Johns Hopkins University Applied Physics Laboratory
The Composition of Titan's Ionospheric Outflow: Mass Resolved INMS Observations of Exospheric Ions From the T40 Flyby
We report on Cassini Ion and Neutral Mass Spectrometer (INMS) observations above Titan's exobase at altitudes of 2225 km to 3034 km. During the January 2008 T40 Cassini flyby of Titan the Ion and Neutral Mass Spectrometer (INMS) serendipitously observed nonthermal, exospheric ions. This is the first mass-resolved observation of Titan's exospheric ions. We observe significant densities of CH5+, HCNH+ and C2H5+ that require ion-molecule reactions to be produced in the quantities observed. During the observation the Cassini spacecraft was executing a roll causing the INMS to observe several locations in velocity space. Ions are observed with velocities of about 1 km/s. The measured composition and ion velocity suggest that the observed ions must have been created deep inside Titan's ionosphere (below the exobase) and then transported to the detection altitude. Plasma motion from below Titan's exobase to large distances can be driven by a combination of thermal pressure and magnetic forces. The observed outward flows may link the main ionosphere with the more distant wake and provide a source of hydrocarbon ions in the Saturnian system.