What sets the magnetic field strength and cycle period in solar-type stars?

TL;DR

This study uses global dynamo simulations to investigate how stellar rotation influences magnetic field strength and cycle periods in solar-like stars, revealing distinct dynamo regimes related to Rossby number and the role of shear layers.

Contribution

It demonstrates the importance of Rossby number and shear layers in determining magnetic properties, highlighting a bifurcation in dynamo behavior around Ro~1.

Findings

Magnetic field strength is independent of rotation period at the surface.

Magnetic cycle period increases with rotation period.

A bifurcation at Ro~1 separates different dynamo regimes.

Abstract

Two fundamental properties of stellar magnetic fields have been determined by observations for solar-like stars with different Rossby numbers (Ro), namely, the magnetic field strength and the magnetic cycle period. The field strength exhibits two regimes: 1) for fast rotation it is independent of Ro, 2) for slow rotation it decays with Ro following a power law. For the magnetic cycle period two regimes of activity, the active and inactive branches, also have been identified. For both of them, the longer the rotation period, the longer the activity cycle. Using global dynamo simulations of solar like stars with Rossby numbers between ~0.4 and ~2, this paper explores the relevance of rotational shear layers in determining these observational properties. Our results, consistent with non-linear alpha^2-Omega dynamos, show that the total magnetic field strength is independent of the rotation period. Yet at surface levels, the origin of the magnetic field is determined by Ro. While for Ro<1 it is generated in the convection zone, for Ro>1 strong toroidal fields are generated at the tachocline and rapidly emerge towards the surface. In agreement with the observations, the magnetic cycle period increases with the rotational period. However, a bifurcation is observed for Ro~1, separating a regime where oscillatory dynamos operate mainly in the convection zone, from the regime where the tachocline has a predominant role. In the latter the cycles are believed to result from the periodic energy exchange between the dynamo and the magneto-shear instabilities developing in the tachocline and the radiative interior.