On the use of horizontal components of magnetic field in determining core motions

Abstract

SUMMARY Geomagnetic secular variation is caused by flow of fluid in the Earth's core. If electrical diffusion can be ignored, field lines are frozen to the core fluid and can be used as tracers for the flow at the top of the free stream, which lies beneath the diffusive boundary layer. In this paper we assume the flux is frozen-in and investigate the jump in the horizontal field across the boundary layer. Theoretical studies indicate that such a jump will be small (about 1000 nT); we test the hypothesis that the jump is zero within the errors of measurement of the magnetic field. Integral constraints, which are conserved if the horizontal components of field at the base of the mantle can be equated to the components at the top of the free stream, are derived and computed using seven main-field models from epochs 1935.5 to 1980.0. These main-field models have been forced to satisfy all the conditions of frozen-flux theory. The integral constraints are found to be constant within their expected errors, and we conclude that the observations are consistent with zero jump in horizontal field across the boundary layer. This result implies that use can be made of the horizontal components of field in determining core flow. We attempt to compute the flow along null-flux curves, which is not determined at all by the radial component of the field, for epoch 1970. The result is very noisy because of its reliance on gradients of the secular variation at the core-mantle boundary, which are poorly determined. We do not attempt to compute reasonable core flows consistent with these observations, although presumably many such flows exist, because they would be so poorly constrained by observation.