Determining the Nominal Thickness and Variability of the Magnetodisc Current Sheet at Saturn

The thickness and variability of the Saturnian magnetodisc current sheet is investigated using the Cassini magnetometer data set. Cassini performed 66 fast, steep crossings of the equatorial current sheet where a clear signature in the magnetic field data allowed for a direct determination of its thickness and the offset of its center. The average, or nominal, current sheet half-thickness is 1.3 R-S, where R-S is the equatorial radius of Saturn, equal to 60,268 km. This is thinner than previously calculated, but both spatial and temporal dependencies are identified. The current sheet is thicker and more variable by a factor similar to 2 on the nightside compared to the dayside, ranging from 0.5-3 R-S. The current sheet is on average 50% thicker in the nightside quasi-dipolar region (<= 15 R-S) compared to the dayside. These results are consistent with the presence of a noon-midnight electric field at Saturn that produces a hotter plasma population on the nightside compared to the dayside. It is also shown that the current sheet becomes significantly thinner in the outer region of the nightside, while staying approximately constant with radial distance on the dayside, reflecting the dayside compression of the magnetosphere by the solar wind. Some of the variability is well characterized by the planetary period oscillations (PPOs). However, we also find evidence for non-PPO drivers of variability. Plain Language Summary A key discovery of the Cassini orbital mission at Saturn was that the small moon Enceladus was geologically active. Water from a subsurface ocean escapes as vapor through geysers in the moon's southern hemisphere. A significant portion becomes ionized and interacts with the magnetic environment that surrounds Saturn, called its magnetosphere. Due to the rapid rotation rate of the planet, this layer of charged particles sourced from Enceladus is stretched into a thin disc around Saturn, known as the magnetodisc current sheet. In this study, we explore the thickness of the current sheet, identifying its average structure and potential sources of variability. We find that the current sheet thickness depends on both the distance from Saturn and position in local time with respect to the Sun. We also find that some of the variability about its average state is somewhat described by the presence of a magnetic perturbation system that occurs close to the rotation period of Saturn. These results are essential for accurately modeling the magnetosphere of Saturn and understanding how the system responds to different drivers of variability. Key Points The median current sheet half-thickness is 1.3 Saturn radii, but spatial and temporal variability is identified e current sheet is 50% thicker on the nightside close to the planet but reduces more significantly with radial distance than the dayside The planetary period modulations organize some of the variability, but there is evidence of other temporal drivers