Electron Density Distributions Inside and Outside the Enceladus L-Shell

Electron densities derived from measurements of the upper hybrid emissions by Cassini's Radio and Plasma Wave Science (RPWS) instrument have been previously used to map the distribution of plasma in Saturn's inner magnetosphere. With the exception of orbit insertion on June 30-July 1, 2004, few density measurements were available inside the orbit of Enceladus before 2008 and early density models primarily focused on the plasma distribution beyond 5 Saturnian radii (R_S). High-inclination orbits in 2008 have provided RPWS density measurements down to 2.6 R_S and are used here to map the plasma distribution in the innermost part of the plasma torus where the densities have been shown to be highly variable. RPWS electron densities are used whenever the upper hybrid resonance emission band could be identified from October 28, 2004 through November 7, 2011. This time frame includes high- and mid-inclination orbits as well as equatorial orbits. The data are fitted to a simple exponential scale height model for a single-species plasma for latitudes ≤8°. The model yields a plasma scale height which varies with increasing L-shell as L^1.5. The equatorial electron density measurements are fitted to a two-part radial dependence model which yields a peak density value of 72 cm^-3 at 4.6 R_S and shows the plasma increasing sharply as R^4.1 inside 4 R_S and decreasing as R^-4.8 outside 5 R_S. The modeled equatorial density measurements and the plasma scale height are both used in the exponential scale height model to develop an electron density contour map for 2.6 ≤ L ≤ 10.