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Séminaire MHD (SEMHD)

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Modèles MHD du noyau terrestre contraints par les observations récentes du champ magnétique terrestre – 2. Revue théorique
Dominique Jault (CNRS)

22 juin 2009

Until very recently, the inversion of geomagnetic field models for core surface flows has been treated as a purely kinematic problem, using the core surface radial field as a passive tracer for the flow. We will review this classical approach and its main results. Recent satellite missions boosted an increasing complexity of computed magnetic-field models, where the separation of different contributing sources seems more and more realistic. Unfortunately we still have a blurred view of the magnetic field at the core-mantle boundary. This is an important issue as core flows are detected from the motion of magnetic flux lines leaving the Earth’s core. We will discuss the strategies that have been developped to fully account for this difficulty.
In an effort to use recent geomagnetic models to explore the Earth’s core dynamics, we have been testing geostrophic/quasi-geostrophic flows. To sustain the relevancy of these flows, we will argue that motions evolving on timescales shorter than the magnetic diffusion time are almost quasi-geostrophic and thus axially invariant: they are completely determined from a stream-function, which is independent of the z-coordinate along the rotation axis. We will then report how a planetary-scale anticyclonic gyre has been inferred within the Earth’s fluid core.
It is likely that magnetic fields are much stronger inside the core that at the core surface. The detection of a 6-yr periodicity in calculated core flows may be the signature of Alfvén waves propagating inside the core. That gives us an estimate of the magnetic-field intensity. We will introduce the data assimilation scheme that we are now using to further constrain the geometry of the magnetic field in the core interior.

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Dominique Jault Dominique Jault (CNRS)
Directeur de recherches au CNRS – Institut des Sciences de la Terre (ISTerre, anciennement LGIT).