Particle-in-cell simulations of the Cassini spacecraft’s interaction with Saturn’s ionosphere during the Grand Finale

Abstract

A surprising and unexpected phenomenon observed during Cassini’s Grand Finale was the spacecraft charging to positive potentials in Saturn’s ionosphere. Here, the ionospheric plasma was depleted of free electrons with negatively charged ions and dust accumulating up to over 95 % of the negative charge density. To further understand the spacecraft-plasma interaction, we perform a three dimensional Particle-In-Cell study of a model Cassini spacecraft immersed in plasma representative of Saturn’s ionosphere. The simulations reveal complex interaction features such as electron wings and a highly structured wake containing spacecraft-scale vortices. The results show how a large negative ion concentration combined with a large negative to positive ion mass ratio is able to drive the spacecraft to the observed positive potentials. Despite the high electron depletions, the electron properties are found as a significant controlling factor for the spacecraft potential together with the magnetic field orientation which induces a potential gradient directed across Cassini’s asymmetric body. This study reveals the global spacecraft interaction experienced by Cassini during the Grand Finale and how this is influenced by the unexpected negative ion and dust populations.

Implications from secondary emission from neutral impact on Cassini plasma and dust measurements

“…One should remember that the LP estimates, which are sampled from a small volume immediately surrounding a probe close to the spacecraft, may be different from the plasma parameter in an undisturbed ionosphere. In contrast to fast-flowing ions, neutrals, and dust, plasma electrons could readily be perturbed from their orbit to the LP instrument by spacecraft-plasma interactions (Odelstad et al 2017 ;Johansson et al 2020Johansson et al , 2021Zhang et al 2021 ). As the spacecraft is charged (in volts) much abo v e the electron energy (in eV), we cannot rule out that this could significantly affect our measurements.…”

Simulating Secondary Electron and Ion Emission from the Cassini Spacecraft in Saturn’s Ionosphere

“…Each species has mass and charge normalized to the proton scale with a real ion-to-electron mass ratio. The spacecraft is treated as a perfect conductor, and a detailed description of conductors and the numeric can be found in the previous work (Zhang et al 2021a).…”

“…We model Cassini at a representative altitude of 2500 km during Rev 292, as in the previous study (Zhang et al 2021a), but instead of including "dust" particles, we evaluate the hypothesis that SEE and SIE currents present a viable alternative to charged dust currents. To thus compare the effect of dust and secondary electron emissions accurately and independently, we use similar environment parameters as in our previous study where the dust was investigated, but replace the dust populations with secondary ion and electron emissions.…”

“…Two distinct interpretations of the LP data thus exist within the literature. Morooka et al (2019) first reported the apparent current discrepancies as arising from significant populations of charged dust, an interpretation that has formed the basis for sequential studies of Saturn's ionosphere (e.g., Wahlund et al 2018;Hadid et al 2019;Shebanits et al 2020;Zhang et al 2021a). Johansson et al (2022), however, recently suggested that less dust is present and that the LP current imbalance is caused by secondary electrons and ions, emitted due to impacting gas molecules.…”

“…The two contrasting interpretations of the LP data have the commonality that they both identify Cassini as having charged to positive floating potentials. Zhang et al (2021a) examined the role of charged dust in charging Cassini and showed that this could charge Cassini to positive potentials when the negatively charged ion/dust mass was significantly greater than that of the positive ions/dust and when the electrons constitute less than 10% of the total electron density. In this study, we evaluate the hypothesis that secondary electron emissions in Saturn's ionosphere might have induced a similar effect.…”