Electromagnetic Core-mantle Coupling

Abstract

Summary The work of Bullard (1950) and Rochester (1960) on the geomagnetic westward drift and its effects on the Earth’s rotation is extended to investigate the effects of assuming various distributions of electrical conductivity in the mantle. By a proper choice of conductivities, one is able to increase the theoretical value for the tightness of the coupling by a factor of at least six over that afforded by Rochester’s model, without sacrificing agreement with observations on the rapidity with which changes in the secular variation are established at the Earth’s surface. It is shown that it is reasonable to attribute the observed random changes in the length of the day to perturbations in the electromagnetic coupling. I. Introduction . The most significant hypotheses brought forward to date for the explanation of the existence and maintenance of the terrestrial magnetic field have been reviewed by Jacobs (1956). Of these, only the electromagnetic dynamo theory is able to stand in the light of present information. Elsasser (1956) has reviewed the models suggested for a dynamo mechanism in the core capable of maintaining the dipole part of the field. The analytical details of dynamo operation in the core are irrelevant to the theoretical development in this paper. We need only assume, as did Rochester (1960), that toroidal fields of the correct character are produced in the deep interior and manifest themselves at the core boundary. Convective motion (more or less radial) in the fluid core is fundamental to the maintenance of dynamo action in all current theories. Where the velocity of the convective motion possesses a radial component, conservation of angular momentum requires that the inner fluid core rotate more rapidly than the outer layers. Evidence for this differential rate of rotation is given by observation of a westward drift of the non-dipole part of the field and of the centres of secular change (Bullard & others 1950). Since the high electrical conductivity of the core requires that the magnetic field lines be forced to move bodily with the fluid, the average value of the geomagnetic westward drift can be interpreted as the rate of rotation of the outer layers of the fluid core relative to an observer stationed on the mantle. The non-uniform rotation, together with the high electrical conductivity of the core, leads to the conclusion that a large toroidal field must be induced at the core boundary from the poloidal fields observed at the Earth’s surface. The toroidal fields leak into the conducting part of the mantle, vanishing at the boundary between the conducting and insulating parfs. The existence of a 361