  We consider the effects of vacuum polarization and proton cyclotron
resonances on the propagation of radiation through a strongly magnetized
plasma. We analyze the conditions under which the photons evolve adiabatically
through the resonant density and find that the adibaticity condition is
satisfied for most photon energies of interest, allowing for a normal-mode
treatment of the photon propagation. We then construct radiative equilibrium
atmosphere models of strongly magnetized neutron stars that includes these
effects, employing a new numerical method that resolves accurately the sharp
changes of the absorption and mode-coupling cross sections at the resonant
densities. We show that the resulting spectra are modified by both resonances
and are harder at all field strengths than a blackbody at the effective
temperature. We also show that the narrow absorption features introduced by the
proton cyclotron resonance have small equivalent widths. We discuss the
implications of our results for properties of thermal emission from the
surfaces of young neutron stars.
