MAGSAT and Geodynamo

Abstract

Accurate, uniform and global vector magnetic field data obtained by MAGSAT provided a high resolution model of the earth’s magnetic field. It became possible to separate the core and crustal parts in spherical harmonic representation. Such a separation also permitted a reliable projection of the field and its secular variation at the core mantle boundary (CMB). The results show that quadrupole and higher-order multipoles originate from currents at the CMB. The dipole term is about 80% from currents involving a part or whole of the core and the remaining 20% from the surface currents. The vertical component(Bϒ) and its secular variation at CMB were used to estimate the field velocity under the assumption of frozen-field core (FFC) approximation. Purely toroidal flow does not fit the observed data as satisfactorily as the combined poloidal and toroidal flows. Poloidal motion represents upwelling and downwelling hence vertical motions. This indicate that either the top of the core is not stably stratified or that such stratification is insufficient to stop vertical motion at depth. The fluid flow has a bulk westward drift with superposed jets and vortices. The ratio of westward velocity and the radial derivative of vertical velocity supports gravitationally and/or thermally powered dynamo. The FFC approximation itself is found to be violated over patches at the CMB. Features like sun-spots are seen under southern Africa and South America. Postulates for occurrence of such spots are discussed and it is argued that it may now be desirable to study solar and geo-dynamo jointly.