Ionospheric plasma in Titan's tail: CAPS observations and interactions with Saturn's magnetosphere

Titan's nitrogen and methane upper atmosphere provides an interesting ionosphere consisting of positive ions, neutrals and, surprisingly, negative ions. A number of production and loss processes are at work in this complex environment, including plasma escape which contributes to atmospheric loss. Orbiting at 20 Saturn radii, Titan is usually surrounded by Saturn's magnetospheric environment which is variable between encounters; this then affects the plasma interaction with the ionosphere. Cassini's three tail flybys T9, T75 and T63, each at several Titan radii, provided unique measurements in the tail region. Remarkably, each of these encounters provided at least two distinct intervals where ionospheric plasma is seen well away from its production region. Measurements of electron density and mass-separated ion velocity allow estimates of the plasma escape rate from Titan. We show that the plasma loss from Titan's ionosphere is ~7 tonnes per day based on these encounters, and also summarise other recent observations of ionospheric plasma in Titan's tail and on negative ions at Titan.