The geomagnetic dynamo ― Elementary energetics and thermodynamics

The basic energetics and thermodynamics of dynamo models of the geomagnetic field are examined in a general nonmathematical review. The properties of the earth core are summarized; the powers needed to drive the dynamo and account for the observable field, an unobserved 10-nT toroidal field, or a 50-nT field are estimated as 200 MW, 10 GW, and 250 GW, respectively; the impossibility of obtaining such powers from rotation (and precession) is demonstrated; and thermal and gravitational sources are considered in detail. Consideration is given to the release of up to 10 TW of heat by K-40 decay (assuming K is concentrated in the core), heat release of up to 2 TW by cooling of the core over 4 Gyr, and the release of about 600 GW of gravitational potential energy through compositional convection from the freezing out at the surface of the inner core (moving mass toward the center). The latter mechanism is considered the most plausible at present.