Numerical models of zonal flow dynamos: an application to the ice giants

The weakly dipolar and strongly tilted magnetic fields of Uranus and Neptune are apparently generated by a dynamo process distinct from that which produces axial dipoles. We study a suite of numerical dynamos driven by convection in a rapidly rotating spherical shell and focus on cases with relatively high magnetic diffusivity. Models are presented with magnetic Prandtl number Pm = 0.1–5.0 and Rayleigh numbers between 10 and 80 times the critical Rayleigh number for convection. In the cases with high magnetic diffusivity, the fluid flow has a dominant effect over the magnetic fields, which are characteristically quadrupolar and octupolar and strongly variable in time. The dipolar component is typically weak, and strongly tilted from the axis of rotation. Most of the cases we present result in low Alfvén and Elsasser numbers, in agreement with previous studies of nondipolar dynamos. Our results suggest that the peculiar magnetic fields of Uranus and Neptune result from dynamo action driven by convectively generated, strong zonal flow in the electrolytic fluid envelope.