanisms.

Contribution

It introduces a detailed nonlinear mean-field dynamo model for a fully convective star, highlighting regime transitions and magnetic field structures influenced by nonlinear feedback.

Findings

Oscillating dynamo regimes with ~40-year periods at intermediate Pm_T

Transition from axisymmetric to non-axisymmetric regimes due to nonlinear alpha-effect

Dominance of non-axisymmetric m=1 mode with Yin-Yang magnetic pattern

Abstract

The mean-field dynamo model is employed to study the non-linear dynamo regimes in a fully convective star of mass 0.3$M_{\odot}$ rotating with period of 10 days. For the intermediate value of the parameter of the turbulent magnetic Prandl number, $Pm_{T}=3$ we found the oscillating dynamo regimes with period about 40Yr. The higher $Pm_{T}$ results to longer dynamo periods. If the large-scale flows is fixed we find that the dynamo transits from axisymmetric to non-axisymmetric regimes for the overcritical parameter of the $\alpha$effect. The change of dynamo regime occurs because of the non-axisymmetric non-linear $\alpha$-effect. The situation persists in the fully non-linear dynamo models with regards of the magnetic feedback on the angular momentum balance and the heat transport in the star. It is found that the large-scale magnetic field quenches the latitudinal shear in the bulk of the star. However, the strong radial shear operates in the subsurface layer of the star. In the nonlinear case the profile of the angular velocity inside the star become close to the spherical surfaces. This supports the equator-ward migration of the axisymmetric magnetic field dynamo waves. It was found that, the magnetic configuration of the star dominates by the regular non-axisymmetric mode m=1, forming Yin Yang magnetic polarity pattern with the strong (>500 G) poloidal magnetic field in polar regions.