HAL : insu-00652996, version 3

arXiv : 1112.3879

DOI : 10.1111/j.1365-246X.2012.05611.x

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Geophysical Journal International 191, 2 (2012) pages 508-516

Versions disponibles :

v1 (16-12-2011)

v2 (19-12-2011)

v3 (04-07-2012)

On the reflection of Alfvén waves and its implication for Earth's core modeling

Nathanaël Schaeffer (  Auteur correspondant ) 1, Dominique Jault 1, Philippe Cardin 1, Marie Drouard 1

(11/2012)

Alfvén waves propagate in electrically conducting fluids in the presence of a magnetic field. Their reflection properties depend on the ratio between the kinematic viscosity and the magnetic diffusivity of the fluid, also known as the magnetic Prandtl number Pm. In the special case Pm=1, there is no reflection on an insulating, no-slip boundary, and the wave energy is entirely dissipated in the boundary layer. We investigate the consequences of this remarkable behaviour for the numerical modeling of torsional Alfvén waves (also known as torsional oscillations), which represent a special class of Alfvén waves, in rapidly rotating spherical shells. They consist of geostrophic motions and are thought to exist in the fluid cores of planets with internal magnetic field. In the geophysical limit Pm << 1, these waves are reflected at the core equator, where they are entirely absorbed for Pm=1. Our numerical calculations show that the reflection coefficient at the equator of these waves remains below 0.2 for Pm > 0.3, which is the range of values for which geodynamo numerical models operate. As a result, geodynamo models with no-slip boundary conditions cannot exhibit torsional oscillation normal modes.

1 :	Institut des sciences de la Terre (ISTerre)
	CNRS : UMR5275 – IFSTTAR – Université de Savoie – Université Joseph Fourier - Grenoble I – INSU – OSUG – Institut de recherche pour le développement [IRD] : UR219

Équipe de recherche : Geodynamo

domaine

:

Physique/Physique/Géophysique Planète et Univers/Sciences de la Terre/Géophysique Sciences de l'environnement/Milieux et Changements globaux Physique/Physique/Dynamique des Fluides Physique/Mécanique/Mécanique des fluides Sciences de l'ingénieur/Mécanique/Mécanique des fluides

insu-00652996, version 3

http://hal-insu.archives-ouvertes.fr/insu-00652996

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Contributeur : Nathanaël Schaeffer <>

Soumis le : Mercredi 4 Juillet 2012, 01:25:41

Dernière modification le : Mardi 16 Octobre 2012, 10:00:13