The earth’s magnetic field is evolving and a clear illustration of this is the shifting of the magnetic north pole by several tens of kilometers a year. Its most spectacular manifestation however, the inversion of the magnetic poles which has occurred a number of times in the history of the Earth, remains a mystery. In addition, observations (terrestrial and from space) reveal an intense dynamic, the major part of which is internally produced, i.e. produced by the dynamo effect of the convection movements of liquid iron in the earth’s core, 3,000 km below our feet. An accurate understanding of the magnetic field and its variations over time is a major challenge for a number of areas of application: aeronautics and space, as well as guidance for deep drilling. To improve our predictions and understanding, we use numerical simulations of the geodynamo, i.e. we resolve the fundamental equations of fluid dynamics, coupled with those of electromagnetism, in a rapidly rotating sphere representing the Earth’s core. Following a great deal of work on the optimization and parallelization of the computer code, we managed for the first time to produce a high-resolution simulation of the turbulent terrestrial core (see Schaeffer+ GJI 2017).

These simulations present many of the characteristics of the Earth’s magnetic field that had also been observed, or predicted by the simplified geodynamo theories, all in the same high definition simulation.

In particular, our simulation revealed a drift of the magnetic field to the west associated with a large scale circulation (3,000 km), as for the Earth. These large scale turbulences are largely aligned with the rotation axis of the planet, confirming an hypothesis that has already been used in other studies. In addition, towards the poles, giant storms agitate the core and are associated with a strong magnetic field. These magnetic storms could play a role in the inversions of the magnetic poles. Our simulations also revealed a very heterogeneous magnetic field : although it does - as is the case in the Earth’s core - have a high average intensity of zones where the field is very strong alternating with zones in which the field is almost zero, going alongside a dynamic favoring relatively large (strong magnetic field) or small (weak field) turbulences. These simulations, focused on the dynamic at a scale of a year to a hundred years, will be references for other studies on the Earth’s actual magnetic field and its evolution. Further simulations are however needed for an understanding of the inversions of the Earth’s magnetic field. This is a significant challenge because we need to be able to simulate the Earth’s core in a realistic way over geological time (the last inversion occurred 780,000 years ago).

 

Radial magnetic field on the surface of the core showing the dominance of the dipole (globally gold in the north, blue in the south) as well as the small structures and spots of inverse flows.

Visualization of the speed of molten iron flows in the core (yellow : fast, black : slow). At the surface, the large blue area indicates the summit of the magnetic storm (high speed towards the west).