The dynamics and excitation of torsional waves in geodynamo simulations

TL;DR

This study uses 3D dynamo simulations to explore torsional waves in planetary cores, revealing their propagation characteristics, driving mechanisms, and potential influence on Earth's rotational variations.

Contribution

The paper demonstrates the presence and properties of torsional oscillations in dynamo simulations, highlighting their dependence on parameters and identifying key driving forces.

Findings

Torsional waves propagate at the Alfvén speed in simulations.

Waves are observed within and passing through the tangent cylinder.

Core travel times of waves are approximately 4 to 6 years.

Abstract

The predominant force balance in rapidly rotating planetary cores is between Coriolis, pressure, buoyancy and Lorentz forces. This magnetostrophic balance leads to a Taylor state where the spatially averaged azimuthal Lorentz force is compelled to vanish on cylinders aligned with the rotation axis. Any deviation from this state leads to a torsional oscillation, signatures of which have been observed in the Earth's secular variation and are thought to influence length of day variations via angular momentum conservation. In order to investigate the dynamics of torsional oscillations, we perform several three-dimensional dynamo simulations in a spherical shell. We find torsional oscillations, identified by their propagation at the correct Alfv\'{e}n speed, in many of our simulations. We find that the frequency, location and direction of propagation of the waves are influenced by the choice of parameters. Torsional waves are observed within the tangent cylinder and also have the ability to pass through it. Several of our simulations display waves with core travel times of 4 to 6 years. We calculate the driving terms for these waves and find that both the Reynolds force and ageostrophic convection acting through the Lorentz force are important in driving torsional oscillations.