Core (M)HD: the surprisingly weak role of magnetic fields in planetary dynamo models

It is generally assumed that the flow within Earth's liquid metal outer core is largely governed by two dominant forces: the Coriolis force, resulting from rapid planetary rotation; and the Lorentz force, caused by the resistance of geomagnetic field lines to bending by flow. Nominally, these two forces reach an equilibrium state known as the magnetostrophic balance, which promotes large scale convection. Analysis of calculations from a broad suite of geodynamo simulations, however, shows that convection remains small scale regardless of magnetic field strength. The particular scaling behavior of characteristic convective cell sizes indicates that it is viscosity, not Lorentz forces, that balances the Coriolis force in the geodynamo simulations. Direct comparisons between non-magnetic and dynamo simulations confirm that magnetic fields only weakly influence convection dynamics in the models. The persistently important role of viscosity in the simulations hint at one of two conclusions: 1) the models, which are not in magnetostrophic balance, are not true to core physics; or 2) the models faithfully replicate core convection, and by extrapolation, core convection occurs on very small length scales. Unfortunately, both of these conclusions indicate that the processes of field generation in models and reality are rather different.