Time scales separation for dynamo actionE. Dormy1, 2 and D. Gérard-Varet3, 2
1 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
received 13 September 2007; accepted in final form 21 January 2008; published March 2008
published online 21 February 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.
47.65.-d - Magnetohydrodynamics and electrohydrodynamics.
47.65.Md - Plasma dynamos.
47.20.-k - Flow instabilities.
© EPLA 2008