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

• Published in: arXiv.org 2024-10
• Main Authors: Barrois, Olivier, Aubert, Julien
• Format: Article
• Language: English
• Subjects: Coriolis force; Drift estimation; Earth core; Geomagnetic field; Geomagnetism; Hydrodynamic equations; Magnetic fields; Magnetic signals; Magnetohydrodynamic waves; Magnetohydrodynamics; Phase velocity; Pulse propagation; Velocity distribution; Wave dispersion; Wave fronts; Wave packets; Wave propagation
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2410.17914

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 Alfvén 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 Alfvén 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.