During the first flyby of the BepiColombo composite spacecraft at Mercury in October 2021 ion spectrometers observed two intense spectral lines with energies between 10 and 70eV. The spectral lines persisted also at larger distances from Mercury and were ob- served again at lower intensity during cruise phase in March 2022 and at the second and third Mercury flyby as a single band. The ion composition indicates that water is the dominant gas source. The outgassing causes the composite spacecraft to charge up to a negative potential of up to -50V. The distribution and intensity of the lower energy signal depends on the intensity of low energy electron fluxes around the spacecraft which again depend on the magnetic field orientation. We interpret the observation as being caused by water outgassing from different source locations on the spacecraft being ion- ized in two different regions of the surrounding potential. The interpretation is confirmed by two dimensional particle-in-cell simulations.

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.

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.