Inversion of torsional oscillations for the structure and dynamics of Earth's core

Abstract

SUMMARY Oscillations in Earth's liquid core with periods of several decades are inferred from variations in the magnetic field. The observed periods are consistent with a type of hydromagnetic wave known as torsional oscillations. These oscillations represent a set of very-low-frequency normal modes in which the internal magnetic field provides the primary restoring force. By adapting the methods of normal-mode seismology, we construct estimates for the internal structure of the magnetic field and several other key parameters, including the viscosity of the inner core. The structure of the recovered field provides useful insights into the nature of convection. We find evidence of columnar convection in the core, and estimate the strength of the field generated by these flows (≈0.3 mT). We also use the normal modes to recover the excitation source for the oscillations. Much of the excitation appears to originate near the surface of a cylinder that is tangent to the equator of the inner core. Distinct events rise above a background level of excitation, and may be related to instabilities in the geodynamo.