Superadiabatic-type magnetic amplification in conventional cosmology

We consider the evolution of cosmological magnetic fields in Friedmann-Robertson-Walker models and outline a geometrical mechanism for their superadiabatic amplification on large scales. The mechanism operates within standard electromagnetic theory and applies to Friedmann-Robertson-Walker universes with open spatial sections. We discuss the general relativistic nature of the effect and show how it modifies the adiabatic magnetic evolution. Assuming a universe that is only marginally open today, we estimate the main features of the superadiabatically amplified residual field.