Ionization equilibrium model predictions for the [N II] 658 nm/[O I] 630 nm ratio for collisions with electrons, plotted as a function of the x_e = n_e/n_H ratio. Different lines refer to different values of electron temperature and density as indicated in the legend. In the upper panel, horizontal lines indicate some of the more stringent upper limits of the [N II] 658 nm/[O I] 630 nm ratio in the NLVC, while in the lower panel the coloured region indicates the range of the [N II] 658 nm/[O I] 630 nm ratio observed in the HVC.