We investigate the possibility that gauge fields are amplified in an expand
ing universe by parametric resonance, during the oscillatory regime of a sc
alar field to which they are coupled. We investigate the coupling of gauge
fields to a charged scalar field and to an axion. For both couplings, gauge
fields 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 mec
hanism could have some relevance for the problem of large scale primordial
magnetic fields. (C) 2001 Published by Elsevier Science B.V.