Analyse de données géomagnétiques

Fig 1 -

Rate of change of the axial dipole (in nT/yr) for an ensemble of realizations of the COV-OBS model over the period 1840-2010 : the dispersion in the ensemble gives a measure of the accuracy in the model parameter (after Gillet, Jault, Finlay & Olsen, 2013)

The main geophysical observation related to the fluid changes inside the Earth’s outer core comes from geomagnetic records. We are developing data analysis tools that aim at extracting as much as possible the information contained in magnetic measurements over millennia (magnetic field recorded in archaeological artifacts), centuries (marine data), decades (observatory data since 1840) and years (continuous satellite measurements after 1999) in order to better understand the core dynamics and to forecast the future evolution of the field.


Our approach aims at assimilating data into models of the fluid outer core, based on either deterministic or stochastic reconstruction techniques. With the former, the state of the core (motions and magnetic field inside the core) is retrieved using observations together with prognostic dynamical equations for the core. The latter instead relies on some statistical a priori knowledge about the variables describing the core, and allows to retrieve not only the most probable state (expectation) given our prior and observations, but also its associated error covariances. Following this avenue of research, we have proposed to the community the geomagnetic model COV-OBS, which covers the full observatory era from 1840 to 2010 (Figure 1).

Fig 2 -

Meridional cross-section of three-dimensional, forward numerical calculations in presence of an imposed magnetic field, for Earth-like parameters (after Gillet, Schaeffer & Jault, 2011)

Fig 3 -

Equatorial map of the westward, eccentric anticyclonic gyre, as imaged from satellite magnetic measurements (after Gillet, Pais & Jault, 2008)

We initiated with the VSQG project (ANR grant, 2006-2009) the assimilation of geomagnetic data into a quasi-geostrophic (QG) model of the Earth’s core. The QG hypothesis, which states that the flow is invariant along the rotation axis, is supported by our numerical calculations for large length-scales (> 50 km) transient motions in the core (Figure 2). Using this assumption, we interpret the observed field changes with a planetary scale, eccentric, anti-cyclonic gyre (Figure 3). By developing ensemble flow inversions techniques, we have detected torsional waves inside the core (Figure 4). This translates into a rms value of about 3 mT for the magnetic field in the bulk of the fluid, and explains a 6 yrs period signal in the independent length-of-day series measured by astronomers.

This research program is continued with the AVS-geomag project (ANR grant, 2011-2015) coordinated from IPGP by A. Fournier, where we extend the stochastic approach to the longer historical and archaeomagnetic eras. We also participate in the construction of synthetic numerical data series, designed for geomagnetic data assimilation benchmarking. We plan to test the capacity of the QG model to reanalyse and predict the recent satellite observations, imaging magnetic forces inside the fluid. Finally, we are expecting the launch of the three satellites of the Swarm constellation of ESA. Our research program involving Swarm data is supported by CNES (French National Space Agency).

Fig 4 -

Sketch of torsional (zonal and geostrophic) waves in the core (courtesy A. Fournier, IPGP), and profile of the cylindrical raidal magnetic field retrieved from data of the observatory era (after Gillet, Jault, Canet & Fournier, 2010)