  Frequency dependence of pulsar linear polarization is investigated by
simulations of emission and propagation processes. Linearly polarized waves are
generated through curvature radiation by relativistic particles streaming along
curved magnetic field lines, which have ordinary mode (O-mode) and
extra-ordinary mode (X-mode) components. As emitted waves propagate outwards,
two mode components are separated due to re- fraction of the O mode, and their
polarization states are also modified. According to the radius to frequency
mapping, low frequency emission is generated from higher magnetosphere, where
significant rotation effect leads the X and O modes to be sepa- rated. Hence,
the low frequency radiation has a large fraction of linear polarization. As the
frequency increases, emission is generated from lower heights, where the
rotation effect becomes weaker and the distribution regions of two modes are
more overlapped. Hence, more significant depolarization appears for emission at
higher frequencies. In addition, refraction effect of the O mode becomes
serious in very deep magnetosphere, which bends the O mode emission towards
outer parts of a pulsar beam and also causes the separation of mode
distribution regions and hence the fractional linear polariza- tion increasing
with frequency. If emission of different frequencies is generated from a region
of the same height, serious O mode refraction can result in the decrease of
both profile width and fractional linear polarization. The observed frequency
dependence of linear polarization for some pulsars can be naturally explained
within the scope of our scenario.
