Abstract
We discuss the planetary period oscillations (PPOs) observed by the Cassini spacecraft in Saturn's magnetosphere, in particular the relationship between the properties of the PPOs in the post-equinox interval as observed in magnetic field data by Andrews et al. (2012) and Provan et al. (2013, 2014) and in Saturn kilometric radiation (SKR) emissions by Fischer et al. (2014, 2015), whose results are somewhat discrepant. We show that differences in the reported PPO periods, a fundamental property which should be essentially identical in the two data sets, can largely be accounted for by the phenomenon of dual modulation of the SKR emissions in polarization-separated data, in which the modulation associated with one hemisphere is also present in the other. Misidentification of the modulations results in a reported reversal in the SKR periods in the initial post-equinox interval, south for north and vice versa, relative to the magnetic oscillations whose hemispheric origin is more securely identified through the field component phase relations. Dual modulation also results in the apparent occurrence of phase-locked common periods in the northern and southern SKR data during later intervals during which two separate periods are clearly discerned in the magnetic data through beat modulations in both phase and amplitude. We further show that the argument of Fischer et al. (2015) concerning the phase relation between the magnetic field oscillations and the SKR modulations is erroneous, the phase difference between them revealing the local time (LT) of the upward field-aligned current of the PPO current system at times of SKR modulation maxima. Furthermore, this LT is found to vary significantly over the Cassini mission from dawn, to dusk, and to noon, depending on the LT of apoapsis where the spacecraft spends most time. These variations are consistent with the view that the SKR modulation is fundamentally a rotating system like the magnetic perturbations, though complicated by the strong LT asymmetry in the strength of the sources, and rule out a mainly clock-like (strobe) modulation as argued by Fischer et al. (2015), for which no physical mechanism is suggested. We also elucidate the nature of the magnetic periods, criticized by Fischer et al. (2015), which have previously been derived in ∼100–200 day post-equinox intervals between abrupt changes in PPO properties, and further show that their argument that the magnetic phase data provide evidence for the occurrence of common phase-locked magnetic oscillations in some intervals is fallacious. The most important consequence of our results, however, is that they demonstrate the essential compatibility of the post-equinox magnetic field and SKR data, despite the contrary results published to date. They also show that due to the dual modulation effect in polarization-separated SKR data, analysis and interpretation may contain more subtleties than previously realized. Joint examination of the combined magnetic and SKR data clearly provides greater insight and enhanced confidence compared with analyses of these data sets individually.
Highlights
Investigation of the phase relationship between the magnetic oscillations, which are known to form perturbations that rotate around the planet at the planetary period oscillations (PPOs) period (Cowley et al, 2006; Andrews et al, 2010a), and that of the Saturn kilometric radiation (SKR) modulations, provides insight into the origins of the latter modulations, and whether these form a rotating system like the magnetic, or whether they are mainly clock-like as suggested by Fischer et al (2015)
Our principal results are summarized as follows. (a) On the basis of physical principle we expect the periods of the magnetic oscillations and SKR modulations to be identical, apart from small differences (∼10 s) resulting from the neglect of rotation effects in deriving SKR periods (see point (e) below), and have been shown to be so in the pre-equinox interval studied by Andrews et al (2011, 2012)
Our analysis supports the previous suggestion by Cowley and Provan (2015) that these results are due to dual modulation of the polarization-separated SKR emissions as previously discussed by Lamy (2011), with one modulation, northern or southern, dominating the other in both polarization channels, further illustrating the comments under point (a) above concerning the analysis and interpretation of SKR data post-equinox. (c) The derived magnetic and SKR periods are in better overall agreement after mid-February 2011, when abrupt changes in oscillation amplitudes and periods occur in the magnetic data at ∼100-200 day intervals (Provan et al, 2013)
Summary
‘planetary period oscillations’ (PPOs) (e.g., Cowley et al, 2006; Southwood and Kivelson, 2007; Andrews et al, 2008, 2010a; Carbary et al, 2007; Provan et al, 2009; Burch et al, 2009; Kurth et al, 2008; Ye et al, 2010), in which modulations of the magnetic field, plasma populations, plasma waves, and radio emissions are observed throughout the system near the planetary rotation period of ∼10.6 h despite the close axial symmetry of the planetary internal magnetic field (Burton et al, 2010). The seasonally-varying properties of the PPOs have been monitored during the Cassini mission to date through measurements of the amplitude and phase of the magnetic oscillations associated with the rotating quasi-uniform perturbation fields observed on few-day periapsis passes through the quasi-dipolar ‘core’ region of the magnetosphere (dipole L ≤ 12) on near-equatorial orbits, combined with corresponding measurements of the rotating quasidipolar perturbation fields on open field lines over the two polar regions on orbits that are sufficiently inclined These data show that the periods were well-separated and slowly varying during Saturn southern summer early in the Cassini mission 2005–2008, at ∼10.8 h for the dominant southern oscillations and ∼10.6 h for the weaker northern oscillations, but converged towards a common period with near-equal amplitudes over a ∼1.5 year interval centered near Saturn vernal equinox in mid-August 2009 (Andrews et al, 2008, 2010b, 2012). We will discuss the evidence concerning this conclusion, and will show their arguments to be erroneous
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