The recently constructed theory of radio wave propagation in the pulsar magnetosphere outlines the general aspects of the radio light curve and polarization formation. It allows us to describe general properties of mean profiles, such as the position angle ($PA$) of the linear polarization, and the circular polarization for the realistic structure of the pair creation region in the pulsar magnetosphere. In this work, we present an application of the radio wave propagation theory to the radio observations of pulsar PSR J1906+0746. This pulsar is particularly interesting because observations of relativistic spin-precession in a binary system allows us to put strong constraints on its geometry. Because it is an almost orthogonal rotator, the pulsar allows us to observe both magnetic poles; as we show, this is crucial for testing the theory of radio wave propagation and obtaining constraints on the parameters of magnetospheric plasma. Our results show that plasma parameters are qualitatively consistent with theories of pair plasma production in polar cap discharges. Specifically, for PSR J1906+0746, we constrain the plasma multiplicity $lambda sim 10^3$ and the Lorentz-factor of secondary plasma $gamma sim $ a few hundred.
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