Polarization Sweeps in Rotation Powered Pulsars
  We re-examine the characteristic polarization angle sweep of rotation-powered pulsars and calculate the expected deviations from this sweep caused by aberrational effects and by polar-cap current flow. We find that in addition to the previously known phase shift of the entire sweep by $\Delta \Phi = -4 r/R_L$, aberration shifts the polarization angle itself by $\Delta \Psi = -(10/3) (r/R_L) \cos \alpha$. Similarly, current flow above the polar cap shifts the polarization sweep by $\Delta \Psi = (10/3) (r/R_L) (J/J_{GJ}) \cos \alpha$, potentially providing a method of directly measuring the magnitude of the current. The competition between these two effects produces a potentially observable signature in the polarization angle sweep. Although these effects may appear similar to orthogonal mode shifts, they are an independent phenomenon with distinct observational characteristics. 
Johann A. Hibschman and Jonathan Arons