Abstract
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.
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