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Given the apparent correlation between the critical frequencies in the spectra of pulsar pulse width and fractional linear polarization, we develop a model of subpulse broadening and depolarization due to the birefringence of the plasma above pulsar polar caps. The broadening at low radio frequency is caused by the divergence of the individual beams of the two propagation modes, and the depolarization at high frequency results from the merger of their orthogonal polarizations. In addition to the critical frequency correlation, the model predicts rapid depolarization with frequency when the linear polarizations of the beams are comparable. The predictions are generally supported by multi-frequency observations of pulsar linear polarization and studies of individual pulse polarization. An interpretation of more recent observations in terms of the model suggests that they are consistent with an emission mechanism which is broad-band in frequency over a narrow range of emission altitudes. Old pulsars are found to depolarize faster than young ones, implying that the relative strength of the orthogonal polarization modes evolves with time.
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