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|Title:||Planetary period oscillations in Saturn's magnetosphere: Coalescence and reversal of northern and southern periods in late northern spring|
Cowley, S. W. H.
Bunce, E. J.
Hunt, G. J.
Dougherty, M. K.
|Publisher:||American Geophysical Union (AGU), Wiley|
|Citation:||Journal of Geophysical Research: Space Physics, 2016 121|
|Abstract:||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 of2015, 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 thesouthern 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.|
|Rights:||©2016. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.|
|Description:||Calibrated magnetic field and radio data from the Cassini mission are available from the NASA Planetary Data System at the Jet Propulsion Laboratory (https://pds.jpl.nasa.gov/). Daily sunspot numbers in Figure 11 were obtained from the OMNIWEB Space Physics Data Facility at Goddard Space Flight Center. This work benefitted from discussions held during meetings of the International Space Science Institute team on “Rotational phenomena in Saturn's magnetosphere.”|
|Appears in Collections:||Published Articles, Dept. of Physics and Astronomy|
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