Berangere Deleplace & Philippe Cardin,  LGIT Grenoble

   
Differential rotation between the liquid core and the solid mantle generates a thin layer at the
top of the core where the lorentz and viscous forces may balance the coriolis forces and play a
major role. We solve the induction and the momentum equation to compute the velocity and the
magnetic field in boundary layer.
Different regimes are possible. On a hand, when the difference of conductivity between the mantle and the core is
small, a pure magnetic case may take place where induced electrival currents are produced in a skin layer and loop
into a conductive solid layer in the mantle. On an other hand , given that the fluid in the outer
core is likely to be subject to high convection, we can assume an Ekman layer based on eddy
viscosity of $10^{-1} m^{2}/s$, such a pure viscous case where an Ekman layer generate a vicous skin at the base of the
mantle is possible as well. A visco-magnetic regime where both, viscous and magnetic torques
work togheter to balance the change in angluar momentum and influence the Earth's axis of rotation
is also investigated.

Using the result of the nutation theory (Mathews et al, 2002)
we show that in order to retrieve VLBI (Very Long Baseline Interferometry) data,
the magnetic field at the C.M.B has to be smaller than the value find by previous authors
with a similar inviscid analysis(Buffet et al, 2002).