Circular polarization in pulsar radio emission due to intrinsically relativistic effects

Observations of single pulses imply that the circular polarization (CP) at a fixed pulsar phase, in some pulsars, can be large and variable from pulse to pulse. In many cases the variations can include changes in the handedness at a fixed phase. Possible explanations for the variable CP are sought in terms of elliptical polarization of the natural wave modes. The pulsar plasma is assumed to be a highly relativistically streaming, one-dimensional, strongly magnetized, electron-positron gas with a net charge density and intrinsically relativistic random motions in its rest frame. It is shown that in such a plasma the polarization of the natural modes includes two regimes where intrinsically relativistic effects determine the CP. One regime arises from aberration causing a small cone of CP about the backward (anti-streaming) direction in the rest frame to expand into the forward hemisphere in the pulsar frame. This mechanism can lead to significant CP at relatively high frequencies. The other mechanism arises from the intrinsically relativistic random motions, which allow a new regime of CP that has no counterpart in a cold plasma; however, for typical parameters this mechanism is effective only at frequencies higher than is consistent with our neglect of the cyclotron resonance. It is suggested that the explanation for the observed CP requires that the CP arise as a propagation effect, and be determined by the properties of the wave modes at a polarization limiting region.