On the thermal electron balance in Titan's sunlit upper atmosphere

E. Vigren, M. Galand, R. V. Yelle, J. Cui, J. E. Wahlund, K. Ågren, P. P. Lavvas, I. C.F. Mueller-Wodarg, D. F. Strobel, V. Vuitton, A. Bazin

Research output: Contribution to journalArticle

27 Scopus citations

Abstract

The Cassini mission has investigated Titan's upper atmosphere in detail and found that, under solar irradiation, it has a well-developed ionosphere, which peaks between 1000 and 1200km. In this paper we focus on the T40, T41, T42 and T48 Titan flybys by the Cassini spacecraft and use in situ measurements of N2 and CH4 densities by the Ion Neutral Mass Spectrometer (INMS) as input into a solar energy deposition model to determine electron production rates. We combine these electron production rates with estimates of the effective recombination coefficient based on available laboratory data for Titan ions' dissociative recombination rates and electron temperatures derived from the Langmuir probe (LP) to predict electron number densities in Titan's upper atmosphere, assuming photochemical equilibrium and loss of electrons exclusively through dissociative recombination with molecular ions. We then compare these predicted electron number densities with those observed in Titan's upper atmosphere by the LP. The assumption of photochemical equilibrium is supported by a reasonable agreement between the altitudes where the electron densities are observed to peak and where the electron production rates are calculated to peak (roughly corresponding to the unit optical depth for HeII photons at 30.38nm). We find, however, that the predicted electron number densities are nearly a factor of two higher than those observed throughout the altitude range between 1050 and 1200km (where we have made estimates of the effective recombination coefficient). There are different possible reasons for this discrepancy; one possibility is that there may be important loss processes of free electrons other than dissociative recombination in Titan's upper atmosphere.

Original languageEnglish (US)
Pages (from-to)234-251
Number of pages18
JournalIcarus
Volume223
Issue number1
DOIs
StatePublished - Mar 1 2013

Keywords

  • Atmospheres, Chemistry
  • Ionospheres
  • Titan

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Vigren, E., Galand, M., Yelle, R. V., Cui, J., Wahlund, J. E., Ågren, K., Lavvas, P. P., Mueller-Wodarg, I. C. F., Strobel, D. F., Vuitton, V., & Bazin, A. (2013). On the thermal electron balance in Titan's sunlit upper atmosphere. Icarus, 223(1), 234-251. https://doi.org/10.1016/j.icarus.2012.12.010