GEOMAGNETIC-FIELD MORPHOLOGIES FROM A KINEMATIC DYNAMO MODEL

THE Earth's magnetic field is generated by flow of liquid iron in the outer core, acting as a dynamo. In the simple kinematic theory a fluid flow is prescribed and tested for its ability to generate magnetic field, no account being taken of the forces required to drive the flow. Although incomplete, kinematic theory gives valuable insight into the more difficult dynamical problem and produces field morphologies which can be compared with observations. We are attempting a comprehensive study of three-dimensional kinematic dynamo action for a class of fluid flow typical of that driven by convection (G.S. and D.G., manuscript in preparation), and have already found similarities between steady dipole solutions and the geomagnetic field(1). Here we examine the effect of meridian circulation in determining the stability of steady and oscillatory solutions. The magnetic field morphology on both sides of the stability boundary is determined by magnetic flux concentration by downwelling fluid(2). The oscillatory dipole reverses polarity through the appearance and poleward migration of patches of reversed flux, similar to those seen in today's geomagnetic field(3). Virtual geomagnetic poles computed from the reversing field follow paths that are concentrated along the longitudes of maximum flux, suggesting a link with recent palaeomagnetic results(4-7).