Observations (top panel) compared to the predictions of PDR (middle panel) and TDR (bottom panel) models. Data (open circles) − The CH⁺ and total hydrogen column densities inferred from visible absorption lines and extinction measurements are shown in red and are from Crane et al. (1995), Gredel (1997), and Weselak et al. (2008). The CH⁺ and total hydrogen column densities inferred from the far-infrared absorption lines of CH⁺, HF, and CH and the 21 cm emission lines of HI are shown in blue and are from Falgarone et al. (2010a), Gerin et al. (2010b), Neufeld et al. (2010b), Sonnentrucker et al. (2010), and Godard et al. (2012). The red and blue lines correspond to the mean ratio N(CH⁺) /N_H computed with the visible and submillimetre data, respectively. PDR and TDR model predictions (filled symbols) − Computed for several densities: 10 (crosses), 30 (triangles), 50 (squares), and 100 cm^-3 (circles). The TDR models are computed for (green) and 10^-23 (magenta) erg cm^-3 s^-1, all the other parameters are set to their standard values.