A Dynamical Prospective on Interannual Geomagnetic Field Changes

Abstract

Geomagnetic observations from satellites have highlighted interannual variations in the rate of change of the magnetic field originating from Earth's core. Downward continued to the core surface, these variations primarily show up in the equatorial belt. First, we recall the main characteristics of these patterns, addressing their spatio-temporal resolution, as seen from field models. We then review the several dynamical frameworks proposed so far to understand and model these observations, which populate the frequency spectrum on time scales close to the Alfvén time A ≈ 2 yr, much shorter than the vortex turnover time U ≈ 150 yr in Earth's core. Magnetic-Archimedes-Coriolis (MAC) waves in a stratified layer below the core surface constitute a first possibility in the case of a sub-adiabatic heat flux at the top of the core. Their period may reach the interannual range for a layer thickness less than ≈ 30 km, for a buoyancy frequency of the order of the Earth's rotation rate. An alternative has been proposed in a context where the Coriolis force dominates the momentum balance, rendering transient motions almost invariant along the rotation axis (quasi-geostrophy, QG). Torsional Alfvén waves, consisting of axisymmetric QG motions, operate at periods similar to the Alfvén time, but are not sufficient to explain the interannual field changes, which require non-axisymmetric motions. QG Alfvén waves (involving the Coriolis and magnetic forces) constitute another possibility, with inertia playing an important role. They have been detected in the latest generation of geodynamo simulations, propagating in a ubiquitous manner at a speed slightly less than the Alfvén velocity. They are localized in longitude and as a result their description requires high azimuthal wavenumber. But the branch of QG waves with large extent in azimuth is also worth considering, as it reaches interannual periods as their radial wavenumber is increased. The excitation of such high-frequency dynamics is discussed with respect to the temporal spectrum of the core field, which presents a slope ∼ f −4 for periods approximately between A and U . We finally summarize the main geophysical implications of the existence of this interannual dynamics on core and lower mantle structure, properties, and dynamics.

A taxonomy of simulated geomagnetic jerks

“…The spatio-temporally filtered time-longitude plots show evidence of structures rapidly drifting east and preferentially westwards at speeds in excess of 100 km yr −1 (Figs 7b, d and 8). Such longitudinal drifts have been reported in several analyses of geomagnetic field models, with varying, but generally remarkably high velocities (Chulliat et al 2015;Chi-Durán et al 2020;Gillet et al 2022a). Filtering at two different levels of spatio-temporal resolution however shows that drifts observed on coarse images are not necessarily preserved on finer representations (Fig.…”

“…7) illustrates how this can be circumvented at sufficiently small (cylindrical radial) wavelengths. For further details on the connections between interannual geomagnetic variations, waves in the core and in numerical simulations, the reader may consult the recent review of Gillet et al (2022a). Here we will focus on specific situations where waves naturally forced by convection reach high amplitudes and compare the resulting magnetic signals to the characteristics of observed geomagnetic jerks.…”

Structure, Materials and Processes in the Earth’s Core and Mantle

“…The study of LOD variations at decadal timescale can also bring insight on the coremantle boundary coupling mechanisms as explained by Gross (2007 and; see also Buffett 2010;Buffett and Seagle 2010;Buffett 2014;Glane and Buffett 2018; as well as Gillet et al 2017Gillet et al , 2021.…”

On the Model of Equatorial Magnetic‐Archimedes‐Coriolis Waves Propagating at Earth's Core Surface and the Potential Implications

“…Because of these small Q values, if without the continuous excitations, these waves will rapidly disappear due to the strongly ohmic dissipation. However, the exact excitation mechanism responsible for the continual generation of these eMAC waves is still unclear (e.g., Gillet et al., 2021), though the convection within the Earth's core may provide a mainly stochastic excitation source (e.g., Buffett & Knezek, 2018; Gillet et al., 2021). Additionally, BM19 referred to an alternatively physical mechanism responsible for these excitations, that is, the westward drift of buoyance plumes in the equatorial areas, which may generate the eMAC waves through either the influence of fluid rising into the stratification layer atop of Earth's core or magnetic disturbances at the bottom of this stratified layer.…”