Westward drift of the Earth's magnetic field

While the Earth's magnetic field has a mainly dipole-like structure, it also has non-axisymmetric features, and these change slowly with time, on a scale of 1000 years or so. In particular, there is a significant westward drift, first observed by Halley in the 17th century.

One possible explanation for this drift lies in magnetic `field-gradient' instabilities in the Earth's liquid core. This is because of one striking property of the resulting waves, which I proved in my 1972 paper: they always travel `westward' relative to the rotating fluid.

This whole idea is loosely related to Hide's well-known wave theory of the geomagnetic westward drift, though the westward selection mechanism itself is quite different. In Hide's theory this stems from the (spherical shell) geometry of the core, but the westward propagation of these `field-gradient' instabilities seems insensitive to the boundary geometry, and stems (somehow) from the nature of the instability mechanism itself.

Rep. Prog. Phys. Vol 36, pp159-221, 1973 (with R. Hide)
J. Fluid Mech. Vol 61, pp 609-624, 1973

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