Energetic electron observations of Rhea's magnetospheric interaction
Energetic electron observations of Rhea's magnetospheric interaction
复制标题
土卫五磁层相互作用的高能电子观测
DOI：
10.1016/j.icarus.2012.07.006
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发表时间：
2012
期刊：
影响因子：
3.2
通讯作者：
M. K. G. Holmberg
中科院分区：
文献类型：
--
作者：
Roussos;P. Kollmann;N. Krupp;C. Paranicas;S. M. Krimigis;D. G. Mitchell;A. M. Persoon;D. A. Gurnett;W. S. Kurth;H. Kriegel;S. Simon;K. K. Khurana;G. H. Jones;J. E. Wahlund;M. K. G. Holmberg
土星的卫星土卫五被认为是一个简单的等离子体吸收体，然而，在其附近的高能粒子观测显示出各种意想不到的和复杂的相互作用特征，这与我们目前对等离子体吸收相互作用的理解不一致。高能电子数据特别有趣，因为它们包含了月球尾流两侧一系列宽的和窄的通量消耗。这些消失与土卫五希尔球轨道上的尘埃和巨石的吸收有关，但后来没有得到证实，所以在这项研究中，我们回顾了迄今为止卡西尼四次飞越土卫五的数据，寻找在卫星相互作用区域内运行的其他过程的证据。我们专注于高能电子观测，我们将其与磁力计，冷等离子体密度和高能离子数据结合起来。所有的飞掠都有独特的特征，但在这里，我们只关注几个一直被观察到的结构。最有趣的共同特征是高能电子通量的窄差，在尾流侧翼附近可见。这些通常与尾流内部的窄通量增强一起出现。对土卫五第一次飞掠（R1）的这些结构进行的相空间密度分析表明，Liouville定理成立，表明它们可能是由于高能电子通过狭窄的通道从磁层快速传输到尾流而形成的。考虑了一系列的可能性来解释这个传输过程。我们研究了在等离子体吸收卫星（通过混合模拟代码建模）的相互作用区域中复杂的高能电子漂移是否可能允许这样的传输。除了几个特征（如随着电子能量的增加，中心尾迹变宽）外，大多数常见的高能电子相互作用特征（包括窄结构）都没有重现。为了解释数据，应考虑混合代码未模拟的其他动态过程。对于小尺度特征，讨论了凹槽（交换）不稳定性作用于电子的可能性。这种不稳定性可能是由等离子体压力和磁场大小的强梯度驱动的：磁力计观测显示出与（预期的）雷亚离子吸收引起的等离子体压力损失相一致的清晰特征。这种不稳定性的另一个潜在驱动因素可能是冷等离子体密度的梯度，然而，令人惊讶的是，在土卫五等离子体尾流的大多数交叉点上都没有这种梯度。土卫五的尾流中没有密度损耗，这表明存在一个局部冷等离子体源区。混合等离子体模拟表明，这个源不可能是土卫五弱外逸层的电离成分。这可能与来自土卫五带负电荷表面的加速光电子有关，这表明表面电荷可能在土卫五磁层相互作用区域的形成中起着非常重要的作用。
Saturn’s moon Rhea is thought to be a simple plasma absorber, however, energetic particle observations in its vicinity show a variety of unexpected and complex interaction features that do not conform with our current understanding about plasma absorbing interactions. Energetic electron data are especially interesting, as they contain a series of broad and narrow flux depletions on either side of the moon’s wake. The association of these dropouts with absorption by dust and boulders orbiting within Rhea’s Hill sphere was suggested but subsequently not confirmed, so in this study we review data from all four Cassini flybys of Rhea to date seeking evidence for alternative processes operating within the moon’s interaction region. We focus on energetic electron observations, which we put in context with magnetometer, cold plasma density and energetic ion data. All flybys have unique features, but here we only focus on several structures that are consistently observed. The most interesting common feature is that of narrow dropouts in energetic electron fluxes, visible near the wake flanks. These are typically seen together with narrow flux enhancements inside the wake. A phase-space-density analysis for these structures from the first Rhea flyby (R1) shows that Liouville’s theorem holds, suggesting that they may be forming due to rapid transport of energetic electrons from the magnetosphere to the wake, through narrow channels. A series of possibilities are considered to explain this transport process. We examined whether complex energetic electron drifts in the interaction region of a plasma absorbing moon (modeled through a hybrid simulation code) may allow such a transport. With the exception of several features (e.g. broadening of the central wake with increasing electron energy), most of the commonly observed interaction signatures in energetic electrons (including the narrow structures) were not reproduced. Additional dynamical processes, not simulated by the hybrid code, should be considered in order to explain the data. For the small scale features, the possibility that a flute (interchange) instability acts on the electrons is discussed. This instability is probably driven by strong gradients in the plasma pressure and the magnetic field magnitude: magnetometer observations show clearly signatures consistent with the (expected) plasma pressure loss due to ion absorption at Rhea. Another potential driver of the instability could have been gradients in the cold plasma density, which are, however, surprisingly absent from most crossings of Rhea’s plasma wake. The lack of a density depletion in Rhea’s wake suggests the presence of a local cold plasma source region. Hybrid plasma simulations show that this source cannot be the ionized component of Rhea’s weak exosphere. It is probably related to accelerated photoelectrons from the moon’s negatively charged surface, indicating that surface charging may play a very important role in shaping Rhea’s magnetospheric interaction region.
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DOI：
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