Top panel: ion mass 19/mass 18 (H₃O⁺/H₂O⁺) and bottom panel: ion mass 29/mass 28 ((HCO⁺ and possibly C₂H)/CO⁺) versus total neutral density at the Rosetta spacecraft. All measurements were made 30 km from the comet for the time intervals in Table 3. The H₃O⁺/H₂O⁺ ratio is mostly lower than 1 for neutral densities lower than about 2.5 × 10⁷ cm^-3. Similarly, the HCO⁺/CO⁺ ratio is much lower than 1 for total neutral densities near 1 × 10⁷ cm^-3. The low number densities of protonated molecules indicate that, at low outgassing rates, ion-neutral chemistry is weak in the coma. For higher outgassing rates, the HCO⁺/CO⁺ ratio can be much greater than 1, possibly indicating enhanced ion-neutral chemistry. The blue lines show results from the model in Sect. 5. The observed mass 19/mass 18 ratio is consistently lower than the predicted ratio while the observed mass 29/mass 28 ratio agrees well with the predicted ratio, particularly at higher total neutral densities.