Geomagnetic Secular Variation and Fluid Motion at the Core Surface [and Discussion]

The radial component of geomagnetic secular variation at the core surface is examined. Discrepancies between models for the same epoch are attributed in part to short-wavelength instabilities, and methods for producing smooth models are illustrated. Good core secular variation models are essential for inferring features of the core fluid flow, and two aspects are investigated here. Initially, a toroidal, azimuthal core-mantle boundary velocity field is assumed, which is tested for compatability with the geomagnetic data. Such a flow could be uniquely determined from the magnetic field and its secular variation. Purely toroidal motion implies no fluid upwelling at the core surface, and the data are consistent with this. The pre-dominantly westward drift of the geomagnetic field prompted the assumption of azimuthal flow. However, this is notjustified, as the magnitudes of the azimuthal and meridional velocity components are comparable at the few points for which this information is available.