The gravitationally powered dynamo

Summary. The energetics of the gravitationally powered dynamo have been studied with the aid of a compressible-earth model which allows for the growth of the solid inner core. The basic premise of this study is that as the Earth gradually cooled over geological time the solid inner core continually accreted dense material which crystallized from an outer core composed of a molten binary alloy. This process requires a continual rearrangement of matter which generates the fluid motions needed to sustain the dynamo. These motions maintain the outer core in a well-mixed state, in apparent contradiction to Higgins & Kennedy’s hypothesis that the outer core is stably stratified. The vigour of these motions is dependent primarily upon the composition of the solid inner core, but is surprisingly independent of the density of the light constituent in the core. If the solid core is composed entirely of heavy metal, then as much as 3.7 x 1OI2 W may be transferred from the core to the mantle as a result of cooling and gravitational settling. This is roughly equal to estimates of the amount of heat conducted down the adiabat in the core, but it is argued that there is no direct relation between the amount of heat conducted down the adiabat and the amount transferred to the mantle if the convection is driven non-thermally. The gravitational energy released per unit mass of the solid inner core is remarkably constant and may be as much as 2 x 106J/kg, roughly five times the value of the latent heat of iron. These values are reduced if the solid inner core contains some light constituents. It was found that the efficiency of the gravitationally powered dynamo may exceed 50 per cent, a much higher figure than is possible for either the thermally or precessionally driven dynamo. Also, the amount of gravitational energy available to drive the dynamo in the future is many times that expended so far. The size of the magnetic field sustained by gravitational settling was related to the density jump at the inner-outer core boundary and the field strength was estimated to lie between 390 and 685 G, strongly suggesting that the dynamo is of the nearly-axisymmetric type developed by Braginsky.

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