Reference frame for our problem. Γ ∈ [ − π/2, π/2] is the inclination angle of the magnetic field vector B with respect to the plane of the sky (yz), and ϕ ∈ [0, 2π] is the angle, counted positively clockwise from the north, between the z axis and the projection of the magnetic field vector onto the plane of the sky. The polarization direction is also in the plane of the sky and perpendicular to that projection, making with the z axis an angle ψ = ϕ − π/2 [π] ∈ [−π/2, π/2]. All constructions except B and Γ are in the plane of the sky.