Local Time Asymmetry of the Electron Density Distribution in Saturn's Inner Magnetosphere

Equatorial electron densities derived from measurements of the upper hybrid emissions by Cassini's Radio and Plasma Wave Science (RPWS) instrument and equatorial electron densities derived from the RPWS Langmuir Probe voltage sweeps are used here to study the large variability in the density distribution of electrons inside 5 R_S in Saturn's inner magnetosphere. Equatorial densities are defined to be all density measurements within 0.05 R_S of Saturn's magnetic equatorial plane. The measurements are taken over a seven-year period from October 28, 2004 through November 7, 2011. This time frame includes high- and mid-inclination orbits as well as equatorial orbits. Because the highly variable density distribution of electrons inside 5 R_S is due, in part, to strong radial and longitudinal dependencies, the electron densities have been normalized to remove these dependencies. But even after normalizing the densities, there still remain large density fluctuations inside and outside the orbit of Enceladus, the primary plasma source in Saturn's magnetosphere. We have investigated the local time variability of the normalized electron density distribution inside 5 R_S and have found that there is a strong density depletion for the 4-hour time interval centered approximately at dawn. There is no discernible electron density variation with local time at dusk. The highest electron densities are observed in a 6-hour time interval centered on 2300 LT.