a) Vertical section of the toy-model magnetic concentration used in the paper (the thick solid line). It represents a concentration of width a, tilted an angle ϕ with respect to the local vertical and observed at heliocentric angle θ. The symbols h and l are, respectively, the geometrical depth of the concentration and the portion of the bottom accessible to observation. b) For a given tilt angle ϕ, Θ and Θ′ represent the extreme inclination angles that allow us to observe the bottom of the magnetic concentration. The encircled plus symbol between the figures shows the positive sense of rotation used in our equations.