Titan's upper ionosphere

As a result of electron impact ionization and photoionization, an ionosphere should be built up at the high-altitude region of Titan's atmosphere. From a simple consideration of the Chapman layer theory, the peak electron density can be determined to be ≈5 × 10³ cm^-3 at about 1200 km altitude. The complex ionmolecule reactions in the N₂-CH₄ atmosphere would lead to a mixture of N₂⁺, CH₃⁺, C₂H₅⁺, and H₂CN⁺ ions as the major ion species in the upper ionosphere. The new calculations presented here also predict the presence of significant amounts of CH⁵⁺, N₂H⁺, C₂H₂⁺, and C₂H₃⁺ ions (i.e., n_i, ≈ 10-100 cm^-3) near the ionospheric peak. Because of fast ion-neutral reactions with CH₄ and N₂, the number density of the Ar⁺ ions would not exceed a value of more than 10 cm^-3 in the upper ionosphere if argon has a mixing ratio about 10^-2 near the homopause. Interaction with the partially corotating magnetospheric plasma could lead to a significant modification of the ionospheric profile in the high altitudes. The ionospheric flow dynamics are studied in the limits of one-fluid mass-loaded flow and of diffusion-convection approximation. Both approaches show that ion-neutral friction acts as a cushion against plasma streaming effects.