The present paper continues to study the propagation of the natural waves in an ultrarelativistic highly magnetized pulsar plasma. We have considered the conversion of the ordinary superluminous waves into the extraordinary waves which takes place in the region of quasi-longitudinal propagation with respect to the magnetic field. Although the wave propagation in pulsar magnetospheres is generally quasi-transverse, for a small segment of the wave trajectory the regime of quasi-longitudinal propagation can be met if one takes into account the effect of refraction in the plasma. As a result of the conversion of the ordinary wave, both the ordinary and extraordinary waves arise. It has been found that at typical pulsar conditions the conversion can be significant, so that a considerable part of the energy of the ordinary waves can be transmitted to the extraordinary ones.
Since the natural waves considered are orthogonally polarized, the position angle of the outgoing radiation is that of the dominant mode. The waves resulting from the conversion can compete in intensity, giving rise to orthogonal transitions in the polarization state of pulsar radiation. The mode switching is expected to occur either if refraction becomes too weak to provide quasi-longitudinal propagation of the waves or if the conditions in the region of conversion are changed significantly. Both these possibilities follow directly from the temporal variations in the plasma density distribution which have the observational support. Thus the conversion can account for the OPM-phenomenon characteristic of pulsar radiation.
Since at higher frequencies refraction is stronger, the regime of quasi-longitudinal propagation becomes realizable for most of the rays. Then the longitude ranges corresponding to the prominent OPM-transitions are enlarged and cover most of the pulse. As a result, the outgoing radiation can be substantially depolarized. The high-frequency depolarization of pulsar radiation is a well-known observational fact. As a rule, it reveals itself beyond some critical frequency. Within the framework of our considerations, the latter is thought to be the frequency at which refraction becomes efficient for most of the rays.
The waves escaping from the region of conversion are subsequently subjected to the polarization-limiting effect as they leave the magnetospheric plasma. Owing to this effect the waves acquire some circular polarization, with the sense being opposite for the initially ordinary and extraordinary waves. Therefore the resultant sense of circular polarization is determined by the dominant mode. Hence, the OPM-transitions should be accompanied by change in the sense of the circular polarization. This, too, is commonly observed.
In addition, the polarization-limiting effect leads to the shift in the position angle of linear polarization. For fixed conditions it is equal for both natural waves. However, due to temporal variations in the plasma density distribution the shift in position angle is altered, so that the distribution of the observed OPMs in position angle should be somewhat broadened. This indeed agrees with observations. In some cases the observed OPMs are clearly non-orthogonal, indicating that the variations of the conditions in the region of conversion are strongly correlated with those in the polarization-limiting region; then the OPM-switching is accompanied by significant change in the shift of position angle. Since the polarization-limiting effect can be strong enough to reveal itself in the observational data, any technique of mode separation should allow for the consequences of this effect.
Thus, the conversion of the natural waves in the magnetospheric plasma can explain the main features of the OPM-phenomenon observed in pulsar radiation.
Copyright ESO 2001