Turbulent transport coefficients in spherical wedge dynamo simulations of solar-like stars

TL;DR

This study uses advanced simulations and the test-field method to analyze turbulent transport coefficients in a solar-like dynamo, revealing complex behaviors and nonlinear feedback mechanisms affecting magnetic field generation.

Contribution

First application of the test-field method to fully compressible solar-like dynamo simulations in a spherical wedge, providing detailed insights into turbulent transport coefficients.

Findings

The $eta$ tensor component does not match kinetic helicity expectations.

Turbulent pumping significantly influences mean magnetic field flows.

Coefficients are modulated by magnetic activity cycles, showing quenching and enhancement.

Abstract

We investigate dynamo action in global compressible solar-like convective dynamos in the framework of mean-field theory. We simulate a solar-type star in a wedge-shaped spherical shell, where the interplay between convection and rotation self-consistently drives a large-scale dynamo. To analyze the dynamo mechanism we apply the test-field method for azimuthally ($\phi$) averaged fields to determine the 27 turbulent transport coefficients of the electromotive force, of which six are related to the $\alpha$ tensor. This method has previously been used either in simulations in Cartesian coordinates or in the geodynamo context and is applied here for the first time to fully compressible simulations of solar-like dynamos. We find that the $\phi\phi$-component of the $\alpha$ tensor does not follow the profile expected from that of kinetic helicity. The turbulent pumping velocities significantly alter the effective mean flows acting on the magnetic field and therefore challenge the flux transport dynamo concept. All coefficients are significantly affected by dynamically important magnetic fields. Quenching as well as enhancement are being observed. This leads to a modulation of the coefficients with the activity cycle. The temporal variations are found to be comparable to the time-averaged values and seem to be responsible for a nonlinear feedback on the magnetic field generation. Furthermore, we quantify the validity of the Parker-Yoshimura rule for the equatorward propagation of the mean magnetic field in the present case.