  We consider the evolution of electromagnetic fields in curved spacetimes and
calculate the exact wave equations of the associated electric and magnetic
components. Our analysis is fully covariant, applies to a general spacetime and
isolates all the sources that affect the propagation of these waves. Among
others, we explicitly show how the different parts of the gravitational field
act as driving sources of electromagnetic disturbances. When applied to
perturbed FRW cosmologies, our results argue for a superadiabatic-type
amplification of large-scale cosmological magnetic fields in Friedmann models
with open spatial curvature.
