Abstract
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 per cent 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.
Highlights
Spacecraft immersed within a plasma will become electrically charged due to currents of incident electron and ion species (Garrett 1981; Whipple 1981)
When the net current induced by these species are not zero, net charge is accumulated and there is a potential difference between the spacecraft and the surrounding plasma
Since the plasma species have different charges, each current is either decreased or increased by the change of the spacecraft potential, which continues until an equilibrium potential is reached, where the net current of all plasma species sum up to zero
Summary
Spacecraft immersed within a plasma will become electrically charged due to currents of incident electron and ion species (Garrett 1981; Whipple 1981). The presence of negative ions and dust grains in Saturn’s ionosphere is evident from the Langmuir Probe (LP) measurements (Morooka et al 2019), which showed significant concentrations up to over 95 per cent of the negative charge density, at altitudes of 3200 km down to the closest measurement at ≈1600 km. These appear an intrinsic part of the giant planet’s ionosphere and distinct from electrons depletions associated with Saturn’s main rings
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
