Ionization balance in Titan’s nightside ionosphere E. Vigren a,f,⇑ , M. Galand a , R.V. Yelle b , A. Wellbrock c , A.J. Coates c , D. Snowden b , J. Cui d , P. Lavvas e , N.J.T. Edberg f , O. Shebanits f , J.-E. Wahlund f , V. Vuitton g , K. Mandt h a Department of Physics, Imperial College London, London SW7 2AZ, UK b Lunar and Planetary Laboratory, University of Arizona, Tucson 85721-0092, USA c Mullard Space Science Laboratory, University College London, Dorking, Surrey RH5 6NT, UK d Key Laboratory of Lunar and Deep Space Exploration, Chinese Academy of Sciences, Beijing 100012, China e Groupe de Spectrométrie Moléculaire et Atmosphérique, Université Reims Champagne-Ardenne, UMR 7331, 51687 Reims, France f Swedish Institute of Space Physics, Uppsala, Sweden g Univ. Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France h Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX 78228, USA article info Article history: Received 26 March 2014 Revised 27 August 2014 Accepted 7 November 2014 Available online 22 November 2014 Keywords: Titan Ionospheres Titan, atmosphere abstract Based on a multi-instrumental Cassini dataset we make model versus observation comparisons of plasma number densities, n P =(n e n I ) 1/2 (n e and n I being the electron number density and total positive ion num- ber density, respectively) and short-lived ion number densities (N + , CH 2 + , CH 3 + , CH 4 + ) in the southern hemi- sphere of Titan’s nightside ionosphere over altitudes ranging from 1100 and 1200 km and from 1100 to 1350 km, respectively. The n P model assumes photochemical equilibrium, ion–electron pair production driven by magnetospheric electron precipitation and dissociative recombination as the principal plasma neutralization process. The model to derive short-lived-ion number densities assumes photochemical equilibrium for the short-lived ions, primary ion production by electron-impact ionization of N 2 and CH 4 and removal of the short-lived ions through reactions with CH 4 . It is shown that the models reason- ably reproduce the observations, both with regards to n P and the number densities of the short-lived ions. This is contrasted by the difficulties in accurately reproducing ion and electron number densities in Titan’s sunlit ionosphere. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Titan, the largest satellite of Saturn, has a dense and extended atmosphere dominated by N 2 and CH 4 . The Cassini mission has revealed a chemically complex ionosphere around Titan. N 2 and CH 4 are ionized and/or dissociated by solar photons or particle irra- diation marking the onset of a chain of chemical reactions, which produce hydrocarbon and nitrile ions, heavy positive and negative ions, and eventually aerosols (e.g., Vuitton et al., 2007; Coates et al., 2007, 2009; Waite et al., 2007; Wahlund et al., 2009; Crary et al., 2009; Ågren et al., 2012; Shebanits et al., 2013; Lavvas et al., 2013; Wellbrock et al., 2013). However, Titan dayside ionospheric models have shown difficulties in reproducing observed electron number densities (e.g., Vigren et al., 2013), as well as the observed number densities of HCNH + , the dominant ion in the main ionosphere (e.g., Vuitton et al., 2009; Westlake et al., 2012). The sunlit side electron number densities derived in the Cassini multi-instrumental study by Vigren et al. (2013) are systematically a factor of 2 higher than the values deduced from the Radio Plasma Wave Science/Langmuir Probe (RPWS/LP) mea- surements. From the latter, the dayside electron number densities are found to peak typically at values 2000–5000 cm 3 in the alti- tude range 1000–1200 km. The model predicts the observed shape of the electron number density in altitude and both the observa- tions and the model show that a decreased solar zenith angle decreases the altitude and increases the magnitude of the electron number density peak. Whether the cause of the discrepancy in magnitude is overestimated plasma production, underestimated plasma loss or a combination of the two is an open question. There are different levels of agreement in existing model-observation comparisons of short-lived ions in Titan’s dayside ionosphere (short-lived ions include e.g., N + ,N 2 + and CH x + with x < 5; ions that are reactive with CH 4 and typically lost in 5–200 s upon forma- tion in the altitude range 1000–1350 km). On the one hand, Robertson et al. (2009), Westlake et al. (2012) and Richard (2013) obtain a good agreement with the Ion Neutral Mass http://dx.doi.org/10.1016/j.icarus.2014.11.012 0019-1035/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author at: Swedish Institute of Space Physics, Box 537, SE-75121 Uppsala, Sweden. E-mail address: erik.vigren@irfu.se (E. Vigren). Icarus 248 (2015) 539–546 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus