Properties of a cutout along the x-axis of the lower part of the EB (indicated as a white box in Figs. 1 and 3). Top left: destruction probability ϵ of He I D₃. Bottom left: same for He I 10 830 Å. Top right: thermalization length 1/ϵ (see Eq. (16)) in units of optical depth as a function of height at location L_em (see Fig. 5), plotted together with the optical depth τ_ν at ν_em (see Fig. 8). h_em as defined in Fig. 8 is overplotted as a vertical line. Bottom right: same for He I 10 830 Å.