Structure and dynamics of the polar vortex in the Earth's core

Observations of the Earth's magnetic field suggest that there are anticyclonic polar vortices in the core (Olson and Aurnou, 1999; Hulot et al., 2002). Some geodynamo simulations have also shown the existence of an anticyclonic flow in the polar region of the Earth's outer core. The polar vortices are investigated using a spherical convection-driven dynamo model. In a fully three-dimensional model where no longitudinal symmetry is imposed, we find that the polar vortex core is offset from the pole itself by approximately 10 degrees from the axis of rotation. It is therefore non-axisymmetric, and can drift considerably in longitude during a magnetic diffusion time. We also find that the strong polar vortex depends crucially on the magnetic field in the core. The simulation results are compared with the polar core flow reconstructed from secular variation observations.