Asymptotic behaviour of rotating convection-driven dynamos in the plane layer geometry

Abstract

Dynamos driven by rotating convection in the plane layer geometry are investigated numerically for a range of Ekman number ($E$), magnetic Prandtl number ($Pm$) and Rayleigh number ($Ra$). The primary purpose of the investigation is to compare results of the simulations with previously developed asymptotic theory that is applicable in the limit of rapid rotation. We find that all of the simulations are in the quasi-geostrophic regime in which the Coriolis and pressure gradient forces are approximately balanced at leading order, whereas all other forces, including the Lorentz force, act as perturbations. Agreement between simulation output and asymptotic scalings for the energetics, flow speeds, magnetic field amplitude and length scales is found. The transition from large scale dynamos to small scale dynamos is well described by the magnetic Reynolds number based on the small convective length scale, $\widetilde{Rm}$, with large scale dynamos preferred when $\widetilde{Rm} \lesssim O(1)$. The magnitude of the large scale magnetic field is observed to saturate and become approximately constant with increasing Rayleigh number. Energy spectra show that all length scales present in the flow field and the small-scale magnetic field are consistent with a scaling of $E^{1/3}$, even in the turbulent regime. For a fixed value of $E$, we find that the viscous dissipation length scale is approximately constant over a broad range of $Ra$; the ohmic dissipation length scale is approximately constant within the large scale dynamo regime, but transitions to a $\widetilde{Rm}^{-1/2}$ scaling in the small scale dynamo regime.