Abstract
AbstractWe compare the properties of planetary period oscillations observed in Saturn kilometric radiation (SKR) and magnetospheric magnetic field data from Saturn equinox in August 2009 to July 2013. As shown previously, the southern and northern oscillation periods converged across equinox from ~10.8 h and ~10.6 h, respectively, during southern summer, to closely common values ~10.7 h approximately 1 year after equinox. Near coalescence is judged to have occurred approximately 3 months earlier in the SKR data, centered in late June 2010, than in the magnetic data, in late September, though SKR periods were particularly difficult to determine during this interval due to less clearly modulated emissions. Both data sets agree, however, that by early November 2010 the two periods had separated again but remained closely spaced with a difference in period of ~3 min about a mean of ~10.67 h, with the southern period remaining longer than the northern. Thus, no enduring reversal of the northern and southern periods took place following near coalescence in mid‐2010, the periods remaining uncrossed to the end of the interval studied here. The SKR modulations also show effects related to the sharp amplitude changes observed in the magnetic oscillation data at ~100–200 day intervals since February 2011, though the correspondences are not exact, indicating that other factors such as “seeing” effects on the variable Cassini orbit are also involved. Postequinox variations in the relative phase between the magnetic and SKR oscillations are also shown to be related to changes in orbit apoapsis orientation.
Highlights
Despite the near-perfect axisymmetry of Saturn’s internal planetary magnetic field [e.g., Burton et al, 2010], observations from Pioneer-11, Voyager-1 and Voyager-2, and Cassini have shown that rotating modulations near the ~11 h planetary period are ubiquitous in Saturn’s magnetosphere
Cassini observations of modulations in the powerful Saturn kilometric radiation (SKR) emissions have shown that there are two such planetary period oscillation (PPO) systems present, one related to each polar hemisphere, that rotate about the planetary axis with slightly different periods [Kurth et al, 2008; Gurnett et al, 2009a, 2009b]
In this paper we have newly compared the Saturn PPO properties determined from Cassini magnetic field data in the postequinox interval with the related modulations observed in the SKR emission data
Summary
Despite the near-perfect axisymmetry of Saturn’s internal planetary magnetic field [e.g., Burton et al, 2010], observations from Pioneer-11, Voyager-1 and Voyager-2, and Cassini have shown that rotating modulations near the ~11 h planetary period are ubiquitous in Saturn’s magnetosphere. Cassini observations of modulations in the powerful Saturn kilometric radiation (SKR) emissions have shown that there are two such planetary period oscillation (PPO) systems present, one related to each polar hemisphere, that rotate about the planetary axis with slightly different periods [Kurth et al, 2008; Gurnett et al, 2009a, 2009b] These periods are found to vary slowly with Saturn’s seasons, being more widely separated during southern summer conditions, ~10.8 h for the southern and ~10.6 h for the northern system, but converging toward ~10.7 h across equinox [Galopeau and Lecaheux, 2000; Gurnett et al, 2010b, 2011; Lamy, 2011]. We examine and compare the magnetic field and SKR oscillations in the postequinox interval, extending both data sets to mid-2013
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