Planetary period oscillations in Saturn's magnetosphere: Further comments on the relationship between post-equinox properties deduced from magnetic field and Saturn kilometric radiation measurements
Planetary period oscillations in Saturn's magnetosphere: Further comments on the relationship between post-equinox properties deduced from magnetic field and Saturn kilometric radiation measurements
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DOI：
10.1016/j.icarus.2016.02.051
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发表时间：
2016-07
期刊：
影响因子：
3.2
通讯作者：
S. Cowley;G. Provan
中科院分区：
文献类型：
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作者：
S. Cowley;G. Provan
我们讨论了卡西尼号航天器在土星磁层中观测到的行星周期振荡（PPO），特别是安德鲁斯等人在磁场数据中观测到的春分后间隔中 PPO 特性之间的关系。 (2012) 和 Provan 等人。 (2013, 2014) 和 Fischer 等人的土星千米辐射 (SKR) 排放。 （2014，2015），其结果有些出入。我们表明，报告的 PPO 周期的差异（两个数据集中应该基本相同的基本属性）在很大程度上可以通过偏振分离数据中 SKR 发射的双重调制现象来解释，其中与一个半球相关的调制也存在于另一个半球中。调制的错误识别导致据报告在最初的春分后间隔中 SKR 周期发生逆转，即南为北，反之亦然，相对于磁振荡，磁振荡的半球起源可以通过场分量相位关系更安全地识别。双调制还导致在后期间隔期间北部和南部 SKR 数据中明显出现锁相公共周期，在此期间通过相位和幅度的拍频调制在磁数据中清楚地辨别出两个单独的周期。我们进一步表明费舍尔等人的论点。 （2015）关于磁场振荡和 SKR 调制之间的相位关系的观点是错误的，它们之间的相位差揭示了 SKR 调制最大值时 PPO 电流系统的向上场对准电流的本地时间（LT）。此外，在卡西尼号任务中，从黎明、黄昏到中午，该 LT 变化很大，具体取决于航天器在远地点停留时间最长的 LT。这些变化与 SKR 调制本质上是一个像磁扰动一样的旋转系统的观点是一致的，尽管由于源强度的强 LT 不对称性而变得复杂，并且排除了 Fischer 等人所主张的主要类时钟（选通）调制。 （2015），没有提出任何物理机制。我们还阐明了费舍尔等人批评的磁周期的本质。 (2015)，这些数据先前是在 PPO 特性突然变化之间的春分后约 100-200 天的间隔中得出的，并进一步表明，他们关于磁相位数据为某些间隔中常见锁相磁振荡的发生提供证据的论点是错误的。然而，我们的结果最重要的结果是，它们证明了春分后磁场和 SKR 数据的基本兼容性，尽管迄今为止发布了相反的结果。他们还表明，由于偏振分离 SKR 数据中的双重调制效应，分析和解释可能包含比以前意识到的更多微妙之处。与单独分析这些数据集相比，对组合磁性数据和 SKR 数据进行联合检查显然可以提供更深入的见解并增强信心。
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.