Electron Density Depletions in Saturn's Inner Magnetosphere

Upper hybrid resonance bands observed with Cassini's Radio Plasma Wave Science (RPWS) instrument have been used to derive electron densities in Saturn's magnetosphere inside 10 RS. These bands occasionally show short-lived, abrupt decreases in frequency, which are indicative of localized depletions in the electron density. More frequently, the intensity of the upper hybrid band drops abruptly, close to the background noise level of the receiver. Although changes in the upper hybrid resonance amplitudes can be attributed to temperature variations in the plasma, these amplitude variations are most likely due to density depletions. Examples of these RPWS upper hybrid amplitude drop-outs are shown with coincident examples of plasma density depletions from the Cassini Plasma Spectrometer instrument for October 28, 2004. A survey of the RPWS density depletions and the distribution and frequency of occurrence as functions of radial distance, latitude and local time are presented for the period beginning with Cassini's arrival at Saturn on July 1, 2004 through May, 2006. The distribution of the density depletions with radial distance shows that the depletions rarely occur inside 5 RS and the occurrence frequency increases with increasing radial distance out to 10 RS. The largest density depletions are found near the equatorial plane.