Observation of liquid motion in a precessing oblate spheroid

Abstract

We summarize experimental results for transient flow patterns in a liquid-filled oblate spheroid, and compare the observed flows with existing geodynamo theories and evidence. Flow patterns are traced from a state near inertial rest through spin-up at a constant cavity spin rate, to forced precession where the cavity spin axis precesses at a constant rate about an inertially fixed direction displaced from the cavity spin axis by a constant half-coning angle. Space- and time-dependent flow with various instabilities are recorded for precession-induced boundary layer and deep interior flow velocities. Slow precession induces cylindrical shear layers and mostly closed meridional convection in the bulk of the core, the same features as found in buoyancy-driven convection models and in a ‘weak Taylor state’ solution for a numerical αω dynamo with small non-zero viscous coupling. Observed precessional flows exhibit significant departures from geostrophic equilibrium.