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

Abstract

[1] 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 8 magnetic field components imply the presence of a quasi-uniform equatorial field rotating near the SKR period, while the sense of the q 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 q 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 8 strongly correlated, but not q. 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.

Field‐aligned currents in Saturn's northern nightside magnetosphere: Evidence for interhemispheric current flow associated with planetary period oscillations

“…Each of these two systems then rotates at similar but separate periods close to the planetary rotation period, that have been found, initially through tracking related modulated radio wave emissions, to vary slowly with Saturn's seasons [Galopeau and Lecacheux, 2000;Kurth et al, 2008;Gurnett et al, 2009aGurnett et al, , 2009bGurnett et al, , 2011Lamy, 2011]. Essentially the same rotation periods have been derived by following the phase of the resulting magnetic field oscillations observed during periapsis passes of the Cassini spacecraft [Andrews et al, 2008[Andrews et al, , 2010bProvan et al, 2014]. During the interval of interest here spanning 2008, occurring somewhat before Saturn equinox in mid-August 2009, the northern and southern periods were well separated at~10.60 and 10.82 h, respectively.…”

“…During the interval of interest here spanning 2008, occurring somewhat before Saturn equinox in mid-August 2009, the northern and southern periods were well separated at~10.60 and 10.82 h, respectively. At a given point in the equatorial region inside the main field-aligned current sheets the azimuthal field of each system thus oscillates in lagging quadrature with the radial component, this being the signature of a quasi-uniform perturbation field in the near-equatorial region, while outside the current sheets the sense of the azimuthal field is reversed so that it oscillates in leading quadrature with the radial, this being the signature of a quasi-dipolar field [Espinosa and Dougherty, 2000;Espinosa et al, 2003;Cowley et al, 2006;Southwood and Kivelson, 2007;Andrews et al, 2008Andrews et al, , 2010aProvan et al, 2009]. The colatitudinal component then oscillates in antiphase with the radial in the northern system and in-phase with the radial in the southern, as is evident in Figures 1e and 1h.…”

Planetary period oscillations in Saturn's magnetosphere: Examining the relationship between abrupt changes in behavior and solar wind‐induced magnetospheric compressions and expansions

“…Below we summarize the form of the PPO magnetic systems previously reported in other publications [see, e.g., Andrews et al, 2008Andrews et al, , 2010aAndrews et al, , 2010bAndrews et al, , 2012Provan et al, 2011Provan et al, , 2013. There are two magnetic perturbation systems which take the form of perturbation field loops, one closing over the northern (N) pole and the other over the southern (S) pole.…”

“…Despite this, however, oscillations with a period near thẽ 11 h planetary period are observed throughout Saturn's magnetosphere. Such modulations are observed in the magnetic field, Saturnian kilometric radiation (SKR), plasma parameters, energetic particle fluxes, and associated neutral atom emissions, as well as auroral ultraviolet, infrared, and radio emissions [e.g., Warwick et al, 1981Warwick et al, , 1982Desch and Kaiser, 1981;Gurnett et al, 1981Gurnett et al, , 2007Gurnett et al, , 2010aSandel and Broadfoot, 1981;Sandel et al, 1982;Carbary and Krimigis, 1982;Espinosa and Dougherty, 2000;Krupp et al, 2005;Cowley et al, 2006;Kurth et al, 2007;Southwood and Kivelson, 2007;Carbary et al, 2007Carbary et al, , 2008aCarbary et al, , 2008bZarka et al, 2007;Andrews et al, 2008;Nichols et al, 2008Nichols et al, , 2010aNichols et al, , 2010bBurch et al, 2009;Provan et al, 2009aProvan et al, , 2009bClarke et al, 2006;Badman et al, 2012;Lamy et al, 2013]. Those oscillations were first observed in Voyager measurements of the SKR emissions, around the time of Saturn's northern hemisphere spring equinox [Kaiser et al, 1980].…”

“…Andrews et al [2008] have described how ψ i is determined for each of the three components of the magnetic field on an orbit-by-orbit basis by a cross-correlation analysis between the observed oscillations and an oscillation at a guide period selected to be close to the period of the oscillations. The ψ i phases are then plotted versus time, relative to a suitable guide phase, in the S and N format.…”