PHYSICAL-PROPERTIES AT THE TOP OF THE CORE AND CORE SURFACE MOTIONS

Changes of the Earth's magnetic field, with time constants of 5-100 years, have been used to constrain the fluid motions at the top of the core. We recall arguments to support the hypothesis that the horizontal components of the Navier-Stokes equation reduce, at the top of the core, to a geostrophic equilibrium. The Navier-Stokes equation then allows us to derive the fluid pressure from the flow-upsilon, and the density heterogeneity from both the flow-upsilon and its shear partial-upsilon/partial-tau at the top of the core. The radial shear of the flow cannot be inferred from the secular variation of the magnetic field; only the radial component of the induction equation can be used to constrain the motions and the shear plays no part in this radial component, so the density heterogeneities linked with the motion cannot be calculated this way. Finally, the geostrophic formalism is shown to be compatible with a possible stable stratification of the upper core which might imply strong dependence of the flow on the radius.