Volume 81, Number 6, March 2008
|Number of page(s)||5|
|Section||Electromagnetism, Optics, Acoustics, Heat Transfer, Classical Mechanics, and Fluid Dynamics|
|Published online||21 February 2008|
Time scales separation for dynamo action
MAG (Ecole Normale Supérieure/Institut de Physique du Globe de Paris), Département de Physique, LRA, Ecole Normale Supérieure - 24, rue Lhomond, 75231 Paris Cedex 05, France
2 CNRS - France
3 Département de Mathématiques et Applications, Ecole Normale Supérieure - 45, rue d'Ulm, 75230 Paris Cedex 05, France
Accepted: 21 January 2008
The study of dynamo action in astrophysical objects classically involves two time scales: the slow diffusive one and the fast advective one. We investigate the possibility of field amplification on an intermediate time scale associated with time-dependent modulations of the flow. We consider a simple steady configuration for which dynamo action is not realised. We study the effect of time-dependent perturbations of the flow. We show that some vanishing low-frequency perturbations can yield exponential growth of the magnetic field on the typical time scale of oscillation. The dynamo mechanism relies here on a parametric instability associated with transient amplification by shear flows. Consequences on natural dynamos are discussed.
PACS: 47.65.-d – Magnetohydrodynamics and electrohydrodynamics / 47.65.Md – Plasma dynamos / 47.20.-k – Flow instabilities
© EPLA, 2008
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