Magnetic fields of low-mass main sequences stars: Nonlinear dynamo theory and mean-field numerical simulations

TL;DR

This study uses mean-field simulations and nonlinear theory to analyze magnetic field generation in low-mass stars, revealing how differential rotation influences magnetic cycle behavior and complexity.

Contribution

It introduces a detailed mean-field dynamo model for low-mass stars, highlighting the impact of weak differential rotation on magnetic activity regimes and cycle periods.

Findings

Magnetic activity shifts from aperiodic to chaotic with increased differential rotation.

Magnetic cycle periods range from tens to thousands of years.

Weak differential rotation modifies the dynamo behavior significantly.

Abstract

Our theoretical and numerical analysis have suggested that for low-mass main sequences stars (of the spectral classes from M5 to G0) rotating much faster than the Sun, the generated large-scale magnetic field is caused by the mean-field $\alpha^2\Omega$ dynamo, whereby the $\alpha^2$ dynamo is modified by a weak differential rotation. Even for a weak differential rotation, the behaviour of the magnetic activity is changed drastically from aperiodic regime to non-linear oscillations and appearance of a chaotic behaviour with increase of the differential rotation. Periods of the magnetic cycles decrease with increase of the differential rotation, and they vary from tens to thousand years. This long-term behaviour of the magnetic cycles may be related to the characteristic time of the evolution of the magnetic helicity density of the small-scale field. The performed analysis is based on the mean-field simulations (MFS) of the $\alpha^2\Omega$ and $\alpha^2$ dynamos and a developed non-linear theory of $\alpha^2$ dynamo. The applied MFS model was calibrated using turbulent parameters typical for the solar convective zone.