Torsional Alfvén waves in the Earth's core

Torsional Alfvén waves are theoretically predicted to exist in Earth's outer core, have been inferred from geophysical data and observed in geodynamo simulations. They provide an indirect means of investigating core dynamics, core properties and core-mantle coupling mechanisms. In this study, we produce 1-D forward models of torsional waves in Earth's core and study the wave-induced secular variation (SV). We find that torsional waves undergo significant dispersion during propagation that arises due to their geometric setting, with long wavelength features being more dispersive than short wavelength features. Other key propagation features observed in our models are phase shifts at Earth's rotation axis, low amplitude wakes trailing behind sharply defined pulses, reflections from the tangent cylinder and internal wave reflections caused by strong magnetic field gradients. These combined dispersive effects may lead to difficulties in resolving the excitation mechanism of any torsional waves identified in geophysical data. Fast torsional waves with amplitudes and timescales consistent with a recent study of the 6 yr $\Delta$ LOD signal (Gillet et al., 2010) induce very rapid, small (maximum ~2 nT/yr at Earth's surface) SV signals that likely could not be resolved in observations of the Earth's SV. Slow torsional waves with amplitudes and timescales consistent with, for example, Zatman & Bloxham (1997), Hide et al. (2000) produce larger SV signals that reach amplitudes of ~20 nT/yr at Earth's surface. We applied the two-part linear regression jerk detection method developed by Brown et al. (2013) to the SV induced by slow torsional waves, using the same parameters as used on real SV, which identified several synthetic jerk events. As the local magnetic field morphology dictates which regions are sensitive to zonal core flow, and not all regions are sensitive at the same time, the modelled waves are not able to induce global contemporaneous jerk events such as that observed in 1969. The synthetic jerks are only observed on regional scales and generally occur in a single SV component. Also, the identified events are periodic due to waves passing beneath locations periodically and the SV signals are smoothly varying. These smooth signals are more consistent with the geomagnetic jerks envisaged by Demetrescu & Dobrica (2005, 2014), than the sharp 'V' shapes that are typically associated with geomagnetic jerks.