EFFECTS OF A HETEROGENEOUS MANTLE ON THE VELOCITY AND MAGNETIC-FIELDSAT THE TOP OF THE CORE

Magnetic induction due to a steady how of the core past a bumpy, heter ogeneous mantle is investigated numerically. This study emphasizes sho rt-wavelength phenomena so that a Cartesian approximation of the bound ary region can be used to identify the main processes without introduc ing the additional complications of spherical geometry. Conductivity v ariations in the mantle are confined to a thin layer, coinciding rough ly with the D '' region, while boundary irregularities are assumed to be small so that perturbation methods are applicable. Solutions for th e full hydromagnetic disturbance in the fluid core are calculated usin g a spectral method in which the unknown velocity and magnetic perturb ations are expanded in Laguerre polynomials. The most significant pert urbations 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 m antle heterogeneity. Such large perturbations would represent a signif icant fraction of the total field at the CMB and may account for those features in the held which appear stationary. These magnetic perturba tions would also affect the mechanical coupling between the core and t he mantle. Estimates of the magnetic shear stress on the mantle increa se by a factor of two with the additional held induced by the effects of 2 km boundary topography. Fluid pressure and magnetic normal stress es act on the boundary topography to make an additional contribution t o the force couple.