Abstract
We examine magnetic field data from 10 apoapsis passes of the Cassini spacecraft during 2006 when the spacecraft explored the midnight and dawn sectors of Saturn's magnetotail to down‐tail distances of ∼65 RS (Saturn radius, RS, is 60,268 km). Oscillations in the radial component of the field near the ∼11 hour planetary period associated with north‐south motions of the current sheet are ubiquitous in these data. Here, we examine and model the phase of these oscillations throughout the interval, taking account of both local time and radial propagation effects, and show that the oscillations exhibit dual periodicities. Those observed at distances exceeding ∼3 RS north of the modeled average center of the current sheet are found to oscillate near the modulation period of the northern Saturn kilometric radiation (SKR) emissions, while those observed south of this location oscillate near the modulation period of the southern SKR emissions. The phasing in both cases is consistent with the sense of the associated rotating quasi‐uniform perturbation fields within the quasi‐dipolar “core” region. We determine the structure of the current sheet as a function of the modeled phases, the results implying that the form of the modulation varies significantly over the beat cycle of the two oscillations. When the two field oscillations are in phase, the current sheet oscillates north‐south with a peak‐to‐peak amplitude of ∼3 RS. When they are in antiphase, however, the thickness of the current sheet is also strongly modulated during the oscillation by factors of ∼2.
Highlights
[2] Despite the close axial symmetry of Saturn’s planetary magnetic field, oscillations in field and plasma parameters near the planetary rotation period have been found to be ubiquitous throughout Saturn’s magnetosphere
Auroral hiss and magnetic field modulations in the polar regions are found to have the same rotation period as the Saturn kilometric radiation (SKR) modulations in the corresponding hemisphere [Gurnett et al, 2009b; Andrews et al, 2010a; Southwood, 2011], with evidence recently being presented that the SKR modulations rotate around the auroral ovals in each hemisphere [Andrews et al, 2011; Lamy, 2011]
[4] Given the dual periodicities observed in the magnetic field oscillations, in the two high-latitude polar regions, it is of interest to examine their consequences for oscillations in Saturn’s magnetic tail, the lobes of which map into the two polar regions at the planet
Summary
[2] Despite the close axial symmetry of Saturn’s planetary magnetic field, oscillations in field and plasma parameters near the planetary rotation period have been found to be ubiquitous throughout Saturn’s magnetosphere. The first is the displacement of the polar region of open field lines shown on the right in Figures 1c and 1d, as previously discussed by Provan et al [2009b] The implication of these directions is that the auroral oval is displaced sunward at SKR maxima in corresponding hemispheres and tailward at SKR minima, consistent with the results in the study by Nichols et al [2008, 2010b]. Half a beat period later, the magnetic perturbations of the two systems will be in antiphase, producing oppositely directed field perturbations in the northern and southern tails that successively enhance and reduce the tail field simultaneously in both hemispheres, leading to modulation of the current and plasma sheet According to this picture, field enhancement in both hemispheres will correspond to SKR minima in both hemispheres, while field reduction in both hemispheres will correspond to SKR maxima. The following analyses constitute a search in the Cassini data for evidence of such behavior
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