The large-scale structure of the Saturnian magnetosphere

Abstract

The Cassini mission offered a unique opportunity to study the global characteristics of Saturn and its magnetosphere due to its significant spatial and temporal coverage. The mission that spanned almost half a Saturnian year covered elements of both seasons and two solar cycles. Now it is complete we can better understand how the magnetosphere of Saturn evolves over time and can identify what the key drivers are that shape its large-scale structure.

A global stress state of the Saturnian magnetosphere is determined to identify when the system is compressed, expanded, or near its ground state. The magnetic field is found to exhibit an underlying local time asymmetry, where the field is more compressed at dusk and expanded at dawn. The system remains close to its ground state throughout the Cassini era, but examples of significant perturbations to the system are identified, revealing a highly variable global structure.

The thickness of the magnetodisc current sheet that is populated by plasma sourced from the moon Enceladus is determined using fast, steep crossings of the current layer. A local time and radial structure are found that reflect the different particle motions and field responses due to magnetopause confinement. These results are essential for developing models of the Saturnian system and highlight the importance of incorporating local time variability when analysing data.

The lack of an observed cushion region in the outer magnetosphere of Saturn has been a mystery since this region exists at Jupiter and the two share similar features. Using the entire Cassini orbital dataset, we find that this phenomenon does occur but that it is rare, finding only a handful of examples. The evidence of a cushion forming at dusk at Saturn rather than dawn, as it is at Jupiter, reveals significant differences in configuration and dynamics between these two systems.