Transient Change of Io's Neutral Oxygen Cloud and Plasma Torus Observed by Hisaki
Transient Change of Io's Neutral Oxygen Cloud and Plasma Torus Observed by Hisaki
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久崎观测到的木卫一中性氧云和等离子体环的瞬态变化
DOI：
10.1029/2019ja026877
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Bagenal Fran
中科院分区：
文献类型：
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作者：
Koga Ryoichi;Tsuchiya Fuminori;Kagitani Masato;Sakanoi Takeshi;Yoshioka Kazuo;Yoshikawa Ichiro;Kimura Tomoki;Murakami Go;Yamazaki Atsushi;Smith H. Todd;Bagenal Fran
艾奥的大气氧原子被大气溅射加热并逃离艾奥的引力，在艾奥的轨道周围形成中性氧云。这种中性云作为 Io 氧等离子体环面的来源很重要。先前的研究得出了平衡中性氧云的分布和密度。然而，人们对中性云的演变知之甚少。在这项研究中，我们分析了Hisaki卫星对2015年环面瞬态强密度增强期间（称为高密度期）围绕木卫一轨道的OI 130.4 nm发射的空间分布的观测结果。比较OI和OII 83.4 nm发射的时间变化，我们估计该时期O+的寿命在高密度期约为21天，在正常密度期约为41天。 Hisaki 的观察结果与当环面密度增加时 O+ 的寿命缩短是一致的。径向分布显示，在高密度时期，中性氧云向外扩散至8.6个木星半径。我们还发现，在高密度时期，木卫一轨道（假设南北厚度为1.2木星半径）的中性氧数密度增加到cm−3，是正常密度时期（cm−3）的三倍多。方位角分布显示出木卫一周围有一个致密区域，以及沿木卫一轨道分布的纵向均匀的弥散区域，该区域在高密度期间扩大。
Io's atmospheric oxygen atoms are heated by atmospheric sputtering and escape from Io's gravity, forming a neutral oxygen cloud around Io's orbit. This neutral cloud is important as a source of the Io oxygen plasma torus. Previous studies derived the distribution and density of the equilibrium neutral oxygen cloud. However, little is known about the evolution of the neutral cloud. In this study, we analyzed Hisaki satellite observations of the spatial distribution of OI 130.4 nm emissions around Io's orbit during transient strong density enhancement in the torus in 2015 (called high density period). Comparing time variations of OI and OII 83.4 nm emissions, we estimated that the lifetimes of O+in this period were about 21 days in the high density period and 41 days in the normal density period. Hisaki observations are consistent with a decrease in the lifetime of O+when the density in the torus increases. The radial distribution showed the neutral oxygen cloud spread outward up to 8.6 Jupiter radii during the high density period. We also show that during the high density period, the neutral oxygen number density at Io's orbit (where north‐south thickness is assumed to be 1.2 Jupiter radii) increased to cm−3, more than three times the value during the normal density period ( cm−3). The azimuthal distribution showed a dense region around Io and a longitudinally uniform, diffuse region distributed along Io's orbit that enlarges during the high density period.