Illustration of the Stokes vector conventions recommended by IAU (1974) and used in this paper. The radiation beam travels along its propagation direction, k, out of the page. The electric field vector describes an ellipse over time. The linear polarization angle, γ, is given by the angle between the primary axis of the ellipse (green line) and the reference direction, d. The position angle of the electric field vector increases with time, the beam has right-handed circular polarization.