Re-Analysis of the Cassini RPWS/LP Data in Titan's Ionosphere: 2. Statistics on 57 Flybys

Abstract

The ionosphere of Titan hosts a complex ion chemistry leading to the formation of organic dust below 1200 km. Current models cannot fully explain the observed electron temperature in this dusty environment. To achieve new insight, we have re-analyzed the data taken in the ionosphere of Titan by the Cassini Langmuir probe (LP), part of the Radio and Plasma Wave Science package. A first paper (Chatain et al., 2021) introduces the new analysis method and discusses the identification of 4 electron populations produced by different ionization mechanisms. In this second paper, we present a statistical study of the whole LP dataset below 1200 km which gives clues on the origin of the 4 populations. One small population is attributed to photo- or secondary electrons emitted from the surface of the probe boom. A second population is systematically observed, at a constant density (~500 cm-3), and is attributed to background thermalized electrons from the ionization process of precipitating particles fom the surrounding magnetosphere. The two last populations increase in density with pressure, solar illumination and EUV flux. The third population is observed with varying densities at all altitudes and solar zenith angles except on the far nightside (SZA > ~140{\deg}), with a maximum density of 2700 cm-3. It is therefore certainly related to the photo-ionization of the atmospheric molecules. Finally, a fourth population detected only on the dayside and below 1200 km reaching up to 2000 cm-3 could be photo- or thermo-emitted from dust grains.