MAGNETOCONVECTION IN A ROTATING SPHERICAL-SHELL - STRUCTURE OF FLOW IN THE OUTER CORE

The structure of thermal convection in a three-dimensional rotating spherical shell of electrically conducting fluid with an imposed toroidal magnetic field depends on the Elsasser number, the ratio of Lorentz to Coriolis forces, and on the geometry of the shell. We present results of numerical calculations demonstrating that, for Elsasser numbers Lambda much less than 1, fully developed convection consists of narrow geostrophic columns outside the inner-core tangent cylinder and weaker three-dimensional flow inside the tangent cylinder. For Elsasser numbers Lambda greater than or equal to 1 the fully developed convection consists of magnetostrophic flow inside the tangent cylinder and-large-scale spiraling geostrophic flow outside. Interaction of the geostrophic flow with toroidal field induces patches of concentrated poloidal field, which combine to produce the external dipole. The magnetostrophic component of the flow induces very little poloidal field. The external dipole field is nearly axial for small Lambda A, whereas for large Lambda the external dipole is inclined and precesses about the rotation axis.