2004ApJ...606.1167R

We report here the discovery of a significant source of systematic error in the determinations of pulsar rotation measures. Conventional analysis of high-sensitivity polarimetric observations of PSR B2016+28 display variation in the rotation measure of ±15 rad/m2 (around the mean value of -34.6 rad/m2) across the pulse profile. Analysis of single pulse data shows that this variation is an artifact of the incoherent superposition of quasi-orthogonal polarization modes along with the frequency dependence of relative strength and/or quasi-orthogonality of the modes. Quasi-orthogonal polarization is common among pulsars, and therefore this effect needs to be taken into account in the interpretation of pulsar rotation measures.