A Dynamical Prospective on Interannual Geomagnetic Field Changes

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 Alfven time tau(A) approximate to 2 yr, much shorter than the vortex turnover time tau(U) approximate to 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 approximate to 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 Alfven waves, consisting of axisymmetric QG motions, operate at periods similar to the Alfven time, but are not sufficient to explain the interannual field changes, which require non-axisymmetric motions. QG Alfven 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 Alfven 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 similar to f(-4) for periods approximately between tau(A) and tau(U). We finally summarize the main geophysical implications of the existence of this interannual dynamics on core and lower mantle structure, properties, and dynamics.

Automated summary

Satellite geomagnetic observations have revealed interannual variations in the rate of change of Earth's core-originating magnetic field, primarily in the equatorial belt. Various dynamic frameworks have been proposed to understand and model these observations. The excitation of high-frequency dynamics has significant implications for the structure, properties, and dynamics of the core and lower mantle.