Geomagnetic data and core motions

Abstract

Backus has observed that infinite core conductivity implies the vanishing of the time derivative of the magnetic flux through any patch on the core surface bounded by a 'null-flux curve', on which the radial magnetic field vanished. Field model GSFC (12/66) is consistent with this criterion only if features with scales smaller than angular degree 7 are important or if the dipole time derivatives are deleted. Deleting the dipole is reasonable if the dipole decays in its 3rd radial mode or core conductivity is 4 x 4$^4$ mho/m. If the data admit infinite conductivity, Backus has also shown that there is an infinite-dimensional affine space of 'eligible' surface velocity fields which will produce the observed secular variation from the observed geomagnetic field, but that at any point on any null-flux curve all eligible flows have the same component normal to the curve. Using only secular variation harmonic coefficients with angular degrees 2 to 6, we obtain velocity components normal to the null-flux curves which are compatible with primarily latitude-dependent westward drift, but not with the velocity field recently proposed by Kahle, Ball & Vestine (1967b).