The geodynamo

 

At the heart of our planet, the Earth’s core is mainly composed of liquid (outer core) and solid (inner core) iron. Its formation and differentiation is one of the significant events in the early history of our planet. 

 

The planetary accretion and radioactive heat is evacuated from the core through thermo-chemical convection. The solid inner core has appeared at least a few hundred million years ago, and is constantly growing as the planet cools.

 

Convection in the outer core is believed to be at the origin of the Earth’s magnetic field, through a dynamo effect: fluid motion within a pre-existing magnetic field creates electric currents, which in turn induce a magnetic field that reinforces the pre-existing magnetic field. The central research questions are

 

- How does the geodynamo work?

 

- Despite the huge computational difficulties that are involved, can we model this process accurately, from the fast dynamics of geomagnetic jerks occurring over years and less to the longest features such as geomagnetic reversals occurring once every few hundred thousand years?

 

- Can we use the geomagnetic signal together with numerical models to probe the structure, history and dynamics of our planet?

 

- How does the outer core couple with its adjacent layers, the inner core and the mantle?

 

- Can we forecast the future evolution of the geomagnetic field?

 

 

The geomagnetic signal

 

We know that the magnetic field of the Earth has been sustained for at least three billion years because of the magnetic signature embedded in minerals present at the surface of the planet. Over more recent epochs, lava flows and lake sediments also provide a way to sample the evolution of the geomagnetic field. Archaeomagnetic artifacts are also used in order to infer the field over the last centuries. The use of the compass marks the start of the historical geomagnetic era. Invaluable data can be gathered from ship logs over the last four centuries. Starting with the discovery of how to measure the intensity of the Earth’s magnetic field (Gauss, 1833), the observatory era has seen the construction and operation of numerous magnetic observatories over the land surface of the Earth. Since a few decades, magnetic satellites provide a global and continuous coverage of the geomagnetic field evolution. 

 

When measured with proper coverage, the magnetic field at the Earth surface can be downward continued to the boundary between the liquid core and the mantle (the core-mantle boundary). Highly detailed images of the magnetic field close to the generation region can then be constructed over the observatory and satellite era. The most notable features of such geomagnetic field models are the westward drift of equatorial magnetic flux patches, and the presence of high-latitude flux patches with an oscillatory behavior around an equilibrium position. 

 

Our main challenge is to understand the physical reasons and the fluid flows underlying this pattern of surface magnetic evolution. Physical models of the geodynamo can be constructed and numerical computer simulations are carried out in order to achieve this goal.