Chemical stratification at the top of Earth's core: Constraints from observations of nutations

Dissipation due to tidally driven flow in the Earth's liquid core is detected in observations of nutations. One source of dissipation is due to electromagnetic core-mantle coupling, but this mechanism requires a high electrical conductivity on the mantle side of the boundary and a strong radial magnetic field. An alternative mechanism is viable in the presence of fluid stratification at the top of the core. Stratification causes the fluid close to the core-mantle boundary to be trapped by the effects of topography. Further from the boundary the stratified fluid is swept past the mantle with the underlying tidal flow. Shear in the flow induces electric currents where the fluid is permeated with a radial magnetic field. The resulting dissipation is only weakly dependent on the electrical conductivity of the mantle and the required strength of the radial magnetic field can be lowered. For a representative calculation we explain the observed dissipation with a radial field of 0.5 mT and a mantle conductivity of 1000 S/m, provided the buoyancy frequency in the stratified layer is 0.09 s(-1). Such a strong stratification has recently been proposed on the basis of chemical interactions between the core and the mantle. Nutation observations support the presence of stratification to the extent that the resulting dissipation mechanism is more compatible with conventional estimates for the radial magnetic field and the mantle conductivity. (C) 2010 Elsevier B.V. All rights reserved.