Variations in length of day (equivalently, the rate of rotation of the Earth's surface) are observed on timescales from subdaily to millions of years. They result either from external torques (for example, the tidal torques exerted on the Earth by the moon and sun), or from the exchange of angular momentum between different reservoirs within the Earth system. Of these, the fluid core is by far the most massive, and there is convincing evidence that at least decadal variations in length of day (ΔLOD) (q.v.) result from the exchange of angular momentum between the core and the mantle. Such exchange requires a coupling mechanism between the core and mantle. The mechanisms that have been suggested fall into four classes: viscous coupling, topographic coupling (q.v.), gravitational coupling, and electromagnetic coupling.
Although the mantle is a poor electrical conductor in comparison with the molten iron core, it is nonetheless not a perfect insulator. Currents flowing in the core can...
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Holme, R. (2007). CORE‐MANTLE COUPLING, ELECTROMAGNETIC. In: Gubbins, D., Herrero-Bervera, E. (eds) Encyclopedia of Geomagnetism and Paleomagnetism. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-4423-6_52
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