Re-analysis of the Cassini RPWS/LP data in Titan’s ionosphere: electron density and temperature of cold electron populations

The Cassini Langmuir Probe (LP) data acquired in the ionosphere of Titan are re-analysed to finely study the electron behaviour in the birthplace of Titan’s aerosols (900-1200 km) [Waite et al 2007].

The detailed analysis of the complete Cassini LP dataset below 1200 km (57 flybys) shows the systematic detection of 2 to 4 electron populations (further named P₁, P₂, P₃, P₄), with reproducible characteristics depending on altitude and solar illumination. Populations P₁ and P₂ are always present, contrarily to P₃ and P₄. Due to their low density and low potential, P₁ electrons are suspected to be photo-electrons [Wahlund et al 2009] or secondary electrons emitted on the probe stick.

The electron populations densities and temperatures are deduced from the Orbital Motion Limited theory and the Sheath Limited theory [Wahlund et al 2009, Whipple 1965]. We observe that electron temperatures do not vary much with altitude between 1200 and 950 km, except for P₄. Statistical correlations with other quantities measured by Cassini are investigated. In particular, we observe that P₃ and P₄ densities are correlated with the extreme UV flux.

From our results we suggest possible origins for the three populations P₂, P₃ and P₄, coming from the plasma surrounding the probe:

-P₂ is detected in all cases, at rather low density (~500 cm^-3) and temperature (~0.04 eV). These are possibly induced by particle precipitation.

-P₃ electrons are denser with stronger solar illumination and higher pressure (up to 3000 cm^-3). Therefore, they are likely to be related to photo-ionization. They are hotter than P₂ electrons (~0.06-0.07 eV).

-P₄ electrons are only observed on dayside and below 1200 km, in the place where heavy negative ions and aerosols are present. They are then plausibly linked to dusty plasma effects. We suggest two possible formation processes: (1) the photo-emission of electrons from grains could be triggered by photons of a few eV due to the negative charge born by the aerosols [Shebanits et al 2016; Tigrine et al 2018] ; (2) electrons could also be thermo-emitted from the grains, as a result of their heating by diverse processes such as heterogeneous chemistry, sticking of electrons or recombination of radicals [Woodard et al 2020].