Reversal models from dynamo calculations

Abstract

A numerical model of convection driven dynamo action, exhibiting a dominantly dipole magnetic field subject to intermittent reversals, is described. This model was originally restricted to two azimuthal modes, but has now been extended towards full three–dimensional resolution. The hydrodynamic state of this model is explained by a thermal wind mechanism, which generates strong zonal flows and secondary meridional circulation from a characteristic temperature profile consistent with highly supercritical convection. The zonal flows generate strong zonal magnetic fields via an omega–effect, and the equilibration of the dynamo relies heavily on a reduction in this effect, as might be parametrized as an ‘omega–quenching’. The time–behaviour of the model is complex, but the importance of meridional circulation in this respect is clear; fluctuations in this component of flow provide a simple kinematic mechanism for reversals. Some parallels between model solutions and geomagnetic observations are noted, although detailed comparisons remain premature. Although the model remains far from the appropriate physical regime, the thermal wind and kinematic reversal mechanisms described are quite general, and may continue to play a role in the geodynamo.