Electric field, observed EVPA, and Stokes Q and U. All quantities are defined in the observer’s frame, as measured by the distant observer. The observed electric field associated with the wave received at the observer’s screen is the black arrow with a position angle east of north corresponding to the observed EVPA. For a fully linearly polarized wave, there is a bijection (up to a sign ambiguity) between providing the electric vector magnitude and direction on screen and the pair of Stokes parameters (Q, U). The electric vector magnitude is given by Eq. (18), while its orientation follows χ_o = 1/2 atan2(Q, U), see Eq. (17). It is easy to check from the definitions of Eq. (14) that the north-south and east-west directions coincide with positive and negative Stokes Q (and zero Stokes U), respectively, while the diagonals correspond to positive and negative Stokes U (and zero Stokes Q), respectively.