Turbulent transport coefficients in spherical wedge dynamo simulations of solar-like stars
1 Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
2 ReSoLVE Centre of Excellence, Department of Computer Science, Aalto University, PO Box 15400, 00 076 Aalto, Finland
3 Physics Department, Gustaf Hällströmin katu 2a, PO Box 64, 00014 University of Helsinki, Finland
4 Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
5 Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, 10691 Stockholm, Sweden
6 Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
7 JILA and Department of Astrophysical and Planetary Sciences, Box 440, University of Colorado, Boulder, CO 80303, USA
8 Laboratory for Atmospheric and Space Physics, 3665 Discovery Drive, Boulder, CO 80303, USA
Received: 14 January 2016
Accepted: 4 July 2017
Aims. We investigate dynamo action in global compressible solar-like convective dynamos in the framework of mean-field theory.
Methods. 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 (φ) averaged fields to determine the 27 turbulent transport coefficients of the electromotive force, of which six are related to the α 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.
Results. We find that the φφ-component of the α 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.
Key words: magnetohydrodynamics (MHD) / turbulence / dynamo / Sun: magnetic fields / Sun: rotation / Sun: activity
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