The rotation of the Earth's inner core

Abstract
The inner core rotates under the influence of inertial, frictional, electromagnetic and gravity torques. When its symmetry axis is inclined by an angle to the mantle symmetry axis, calculation shows the gravity restoring torque to be 2.50 x 1024cos01sin01N m, several orders of magnitude larger than the other torques combined. Investigation of its rotational dynamics under this torque shows that it can have two modes of steady precession. One mode involves a slow precession in space and is presumably ruled out because of the nearly diurnal relative motion required with respect to the outer core. The second steady precessional mode involves a rapid, nearly diurnal prograde motion in space and hence is slow in the mantle frame. Because the inner core is a good electrical conductor and because of its large rotational energy it is likely to have a strong interaction with the main magnetic field. In particular, if it is in the second steady precessional mode it may be carrying magnetic fields bodily for periods of several thousand years (its magnetic diffusion time) and the ‘dipole wobble’ of the main field, seen in historical measurements, and apparently extending into the palaeomagnetic record, may be a reflection of the influence of this motion on the outer core dynamo.

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