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Theses

Dynamos numériques planétaires générées par cisaillement en surface ou chauffage interne

Abstract : In the first part of this thesis, we develop a hybrid numerical code based on a quasigeostrophic model for the flows inside the planetary cores driven by internal heating. The code calculates the velocity field in the equatorial plane, and the temperature and magnetic fields in three dimensions in the sphere. This hybrid approach allows us to model turbulent convective flows (high Reynolds numbers, Re > 10000) under a strong influence of rotation (small Ekman numbers) for small Prandtl numbers, P = 0.1 − 0.01. Large amplitude geostrophic zonal circulations are a robust feature of these flows. The scale and the amplitude of the zonal motion is controlled by potential vorticity mixing and boundary friction. We identify the presence of large-scale Rossby waves propagating in the vigourously convecting region. The flows produce kinematic dynamos, with a small-scale poloidal magnetic field and a mainly axisymmetric toroidal field. The critical magnetic Reynolds numbers are of order 1000. We find that the impact of the thermal wind on the dynamo threshold is not significant. In the second part of this thesis, we study dynamos generated by surface shears. Spherical Couette flow (the flow between two spheres in differential rotation) produces dynamos with a high critical Reynolds number. By breaking the axial symmetry of the flow, the shear instability (in the form of a wave) plays a crucial role. The toroidal magnetic field is large compared with the poloidal field, suggesting the role of the omega effect in the dynamo process. We study the dynamics and the dynamo action produced by zonal jets, i.e., produced by differential rotation that is alternately westward and eastward with latitude. The zonal jets imposed at the outer surface are modified by Rossby waves, which widen the jets and lower their amplitude. The dynamo mechanism relies on the propagation of the Rossby waves. The amplitude of the axisymmetric poloidal magnetic field depends on the lengthscale of the Rossby waves through their phase speed. As the width of the jets fixes the lengthscale of the Rossby waves, we can establish a link between production of the axisymmetric poloidal magnetic field and the width of the jets.
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https://tel.archives-ouvertes.fr/tel-00576177
Contributor : Céline Guervilly <>
Submitted on : Sunday, March 13, 2011 - 5:13:13 AM
Last modification on : Tuesday, September 7, 2021 - 3:54:30 PM
Long-term archiving on: : Saturday, December 3, 2016 - 8:25:45 PM

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  • HAL Id : tel-00576177, version 1

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Céline Guervilly. Dynamos numériques planétaires générées par cisaillement en surface ou chauffage interne. Géophysique [physics.geo-ph]. Université Joseph-Fourier - Grenoble I, 2010. Français. ⟨tel-00576177⟩

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