An Investigation of the Rotational Periodicities of Saturn's High-Latitude Density Boundary

Electron density measurements from the Cassini RPWS Langmuir Probe instrument have identified a region of plasma densities that are strongly periodic beyond a dipole L-shell of 15 in Saturn's magnetosphere. The higher electron densities in this region, which are typically more than two orders of magnitude higher than the low densities, are anti-correlated with the occurrence of intense auroral hiss emissions. Sharp density gradients define a plasma boundary that separates the region of higher plasma density from the region of low plasma density. During the highly-inclined orbits of late 2006, both the auroral hiss emissions and the high-latitude plasma densities exhibit a clear modulation near the 10.6-10.8 hour rotation period of Saturn. This study uses the newly-derived SLS4 longitude system to organize the boundary of the higher density region and determine the phase of the plasma rotation. Over seven consecutive orbits in late 2006, the location of the higher density region is found to be well-organized by the SLS4 longitude system within each orbit, but the phase difference between successive orbits can be highly variable. One possible explanation for the phase shifts could be the occurrence of substorm-like events observed in Saturn's magnetotail, which have been shown to be related to low-frequency extensions of the SKR emissions observed just prior to the phase shifts. We also use thirty consecutive orbits in 2008, when Cassini crossed the high-latitude density boundary inside 5 RS, to determine asymmetries between the northern and southern boundaries and to examine phase differences over the seven month interval.