Abstract
AbstractWe investigate planetary period oscillations (PPOs) in Saturn's magnetosphere using Cassini magnetic field and Saturn kilometric radiation (SKR) data over the interval from late 2012 to the end of 2015, beginning ~3 years after vernal equinox and ending ~1.5 years before northern solstice. Previous studies have shown that the northern and southern PPO periods converged across equinox from southern summer values ~10.8 h for the southern system and ~10.6 h for the northern system and near coalesced ~1 year after equinox, before separating again with the southern period ~10.69 h remaining longer than the northern ~10.64 h. We show that these conditions ended in mid‐2013 when the two periods coalesced at ~10.66 h and remained so until mid‐2014, increasing together to longer periods ~10.70 h. During coalescence the two systems were locked near magnetic antiphase with SKR modulations in phase, a condition in which the effects of the generating rotating twin vortex flows in the two ionospheres reinforce each other via hemisphere‐to‐hemisphere coupling. The magnetic‐SKR relative phasing indicates the dominance of postdawn SKR sources in both hemispheres, as was generally the case during the study interval. In mid‐2014 the two periods separated again, the northern increasing to ~10.78 h by the end of 2015, similar to the southern period during southern summer, while the southern period remained fixed near ~10.70 h, well above the northern period during southern summer. Despite this difference, this behavior resulted in the first enduring reversal of the two periods, northern longer than southern, during the Cassini era.
Highlights
A unique feature of Saturn’s magnetosphere is the ubiquitous modulations observed throughout the system near the ~10.6 h planetary rotation period, despite the close axisymmetry of the internal planetary magnetic field [Burton et al, 2010]
We investigate planetary period oscillations (PPOs) in Saturn’s magnetosphere using Cassini magnetic field and Saturn kilometric radiation (SKR) data over the interval from late 2012 to the end of 2015, beginning ~3 years after vernal equinox and ending ~1.5 years before northern solstice
Periodograms for the right hand (RH) and left hand (LH) emissions are shown in Figures 7a and 7c, respectively, which are repeated in Figures 7b and 7d with the corresponding SKR and magnetic field periods being overplotted as orange and white dotted lines, respectively
Summary
A unique feature of Saturn’s magnetosphere is the ubiquitous modulations observed throughout the system near the ~10.6 h planetary rotation period, despite the close axisymmetry of the internal planetary magnetic field [Burton et al, 2010]. Corresponding analyses of SKR data over this interval have shown a good measure of agreement [Provan et al, 2014; Fischer et al, 2014; Cowley and Provan, 2015] Overall, these SKR and magnetic field results indicate that while the two PPO periods came close to coalescence in the immediate postequinox interval in 2010 and may even have undergone short intervals of weak period reversal as suggested by both polarization-separated SKR data [Lamy, 2011] and magnetic field data [Cowley and Provan, 2015], a clear reversal to well-separated periods similar to that suggested by the Ulysses data following the previous spring equinox did not take place during the interval to at least ~4 years following vernal equinox in mid-2009. We show that following the ~3 year postequinox interval between mid-2010 and mid-2013 in which the periods were closely spaced but with the southern period remaining longer than the northern, as outlined above, the two periods near coalesced at an intermediate value over a ~1 year interval until mid-2014 and enduringly reversed, northern longer than southern, for the first time during the Cassini era
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