Planetary period oscillations in Saturn's magnetosphere: Evidence in magnetic field phase data for rotational modulation of Saturn kilometric radiation emissions ROTATION SKR MODULATION
Planetary period oscillations in Saturn's magnetosphere: Evidence in magnetic field phase data for rotational modulation of Saturn kilometric radiation emissions ROTATION SKR MODULATION
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土星磁层中的行星周期振荡：土星千米辐射发射旋转调制的磁场相位数据证据 旋转 SKR 调制
DOI：
10.1029/2011ja016636
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发表时间：
2011
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影响因子：
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通讯作者：
Andrews D
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文献类型：
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作者：
Andrews D
旅行者号最初对土星千米辐射（SKR）的观测表明，在1011 h行星自转周期附近的发射功率调制是“频闪式”的，其相位变化与观测者的位置无关，而卡西尼号随后对磁层磁场和等离子体参数的相关振荡的研究表明，这些围绕行星旋转的周期接近SKR周期。然而，对2004-2010年期间磁振荡数据的分析揭示了主导南方周期磁振荡和SKR调制相位之间存在可变的长期漂移，这在卡西尼号远地点于2009年中期首次进入黄昏后扇区后变得非常明显。在这里，我们使用一个简单的理论模型来表明，出现这样的相位漂移，如果SKR调制相位也围绕极光椭圆旋转，结合高度限制的SKR源的航天器由于锥形光束的排放。类似频闪的行为发生在黎明前到中午的扇区，在那里航天器几乎连续地看到上午最强烈的SKR源，与旅行者号的发现一致，而在其他地方，SKR调制相位强烈地依赖于航天器当地时间，与黄昏后扇区的上午源近似反相。支持这种情况下的证据是通过一个独立的测定整个6年的时间间隔的南部磁场扰动的旋转周期的变量。
Initial Voyager observations of Saturn kilometric radiation (SKR) indicated that the modulations in emitted power near the ∼11 h planetary rotation period are “strobe like,” varying with a phase independent of observer position, while subsequent Cassini studies of related oscillations in the magnetospheric magnetic field and plasma parameters have shown that these rotate around the planet with a period close to the SKR period. However, analysis of magnetic oscillation data over the interval 2004–2010 reveals the presence of variable secular drifts between the phases of the dominant southern period magnetic oscillations and SKR modulations, which become very marked after Cassini apoapsis moved for the first time into the postdusk sector in mid‐2009. Here we use a simple theoretical model to show that such phase drifts arise if the SKR modulation phase also rotates around the auroral oval, combined with a highly restricted view of the SKR sources by the spacecraft due to the conical beaming of the emissions. Strobe‐like behavior then occurs in the predawn‐to‐noon sector where the spacecraft has a near‐continuous view of the most intense midmorning SKR sources, in agreement with the Voyager findings, while elsewhere the SKR modulation phase depends strongly on spacecraft local time, being in approximate antiphase with the midmorning sources in the postdusk sector. Supporting evidence for this scenario is provided through an independent determination of the variable rotation period of the southern magnetic field perturbations throughout the 6 year interval.