Latitude Dependence of the Angular Dispersion of the Geomagnetic Field

Abstract

Summary Changes in the direction of the Earth's magnetic field at a given site are produced in part by wobble of the main geomagnetic dipole, in part by fluctuations in the intensity and direction of the non-dipole field, and in part by changes in the intensity of the main dipole field. These three processes combine to produce an angular variance that is strongly latitude dependent. A method is presented for isolating the contribution due to variation with latitude of the average intensity of the non-dipole field. The aspect of geomagnetic secular variation most accessible to palaeomagnetic measurement is the angular dispersion of the field over long periods of time. The amount of the angular dispersion is determined by two factors. One is the angular wobble of the main geomagnetic field. The other is the time average of the ratio flF of the non-dipole field to the dipole field. This time average is of general geophysical interest because it provides a measure of the time average of the magnetohydrodynamic processes in the region of the Earth's core beneath the site where the palaeomagnetic samples were collected. The time average of the nondipole field may exhibit a longitude dependence and a latitude (or zonal) dependence. Existence of the former implies that lateral differences exist in the boundary conditions at the core-mantle interface. Lateral temperature variations (Cox & Doell 1964) or lateral differences in relief along the core-mantle interface (Hide 1966) may lead to longitudinal differences in the operation of the geomagnetic dynamo and hence to longitudinal differences in the time average of the non-dipole field. Even if the core-mantle boundary conditions were completely uniform, however, the angular dispersion of the geomagnetic field would still vary with latitude for two reasons. The first is that the Coriolis forces of the geomagnetic dynamo vary with latitude, hence it is reasonable that the nondipole field may also vary. The second is that because of the geometry of the vector addition of dipole and nondipole fields on a sphere, the geomagnetic angular dispersion will vary with latitude even if the non-dipole field is the same at all latitudes. The objective of the present study is to develop models that describe the latitude dependence of geomagnetic