The influence of fluid properties on the morphology of core turbulence and the geomagnetic field

Here we investigate the effects of fluid properties on the morphology and dynamics of convection in the Earth's outer core. The results of two quasi-geostrophic convection simulations are carried out at comparable convective velocities for fluids in which the ratio between the kinematic viscosity and thermal diffusivity (the Prandtl number, Pr) is 0.1 and 10. The Pr=0.1 case is representative of thermal convection in a liquid metal, whereas the Pr=10 case is representative of chemical convection. We find the influence of the Prandtl number to be significant; low Prandtl number fluids have a propensity for large-scale coherent vortex formation and slowly varying dynamics. Conversely, the high Prandtl case is dominated by significantly smaller length scales and more rapidly varying dynamics. However, both cases have zonal flows with similar strength, demonstrating that Reynolds stresses in high Prandtl number convection can be large when the buoyancy forcing is strong. By using a simple kinematic magnetic induction model we show that the structure of the magnetic field is not a direct indication of the underlying convective morphology when the magnetic diffusivity is large, as in Earth's core. Thus, our simulation results imply that the convective turbulence differs between thermally and chemically dominated convection, but that it may be difficult to determine the dominant forcing from geomagnetic field structure alone.

► Low Prandtl number fluids form coherent vortices with slowly varying dynamics. ► High Prandtl number fluids have small length scales and rapidly varying dynamics. ► Magnetic diffusion gives similar magnetic signature for high and low Prandtl numbers.