  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.
