Towards a theory of irregular variations in the length of the day and core-mantle coupling

Abstract
Determinations of fluctuations in the length of the day reveal changes due to the transfer of angular momentum between the Earth’s ‘solid’ mantle and the overlying atmosphere on time scales upwards of a few weeks, as well as the slower but more pronounced ‘decade variations’ due largely (according to current ideas) to angular momentum transfer between the mantle and the Earth’s liquid core. Improvements in techniques for monitoring the Earth’s rotation, such as those afforded by recent advances in methods of ranging to artificial satellites and the Moon and of very long baseline interferometry, should therefore lead to results of interest to meteorologists concerned with planetary-scale motions in the atmosphere and to geophysicists con­cerned with the magnetohydrodynamics of the core and the origin of the main geo­magnetic field. The consideration of the stresses at the Earth’s surface and at the core­mantle interface that bring about angular momentum exchange between the solid and fluid parts of the Earth raises a number of basic hydrodynamical questions requiring further experimental and theoretical research. In the case of the core, quantitative difficulties encountered by the suggestion that the stresses are electromagnetic in origin led to the idea of topographic coupling associated with hypothetical undulations of the core-mantle interface.

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