A coupled model of Titan's atmosphere and ionosphere

The paper deals with a coupled model of the upper atmosphere and ionosphere of Titan. The neutral atmosphere is described by a comprehensive photochemical model for 30 degrees solar zenith angle with molecular and turbulent diffusive transport taken into account. At the upper boundary (1430 km) escaping flux of light molecules is included in boundary conditions, while near the lower boundary (32 km) condensation of less volatile species is introduced. The atmosphere is coupled with the ionosphere via a large set of bi- and termolecular ion-neutral reactions. The ionosphere is considered to be in chemical equilibrium; in addition to ion-neutral reactions, recombination loss and ionization by solar photons as well as by saturnian magnetospheric electrons and photoelectrons appear in the mass balance equations. In the most advanced models, from several considered, heavy hydrocarbon ions and heavy nitriles are grouped and represented as two pseudoions. The peak ion density of 5200-6000 cm(-3), depending on the employed model, is obtained at a height of 970-1000 km, i.e., slightly lower than in previous ionospheric studies, which, however, were carried out for 60 degrees solar zenith angle. The main ionospheric component near the peak is HCNH+; but at lower altitudes heavy nitriles followed by heavy hydrocarbons are more abundant, while near the upper boundary (>1400 km) CH5+ and C2H5+ compete with HCNH+. When termolecular reaction rates are increased to about 5 x 10(-23) cm(6) s(-1) as suggested by V. G. Anicich and M. J. McEwan (1997, Planet. Space Sci. 45, 897-923) for a nitrogen-dominated atmosphere, ion density profiles change significantly (50%) in the lower part of the ionosphere, at 700-800 km. The astonishing result by J. L. Fox and R. V. Yelle (1997, Geophys. Res. Lett. 24, 2179-2192), whose ionospheric model prefers heavy pseudoions as main constituents near the peak, can, in part, be reproduced if the density of C4H2, a neutral molecule that controls heavy ion production rate, is increased by a factor of 100. The ionosphere is quite sensitive (up to 30% change in density) to variation in recombination rates and/or electron temperature profile. As it can be expected, densities of certain minor neutral components are severely influenced by coupling with ions, e.g., by an order of magnitude for CH3C2H. Surprisingly enough, even some major neutral species, like CH4, show a 50% change in the thermospheric density profile when ion-neutral reactions are introduced. This proves that for molecules in a shallow chemical equilibrium with other neutral species, even a small perturbation caused by reactions with ions can result in a significant change of the density. An interesting and reassuring conclusion has been obtained for HCN: to reach the agreement with the observed mixing ratio profile in the thermosphere, the loss due to polymerization should be considered together with a strongly productive interaction with ions. In view of the results obtained for the neutral atmosphere it seems that coupling with ions can be as important as other factors that shape the neutral density profiles and should be taken into account in future models that will aim at data interpretation from the Cassini/Huygens mission. (C) 2000 Academic Press.