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

Abstract

We investigate the magnetic field generation in global 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 large-scale dynamo. To analyze the dynamo mechanism we apply the test-field method for azimuthally (φ) averaged fields to determine the 27 turbulent transport coefficients of the electromotive force, of which 9 are related to the α effect tensor. This method has previously been used either in simulations in Cartesian coordinates or in the geodynamo context and it is applied here for the first time in simulations of solar-like dynamo action. We find that the φφ -component of the $\alpha$ tensor does not follow the profile expected from that of kinetic helicity. Beside the dominant $\alpha$-$\Omega$ dynamo, also an α dynamo is locally enhanced. 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 due to dynamically important magnetic fields with quenching as well as enhancement 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 value 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.