By considering an idealized model of helically forced flow in an extended domain
that allows scale separation, we have investigated the interaction between dynamo
action on different spatial scales. The evolution of the magnetic field is studied
numerically, from an initial state of weak magnetization, through the kinematic and
into the dynamic regime. We show how the choice of initial conditions is a crucial
factor in determining the structure of the magnetic field at subsequent times. For a
simulation with initial conditions chosen to favour the growth of the small-scale field,
the evolution of the large-scale magnetic field can be described in terms of the α-effect
of mean field magnetohydrodynamics. We have investigated this feature further by a
series of related numerical simulations in smaller domains. Of particular significance
is that the results are consistent with the existence of a nonlinearly driven α-effect
that becomes saturated at very small amplitudes of the mean magnetic field.