On the origin of orthogonal polarization modes in pulsar radio emission

This paper is devoted to studying the conversion of ordinary superluminous waves into extraordinary waves in an ultrarelativistic highly magnetized pl asma of pulsars. The conversion appears to take place in the region of quas i-longitudinal propagation with respect to the magnetic field. Although the propagation of waves in pulsar magnetospheres is generally quasi-transvers e, due to refraction the wave vector can for a short while become nearly al igned with the ambient magnetic field. As a result of the conversion, the o riginally ordinary wave turns into two waves, the ordinary and extraordinar y. It is found that at pulsar conditions the conversion of the ordinary wav es can be significant, i.e. a considerable part of the energy can be transm itted to the extraordinary waves. The process is suggested to underlie the observed transitions between the orthogonal polarization states in pulsar r adiation. The connection of conversion with magnetospheric refraction impli es a number of observational consequences which agree well with the observe d features of the orthogonally polarized modes. In particular, it becomes p ossible to explain both the longitudinal location of the prominent orthogon al transitions and the high-frequency depolarization of pulsar radiation. F urther polarization evolution of the natural waves resulting from the conve rsion is investigated as well. It is found that the polarization-limiting e ffect can account for both the change in the sense of circular polarization during the orthogonal transitions and the non-orthogonality of the observe d modes. It is shown that any technique of mode separation should allow for the consequences of the polarization-limiting effect.