Observations of photoelectrons with CAPS ELS at Titan

Photoelectrons are generated in Titan's ionosphere as a result of the strong HeII (30.4nm) solar line ionising N2 [1]. The presence of these photoelectrons is discernible as a discrete peak in the electron energy spectrum observed by the ELS (ELectron Spectrometer) part of the Cassini Plasma Spectrometer (CAPS). They are generally observed in the dayside ionosphere. [2] discussed observations of photoelectrons in Titan's distant tail during the T9 encounter. This study describes other photoelectron observations near Titan. On some encounters photoelectrons are observed at large distances, some of which are near the nightside and in the tail. The origin of these is discussed. In some cases the photoelectrons may have travelled to the observation sites by means of a magnetic connection to lower altitudes on the dayside ionosphere where they could have been produced. To investigate this possibility, T15 observations are compared with different model results ([3], [4], [5]). The prospect of local production is also considered. References: [1] Galand et al (2006), Electron temperature of Titan's sunlit ionosphere, GRL, 33, L21101, doi:10.1029/2006GL027488. [2] Coates, A. J. et al (2007), Ionospheric electrons in Titan's tail: Plasma structure during the Cassini T9 encounter, GRL, 34, L24S05, doi:10.1029/2007GL030919. [3] Sillanpaa, I., Hybrid Modelling of Titan's Interaction with the Magnetosphere of Saturn, Ph.D. dissertation, Yliopistopaino, 2008. [4] Ma, Y. J.,et al (2008), Real-time Global MHD Simulations of Cassini T32 Flyby: from Magnetosphere to Magnetosheath, JGR, submitted. [5] Cravens, T. E. et al (2008), Model-Data Comparisons for Titan's Nightside Ionosphere, Icarus, submitted.