Re-analysis of the Cassini RPWS/LP data in Titan’s ionosphere. Part II:
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 explain
the observed electron temperature in this dusty environment. To get new
clues, we re-analyzed the data taken in the ionosphere of Titan by the
Cassini Langmuir probe (LP), part of the Radio and Plasma Wave Science
(RPWS) package. A first paper (Chatain et al., n.d.) introduces the new
analysis method and discusses the detection of 4 different electron
populations. In this second paper, we present a statistical study of the
whole LP dataset below 1200 km and 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 globally constant density
(~500 cm), and is attributed to background thermalized
electrons. The two last populations increase in density with pressure,
solar illumination and extreme UV flux. The third population is observed
with varying densities at all altitudes (at least up to 1400 km) and
solar zenith angles except on the far nightside (SZA >
~140°), with a maximum density of 2700 cm. It is
therefore certainly related to photo-ionization and its subsequent
active ion chemistry. Finally, a fourth population detected only on the
dayside and below 1200 km reaching up to 2000 cm could be photo- or
thermo-emitted from dust grains.