Particle-In-Cell Simulations of the Cassini Spacecraft's Interaction with Saturn's Ionosphere during the Grand Finale

TL;DR

This study uses 3D Particle-In-Cell simulations to analyze Cassini's unexpected positive charging in Saturn's ionosphere, revealing the influence of negative ions, dust, and magnetic fields on spacecraft-plasma interactions.

Contribution

It provides the first detailed 3D simulation analysis of Cassini's spacecraft charging phenomena in Saturn's ionosphere, highlighting the roles of negative ions and magnetic field orientation.

Findings

Negative ion concentration drives positive spacecraft potentials.

Electron properties significantly influence spacecraft charging.

Magnetic field orientation affects potential gradients.

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.