Can Precession Power the Geomagnetic Dynamo?

Abstract

The power requirement of a stationary geomagnetic dynamo driven by some agency external to the core (e. g. precession) is equal to the ohmic dissipation in the core, Q1. For a dynamo in which differential rotation is important we show that Q1ασ1−2, where σ1≃ 5 × 105 ohm−1 m−1 is the electrical conductivity of the core. Estimates of Q1 for kinematic dynamo models range from 109 to over 1012 Watts, depending on the particular regenerative scheme characterizing the dynamo. Precessional power input to the magnetic field can be estimated in terms of the electromagnetic part of the core-mantle coupling and the tiltover angle (inclination of the core angular momentum vector to that of the mantle). We correct Stacey's estimate of core-mantle coupling to take into account the diurnal frequency of precession-induced flow relative to the mantle, and show that the power available from precession to stir the core cannot exceed 108 W if the core flow remains stable. As a caution against the widespread uncritical acceptance of Malkus’ claims to have demonstrated the energetic adequacy of precession-driven hydromagnetic turbulence, we enumerate the mathematical and physical errors which cast doubt on his theoretical arguments.