I have proposed that the precessional torques acting on the earth can sustain a turbulent hydromagnetic flow in the molten core. A gross balance of the Coriolis force, the Lorentz force, and the precessional force in the core fluid provided estimates of the fluid velocity and the interior magnetic field characteristic of such flow. Then these numbers and a balance of the processes responsible for the decay and regeneration of the magnetic field provided an estimate of the magnetic field external to the core. This external field is in keeping with the observations, but its value is dependent upon the speculative value for the electrical conductivity of core material. The proposal that turbulent flow due to precession can occur in the core was tested in a study of nonmagnetic laboratory flows induced by the steady precession of fluid-filled rotating spheroids. It was found that these flows exhibit both small wavelike instabilities and violent finite-amplitude instability to turbulent motion above critical values of the precession rate. The observed critical parameters indicate that a laminar flow in the core, due to the earth's precession, would have weak hydrodynamic instabilities at most, but that finite-amplitude hydromagnetic instability could lead to fully turbulent flow.

Malkus, W. V. R. (1968). Precession of the Earth as the Cause of Geomagnetism. Science, 160(3825), 259–264.