Magnetoconvection and thermal coupling of the Earth's core and mantle

Numerical calculations of finite amplitude magnetoconvection in a rotating spherical shell are used to investigate the thermal coupling of the Earth's core to the mantle. From the observed distribution of lower mantle seismic velocity heterogeneity we construct a pattern of heat flow on the core-mantle boundary consisting of a spherical average go plus heterogeneity with amplitude Delta q. For Delta q/q(0) = 0, corresponding to a homogeneous lower mantle, convection in the presence of a strong toroidal magnetic field consists of nearly axisymmetric magnetostrophic flow inside the inner-core tangent cylinder and a single large-scale spiraling columnar plume outside the tangent cylinder. Interaction of the columnar plume with the toroidal field induces patches of radial magnetic field distributed symmetrically with respect to the equator, For Delta q/q(0) = 10, corresponding to a strongly heterogeneous lower mantle, stably stratified regions develop below warm mantle and enhanced convection develops below cold mantle. This modulation of the convection pattern breaks the columnar structure of core motions and destroys the equatorial symmetry of the induced magnetic field, without locking it to the mantle. Our results indicate that mantle structure is of secondary importance, compared with rotation, in controlling the structure of the geomagnetic field.