Abstract
This paper is a non-mathematical review, summarising the work in this field.
Estimates are made of the power needed to maintain the electric currents which give the main geomagnetic field. The observed surface field needs at least 2×108 W, but unobservable fields may need much more; a toroidal field of peak value 10 or 50nT would need ∼1010 or 2.5×1011 W.
Ways of obtaining this power from the Earth's rotation, particularly through precession, are considered and rejected.
Thermal power sources have the disadvantages that there is inherent thermodynamic inefficiency in driving the dynamo, and that a significant fraction of the heat input will be carried away by conduction rather than convection. Radioactivity will only be important if there is a substantial amount of potassium in the core. If this is not the case the core might be cooling; cooling at 20K per 109 yr would release specific heat at a rate of ∼1012 W. If the cooling causes the inner core to grow by freezing from the liquid core, then an additional ∼1012 W would be released from the latent heat of freezing. These heat fluxes might support a dynamo having a small toroidal field.
If, as seems likely, the solid inner core is significantly denser than the liquid, such cooling would also release ∼0.6×1012 W of gravitational energy, giving compositional convection which would drive the dynamo very efficiently and give a large toroidal field.
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Lowes, F.J. The geomagnetic dynamo-elementary energetics and thermodynamics. Geophysical Surveys 7, 91–105 (1984). https://doi.org/10.1007/BF01449178
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DOI: https://doi.org/10.1007/BF01449178