Simulating the effect of centrifugal forces in Jupiter's magnetosphere
Simulating the effect of centrifugal forces in Jupiter's magnetosphere
复制标题
模拟木星磁层中离心力的影响
DOI：
10.1002/2013ja019381
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发表时间：
2014
期刊：
影响因子：
--
通讯作者：
E. Bunce
中科院分区：
文献类型：
--
作者：
M. Vogt;M. Kivelson;K. Khurana;R. Walker;M. Ashour‐Abdalla;E. Bunce
木星的大尺寸和快速的行星自转周期相结合，产生了强大的离心力，导致了其磁层的许多独特特性。此前有人提出，这种离心力和非绝热场线拉伸可能会导致观察到的木星等离子体片的黎明-黄昏不对称性，该等离子体片在黄昏时最厚。当通量管在中午和黄昏之间旋转和拉伸时，沿场弹跳的粒子获得平行能量并产生压力各向异性。由于弹跳时间与向外膨胀的时间尺度相比可能很长，因此粒子可能会非绝热响应，并且由此产生的压力各向异性会导致等离子体片不稳定。我们使用大规模动力学模拟来跟踪一组旋转粒子在随时间变化的旋转磁场中移动，该磁场旨在代表木星磁层中的通量管膨胀。该分析通过表征压力各向异性和能量变化来量化捕获粒子的响应。我们比较了非绝热和绝热向外膨胀的结果，发现非绝热情况比绝热膨胀导致大的俯仰角各向异性和更高的总能量。尽管计算处理不完全自洽，但结果支持等离子体压力变化导致磁场结构随局部时间变化的命题。我们的研究结果与木星磁层中的非绝热效应导致场偶极化以及中午和黄昏之间观察到的等离子体片增厚的观点是一致的。
Jupiter's large scale size and rapid planetary rotation period combine to produce the strong centrifugal force responsible for many unique properties of its magnetosphere. It was previously proposed that this centrifugal force and nonadiabatic field line stretching could cause the observed dawn‐dusk asymmetry of Jupiter's plasma sheet, which is thickest near dusk. As flux tubes rotate and stretch between noon and dusk, particles bouncing along the field gain parallel energy and create pressure anisotropy. Because bounce times can be long compared with the outward expansion timescale, particles may respond nonadiabatically, and the resulting pressure anisotropy can drive the plasma sheet to instability. We used a large‐scale kinetic simulation to follow a collection of rotating particles as they move in a time‐varying, rotating magnetic field designed to represent flux tube expansion in Jupiter's magnetosphere. The analysis quantifies the response of trapped particles by characterizing the pressure anisotropy and energy changes. We compare results of nonadiabatic and adiabatic outward expansions and find that the nonadiabatic case leads to a large pitch angle anisotropy and higher total energy than for adiabatic expansion. Although the calculation was not handled fully self‐consistently, the results support the proposition that plasma pressure changes lead to changes in the magnetic field structure with local time. Our findings are consistent with the idea that nonadiabatic effects in Jupiter's magnetosphere contribute to field dipolarization and the observed plasma sheet thickening between noon and dusk.