Geomagnetic westward drift and irregularities in the Earth's rotation

Abstract

Bullard et al.'s (1950) rigid-sphere model for the steady westward drift of the earth’s non-dipole magnetic field is extended to include the magnetic coupling provided by the presence of the higher multipole fields at the core-mantle boundary. If the first six harmonics of the observed surface field are taken into account, the retarding couple on the mantle is increased by a factor of 1-6. Using this resul it would follow from Bullard’s specialized model of the core that the geomagnetic dynamo mechanism generates a toroidal field of several hundred gauss in the deep interior of the core. Time-dependent perturbations of the mantle-core coupling are investigated rigorously, and it is shown that reasonable fluctuations of the fields at the core-mantle boundary are capable of explaining changes in the length of day at the rate of order 1 ms in 10 yr . The tightness of the coupling is increased by 60% over that afforded by Bullard’s model. The argument provides additional evidence that the mean electrical conductivity in the bottom 2000 km of the mantle is at least 10~ 9 e.m.u. A summary of knowledge of the distribution of electrical conductivity with depth in the earth is given in the Introduction.