A Scale Height Model of Saturn's Plasma Disk

In the year since Cassini was placed in orbit around Saturn on July 1, 2004, the spacecraft has passed through Saturn's inner magnetosphere thirteen times over latitudes ranging from +14 degrees to -21 degrees. Electron densities are derived from measurements of the upper hybrid resonance frequency by the Radio and Plasma Wave Science (RPWS) instrument for these thirteen passes. The objective of this study is to develop a model of the plasma density distribution that is valid within 20 degrees of the magnetic equator. We use a centrifugal scale height model to describe the plasma distribution in the region of the inner magnetosphere where the plasma exhibits a nearly source-free outward radial expansion, i.e., over the range 5 < L < 8 where L is the equatorial radius of the magnetic field line in Saturn radii. We show that the best fit to the electron density varies as ne = nO exp[-(1/3)(L2/H2)(1-cos6 λ)] where λ is the latitude and H is the scale height. The scale height is found to vary as L1.94, from H = 0.7 at L = 5, to H = 1.7 at L = 8. The equatorial plasma density varies as nO = 3.8 × 104 (1/R)3.94.