Density structures, dynamics, and seasonal and solar cycle modulations of Saturn's inner plasma disk†

We present statistical results from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe (LP) recorded during the time interval from orbit 3 (2005-02-01) to 237 (2016-06-29). 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 densities 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 towards 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.