Values of the radial profile indexes of vertical optical depth (τ) derived from the n and σ fits displayed in Table 1, when making the simplifying assumption that the radial dependence of τ is the same as that of σ, or is equal to that of n plus one (constant opening angle). The blue domain corresponds to the range of τ(r) indexes between the −1.48 value obtained for our nominal case and the −1.21 value for our bright disc (f_d = 4 × 10⁻³) case (see Fig. 1).