Density Structures, Dynamics, and Seasonal and Solar Cycle Modulations of Saturn's Inner Plasma Disk

Abstract

We present statistical results from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe measurements recorded during the time interval from orbit 3 (1 February 2005) to 237 (29 June 2016). A new and improved data analysis method to obtain ion density from the Cassini LP measurements is used to study the asymmetries and modulations found in the inner plasma disk of Saturn, between 2.5 and 12 Saturn radii (1 RS=60,268 km). The structure of Saturn's plasma disk is mapped, and the plasma density peak, nmax, is shown to be located at ∼4.6 RS and not at the main neutral source region at 3.95 RS. The shift in the location of nmax is due to that the hot electron impact ionization rate peaks at ∼4.6 RS. Cassini RPWS plasma disk measurements show a solar cycle modulation. However, estimates of the change in ion density due to varying EUV flux is not large enough to describe the detected dependency, which implies that an additional mechanism, still unknown, is also affecting the plasma density in the studied region. We also present a dayside/nightside ion density asymmetry, with nightside densities up to a factor of 2 larger than on the dayside. The largest density difference is found in the radial region 4 to 5 RS. The dynamic variation in ion density increases toward Saturn, indicating an internal origin of the large density variability in the plasma disk rather than being caused by an external source origin in the outer magnetosphere.

Saturn’s near-equatorial ionospheric conductivities from in situ measurements

“…For the parallel conductivity, the total electron collision frequency is 14,30,44,45 provide the electron temperature and density as well as the total charge density of ions including heavier (amu > 4) components (published 1 ). The MAG 46,47 provides the total magnetic field strength (Fig.…”

Cassini‐Plasma Interaction Simulations Revealing the Cassini Ion Wake Characteristics: Implications for In‐Situ Data Analyses and Ion Temperature Estimates

“…This was not true later on in the mission when spacecraft calibration rolls started to be performed more often in the denser part of the plasma disk, thus producing more significant and a larger amounts of ion wake measurements, shown in Holmberg et al. (2017).…”

“…We also conclude that the disturbance in the plasma density measurements of the LP, due to the presence of the ion wake, can occasionally be seen already at around 70° angle between the corotation direction and the LP. However, a vast majority of the detectable ion wake encounters are located within 4–6 R S and in the later part of the Cassini mission and they have therefore only caused minor perturbations to the LP data analyses (Holmberg et al., 2017). Less than 15 wake signatures could be found at 7.6 R S , with most of them consisting of only one data point.…”

Enceladus and Titan: emerging worlds of the Solar System

“…Subsequently, transport, photoionization, and electron impact ionization of the neutral material creates a plasma disc at the same location. The disk has been suggested to vary with longitude [29], local time [35], season [112], and the solar cycle [36]. These asymmetries found in the plasma disc are still under investigation.…”