Low inertia reversing geodynamos

Abstract

Convection driven geodynamo models in rotating spherical geometry have regimes in which reversals occur. However, reversing dynamo models are usually found in regimes where the kinetic and magnetic energy is comparable, so that inertia is playing a significant role in the core dynamics. In the Earth's core, the Rossby number is very small, and the magnetic energy is much larger than the kinetic energy. Here we investigate dynamo models in the strong-field regime, where magnetic forces have a significant effect on convection. In the core, the strong field is achieved by having the magnetic Prandtl number $\mathrm{Pm}$ small, but the Ekman number $E$ extremely small. In simulations, very small $E$ is not possible, but the strong-field regime can be reached by increasing $\mathrm{Pm}$. However, if $\mathrm{Pm}$ is raised while the fluid Prandtl number $Pr$ is fixed at unity, the most common choice, the Péclet number becomes small, so that the linear terms in the heat (or composition) equation dominate, which is also far from Earth-like behaviour. Here we increase $Pr$ and $\mathrm{Pm}$ together, so that nonlinearity is important in the heat equation and the dynamo is strong-field. We find that Earth-like reversals are possible at numerically achievable parameter values, and the simulations have Earth-like magnetic fields away from the times at which it reverses. The magnetic energy is much greater than the kinetic energy except close to the reversal times.