Waves in the Earth’s fluid core

– Felix Gerick, CNES, Royal Observatory of Belgium –

Résumé :

Beneath Earth’s silicate mantle lies the core, which consists almost entirely of iron. The majority of the core is liquid and therefore hydrodynamic waves can propagate through it, as in any known liquid. Despite the vastly different environment, the core has a viscosity comparable to that of the water ocean at Earth’s surface. Another similarity is the strong influence of Earth’s rotation, and numerical simulations of the Earth’s core predict a predominantly columnar structure to the flow, reminiscent of the quasi-geostrophic flows of oceans and atmospheres. However, we cannot simply extrapolate everything we know from the hydrosphere to the core. One of the key differences is the influence of the Earth’s magnetic field, which is self-sustainably generated within the core. Since the core is electrically conducting, magnetic field lines can act as a restoring force to perturbations in the flow. New types of waves arise, commonly referred to as magnetohydrodynamic waves, which can travel through the whole bulk of the core.

Seismic waves give us some information on the material properties of the core, such as its density. Magnetohydrodynamic waves within the core are able to change the magnetic field, observable at the Earth’s surface, and are sensitive to the magnetic field strength in the interior of the core and other properties. Therefore they complement the information available from seismology. In addition, some variations in Earth’s length-of-day are attributed to waves in Earth’s core. The understanding of how angular momentum is transferred from the core to the mantle could provide us additional information on properties of the core-mantle boundary. Potentially, wave motions in the Earth’s core can also contribute to changes in Earth’s gravity field.