Evolution of the N₂D⁺/N₂H⁺ ratio when the cosmic ray ionization rate is divided by 3 (bottom row), multiplied by 3 (middle row) and by 10 (top row) for the slow (right) and fast (left) models. The dotted line traces the new versus the reference model ratio for the last epoch (corresponding to a peak density of 2  ×  10⁶ cm^-3) in each frame. The dashed lines trace the reference results (from Fig. 5).