Extreme rotating convection experiments and implications for modeling the dynamo

The magnetic field of Earth is generated by turbulent thermal and chemical convection of molten iron in the outer core. Here we present coupled laboratory and numerical experiments that investigate core-style turbulence in parameter regimes beyond those accessible to present-day planetary dynamo models with spherical shell geometries. Via right cylindrical laboratory experiments and Cartesian doubly periodic numerical simulations we are able to explore Ekman numbers (ratio of viscous / Coriolis forces) as low as 3x10^(-8) and Rayleigh numbers (ratio of buoyancy / diffusion) as high as 10^13. Our results show that the rapidly rotating regime - where well-organized flows manifest as axially aligned convective Taylor columns - breaks down far more easily than has been traditionally argued. Furthermore, this breakdown occurs similarly in present-day dynamo models. Extrapolating our results to planetary settings, we hypothesize that convection in the Earth's core exists near the transition between the weakly rotating (quasi-3D) and rapidly rotating (quasi-2D) regimes. Interestingly, when present-day dynamo models are carried out in this transitional region of parameter space, they are typically unable to generate Earth-like magnetic fields.