On the dynamical implications of models of B(s) in the Earth's core

Abstract

Recent work has shown that the zonal, equatorially symmetric, time-varying part of a model of the flow at the surface of the Earth's core can be well explained by only two standing waves, and that by making certain assumptions these waves may be inverted for rms B(s) (the component of field pointing away from the rotation axis) and a quantity parametrizing friction or excitation (F) of the waves. Here, we discuss the two-wave fit, and describe the implications of models of rms B(s) and friction/excitation for the dynamic state of the core, for the torque balance on axial cylinders, and for recent numerical simulations of the geodynamo. We find several possible explanations for why only two standing waves are needed to fit the data, including the possibility that it is due to the resolution of the core flow model rather than conditions within the core itself. We find that the fits of rms B(s) and F suggest that the role of inertia should not be discounted in the core, and that care should be taken in constructing geodynamo simulations so that the effective friction at the core-mantle boundary does not swamp the inertial term. A ratio of the magnitude of the two appears to be O(1) in the Earth's core: we believe that, ideally, a numerical model of the Earth's core should reproduce this result.