Thermal core–mantle interaction: Exploring regimes for ‘locked’ dynamo action

Possible effects on the geodynamo of lateral variations in heat flux from the core are explored using two different patterns of heat flow from the core. One is based on lower mantle shear wave velocity and the other is the single spherical harmonic Y22. The self-consistent dynamo equations driven by thermal convection in a Boussinesq fluid are solved. Our choice of parameters is guided by earlier work on non-magnetic convection. We have already found a nearly steady solution locked to the tomographic boundary condition that bears a remarkable resemblance to the present day field; here we seek to understand this locked regime. Numerical considerations demand an artificially high-Ekman number; we choose a low-Rayleigh number and a Prandtl number of order 1. In this regime locking occurs when the underlying convection has an azimuthal wavelength similar to that of the boundary conditions, as in the non-magnetic case. This is demonstrated where a drifting non-magnetic flow dominated by m=8m=8 rolls is converted to locked large scale-flow by the presence of a self-generated magnetic field. Large but geophysically reasonable lateral variations in the flux are required for locking. Dynamo action can fail at very large variation, where strong thermal winds disrupt the mechanism in this regime. If the Rayleigh number is too close to critical the dynamo can fail for low lateral variation, where the flow has a larger component driven by the boundary condition. Similarly, increased azimuthal flow at lower Prandtl numbers blurs the effect of the boundary condition.