Mean-field analysis on large-scale magnetic fields at high Reynolds numbers

TL;DR

This paper analyzes the mean-field dynamics of large-scale solar magnetic fields at high Reynolds numbers, revealing decreased turbulent magnetic diffusivity and a cyclic reversal process driven by the alpha-effect.

Contribution

It provides a detailed mean-field analysis of a global solar dynamo simulation, highlighting the role of turbulent magnetic diffusivity and downward pumping in magnetic field maintenance.

Findings

Turbulent magnetic diffusivity decreases at high Reynolds numbers.

Decreased diffusivity suppresses energy transfer to small-scale fields.

Downward turbulent pumping influences magnetic field reversals.

Abstract

Solar magnetic fields comprise an 11-year 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 this field have not been fully understood. This study extracts the alpha-tensor and the turbulent magnetic diffusivity tensor through mean-field analyses. The turbulent magnetic diffusivity exhibits a significant decrease towards 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 alpha-effect and is possibly triggered by downward pumping.