Fig. 8

Results from the simplified coma model for a water-dominated coma and low outgassing rate. All ion number densities decrease with increasing distance from the comet because of competing factors of the 1/r2 density decrease and the increases and decreases due to ionization and ion-neutral chemistry. Whereas the chemical equations that govern the H3O+ and HCO+ number densities have no loss terms in this simple model, the equations that govern the H2O+ and CO+ number densities do have loss terms. Therefore, the H3O+ and HCO+ number densities decrease more slowly than those for H2O+ and CO+. At 30 km from the comet, the H3O+/H2O+ and HCO+/CO+ ratios are both lower than 1, indicating weak ion-neutral chemistry for this low outgassing case.
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