Polarised radio emission from pulsars and the influenceof the magnetosphere

Since the discovery of pulsars 50 years ago, a lot of progress has been made in understanding how the radio emission mechanism of pulsars works. Nevertheless, explaining the wealth of observational phenomenology has proven difficult. In this thesis, we investigated the radio polarisation properties of pulsars. It has become clear that the propagation of radio emission in the pulsar magnetosphere plays a major role in shaping the observed radiation, especially where polarisation is considered. We started by studying the time-dependence of radio polarisation by observing the polarisation properties of individual pulses of PSR B0031-07, a pulsar with multiple stable drift modes. Furthermore, using a larger sample of 31 pulsars, we aimed to explore if it is common for switches between orthogonal polarisation modes (OPMs) to be linked to periodic intensity variability. Eleven pulsars in the sample showed both a mixture of OPMs and drifting subpulses. Out of these, nine showed synchronous switching of OPMs with the drifting subpulses and for the remaining two pulsars we showed that the drifting might well be too complex to reveal the link between polarisation and the periodic intensity variations. So if the pulsar has OPM switches, it is most likely linked to drifting subpulses if present. This is argued to be a consequence of propagation effects in the pulsar magnetosphere. Even if no periodic intensity variations are observed, it was established that changes in polarisation are coupled to variability in intensity. The frequency dependence of radio polarisation was studied by conducting the largest investigation to date into the origin of phase resolved apparent rotation measure (RM) variations in the polarised signals of 98 radio pulsars. A total of 42 pulsars showed significant phase resolved apparent RM variations. Our results are inconsistent with all variations being caused by interstellar scattering. Instead, magnetospheric propagation is argued to be important, and thus leaving a frequency dependent imprint in the observed polarised radiation. Lastly, we investigated the polarisation properties of three newly discovered pulsars from the GMRT High Resolution Southern Sky Survey to get geometrical information about the pulsar and its emission beams. We conclude that the magnetic axis of PSR J0514-4407 is highly inclined with respect to the rotation axis. This is consistent with what we would expect given that is it a gamma-ray emitting pulsar.