TANGENTIALLY GEOSTROPHIC FLOW AT THE CORE MANTLE BOUNDARY COMPATIBLE WITH THE OBSERVED GEOMAGNETIC SECULAR VARIATION - THE LARGE-SCALE COMPONENT OF THE FLOW

We present a method for determining the large-scale component of a tangentially geostrophic flow beneath the core-mantle boundary compatible with magnetic secular variation observations. We use a tangentially geostrophic basis to ensure the geostrophy of the motion. The fit of the secular variation (SV) generated by the motion to the observed SV (in fact SV models) is adequate, taking into account the existing error level. As in any horizontal geostrophic motion, the flow is expressed as a sum of two independent tangentially geostrophic flows: a zonal component, which is toroidal, and a non-zonal component, which is directly linked with the motions deeper in core. The flow derived for the recent epoch (1970-1985) presents interesting symmetry properties: the non-zonal velocities are the same at two antipodal points, while the zonal velocities are the same at two points symmetrical about the Equator. The non-zonal component of the flow is more vigorous than the zonal one; the consoidal ingredient, though weaker than the toroidal ingredient, is essential and indicates strong vertical motion at depth in low-latitude areas. The SV is actually compatible with a geostrophic motion at the core-mantle boundary and appears to be mainly due to the action of the non-zonal component of the flow. © 1990.