Illustration of notations. The z axis is chosen perpendicular to the surface, and the (xz) plane is chosen parallel to the magnetic field lines, which make an angle θ_B with the normal to the surface. The direction of the reflected beam with wave vector k_r is determined by the polar angle θ_k and the azimuthal angle ϕ, and α_r is the angle between the reflected beam and the field lines. Thick solid lines show the reflected beam and magnetic field directions, thin solid lines illustrate the coordinates, and dashed lines show the incident photon wave vector k_i and its quadrant. The lines marked illustrate the basic polarizations adopted for the description of reflectivities: and are perpendicular to the wave vectors k_i and k_i, respectively; are parallel to the surface, and lie in the perpendicular plane. The axes x′ and y′ lie in the plane made by and , x′ being aligned in the plane made by B and k_r.