  Cosmological magnetic fields in open Friedmann universes can experience
superadiabatic amplification within the realm of conventional electromagnetism.
This is possible mathematically, despite the conformal invariance of Maxwell's
equations, because Friedmann spacetimes with non-Euclidean spatial geometry are
not globally conformal to Minkowski space. Physically, this means that even
universes that are marginally open today can sustain large-scale magnetic
fields that are substantially stronger than previously anticipated. In the
present article, we investigate this purely geometric amplification mechanism
in greater detail, focusing on the early evolution of the electromagnetic modes
in inflationary Friedmann models with hyperbolic spatial geometry. This also
allows us to refine the earlier numerical estimates and provide the current
spectrum of the residual, superadiabatically amplified magnetic field.
