Differential rotation of the inner core

Inner core anisotropy (-> one anisotropy axis)
=> variation in seismic wave travel time
rem: an anisotropy axis might be a too simple picture.

Differential rotation from observations:

Eastward motion at depth :
<=> westward motion of outer part of core (angular momentum balance)
<=> westward drift of magnetic field

If differential rotation is real
=> disrupts the axisymmetric flow centred around cylinders

Consistency with coupling mechanisms ?

Consistency with temperatures ?
Convection + dynamo action in outer core
=> excess of temperature inside tangent cylinder surrounding inner core
=> temperature difference with the liquid outside this cylinder
=> prograde thermal wind and strong azimuthal magnetic field inside the cylinder
=> electromagnetic torque at ICB -> consistency with the differential rotation
(a temperature difference of 0.001 K is sufficient)
(Aurnou et al., 1996, GRL, 23, 3401-3404)

Consistency with flow?
The combination of toroidal and poloidal magnetic fields exerts electromagnetic
torques on the solid inner core, resulting in its anomalous spin. Aurnau et al.
(1988) have shown that there is a close relation between the inner core rotation
and the generation of the toroidal magnetic field (which is induced by the
poloidal magnetic field). If the inner core is believed to be rotating in the prograde sense,
columnar flow following geostrophic contours outside the tangent cylinder of the
inner core may be eliminated (induces retrograde inner core rotation). However,
thermal wind azimuthal flow resulting from a positive temperature anomaly whinin
the inner core is compatible with a prograde inner core rotation.