Time scales separation for dynamo action

Emmanuel Dormy (1) , David Gerard-Varet (2)

1 LPS - Laboratoire de Physique Statistique de l'ENS
2 DMA - Département de Mathématiques et Applications - ENS Paris

The study of dynamo action in astrophysical objects classically involves two timescales: the slow diffusive one and the fast advective one. We investigate the possibility of field amplification on an intermediate timescale 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.

Mots clés: Magnetohydrodynamics Plasma dynamos Flow instabilities

Domaines: Mécanique des fluides [physics.class-ph]; Cosmologie et astrophysique extra-galactique [astro-ph.CO]

Emmanuel Dormy, David Gerard-Varet. Time scales separation for dynamo action. EPL - Europhysics Letters, 2008, 64002 p1-5. ⟨hal-00268726⟩