  We investigate the possibility that gauge fluctuations are amplified in an
expanding universe by parametric resonance, during the oscillatory regime of a
scalar field to which they are coupled. We investigate the couplings of gauge
fields to a charged scalar field and to an axion. For both couplings, gauge
field fluctuations undergo exponential instabilities. We discuss how the
presence of other charges or currents may counteract the resonance, but we
argue that in some cases the resonance will persist and that hence this
mechanism could have some relevance for the problem of large scale primordial
magnetic fields.
