On the passage of radiation through moving astrophysical plasmas

Consideration is given to the passage of radiation through dynamic astrophysical plasmas. It is assumed that the plasma is homogeneous, isotropic, transparent, and lossless (i.e., has negligible absorption). A pencil beam of quasi-monochromatic radiation is incident normally on the plane surface of the medium. The medium moves at a constant rate in the direction parallel to the surface. Three cases are noted: (1) for a nonrelativistic, warm isotropic plasma with both spatial and frequency dispersion, the beam is bent in the upstream direction, (2) for a medium with only frequency dispersion and satisfying the Landau and Lifshitz transparency criterion, the refracted beam cannot be bent in the upstream direction, and (3) for a cold isotropic plasma, the motion does not affect the motion of the beam. It is noted that the direction of energy flow coincides with that of the group velocity (and not with that of the Poynting vector) for a plane, quasi-monochromatic wave propagating in a dynamic homogeneous dispersive medium of infinite extent.