Schematic diagram of the model geometry. The coordinate system (x, y, z) is that in which the PRT equations are integrated. The symbols have the following meanings: k is the spatial component of the wavevector, B is the three-vector of the magnetic field, u is the spatial component of the plasma bulk four-velocity, θ is the angle between the x-axis and the B-field vector measured in the frame of the adopted coordinate system; it has different values when measured in the rest-frame co-moving with the plasma. Finally, the coordinate system (x⁰, y⁰, x⁰) is the frame in which the Stokes parameters are observed.