Longitudinal and Local Time Asymmetries of Magnetospheric Turbulence in Saturn's Plasma Sheet

Abstract:

Based on earlier studies that have shown Saturn's middle magnetosphere to contain turbulent magnetic field fluctuations, we analyze the spatial and temporal variations of these magnetic fluctuations, their relative spectral power on kinetic scales, the spectral index of ion kinetic turbulence, and turbulent heating rate as a function of local time and Saturn longitude (SLS4-South). The region of study is Saturn's plasma sheet at a distance of 6-20 R_s. The data set consists of magnetic field data measured during 97 orbits (Revolutions) from the equatorial phases of Cassini covering nine years from 2004 to 2012. We find asymmetries in the magnetosphere with enhanced spectral power around noon and ~270° SLS4-S longitude (for 2004-2010 data), which leads to an increased turbulent heating rate in these regions. Analysis of single orbits reveals that the heating rate of 63% of all analyzed orbits is significantly (p<0.01) modulated with a sinusoid of order m=1. For these orbits the turbulent heating rate peaks at a mean longitude of 300°±90°. A significant modulation of the turbulent heating rate is predominantly observed during times when Cassini is located near pre-midnight. This is consistent with an increased heating rate during SKR maximum. The mean spectral index of ion kinetic turbulent spectra for distances of r=9-20 R_s is κ=2.6±0.3 and shows less asymmetry. In total, the turbulent heating amounts to 140-210 GW in Saturn's plasma sheet from 6-20 R_s.