The Structure of Titan’s Ionosphere from 10 Years of Cassini Measurements: Solar Cycle and Saturn Local Time Dependence

Friday, 19 December 2014
Niklas J. T. Edberg1, William S Kurth2, Donald A Gurnett3, David J Andrews1, Erik Vigren1, Oleg Shebanits1, Karin Agren1, Jan-Erik Wahlund1, Hermann J Opgenoorth1, Mika Holmberg4, Caitriona M Jackman5, Thomas Cravens6, Cesar Bertucci7 and Michele Karen Dougherty8, (1)IRF Swedish Institute of Space Physics Uppsala, Uppsala, Sweden, (2)University of Iowa, Iowa City, IA, United States, (3)Univ Iowa, Iowa City, IA, United States, (4)Uppsala University, Uppsala, Sweden, (5)University College London, London, United Kingdom, (6)University of Kansas, Lawrence, KS, United States, (7)University of Buenos Aires, Buenos Aires, Argentina, (8)Imperial College London, Blackett Laboratory, London, United Kingdom
We present measurements from the Cassini Radio and Plasma Wave Science/Langmuir probe (RPWS/LP) instrument of the electron density in the ionosphere of Titan from the first ~100 flybys (2004-2014). After more than 10 years of measurements a good number of measurements exists from Titan’s ionosphere. This allows for statistical studies of the structure of Titan’s ionosphere. The electron density has been shown to vary significantly from one flyby to the next, as well as on longer time scales and here we discern some of the reasons for the observed ionospheric variability. Firstly, following the rise to the recent solar maximum we show how the ionospheric peak density, normalized to a common solar zenith angle, Nnorm clearly varies with the ~11-year solar cycle. Nnorm correlates well with the solar energy flux Fe and we find that Nnorm ∝ Fek, with k = 0.54 ± 0.18, which is close to the theoretical value of 0.5. Secondly, we present results that indicate that the ionospheric density in the topside ionosphere (altitude range 1200-2400 km) are generally significantly increased, roughly by a factor of 2, when Titan is located in the post-midnight sector of Saturn, i.e. at Saturn local times 00 - 03 h, compared to other local time sectors. We suggest that this increase could be caused by additional particle impact ionization from reconnection events in the Saturn tail.