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

Abstract

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 ∼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 SummaryA 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.

Saturn’s nightside ring current during Cassini’s Grand Finale

“…Carbary (2019) determined averaged empirical models of the thickness of the ring current from Cassini's magnetic field data. We find that the Staniland et al (2020) ring current agrees with the more intense ring current region determined by Carbary (2019). The maximum thickness of the ring current detected here on the proximal Revs is slightly smaller than the outer boundary region presented by Carbary (2019).…”

“…This resulted in a modelled ring current with a mean value of <D> = 3.21 ± 0.62 R s . This is identical within errors to our mean thickness reported in Figure 13, meaning that the discrepancy between the thickness of the ring current found in this study compared to that reported by Staniland et al (2020), cannot be explained by the initial assumptions made in our ring current modelling.…”

Saturn's Nightside Ring Current During Cassini's Grand Finale

“…Our results are shown as white lines and colored circles with white edges. The 47 thickness measurements from Staniland et al (2020), presented in Figure 13 as black diamonds, are shown in Figure 2b as black lines, also symmetrically extended to either side of the center of the current sheet. Studying Figure 2b we can see that both sets of results show some features of similarity in the decrease of ring current thickness with radial distance.…”

“…Studying Figure 2b we can see that both sets of results show some features of similarity in the decrease of ring current thickness with radial distance. However, the ring current thicknesses from Staniland et al (2020)…”

“…Studying Figure 2b we can see that both sets of results show some features of similarity in the decrease of ring current thickness with radial distance. However, the ring current thicknesses from Staniland et al (2020) In this study, we modeled the ring current signature observed over a wide radial distance range 4.25 < r < 14 R s , while Staniland et al (2020) determined the current sheet crossings from abrupt deflections in the magnetic field. These deflections may correspond to the width of the most intense current carrying region, and may not capture the weaker current carrying region outside this which still leaves a detectable signature in the magnetic field.…”