Abstract

The morphology of the flow in Earth’s outer core and its magnetic field are thought to be intimately related. Columnar flow usually results in a predominantly dipolar magnetic field, as evidenced from dynamo simulations. The convection columns assume the form of alternating cyclones and anti-cyclones, and carry a large degree of helicity. When the thermal forcing is increased, there is an abrupt transition to a disorganised flow, and the dipole breaks down. The transition in these simulations is marked by a `local’ Rossby number of ~ 0.1. Similarly, in rotating turbulence experiments, there is a sharp transition at a Rossby number of ~ 0.4. For smaller values of the Rossby number, the flow is organised into helical columnar structures, and for larger values the flow appears more three-dimensional. We perform six direct numerical simulations of the flow induced by a layer of buoyant anomalies subject to rotation. As the rotation is weakened, we identify a portion of the flow which is more strongly three-dimensional. We show that the flow in this region is turbulent, and has a Rossby number above a critical value ~ 0.4, consistent with previous findings. We suggest that the discrepancy between the value found here (and in rotating turbulence experiments), and that seen in dynamo simulations ( ~ 0.1), is due to different definitions of the length scale used to define Ro. This in turn suggests that inertial waves, continually launched by buoyant anomalies, sustain the columnar structures in dynamo simulations, and that the transition documented in these simulations is due to the inability of inertial waves to propagate for Ro>0.4.