On the role of tachoclines in solar and stellar dynamos

TL;DR

This paper investigates the role of tachoclines in solar and stellar dynamos through global simulations, revealing how their presence influences magnetic field generation, cycle periods, and dynamo behavior depending on the Rossby number.

Contribution

The study provides new insights into how tachoclines affect dynamo processes and magnetic field evolution in solar-like stars, using comprehensive global simulations that include radiative zones.

Findings

Tachoclines lead to stronger, longer-lived magnetic fields.

Dynamo cycles vary from ~2 years to ~30 years depending on conditions.

Presence of tachoclines results in more complex dynamo dynamics.

Abstract

Rotational shear layers at the boundary between radiative and convective zones, tachoclines, play a key role in the process of magnetic field generation in solar-like stars. We present two sets of global simulations of rotating turbulent convection and dynamo. The first set considers a stellar convective envelope only; the second one, aiming at the formation of a tachocline, considers also the upper part of the radiative zone. Our results indicate that the resulting mean-flows and dynamo properties like the growth rate, saturation energy and mode depend on the Rossby (Ro) number. For the first set of models either oscillatory (with ~2 yr period) or steady dynamo solutions are obtained. The models in the second set naturally develop a tachocline which, in turn, leads to the generation of strong mean magnetic field. Since the field is also deposited into the stable deeper layer, its evolutionary time-scale is much longer than in the models without a tachocline. Surprisingly, the magnetic field in the upper turbulent convection zone evolves in the same time scale as the deep field. These models result in either an oscillatory dynamo with ~30 yr period or in a steady dynamo depending on Ro. In terms of the mean-field dynamo coefficients computed using FOSA, the field evolution in the oscillatory models without a tachocline seems to be consistent with dynamo waves propagating according to the Parker-Yoshimura sign rule. In the models with tachoclines the dynamics is more complex involving other transport mechanisms as well as tachocline instabilities.