  The dispersion characteristics of an circularly polarized electromagnetic
wave of arbitrary amplitude, propagating in a highly (thermally and
kinematically) relativistic plasma, are shown to approach those of a linear
wave in an unmagnetized, non-relativistic plasma. Further aided by high
relativistic temperatures, the cut-off frequency tends to become negligibly
small; as a result, waves with frequencies well below the nominal plasma and
the cyclotron frequencies find the plasma to be essentially transparent. This
relativistic phenomenon may greatly advance our ability to understand and model
the dynamics of a large class of astrophysical and laser-produced high energy
density systems.
