Influence of Mercury's Exosphere on the Structure of the Magnetosphere
Influence of Mercury's Exosphere on the Structure of the Magnetosphere
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DOI：
10.1029/2019ja027691
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发表时间：
2020-07-01
影响因子：
2.8
通讯作者：
Motschmann, Uwe
中科院分区：
文献类型：
--
作者：
Exner, Willi;Simon, Sven;Motschmann, Uwe
汞埋藏在一个稀薄的、高度各向异性的钠外逸层中，主要由等离子体-表面相互作用产生。钠离子密度的绝对值仍在争论中。信使号的快速成像等离子光谱仪(FIPS)的观测结果表明，外层离子的密度比上游太阳风的密度低几个数量级，这表明钠外层对磁层电流系统没有实质性影响。然而，信使号对场线共振的磁场观测显示，钠离子密度与上游的太阳风密度相当。为了研究致密的外圈将如何影响水星磁层内的电流系统，我们应用了一个已建立的混合(动能离子、流体电子)模型，并进行了多个模型运行，外圈密度逐渐增加，从根本没有钠离子到彗星状构型。我们演示了足够致密的外部层如何导致钠离子群体对周围电场的自我屏蔽，以及水星磁层的显著膨胀和对称化，这一点受到偶极偏移的影响越来越小。一旦钠离子密度足够高，区域2的场向电流就会出现在行星附近。模拟的区域2电流位于信使轨道下方，从而为观测中没有这些电流提供了可能的解释。在“引导”磁力线甚至到达行星表面之前，钠系外层也关闭了通过Pedersen和Hall电流的区域1电流的很大一部分。模拟的钠离子和太阳风密度与观测结果吻合得很好。
Mercury is embedded in a tenuous and highly anisotropic sodium exosphere, generated mainly by plasma-surface interactions. The absolute values of the sodium ion density are still under debate. Observations by MESSENGER's Fast Imaging Plasma Spectrometer (FIPS) instrument suggest the density of exospheric ions to be several orders of magnitude lower than the upstream solar wind density, indicating that the sodium exosphere has no substantial influence on the magnetospheric current systems. However, MESSENGER magnetic field observations of field line resonances revealed sodium ion densities comparable to the upstream solar wind density. To investigate how a dense exosphere would affect the current systems within Mercury's magnetosphere, we apply an established hybrid (kinetic ions, fluid electrons) model and conduct multiple model runs with gradually increasing exospheric density, ranging from no sodium ions at all to comet-like configurations. We demonstrate how a sufficiently dense exosphere leads to self-shielding of the sodium ion population from the ambient electric field and a significant inflation and symmetrization of Mercury's magnetosphere, which is decreasingly affected by the dipole offset. Once the sodium ion density is sufficiently high, Region 2 field-aligned currents emerge close to the planet. The modeled Region 2 currents are located below the orbit of MESSENGER, thereby providing a possible explanation for the absence of these currents in observations. The sodium exosphere also closes a significant fraction of the Region 1 currents through Pedersen and Hall currents before the "guiding" magnetic field lines even reach the planetary surface. The modeled sodium ion and solar wind densities agree well with observations.