Titan’s ionosphere hosts a globally distributed non-trivial dusty ion-ion plasma, providing an environment for studies of dusty ionospheres that is in many aspects unique in our solar system. Thanks to the Cassini mission, Titan’s ionosphere also features one of the largest dusty plasma datasets from 126 flybys of the moon over 13 years, from 2004 to 2017. Recent studies have shown that negatively charged dust dramatically alters the electric properties of plasmas, in particular planetary ionospheres. Utilizing the full plasma content of the moon’s ionosphere (electrons, positive ions and negative ions/dust grains), we derive the electric conductivities and define the conductive dynamo region. Our results show that using the full plasma content increases the Pedersen conductivities at ~1300 km altitude by 20% compared to the earlier estimates without charged dust, while the Hall conductivities indicate a reverse Hall effect at ~900 km altitude (closest approach) and below. The dayside conductivities are shown to be factor ~7-9 larger than on the nightside, owing to higher dayside plasma densities.

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.

Cassini plasma wave and charged particle observations are combined with magnetometer measurements to investigate Titan’s induced magnetosphere. Electric field emissions close to Titan are identified as upper hybrid resonance emissions, which provide a density estimate of Titan’s cold plasma. These observations have been combined with electron spectrometer measurements to build an integrated map of electron density in Titan’s near environment using observations from TA to T82 flybys, ie which includes flybys from the Cassini prime, equinox and part of the soltice mission. We identify a dense ionospheric region and an extended plasma wake with values ranging between 10-2 and 103 cm-3. Upstream of the induced magnetosphere, the presence of pickup ions in the positive hemisphere of the kronian plasma convective electric field are detected. The mass of the observed pickup corresponds to methane group ions, N2+ and HCNH+ ions as well as Titan’s protons and molecular hydrogen ions. These ions are progressively accelerated by the kronian background electric field and we estimate its intensity by reconstructing the energization of this population. We find values on the order of 0.7 mV/m , consistent with an average estimate of 0.61 mV/m deduced from ∼ |VxB| computation.

Current models of Titan’s ionosphere have difficulties in explaining the observed electron density and/or temperature. In order to get new insights, 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) instrument. This is the first of two papers that present the new analysis method (current paper) and statistics on the whole dataset. We suggest that between 2 and 4 electron populations are necessary to fit the data. Each population is defined by a potential, an electron density and an electron temperature and is easily visualized by a dinstinct peak in the second derivative of the electron current, which is physically related to the electron energy distribution function (Druyvesteyn method). The detected populations vary with solar illumination and altitude. We suggest that the 4 electron populations are due to photo-ionization, magnetospheric particles, dusty plasma and electron emission from the probe boom, respectively.