Coronal formation and heating efficiencies in Titan's upper atmosphere: Construction of a coupled ion, neutral and thermal structure model to interpret the first INMS Cassini data.

Abstract

The composition and thermal structure of Titan's upper atmosphere are studied between 600 and 2000 km, resulting in the first detailed calculation of Titan's altitude-dependent heating efficiencies, and the first estimate of the extent of the corona produced by exothermic chemical reactions. The mechanics of constructing a one-dimensional coupled model of ion and neutral densities, along with the thermal structure, are described. A rotating method is used to study local time variations at constant latitude. The 35 neutral species modeled undergo vertical transport and are extended into the exosphere using the Liouville theorem. The 47 ion species modeled are assumed to be in photochemical equilibrium. Local-time dependent density results are presented for the latitudinal and solar conditions corresponding to the Cassini <italic>T_A</italic> and <italic>T</italic>₅ Titan flybys. The thermal structure is governed by thermal conduction, solar and magnetospheric heating, and radiative cooling in the <italic>HCN</italic> rotational lines. A detailed calculation of heating rate profiles is provided, considering electron impact excitation of nitrogen and methane, suprathermal electron heating, and ion-neutral exothermic chemistry. Vertical redistribution of the exothermic chemical heat by suprathermal fragments is also taken into account, using a two stream model. Diurnal averaged profiles of heating efficiencies are displayed for the <italic>T_A</italic> and <italic>T</italic>₅ solar and latitudinal conditions, revealing large altitude variations corresponding to 25+/-15% and 23+/-19%, respectively. Results from the two stream calculation, modeling 11 hot species simultaneously traveling through a background mixture of <italic>N</italic>₂, <italic>CH </italic>₄, and <italic>H</italic>₂, also showed the existence of a corona induced by exothermic ion-neutral chemistry in Titan's vicinity. This work contributed to the analysis of the INMS <italic>T_A, T_B</italic>, and <italic>T</italic>₅ data at two levels. (1) The atmospheric structure parameters of Titan's upper atmosphere---temperature and eddy diffusion coefficient---were determined using a least-squares fitted diffusion model. The isothermal temperature was found to be 148.6+/-3.2K for <italic>T_A</italic>, 148.3+/-8.7K for <italic>T_B</italic>, and 158.8+/-2.6K for <italic>T</italic>₅. (2) The presence of significant suprathermal mechanisms was identified at altitudes between 1500 and 2000 km. Using the Liouville theorem, the escape fluxes of hot <italic>N</italic>₂ and <italic>CH</italic>₄ particles were estimated at 1.3+/-0.6 x 10<super>28</super>s<super> -1</super> and 3.5+/-1.0 x 10<super>28</super>s<super>-1</super>, respectively.