Dual periodicities in planetary-period magnetic field oscillations in Saturn's tail

摘要

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 exceedingRs 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 ofRs. When they are in antiphase, however, the thickness of the current sheet is also strongly modulated during the oscillation by factors of ～2.