A Model for Negative Ion Chemistry in Titan's Ionosphere

We developed a one-dimensional photochemical model for the dayside ionosphere of Titan for calculating the density profiles of negative ions under steady-state photochemical equilibrium condition. We concentrated on the T40 flyby of the Cassini orbiter and used the in situ measurements from instruments on board Cassini as input to the model. Using the latest available reaction rate coefficients and dissociative electron attachment cross sections, the densities of 10 anions are calculated. Our study shows CN- as the dominant anion, followed by C3N-, which agrees with the results of previous calculations. We suggest that H- could be an important anion in Titan's ionosphere and is the second most abundant anion at altitudes greater than 1200 km. The main production channel of the major ion CN- is the reaction of H- with HCN. The H- also play a major role in the production of anions C2H-, C6H-, and OH-. We present a comparison of the calculated ion density profiles with the relative density profiles derived using recently reported Cassini CAPS/ELS observations.