Observations (top panel) compared to the predictions of TDR models computed for three values of the maximum ion-neutral velocity drift: starting from the second panel, u_i − u_n = 2.0, 3.0, and 5.0 km s^-1. Data (open circles) − the CH⁺ and SH⁺ column densities and the least-squares power-law fit of their ratio (black line) are from Godard et al. (2012). Model predictions (filled symbols) − computed for A_V = 0.1 (red), 0.2 (magenta), and 0.4 (blue) mag and for several densities: 20 (crosses), 30 (triangles), 50 (squares), and 100 cm^-3 (circles). Along each curve, the rate-of-strain a varies between 10^-11 and 10^-10 s^-1 from right to left.