Abstract
Abstract. We discuss the properties of Saturn planetary period oscillations (PPOs) deduced from analysis of Saturn kilometric radiation (SKR) modulations by Fischer et al. (2014), and from prior analysis of magnetic field oscillations data by Andrews et al. (2012) and Provan et al. (2013), with emphasis on the post-equinox interval from early 2010 to early 2013. Fischer et al. (2014) characterize this interval as showing single phase-locked periods in the northern and southern SKR modulations observed in polarization-separated data, while the magnetic data generally show the presence of separated dual periods, northern remaining shorter than southern. We show that the single SKR period corresponds to the southern magnetic period early in 2010, segues into the northern period in late 2010, and returns to the southern period in mid-2012, approximately in line with changes in the dominant magnetic oscillation. An exception occurs in mid-February to late August 2011 when two periods are again discerned in SKR data, in good agreement with the ongoing dual periods in the magnetic data. Fischer et al. (2014) discuss this change in terms of a large jump in the southern SKR period related to the Great White Spot storm, which the magnetic data show is primarily due instead to a reappearance in the SKR data of the ongoing southern modulation in a transitory interval of resumed southern dominance. In the earlier interval from early April 2010 to mid-February 2011 when Fischer et al. (2014) deduce single phase-locked periods, we show unequivocal evidence in the magnetic data for the presence of separated dual oscillations of approximately equal amplitude. We suggest that the apparent single SKR periods result from a previously reported phenomenon in which modulations associated with one hemisphere appear in polarization-separated data associated with the other. In the following interval, mid-August 2011 to early April 2012, when Fischer et al. (2014) again report phase-locked northern and southern oscillations, no ongoing southern oscillation of separate period is discerned in the magnetic data. However, the magnetic amplitude data show that if a phase-locked southern oscillation is indeed present, its amplitude must be less than ~ 5–10 % of the northern oscillation.
Highlights
The phenomenon of planetary period oscillations (PPOs) has formed an important topic in studies of Saturn’s magnetosphere during the Cassini era
In this paper we have discussed the divergent results on the nature of the PPOs in Saturn’s magnetosphere in the postequinox interval presented from analysis of magnetic field oscillations by Andrews et al (2012) and Provan et al (2013, 2014), and from analysis of Saturn kilometric radiation (SKR) modulations by Fischer et al (2014)
As shown by Andrews et al (2012), the SKR and magnetic data sets are in good agreement in the pre-equinox southern summer interval showing well-separated periods of ∼ 10.6 h for the northern oscillations and ∼ 10.8 h for the southern, converging towards a common period of ∼ 10.7 h over a ∼ 1-year interval centred near equinox in mid-August 2009
Summary
The phenomenon of planetary period oscillations (PPOs) has formed an important topic in studies of Saturn’s magnetosphere during the Cassini era. Kurth et al, 2008; Lamy, 2011; Gurnett et al, 2011) and magnetic field data obtained when the spacecraft is located inside the magnetosphere (e.g. Andrews et al, 2012; Provan et al, 2013, 2014) These analyses are in good general agreement that the two periods were well-separated at ∼ 10.6 h for the northern period and ∼ 10.8 h for the southern period during southern summer conditions early in the Cassini mission, and that they converged towards ∼ 10.7 h over a ∼ 1 Earth-year interval spanning Saturn equinox in mid-August 2009. As we will suggest below, just such conclusions have recently been drawn by Fischer et al (2014) studying SKR modulations in the post-equinox interval, while analyses of the magnetic field oscillations clearly show two oscillations with separated periods generally to be present
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