Determination of the electromagnetic and topographic core-mantle coupling torques based on geomagnetic observations: Influence of the electric conductivity of the Earth's mantle

The determination of the electromagnetic (EM) and topographic (TOP) core-mantle coupling torques is based on the observed geomagnetic field at the Earth's surface and additional assumptions like the electric conductivity profile of the Earth's mantle and the topography of the core-mantle boundary (CMB). For the calculation of the EM torques, we need the poloidal and the toroidal geomagnetic field at the CMB, because the EM torques are described as surface integrals over the CMB, on which this torques act. Therefore, we have to determine the poloidal geomagnetic field at the CMB by a non-harmonic downward continuation (NHDC), which considers the electrically conducting mantle. For the toroidal geomagnetic field, the related initial boundary value problem is formulated and solved numerically. The boundary values at the CMB are dependent on the poloidal geomagnetic field and the surface flow velocities of the fluid outer core at the CMB. This velocity field is determined by fluid flow inversion, based on the frozen-flux approximation and the additional constraint of tangential geostrophy (Wardinski 2004). This surface velocity fields are also consistently used for the calculation of the TOP torques. In addition, we need a model for the topography of the CMB. In our investigation we use different conductivity profiles and CMB topography models to calculate the EM and TOP coupling torques, which are compared with excitation function derived from observed polar motion and length of day variations (taken from the IERS).