On the ionospheric structure of Titan

In this study we present data from 17 Titan flybys showing that solar photons are the main ionisation source of Titan's dayside atmosphere. This is the first comprehensive study of Solar Zenith Angle (SZA) dependence of the electron number density and electron temperature at the ionospheric peak. The results show on average four times more plasma on the dayside compared to the nightside, with typical dayside electron densities of around 2500-3500 cm(-3) and corresponding nightside densities of around 400-1000 cm(-3). We identify a broad transition region between SZA 50 degrees and 100 degrees, where the ionosphere of Titan changes from being entirely sunlit to being in the shadow of the moon. For SZA <50 degrees the ionisation peak altitude increases with increasing SZA, whereas the transition region and the nightside show more scattered ionospheric peak altitudes. Typical electron temperatures at the ionospheric peak are 0.03-0.06 eV (approximate to 350-700 K) for both dayside and nightside. (C) 2009 Elsevier Ltd. All rights reserved.