Characterisation of hydromagnetic waves propagating over a steady, non-axisymmetric background magnetic field

TL;DR

This study models hydromagnetic wave propagation over a steady, non-axisymmetric magnetic background, revealing the nature of rapid geomagnetic signals and their dynamics in Earth's outer core.

Contribution

It introduces a linearized MHD framework for non-axisymmetric backgrounds, identifying the transition from Alfvén to magneto-Coriolis waves and their observed drift.

Findings

Identification of axisymmetric torsional Alfvén waves.

Discovery of non-axisymmetric, quasi-geostrophic Alfvén waves.

Estimated phase speed of core surface waves at about 1100 km/y.

Abstract

Motivated by recent observations of rapid (interannual) signals in the geomagnetic data, and by advances in numerical simulations approaching the Earth's outer core conditions, we present a study on the dynamics of hydromagnetic waves evolving over a static base state. Under the assumption of timescales separation between the rapid waves and the slow convection, we linearise the classical magneto-hydrodynamics equations over a steady non-axisymmetric background magnetic field and a zero velocity field. The initial perturbation is a super-rotating pulse of the inner core, which sets the amplitude and length-scales of the waves in the system. The initial pulse triggers axisymmetric, outward propagating torsional Alfv\'en waves, with characteristic thickness scaling with the magnetic Ekman number as $Ek_M^{1/4}$. Because the background state is non-axisymmetric, the pulse also triggers non-axisymmetric, quasi-geostrophic Alfv\'en waves. As these latter waves propagate outwards, they turn into quasi-geostrophic, magneto-Coriolis waves (QG-MC) as the Coriolis force supersedes inertia in the force balance. The period of the initial wave packet is preserved across the shell but the QG-MC wave front disperses and a westward drift is observed after this transformation. Upon reaching the core surface, the westward drift of the QG-MC waves presents an estimated phase speed of about $1100\,km/y$. This analysis confirms the QG-MC nature of the rapid magnetic signals observed in geomagnetic field models near the equator.