Ion transport in Titan's upper atmosphere

Based on a combined Cassini data set including Ion Neutral Mass Spectrometer, Radio Plasma Wave Science, and Magnetometer measurements made during nine close encounters of the Cassini spacecraft with Titan, we investigate the electron (or total ion) distribution in the upper ionosphere of the satellite between 1250 and 1600 km. A comparison of the measured electron distribution with that in diffusive equilibrium suggests global ion escape from Titan with a total ion loss rate of ∼(1.7 ± 0.4) × 10 ²⁵ s^-1. Significant diurnal variation in ion transport is implied by the data, characterized by ion outflow at the dayside and ion inflow at the nightside, especially below ∼1400 km. This is interpreted as a result of day-to-night ion transport, with a horizontal transport rate estimated to be ∼(1.4 ± 0.5) × 10²⁴ s^-1. Such an ion flow is likely to be an important source for Titan's nightside ionosphere, as proposed in Cui et al. [2009a].