On the sensitivity of magnetic cycles in global simulations of solar-like stars

TL;DR

This study uses 3D magnetohydrodynamical simulations to explore how magnetic cycle periods in solar-like stars depend on rotation and convection, revealing a non-linear dynamo mechanism influenced by Rossby number.

Contribution

It demonstrates that the magnetic cycle period decreases with Rossby number, providing a new theoretical insight into stellar magnetic activity diversity.

Findings

Decadal magnetic cycles occur at moderate Rossby numbers (0.25-1).

Shorter cycles near the equator are observed when local Rossby number is below 1.

Cycle period inversely correlates with Rossby number, explaining activity cycle variety.

Abstract

The periods of magnetic activity cycles in the Sun and solar-type stars do not exhibit a simple or even single trend with respect to rotation rate or luminosity. Dynamo models can be used to interpret this diversity, and can ultimately help us understand why some solar-like stars do not exhibit a magnetic cycle, whereas some do, and for the latter what physical mechanisms set their magnetic cycle period. Three-dimensional non-linear magnetohydrodynamical simulations present the advantage of having only a small number of tunable parameters, and produce in a dynamically self-consistent manner the flows and the dynamo magnetic fields pervading stellar interiors. We conducted a series of such simulations within the EULAG-MHD framework, varying the rotation rate and luminosity of the modeled solar-like convective envelopes. We find decadal magnetic cycles when the Rossby number near the base of the convection zone is moderate (typically between 0.25 and 1). Secondary, shorter cycles located at the top of the convective envelope close to the equator are also observed in our numerical experiments, when the local Rossby number is lower than 1. The deep-seated dynamo sustained in these numerical experiments is fundamentally non-linear, in that it is the feedback of the large-scale magnetic field on the large-scale differential rotation that sets the magnetic cycle period. The cycle period is found to decrease with the Rossby number, which offers an alternative theoretical explanation to the variety of activity cycles observed in solar-like stars.