Evidence for Turbulent Magnetic Diffusion in Earth's Core

Abstract

Fluctuations in the paleomagnetic field suggest that the dipole decay time is shorter than expected, based on current estimates for the molecular magnetic diffusivity in the outer core. Similar behavior is observed in turbulent dynamo simulations, where the short magnetic field decay time cannot be attributed to higher-order decay modes. We interpret the short decay time as a signature of turbulent diffusion and show that mean-field theory can quantitatively account for the dynamo results. The predictions depend on the amplitude and length scale of the flow that interacts with the magnetic field. We rely on the pairwise balance between Lorentz/Coriolis and buoyancy/Coriolis forces to identify the relevant part of the flow, and use the resulting flow properties to reproduce results from numerical dynamo simulations within their uncertainties. Upon extending these predictions to the paleomagnetic field, we find that the inferred decay time requires a bulk root-mean-square velocity less than 0.8–1.2 mm s−1. Somewhat lower velocities have been estimated at the top of the core from observations of secular variation. These results show that velocities in the interior of the core are constrained by paleomagnetic observations, and that the amplitude of this flow cannot substantially exceed estimates at the core surface.