Magnetohydrodynamic Flows Formed on the Earth's Outer Core Boundaries

Abstract

A general asymptotic procedure is suggested for describing the magnetohydrodynamic (MHD) flows in the outer core of the Earth on a time scale larger than 100 years. The asymptotic expansion is based on the small Ekman number in the Earth's outer core. Fluid motion in the core can be divided into two parts: large-scale flows in the bulk of the outer core and small-scale ones concentrated near the boundaries. The small-scale and large-scale flows are consistent with each other, when the ratio of small-scale electric currents to large-scale ones and the ratio of Coriolis to Lorentz forces in the bulk of the outer core are roughly described by the magnetic Reynolds number. The present configuration of the Earth's magnetic field requires a boundary layer beneath the core mantle boundary (CMB) which is much thicker near the equator than at other latitudes. The small-scale electric currents in this layer can produce geomagnetic secular variations. Through the application of the present theory to axisymmetric flows, it is shown that the description of the fluid flow is less complicated with the presence of the magnetic field than is in the purely hydrodynamic case. The MHD coupling removes most of complications associated with the formation of the boundary layers.