Position–velocity diagrams of HCO⁺(4–3) emission along parallel (top) and perpendicular (bottom) cuts shown in Fig. 6 and plotted in the same way as Figs. 7 and 8 using the same angular resolution as for [C II]. Ten linear contours from 0.3 to 6.9 K are shown, and the peak intensity is 7.0 K. To the NE, the HCO⁺ is barely detectable 60′′ from S 1, confirming that the density of the surrounding cloud falls off toward the NE. The southwestern most perpendicular position velocity cut 120′′ west of S 1 does not cross the PDR, but is plotted to illustrate that the emission in the Rho Oph A ridge is more blueshifted than the emission from the S 1 PDR.