Electromagnetic and Topographic Coupling, and LOD Variations

The role played by di erent core processes in the changes in the Earth's rotation is assessed and fully dynamical models of the torsional Alfvén waves inside the uid core are reviewed. These waves, rst studied by Braginsky (1970), consist of geostrophic circulation. They have decadal periods and yield time changes in core angular momentum. They arise from small departures from an hypothetical quasi static state, where the total action of the Lorentz force on the geostrophic cylinders cancels out. They cause torques acting on the mantle. Simple models of the torsional waves that rely only on zonal averages of the magnetic eld have incorporated electromagnetic coupling to the mantle. They, however, need some correction. In addition, only a kinematic approach of the topographic coupling, caused by non axial symmetry of the uid cavity, has been successfully attempted to date. Taking into account uncertainties in the height of the core mantle topography and in the electrical conductivity of the deep mantle, it turns out that, in the present state of core modelling, the pressure, gravity and electromagnetic torques acting on the mantle may all produce decade changes in the length of the day with a magnitude comparable to the observations.