Scale similarity of MHD turbulence in the Earth's core

Abstract

Turbulent motions in the core, being highly anisotropic because of the influence of the Earth's rotation and its magnetic field, cause the eddy diffusion of large-scale fields much more effectively than the molecular diffusion. Reliable estimates of the eddy diffusivities, or the subgrid-scale fluxes, are therefore of significance. In this paper, scale similarity of magnetohydrodynamic turbulence in a rapidly rotating system is investigated to model subgrid-scale processes, as used in large-eddy simulations. The turbulent flux has been computed by taking an ensemble average of results of direct numerical simulations, which are to be employed in this paper, over the computational box which represents a small region in the Earth's core. The anisotropy of turbulent flux computed after averaging over segments into which the box is divided remains unchanged even when the size of segments changes. Dependence of turbulent flux computed from fields to which a spatial filter is applied on its width indicates that subgrid-scale flux can be evaluated through extrapolation. This method will be useful for performing global geodynamo simulations taking into account subgrid-scale processes. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences.