Electromagnetic core-mantle coupling for laterally varying mantle conductivity

Electromagnetic coupling between the Earth's core and mantle is one of the proposed mechanisms to explain length of day (LOD) variations on decadal ti mescales. Mantle conductivity, a, determining parameter in this process, is only poorly known. Earlier work on core-mantle coupling mostly assumed lat erally homogeneous mantle conductivities. However, the lower mantle is a hi ghly inhomogeneous region. Seismic evidence is growing that many of the inh omogeneities must have chemical, compositional, or thermal origins. We cons ider the effect of different laterally varying conductivity models on elect romagnetic mantle torques. Torque amplitudes for a,specific epoch can depen d strongly on the assumed conductivity distribution. However, when comparin g time series of the electromagnetic torque to decadal LOD variations, none of the examined conductivity models improve the agreement significantly. A s in the simplest case of a homogeneous mantle conductivity, a minimum aver age conductance of 10(8) S is always required to make electromagnetic coupl ing efficient.