The Earth’s core is traversed by very special fast waves

The Earth’s magnetic field is generated by the geodynamo, a process that takes place within the outer core of our planet more than 3,000 km below our feet. This huge dynamo terrestrial works thanks to the movements of convection which animate the metallic fluid composing the outer core. These movements are themselves produced by the transfer of heat related to the cooling of the seed, the inner core.

However, the mechanisms governing the magnetic field, and more particularly its variations over time, are still poorly understood because the Earth’s magnetic field is not stable. The convection movements which are at the origin of it are indeed chaotic, leading to a constant evolution with a whole range of variations on very varied time scales. One of the best-known effects of these field variations is the reversal of the magnetic poles. This extreme case, although it does not occur regularly, is observed on a time scale of the order of a million years. However, there are variations of the order of one to a few years which are characterized by oscillations of the magnetic field. This very short-term wave dynamics was until now unexplained by current models.

Variations in the magnetic field over short periods

Scientists from the University of Grenoble have therefore looked into the question. Thanks to geomagnetic data collected continuously for twenty years by satellites, they were able to identify the cause of these interannual variations. Nicolas Gillet and his colleagues were thus able to observe for the first time the propagation, inside the nucleus, of the waves responsible for these oscillations of the magnetic field.

These waves are called “magneto-Coriolis” or “magnetohydrodynamic”. They are generated by the rotation of the earth and are carried by the magnetic force produced by the earth’s dynamo. Until now, this type of waves was only a hypothesis, but the research team succeeded in showing their existence and in characterizing them. Their period is thus about seven years, which is very short on the scale of the Earth, their amplitude of 3 km/year and they move at the level of theequator from the core, towards the west, with a phase velocity approximately 1,500 km/year. These waves also have a particular shape: the wave front is organized in different columns which are not aligned with the axis of rotation of the Earth.

For scientists, these waves are therefore different from the waves of torsion d’Alfvén which have a period of six years and are, for their part, symmetrical to the axis of rotation. Alfvén torsion waves can be imagined as cylinders having differential rotation between them. They are not influenced by the speed of rotation of the earthunlike the new magneto-Coriolis waves characterized by the Grenoble team.

Magnetic waves to probe inside the nucleus

The development of this new wave model was made possible in particular by a Numerical simulation precision and thrust of the terrestrial geodynamo, coupled with magnetic data. The identification of these magneto-Coriolis waves allows us to refine our understanding of the processes brought into play by the geodynamo. The results, published in the journal PNASsuggest that the rapid changes observed in the Earth’s magnetic field can be explained by its radial cylindrical component and do not need to appeal, as previously proposed, to stratification of the outer core.

These results open the way to a better understanding of the magnetic field that reigns inside the Earth’s core, which could in particular make it possible to anticipate changes in the field in the more or less short term.