The Magnetodiscs and Aurorae of Giant Planets
The Magnetodiscs and Aurorae of Giant Planets
复制标题
巨行星的磁盘和极光
DOI：
10.1007/978-1-4939-3395-2_7
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发表时间：
2016
期刊：
影响因子：
--
通讯作者：
Achilleos N
中科院分区：
文献类型：
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作者：
Achilleos N
木星和土星这两颗气态巨行星的快速旋转，导致它们在磁层环境中形成了磁盘区域。在这些区域，相对较冷的等离子体被限制在赤道区域，由方位（环）电流产生的磁场增加了行星偶极子，在赤道平面附近形成了径向膨胀的场线。赤道磁碟中随后的力平衡受到离心应力和热离子群的热压的强烈影响，热离子群的热能与其离心势能的大小相比很大。木星和克罗尼安磁层的等离子体来源是各自的卫星木卫一（一颗火山卫星）和土卫二（一颗冰卫星）。这些源产生的等离子体通过各自的磁层向外传输，最终从系统中消失。研究最多的输运机制之一是磁管交换，这是一种等离子体不稳定性，它取代了质量，但没有取代磁通量——这是任何输运过程的重要观测约束。压力各向异性可能在这些磁层的等离子体损失中起作用。对于木星系统来说尤其如此，在旋转膨胀的磁通管的赤道部分可以容纳强大的平行压力，导致这些区域变得不稳定，被吹开并释放出等离子体。等离子体质量损失也与磁尾区域的磁重联事件有关。在这篇综述中，我们总结了与巨行星磁盘中等离子体的产生和输运有关的一些重要的观测和理论概念。我们首先考虑这些系统中力平衡的各个方面，以及它们与母行星电离层的耦合。然后，我们描述了中性和电离物质之间相互作用的作用，以及它如何决定等离子体质量和动量添加到磁碟的速率。接下来，我们描述了等离子体注入的观测性质，以及由此产生的对全球等离子体输运性质和磁碟稳定性的影响。综述了磁通管交换不稳定性的理论，讨论了重力和磁曲率对磁通管交换不稳定性的影响。讨论了模拟交换等离子体结构与土星卫星土卫六之间的相互作用，并描述了其与观测到的土星周期现象的关系。最后，综述了磁碟区域中与质量加载相关的等离子体波的观测、产生和演化。
The rapid rotation of the gas giant planets, Jupiter and Saturn, leads to the formation of magnetodisc regions in their magnetospheric environments. In these regions, relatively cold plasma is confined towards the equatorial regions, and the magnetic field generated by the azimuthal (ring) current adds to the planetary dipole, forming radially distended field lines near the equatorial plane. The ensuing force balance in the equatorial magnetodisc is strongly influenced by centrifugal stress and by the thermal pressure of hot ion populations, whose thermal energy is large compared to the magnitude of their centrifugal potential energy. The sources of plasma for the Jovian and Kronian magnetospheres are the respective satellites Io (a volcanic moon) and Enceladus (an icy moon). The plasma produced by these sources is globally transported outwards through the respective magnetosphere, and ultimately lost from the system. One of the most studied mechanisms for this transport is flux tube interchange, a plasma instability which displaces mass but does not displace magnetic flux—an important observational constraint for any transport process. Pressure anisotropy is likely to play a role in the loss of plasma from these magnetospheres. This is especially the case for the Jovian system, which can harbour strong parallel pressures at the equatorial segments of rotating, expanding flux tubes, leading to these regions becoming unstable, blowing open and releasing their plasma. Plasma mass loss is also associated with magnetic reconnection events in the magnetotail regions. In this overview, we summarise some important observational and theoretical concepts associated with the production and transport of plasma in giant planet magnetodiscs. We begin by considering aspects of force balance in these systems, and their coupling with the ionospheres of their parent planets. We then describe the role of the interaction between neutral and ionized species, and how it determines the rate at which plasma mass and momentum are added to the magnetodisc. Following this, we describe the observational properties of plasma injections, and the consequent implications for the nature of global plasma transport and magnetodisc stability. The theory of the flux tube interchange instability is reviewed, and the influences of gravity and magnetic curvature on the instability are described. The interaction between simulated interchange plasma structures and Saturn’s moon Titan is discussed, and its relationship to observed periodic phenomena at Saturn is described. Finally, the observation, generation and evolution of plasma waves associated with mass loading in the magnetodisc regions is reviewed.