An analytical model is developed for the electromagnetic torques exerted on the mantle by the poloidal magnetic field of the core interacting with a laterally heterogeneous conducting layer at the base of the mantle. Torques due to changes in both orientation and intensity of a dipolar poloidal field are included. Contrary to earlier suggestions, our calculations predict that the trajectory of the magnetic pole during a polarity reversal is not strongly affected by heterogeneity in mantle electrical conductivity.

Laboratory experiments that combine thermal convection in a rapidly rotating shell with a sudden increase of the shell's rotation rate (spin-up) enable us to study processes related to turbulent viscous coupling between planetary fluid cores and solid mantles. We experimentally measure the large-scale effective viscosity by determining how the synchronisation time between the fluid and the shell (called the spin-up time) is shortened when convective turbulence exists in the bulk of the fluid. Our experiments suggest that viscous core-mantle coupling in planets may be greater than has been previously estimated using molecular viscosity values. © 2003 Elsevier B.V. All rights reserved.