Ionization balance in Titan's nightside ionosphere

Abstract

Based on a multi-instrumental Cassini dataset we make model versus observation comparisons of plasma number densities, n_P = (n_en_I)^1/2 (n_e and n_I being the electron number density and total positive ion number density, respectively) and short-lived ion number densities (N⁺, CH₂⁺, CH₃⁺, CH₄⁺) in the southern hemisphere of Titan's nightside ionosphere over altitudes ranging from 1100 and 1200 km and from 1100 to 1350 km, respectively. The n_P model assumes photochemical equilibrium, ion-electron pair production driven by magnetospheric electron precipitation and dissociative recombination as the principal plasma neutralization process. The model to derive short-lived-ion number densities assumes photochemical equilibrium for the short-lived ions, primary ion production by electron-impact ionization of N₂ and CH₄ and removal of the short-lived ions through reactions with CH₄. It is shown that the models reasonably reproduce the observations, both with regards to n_P and the number densities of the short-lived ions. This is contrasted by the difficulties in accurately reproducing ion and electron number densities in Titan's sunlit ionosphere.