Exchange of Angular Momentum between the Core and the Mantle

Abstract

Exchanges of angular momentum between the core and the mantle responsible of the so-called decade variations in the length of the day (l.o.d.) are reexamined. It is proposed that, for the relevant time constants, the changes in the core angular momentum are carried by cylindrical annuli rigidly rotating around the Earth's rotation axis. Then the possible coupling mechanisms are discussed. The electromagnetic torque is first computed using a full expansion of the flow at the core-mantle boundary as inferred from the secular variation of the geomagnetic field. Its time changes are estimated and compared to the time changes of the torque responsible for the l.o.d. time changes. The topographic torque, i.e. the torque exerted on the non-axisymmetric core-mantle boundary by the pressure field linked with the flow at the core mantle boundary (CMB), is then shown to be too large by two orders of magnitude if the amplitude of the topography is as large as proposed by seismologists. A mechanism is presented which can reduce this torque down to values compatible with l.o.d. data.