Investigation of the electron density close to the rings of Saturn and Prediction of the F-ring plasma characteristics

Abstract: Saturn has the second strongest magnetic field among the planets of the solar system, creating the second biggest magnetosphere, after Jupiter. The magnetosphere is similar to Earth's, as both of them are created by a dipole magnetic field, produced by a planetary dynamo. Despite the similarities, they have some major differences. First, the Kronian inner magnetosphere is being controlled by the fast rotation of the planet, while the Earth’s is being affected majorly by the solar wind. Second, a major Kronian magnetospheric plasma sources are the icy moons and the dusty rings, that are located within the inner magnetosphere, while Earth’s plasma sources are mainly the upper atmosphere and the solar wind. Since Cassini’s arrival at Saturn in 2004, more than ten years of data are collected, thus the magnetosphere and the plasma characteristics have been studied thoroughly. Persoon et al. [2013] made an electron density model, based on the electron density obtained by the upper hybrid resonance frequency (fUHR). The model is consistent with Enceladus as the electron plasma source –hence the denser plasma area is around the moon– and it describes the centrifugally drift of the plasma towards the magnetotail. The model is consistent with Cassini’s discovery that Enceladus expels water vapor and ice grains from the gazers located at its South pole, forming a plume. However, individual orbits data are not in agreement with the model, as the plasma density continues to increase toward the planet, which could be an indicator that there is an additional significant plasma source inside 4RS [Persoon et al., 2015]. Unfortunately, the fUHR is hard to be identified inside ~3RS where the magnetic field becomes stronger, and therefore, the electron plasma characteristics within that region are still unknown. This study is using the electron density data obtained by the Cassini's Langmuir Probe (LP). The LP is the ideal instrument to measure the cold plasma properties. Also, up to 2015, Cassini provided us with many orbits close to and inside 3RS, making a statistical analysis of the region possible. Moreover, its last orbits are planned to go further inside 3RS, where the dusty F and B rings are located. The goal of this study is to investigate the electron density on the plasma disk –especially inside 4RS– and possibly predict the electron density for the oncoming orbits. We focus on the equatorial plane (z < |0.05RS|) of the inner magnetosphere (r < 7RS), and we use data from July 2004 (SOI) until January 2016 (Orbit 231). We first confirm that the general characteristics of the electron density in the inner magnetosphere are similar to what was found by the statistics on the fUHR data. However, based on the number of the electron components and their temperature, we concluded that the estimation of the electron density in the LP measurements can be underestimated beyond ~5RS, where the electron temperature increases along with the distance from Saturn. Inside 5RS, the maximum mean value obtained by the LP data is similar to what it was obtained by the fUHR data, but the location of the maximum is closer to the planet. Also, the exponential slopes describing the density increase (inside the maximum) and decrease (outside maximum) are steeper than using the fUHR data. The most remarkable result of this study is a dawn-dusk asymmetry inside 5RS: the electron density in dusk seem to be higher by a factor of 0.25-3; the asymmetry was stronger closer to the planet, inside 3.5RS. On the other hand, no strong evidence for a day-night asymmetry, previously reported with the ion density data obtained by LP, were found. The electron density peak in the region between noon and dusk (Local Time 12-18) was found at ~3.18RS, which matches the orbital distance of Mimas. However, further analysis did not show direct impact on the creation of the secondary maximum.