Cassini observations of ionospheric photoelectrons at large distances from Titan: Implications for Titan's exospheric environment and magnetic tail

Discrete peaks near 24.1 eV are seen in electron spectra measured in Titan's ionosphere by the ELS (Electron Spectrometer), part of the Cassini Plasma Spectrometer (CAPS), and are interpreted as photoelectrons. These photoelectrons are generated as a result of ionization of N-2 by the strong solar He II (30.4 nm) line. They are generally observed in the dayside ionosphere, because this is where neutral N-2 particles can be ionized by solar radiation. Coates et al. (2007) discussed initial observations of photoelectrons in Titan's distant tail during the T9 encounter. Here, we describe additional photoelectron peak observations at large distances from Titan, where they are unlikely to have originated because of low neutral N-2 densities. We consider the tail structures during the encounters T15, T17, and T40. We infer that the distant photoelectrons may have traveled to the observation sites by means of a magnetic connection from lower altitudes in the dayside ionosphere, where they could have been produced. This idea is supported by results of hybrid modeling. Thus photoelectrons may be used as tracers of magnetic field lines and further improve our understanding of Titan's complex plasma environment.