Large-scale fluid motion in the earth's outer core estimated from non-dipole magnetic field data.

Abstract

Fluid motion in the Earth's outer core can be estimated from magnetic field data at the Earth's surface based on some assumptions, without which no unique solution to this inverse problem is likely to be derived. Our basic standpoint is that the non-dipole magnetic field is generated by the interaction between a strong toroidal magnetic field, created by differential rotation, and convective motion in the outer core. We consider large-scale convective motion and express it in terms of the poloidal velocity field expanded into a series of spherical harmonics of degree up to five. We first estimate the radial distribution of differential rotation from the balance between the effective couple due to angular momentum transfer and the electromagnetic couple. Then the radial dependence of the toroidal magnetic field is derived from the interaction between the differential rotation thus estimated and the dipole magnetic field within the outer core. For magnetic field data we use our secular variation model, in which fluctuations of the standing and the drifting parts of the non-zonal magnetic field are taken into account. The velocity field in the outer core is estimated for two cases: first, the non-zonal magnetic field is considered to be in the steady state at various epochs (the quasi-steady case) and next, timedependence of the non-zonal magnetic field is incorporated into equations to be solved (the non-steady case). It turns out that the pattern of convective motion is generally characterized by large-scale motion for the quasi-steady and the nonsteady cases; a pair of upwelling and downwelling motions is seen at the equator near the core surface for the standing and the drifting non-zonal fields, respectively. In the non-steady case, the magnitude of the velocity field is much larger, indicating a more dynamical feature.