Ba. Buffett, EFFECTS OF A HETEROGENEOUS MANTLE ON THE VELOCITY AND MAGNETIC-FIELDSAT THE TOP OF THE CORE, Geophysical journal international, 125(1), 1996, pp. 303-317
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.