ON THE MODE OF DYNAMO ACTION IN A GLOBAL LARGE-EDDY SIMULATION OF SOLAR CONVECTION

In this paper, we examine the mode of dynamo action in the implicit large-eddy magnetohydrodynamical simulation of solar convection reported upon in Ghizaru et al. Motivated by the presence of a strong and well-defined large-scale axisymmetric magnetic component undergoing regular polarity reversals, we define the fluctuating component of the magnetic field as the difference between the total field and its zonal average. The subsequent analysis follows the physical logic and mathematical formulation of mean-field electrodynamics, whereby a turbulent electromotive force (EMF) is computed by the suitable averaging of cross-correlations between fluctuating flow and field components and expressed in terms of the mean field via a linear truncated tensorial expansion. We use singular value decomposition to perform a linear least-squares fit of the temporal variation of the EMF to that of the large-scale magnetic component, which yields the components of the full alpha-tensor. Its antisymmetric component, describing general turbulent pumping, is also extracted. The alpha-tensor so calculated reproduces a number of features already identified in local, Cartesian simulations of magnetohydrodynamical rotating convection, including an alpha(phi phi) component positive in the northern solar hemisphere, peaking at high latitudes, and reversing sign near the bottom of the convection zone; downward turbulent pumping throughout the convecting layer; and significant equatorward turbulent pumping at mid latitudes, and poleward at high latitudes in subsurface layers. We also find that the EMF contributes significantly to the regeneration of the large-scale toroidal magnetic component, which from the point of view of mean-field dynamo models would imply that the simulation operates as an alpha(2)Omega dynamo. We find little significant evidence of alpha-quenching by the large-scale magnetic field. The amplitude of the magnetic cycle appears instead to be regulated primarily by a magnetically driven reduction of the differential rotation.