Westward drift, core motions and exchanges of angular momentum between core and mantle

The westward drift is one of the most well known features of the geomagnetic field. In this paper we come back to the apparent drift of the main field as seen at the Earth's surface, and show that there is no evidence for a body drift but rather clear evidence for a latitude-dependent drift. Computing the fluid flow at the core mantle boundary (CMB), in the frozen flux and geostrophic hypothesis, we also obtain, among other components, a clear differential zonal rotation of the fluid, symmetrical with respect to the Equator. This differential rotation changes with time, with a time constant of the order of 10 years. Dynamic considerations lead us to think that, during such a time interval, the change in the zonal toroidal flow inside the core consists of motions in which axial cylindrical annuli rotate rigidly about the Earth's axis of rotation. Then, for the 1969–85 time-span, the change in the core angular momentum is shown to balance the change in the mantle angular momentum.

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