Volume 555, July 2013
|Number of page(s)||4|
|Section||Planets and planetary systems|
|Published online||11 July 2013|
Rotational non-LTE in HCN in the thermosphere of Titan: Implications for the radiative cooling
1 Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
2 The Catholic University of America, 620 Michigan Ave., Washington DC 20064, USA
3 NASA Goddard Space Flight Center, Mailcode 674, Greenbelt Rd., Greenbelt, MD 20771, USA
4 Laboratoire d’Astrophysique, UMR 5571 CNRS, Université Joseph-Fourier, BP 53, 38041 Grenoble Cedex 9, France
5 Centre National de la Recherche Scientifique/École Polytechnique, Laboratoire de Météorologie Dynamique, 91128 Palaiseau-Cedex, France
Received: 4 February 2013
Accepted: 6 June 2013
Context. The thermal structure of Titan’s thermosphere is determined by the balance between several heating and cooling processes. These processes must be accurately modeled to correctly interpret the available measurements and enhance our understanding of the formation and evolution of this atmosphere. One of the most important thermospheric cooling process for Titan is emission in the HCN rotational band.
Aims. We aim to determine the validity of local thermodynamic equilibrium (LTE) for the HCN rotational distribution in the thermosphere of Titan and the impact of its breakdown on the HCN radiative cooling rate in the thermosphere.
Methods. A general non-LTE radiative transfer code for rotational lines based on the accelerated lambda iteration (ALI) was used to calculate the excitation of HCN rotational levels in Titan’s atmosphere. These level populations were then used to calculate the associated cooling rate.
Results. We show that the common assumption in the models of Titan’s thermospheric energy balance, namely the LTE distribution of rotational lines of HCN, is generally not valid above about 1100 km, or ~0.025 nbar, which will affect the derived thermospheric cooling rates. The effect of non-LTE is to reduce the cooling rate to 15% of the LTE value at around the exobase altitudes depending on the given density of HCN and collisional partners (N2, CH4, H2, and electrons). Since collision state-to-state quenching rates of HCN rotational levels are poorly known, a sensitivity analysis of our results to these rates is also presented.
Key words: radiation mechanisms: non-thermal / line: formation / planets and satellites: atmospheres
© ESO, 2013
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