  We study the onset of vector instabilities in a post-inflationary epoch of
the Universe as a mechanism for primordial magnetic fields amplification
between the end of inflation and the electroweak (EW) transition. We assume the
presence of a charged spectator scalar field arbitrarily coupled to gravity.
This field is in its vacuum state during inflation, but becomes highly excited
after the transition to the radiation dominance due to the gravitational
particle creation. At the beginning of radiation era the ensuing state admits a
hydrodynamic description. In particular since its high temperature, the fluid
may be regarded as a conformal one. The large quantum fluctuations induced
during reheating now become statistical fluctuations whereby an excess charge
and anisotropic pressure will be observed in any finite domain. Under these
conditions a Weibel instability could be triggered thus opposing the dilution
of a primordial magnetic field because of the expansion of the Universe. The
magnitude of the effect is determined by the size of the domain, the coupling
to curvature of the field and the relaxation time of the fluid. We find that
for scales of the order or smaller than the particle horizon at the EW phase
transition, the Weibel instability can overcome the cosmic expansion provided
that the reheating temperature of the Universe and the coupling of the scalar
field to gravity are small enough.
