Planetary period oscillations in Saturn's magnetosphere: Phase relation of equatorial magnetic field oscillations and Saturn kilometric radiation modulation

Abstract

We examine the planetary‐period oscillations in Saturn's magnetic field observed by the Cassini spacecraft on 23 near‐equatorial periapsis passes in the inner magnetosphere spanning October 2004 to July 2006. Overall, we find that the phase of the magnetic oscillations is well organized by the long‐timescale modulation phase of Saturn kilometric radiation (SKR) determined over the same interval by Kurth et al. (2007), suggesting that the slow period variation of the latter relates to inner magnetosphere processes. The relative phases of the oscillations in the spherical polar r and ϕ magnetic field components imply the presence of a quasi‐uniform equatorial field rotating near the SKR period, while the sense of the θ component indicates that the perturbation field lines form loops with apices in the Northern Hemisphere. No consistent evidence is found for a sign reversal in any field component across the equatorial plane, within ±20° in latitude. The relative SKR phasing is such that the peak radio power occurs when the r and θ component maxima lie at ∼0200 LT ± 2 hours. However, a slow drift of the magnetic phase relative to the SKR phase is also discerned, amounting to ∼75° over the study interval. This drift lies within the envelope of scatter in the SKR phase determinations, suggesting that it represents the refinement of a common periodicity. A revised magnetic phase or longitude model is derived that should form an improved organizational system for oscillatory phenomena observed during this interval of the Cassini mission. The magnetic oscillations are also found to exhibit pass‐to‐pass phase “jitter” about the long‐term variation, of RMS amplitude ∼20°, with r and ϕ strongly correlated, but not θ. The relation with the solar wind‐modulated short‐timescale phase variations reported in SKR data by Zarka et al. (2007) remains to be investigated, though the latter are 5 times larger in magnitude.