On the origin of the circular polarization in radio pulsars

The properties of circularly polarized waves are studied in the pulsar magnetosphere plasma. It is shown that some observational characteristics of the circular polarization observed in the pulsar radio emission can be qualitatively explained in the framework of the model based on anomalous Doppler resonance. Performed analysis provides that if the difference between Lorentz factors of electrons and positrons is relatively high, one of the circularly polarized waves becomes superluminal and therefore cannot be generated by cyclotron instability. We suggest that this case corresponds to pulsars with the domination of one hand of circular polarization through the whole averaged pulse profile at all observed frequencies. For intermediate values of the difference between Lorentz factors, both circularly polarized waves are generated, but the waves of one handedness are much more effectively generated for high frequencies, whereas generation of another handedness dominates for low frequencies. This should correspond to pulsars with strong frequency dependence of the degree of circular polarization. The case of relatively small difference between Lorentz factors corresponds to pulsars with sign reversal of the circular polarization in the centre of averaged pulse profiles.