Left: sketch of the prominence as seen on disk (top view). The blue and red contours outline the filament as seen in STEREO-B/EUVI Fe xii 195 Å and He ii 304 Å band-pass images (see Fig. 2). The arrow points to the line-of-sight (LOS) direction. The dotted lines correspond to constant Stonyhurst heliographic longitude and latitude positions on the solar disk. The prominence sits in the 70° longitude, which gives a scattering angle of θ = 70°. Right: geometry of the problem. The inclination of the magnetic field θ_B is measured from the z-axis (solar vertical) and the azimuth χ_B counterclockwise from the x-axis, contained on the surface plane. The graph shows the angle θ between the LOS direction and the local vertical, which corresponds to the light scattering angle.