Propagation effects on polarization of pulsar radio emission

Abstract

We consider the role of a propagation effect such as stimulated Raman scattering on the polarization of radio pulses. When an intense electromagnetic wave with frequency close to the plasma frequency interacts with the plasma in the pulsar magnetosphere, the incident wave undergoes stimulated Raman scattering. Using typical plasma and magnetic field parameters, we compute the growth rate and estimate the polarization properties of the scattered mode. Under some conditions, we find that the polarization properties of the scattered mode can become significantly different from those of the incident wave. The frequencies at which strong Raman scattering occurs in the outer parts of the magnetosphere fall into the observable radio band. In some pulsars, for example PSR B0628-28 and B1914+13, a large and symmetric type of circular polarization has been observed. We propose that such an unusual circular polarization is produced by the propagation effects.