Zonal Flow Dynamos

The magnetic fields of the ice giants have signature characteristics that set them apart from other planetary dynamos. Unlike the magnetic fields of Earth, Jupiter and Saturn, which are dipolar and more or less axially aligned, Uranus and Neptune exhibit non-dipolar and non-axisymmetric magnetic fields. Using numerical models of dynamo action driven by strong convection in a spherical shell, we show that magnetic field generation in a fluid of low electrical conductivity (such as the proposed electrolytic liquid layer of the ice giants) can result in magnetic fields similar to those found by Voyager II in its flybys of Uranus and Neptune. We present numerical simulations of self-sustained dynamos using relatively low magnetic Prandtl numbers (Pm=0.1 and Pm=0.3). The flow outside the tangent cylinder develops a strong azimuthal prograde jet. Coriolis forces are stronger than convective forces (the convective Rossby number is less than unity). These flows sustain highly time variable magnetic fields with strong non-axisymmetric and non- dipolar components. In our models the toroidal kinetic energies dominate over magnetic and poloidal kinetic energies. Thus, the relatively weak magnetic field has little effect on the flow, resulting in Alfvén numbers as low as A=0.07. These simulations also lead to low Elsasser numbers (Λ=0.03-0.43). Our results suggest that a geostrophic (rather than magnetostrophic) force balance may be present in the dynamo region of the ice giants.