Dynamo cycles in global convection simulations of solar-like stars

TL;DR

This study uses 3D magnetohydrodynamical simulations to explore how rotation influences magnetic activity cycles in solar-like stars, revealing a weak increase in cycle period with rotation and supporting a dynamo wave mechanism.

Contribution

It demonstrates the rotational dependence of dynamo cycles in solar-like stars and applies the test-field method to analyze the alpha effect in these simulations.

Findings

Dynamo with oscillating magnetic fields appears at moderate and rapid rotation.

Cycle periods increase weakly with rotation, explained by a Parker-Yoshimura dynamo wave.

No activity branches as previously suggested, but results align with a transitional branch.

Abstract

Several solar-like stars exhibit cyclic magnetic activity similar to the Sun as found in photospheric and chromospheric emission. We want to understand the physical mechanism involved in rotational dependence of these activity cycle periods. We use three-dimensional magnetohydrodynamical simulations of global convective dynamos models of solar-like stars to investigate the rotational dependency of dynamos. We further apply the test-field method to determine the $\alpha$ effect in these simulations. We find dynamo with clear oscillating mean magnetic fields for moderately and rapidly rotating runs. For slower rotation, the field is constant or exhibit irregular cycles. In the moderately and rapidly rotating regime the cycle periods increase weakly with rotation. This behavior can be well explained with a Parker-Yoshimura dynamo wave traveling equatorward. Even though the $\alpha$ effect becomes stronger for increasing rotation, the shear decreases steeper, causing this weak dependence on rotation. Similar as other numerical studies, we find no indication of activity branches as suggested by Brandenburg et al. (1998). However, our simulation seems to agree more with the transitional branch suggested by Distefano et al. (2017) and Olspert et al. (2017). If the Sun exhibit a dynamo wave similar as we find in our simulations, it would operate deep inside the convection zone.