Planetary Period Oscillations in Saturn's Magnetosphere: Cassini Magnetic Field Observations Over the Northern Summer Solstice Interval

We determine properties of Saturn's planetary period oscillations from Cassini magnetic measurements over the ~2‐year interval from September 2015 to end of mission in September 2017, spanning Saturn northern summer solstice in May 2017. Phases of the northern system oscillations are derived over the whole interval, while those of the southern system are not discerned in initial equatorial data due to too low amplitude relative to the northern, but are determined once southern polar data become available from inclined orbits beginning May 2016. Planetary period oscillation periods are shown to be almost constant over these intervals at ~10.79 hr for the northern system and ~10.68 hr for the southern, essentially unchanged from values previously determined after the periods reversed in 2014. High cadence phase and amplitude data obtained from the short‐period Cassini orbits during the mission's last 10 months newly reveal the presence of dual modulated oscillations varying at the beat period of the two systems (~42 days) on nightside polar field lines in the vicinity (likely either side) of the open‐closed field boundary. The modulations differ from those observed previously in the equatorial region, indicative of a reversal in sign of the radial component oscillations, but not of the colatitudinal component oscillations. Brief discussion is given of a possible theoretical scenario. While weak equatorial beat modulations indicate a north/south amplitude ratio >5 early in the study interval, polar and equatorial region modulations suggest a ratio ~1.4 during the later interval, indicating a significant recovery of the southern system.