Mean-field Analysis on Large-scale Magnetic Fields at High Reynolds Numbers

Abstract

Solar magnetic fields comprise an 11 yr activity cycle, represented by the number of sunspots. The maintenance of such a solar magnetic field can be attributed to fluid motion in the convection zone, i.e., a dynamo. This study conducts the mean-field analyses of the global solar dynamo simulation presented by Hotta et al. (2016). Although the study succeeds in producing coherent large-scale magnetic fields at high Reynolds numbers, the detailed physics of the maintenance of these fields have not been fully understood. This study extracts the α tensor and the turbulent magnetic diffusivity tensor β through mean-field analyses. The turbulent magnetic diffusivity exhibits a significant decrease toward high Reynolds numbers. The decrease in the turbulent magnetic diffusivity suppresses the energy conversion of large-scale field to small-scale field. This implies that the decrease in the turbulent magnetic diffusivity contributes to the maintenance of a large-scale magnetic field at high Reynolds numbers. A significant downward turbulent pumping is observed; it is enhanced in the weak phase of the large-scale field. This study proposes a cyclic reversal process of a large-scale field, which is dominantly driven by the α effect and is possibly triggered by downward pumping.