Polarization and phase of planetary‐period magnetic field oscillations on high‐latitude field lines in Saturn's magnetosphere

Abstract

We examine magnetic field data obtained by the Cassini spacecraft on a sequence of high‐latitude orbits in Saturn's magnetosphere spanning October 2006 to May 2007 to determine whether planetary‐period oscillations are present on polar open field lines, such as have been found previously in near‐equatorial magnetic field data. Such oscillations are found generally to be present with amplitudes ∼0.5–1 nT, somewhat smaller than the few nT amplitudes typical of the quasi‐dipolar equatorial region. The polarization characteristics in the northern and southern polar regions are determined and found to differ significantly from those in the equatorial region. The phases of the oscillations in the northern and southern hemispheres are also determined relative to the equatorial oscillations, and hence relative to each other, requiring extension of the equatorial oscillation phase model to the end of 2007, spanning the interval of high‐latitude orbits. The results show that the overall pattern of field oscillations is not consistent with a rotating external current system that mimics a rotating transverse dipole in the outer regions. Rather, we suggest that the overall field perturbations are associated with a rotating partial ring current and its field‐aligned closure currents, the latter favoring the southern ionosphere during the southern summer conditions examined. A physical picture is presented that links together observed planetary‐period modulations in the middle and outer magnetospheric field, plasma, and radio emissions that may be subject to further test and makes predictions as to how these phenomena will evolve during future Saturn equinox and northern summer conditions.

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

“…The polarization characteristics of the purely northern and purely southern PPOs are described in Table 1, which also presents, γ i S,N , the difference in phase between the r component and component i, where i = r, θ, and φ, such that by definition γ r S ≡0°and γ r N ≡0°. For further explanation of the polarization of the northern and southern oscillations, see Provan et al [2009a].…”

“…The first PPO transition time demarked the start of interval E2 in the old nomenclature. Before this time the periods and the phases were believed to change slowly over time as presented in Andrews et al [2008] and Provan et al [2009a]. We here define this first PPO transition as boundary 1, with interval 1 spanning the time period just prior to the 2009 equinox until boundary 1.…”

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

“…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.…”

Periodic motion of the magnetodisk as a cause of quasi-periodic variations in the Kronian magnetosphere

“…It turned out that many magnetospheric phenomena can be organized around the SKR periodicity. The magnetic field shows similar periodicities (Espinosa and Dougherty, 2000;Andrews et al, 2008, Provan et al, 2009, and southern-northern duality was even discovered in the magnetic field variations , Provan et al 2012. Burch et al (2009) have shown that the heavy (water group) ion population of the Kronian plasma exhibit a cam-like feature (the plasma cam) in a longitude system fixed to SKR.…”

“…Another method to find an approximately known periodicity is to apply a band-pass filter to the time series. Andrews et al (2008Andrews et al ( , 2012 and Provan et al (2009Provan et al ( , 2012 used this method. Although the periodicity and its variation is very clear in the filtered data, some information is inevitably lost in the process.…”