Modèle dynamique et assimilation de données de la variation séculaire du champ magnétique terrestre

Abstract : The changes in the Earth's magnetic field on a wide range of time and space scales reflect the various time and space scales of core processes. I propose a simplified model of the rapid dynamics of the Earth's core, tailored to the study of the rapid variations of the geomagnetic field ranging from years to centuries, referred to as the geomagnetic secular variation. The quasi-geostrophic approximation used in the model relies on the prevalence of rotation forces over magnetic forces on short timescales. The axisymmetric component of the model corresponds to the formalism of Alfvén torsional waves. Within this framework, the magnetohydrodynamics takes place in the equatorial plane. At the core-mantle boundary, the equatorial flow interacts with the radial magnetic field through the radial component of the magnetic induction equation. This component of the model connects the dynamics with the observations. Variational data assimilation allows for the interpretation of the secular variation in terms of the underlying dynamics. It seeks to minimize an objective function by computing its sensitivity to its control variables through the integration of the adjoint model. I illustrate this inversion scheme with twin experiments, performed for a steady flow and in a torsional wave scenario. It enables the retrieval of core state variables which are not directly measured. By assimilating core flow models at the core surface, the inferred magnetic tension (the restoring force of torsional waves) yields a large magnetic field inside the core, at least on the order of 3-4 mT. Such fast torsional waves are consistent with a 6-year signal in the length-of-day variation timeseries.
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Elisabeth Canet. Modèle dynamique et assimilation de données de la variation séculaire du champ magnétique terrestre. Géophysique [physics.geo-ph]. Université Joseph-Fourier - Grenoble I, 2009. Français. ⟨tel-00521437⟩

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