Electromagnetic core–mantle coupling: III. Laterally varying mantle conductance

Abstract

We consider the implications of lateral variations in deep-mantle conductance for electromagnetic core–mantle coupling and the consequent decadal variations in length of day. Large scale variations in conductance can alter long-term trends in the torque, but do not on average enhance its magnitude or alter its decadal variation. Many previous estimates of the torque predict an unobserved negative trend during this century; this trend may result from modelling assuming an incorrect conductance structure. By suitable choice of this structure, the trend can be removed, but there is no correlation between such a structure and seismological observations of lower mantle heterogeneity. Runcorn argued that a highly conducting layer in the deep mantle beneath the Pacific could account for features of the observed secular variation, and also preferred palaeomagnetic VGP reversal paths. We argue that the conductance customarily advocated for this mechanism is inconsistent with core–mantle angular momentum exchange on decadal time scales.