Electromagnetic core-mantle coupling - III. Laterally varying mantle conductance

We consider the implications of lateral variations in deep-mantle conductan ce for electromagnetic core-mantle coupling and the consequent decadal vari ations in length of day. Large scale variations in conductance can alter lo ng-term trends in the torque, but do not on average enhance its magnitude o r alter its decadal variation. Many previous estimates of the torque predic t an unobserved negative trend during this century; this trend may result f rom modelling assuming an incorrect conductance structure. By suitable choi ce of this structure, the trend can be removed, but there is no correlation between such a structure and seismological observations of lower mantle he terogeneity. Runcorn argued that a highly conducting layer in the deep mant le beneath the Pacific could account for features of the observed secular v ariation, and also preferred palaeomagnetic VGP reversal paths. We argue th at the conductance customarily advocated for this mechanism is inconsistent with core-mantle angular momentum exchange on decadal time scales. (C) 200 0 Elsevier Science B.V. All rights reserved.