Assimilation of satellite geomagnetic observations

Laboratory : Isterre
supervisors : Nicolas Gillet and Loic Huder
contact : nicolas.gillet univ-grenoble-alpes.fr
Keywords : geomagnetic field ; Earth’s core ; data assimilation
Prerequisites : Applied Maths or Earth sciences or Physics or Mechanics

Our vision of the Earth’s core dynamics has recently been strongly modified by (i) the era of continuous satellite observations (with currently the Swarm mission of ESA), and (ii) the advent of aOur vision of the Earth’s core dynamics has recently been strongly modified by (i) the era of continuous satellite observations (with currently the Swarm mission of ESA), and (ii) the advent of a new generation of numerical simulations of the geodynamo.

In this context, we develop in the geodynamo group of ISTerre a data assimilation algorithm (called « pygeodyn », written in Python ) that aims at recovering, from geomagnetic observations above the Earth’s surface, the evolution of the dynamics at the core surface [1, 2]. Results of the algorithm are available on https://geodyn.univ-grenoble-alpes.fr/. The model is based on stochastic equations, which parameters are derived from numerical simulations of the geodynamo run at extreme parameters [3]. It ingests as observations either spherical harmonics Gauss coefficients of the magnetic field, or ground-based observatory series plus satellite-based virtual observatory series.

Depending on his/her preferences and on the project timing, the candidate will have to work on several improvements of this assimilation tool :
– a better handling of zonal motions that carry torsional waves within Earth’s core [4] ;
– the co-estimation, on top of the core dynamics, of external (magnetospheric) sources ;
– extending the stochastic model to the bulk of the core (may involve artificial intelligence methods) ;
– improving the treatment of magnetic data that enter the algorithm.

Tackling these issues require scientific and technical developement of the data assimilation algorithms. The candidateshould then have some skills in applied maths (in particular linear algebra) and programming (Python, Matlab, Fortran 90 or C). In addition to being integrated in the Geodynamo team, the candidate is expected to collaborate with the Geodata team for the software development part.

This project is supported by the French National Center for Space Studies (CNES) and by the European Space Agency (ESA).

Figure : radial magnetic field (colormap) and stream lines of the
flow reconstructed at the Earth’s core surface

References :
[1] Huder, L., N. Gillet and F. Thollard, pygeodyn 1.1. 0 : a Python package for geomagnetic data assimilation, Geoscientific Model Development 12(8), 3795-3803 (2019)
[2] Gillet, N., L. Huder and J. Aubert, A reduced stochastic model of core surface dynamics based on geodynamo simulations, Geophys. J. Int. 219(1), 522-539 (2019)
[3] Aubert, J., Gastine, T., & Fournier, A.. Spherical convective dynamos in the rapidly rotating asymptotic regime. J. Fluid Mech. 813, 558-593 (2017)
[4] Gillet, N., Jault, D., Canet, E., & Fournier, A. Fast torsional waves and strong magnetic field within the Earth’s core. Nature 465(7294), 74 (2010)
new generation of numerical simulations of the geodynamo.