Effects of a heterogeneous mantle on the velocity and magnetic fields at the top of the core

Magnetic induction due to a steady how of the core past a bumpy, heterogeneous mantle is investigated numerically. This study emphasizes short-wavelength phenomena so that a Cartesian approximation of the boundary region can be used to identify the main processes without introducing the additional complications of spherical geometry. Conductivity variations in the mantle are confined to a thin layer, coinciding roughly with the D '' region, while boundary irregularities are assumed to be small so that perturbation methods are applicable. Solutions for the full hydromagnetic disturbance in the fluid core are calculated using a spectral method in which the unknown velocity and magnetic perturbations are expanded in Laguerre polynomials. The most significant perturbations arise in the magnetic field at the core-mantle boundary (CMB), which can be as large as 1 - 2 x 10(-4) T for plausible levels of mantle heterogeneity. Such large perturbations would represent a significant fraction of the total field at the CMB and may account for those features in the held which appear stationary. These magnetic perturbations would also affect the mechanical coupling between the core and the mantle. Estimates of the magnetic shear stress on the mantle increase by a factor of two with the additional held induced by the effects of 2 km boundary topography. Fluid pressure and magnetic normal stresses act on the boundary topography to make an additional contribution to the force couple.