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

摘要

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.