A new upper limit to the field-aligned potential near Titan

Neutral particles dominate regions of the Saturn magnetosphere and locations near several of Saturn's moons. Sunlight ionizes neutrals, producing photoelectrons with characteristic energy spectra. The Cassini plasma spectrometer electron spectrometer has detected photoelectrons throughout these regions, where photoelectrons may be used as tracers of magnetic field morphology. They also enhance plasma escape by setting up an ambipolar electric field, since the relatively energetic electrons move easily along the magnetic field. A similar mechanism is seen in the Earth's polar wind and at Mars and Venus. Here we present a new analysis of Titan photoelectron data, comparing spectra measured in the sunlit ionosphere at ~1.4 Titan radii (R_T) and at up to 6.8 R_T away. This results in an upper limit on the potential of 2.95 V along magnetic field lines associated with Titan at up to 6.8 R_T, which is comparable to some similar estimates for photoelectrons seen in Earth's magnetosphere.