Issue A&A Volume 506, Number 2, November I 2009 Page(s) 955 - 964 Section Planets and planetary systems DOI 10.1051/0004-6361/200912371 Published online 03 September 2009

A&A 506, 955-964 (2009)
DOI: 10.1051/0004-6361/200912371

Ionization processes in the atmosphere of Titan

I. Ionization in the whole atmosphere

G. Gronoff¹, J. Lilensten¹, L. Desorgher^{2, 3}, and E. Flückiger<sup>2

1</sup>  Laboratoire de Planétologie de Grenoble, Université Joseph Fourier - CNRS, France
    e-mail: guillaume.gronoff@obs.ujf-grenoble.fr
²  Physikalisches Institut, University of Bern, Switzerland
³  SpaceIT GmbH, Sennweg 15, 3012 Bern, Switzerland

Received 22 April 2009 / Accepted 13 July 2009

Abstract
Context. The Cassini probe regularly passes in the vicinity of Titan, revealing new insights into particle precipitation thanks to the electron and proton spectrometer. Moreover, the Huygens probe has revealed an ionized layer at 65 km induced by cosmic rays. The impact of these different particles on the chemistry of Titan is probably very strong.
Aims. In this article, we compute the whole ionization in the atmosphere of Titan: from the cosmic rays near the ground to the EUV in the upper atmosphere. The meteoritic layer is not taken into account.
Methods. We used the transTitan model to compute the electron and EUV impact, and the planetocosmics code to compute the influence of protons and oxygen ions. We coupled the two models to study the influence of the secondary electrons obtained by planetocosmics through the transTitan code. The resulting model improves the accuracy of the calculation through the transport of electrons in the atmosphere.
Results. The whole ionization is computed and studied in details. During the day, the cosmic ray ionization peak is as strong as the UV-EUV one. Electrons and protons are very important depending the precipitation conditions. Protons can create a layer at 500 km, while electrons tend to ionize near 800 km. The oxygen ion impact is near 900 km. The results shows few differences to precedent models for the nightside T5 fly-by of Cassini, and can highlight the sources of the different ion layers detected by radio measurements.
Conclusions. The new model successfully computes the ion production in the atmosphere of Titan. For the first time, a full electron and ion profile has been computed from 0 to 1600 km, which compares qualitatively with measurements. This result can be used by chemical models.

Key words: planets and satellites: individual: Titan -- atmospheric effects -- Sun: UV radiation -- space vehicles: instruments -- methods: numerical

© ESO 2009

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