Abstract
[1] Saturn's magnetosphere is replete with magnetospheric periodicities; magnetic fields, plasma parameters, energetic particle fluxes, and radio emissions have all been observed to vary at a period close to that of Saturn's assumed sidereal rotation rate. In particular, periodicities in Saturn's magnetotail can be interpreted in terms of periodic vertical motion of Saturn's outer magnetospheric plasma sheet. The phase relationships between periodicities in different measurable quantities are a key piece of information in validating the various published models that attempt to relate periodicities in different quantities at different locations. It is important to empirically extract these phase relationships from the data in order to distinguish between these models, and to provide further data on which to base new conceptual models. In this paper a simple structural model of the flapping of Saturn's plasma sheet is developed and fitted to plasma densities in the outer magnetosphere, measured by the Cassini electron spectrometer. This model is used to establish the phase relationships between magnetic field periodicities in the cam region of the magnetosphere and the flapping of the plasma sheet. We find that the plasma sheet flaps in phase with Br and Bθ and in quadrature with the Bϕ component in the core/cam region. The plasma sheet phase also has a strong local time asymmetry. These results support some conceptual periodicity models but are in apparent contradiction with others, suggesting that future work is required to either modify the models or study additional phase relationships that are important for these models.
Highlights
[2] Despite the widely studied near axisymmetry of Saturn’s internal magnetic field, periodicities have been observed in Saturn’s magnetosphere since the epoch of the Pioneer 11 and Voyager flybys
Perhaps the most studied periodicities in Saturn’s magnetosphere are those observed in kilometric radio emissions emitted from the auroral regions, known as Saturn Kilometric Radiation (SKR)
[4] This phase model has been used to demonstrate that the southern SKR emissions have a distinct phase relationship with periodicities in Saturn’s magnetosphere in the location of the auroral oval [Nichols et al, 2008; Provan et al, 2009b], magnetic fields [e.g., Espinosa et al, 2003a; Southwood and Kivelson, 2007; Andrews et al, 2008; Provan et al, 2009a], energetic particles [e.g., Carbary and Krimigis, 1982; Carbary et al, 2007b], plasma [e.g., Gurnett et al, 2007; Arridge et al, 2008a; Khurana et al, 2009; Burch et al, 2008, 2009], energetic neutral atoms (ENAs) [Carbary et al, 2008], and the location of the magnetopause [e.g., Clarke et al, 2010]
Summary
[2] Despite the widely studied near axisymmetry of Saturn’s internal magnetic field, periodicities have been observed in Saturn’s magnetosphere since the epoch of the Pioneer 11 and Voyager flybys. [17] Since the Kronographic latitude of Cassini remains approximately constant near zero during the whole interval presented in Figure 2 and given the evidence that Cassini is periodically immersed in a thin current sheet, sometimes traversing the center plane of the sheet, one can conclude that Saturn’s plasma sheet is undergoing vertical oscillations that have some phase relationship with southern SKR and cam region fields [e.g., Andrews et al, 2008; Arridge et al, 2008b] and particles [e.g., Gurnett et al, 2007].
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