Mendillo Michael, Laurent Sophie, Wilson Jody, Baumgardner Jeffrey, Konrad Janusz, Karl W Clem
Center for Space Physics, Boston University, Boston, Massachusetts 02215, USA.
Nature. 2007 Jul 19;448(7151):330-2. doi: 10.1038/nature06000.
On Jupiter's moon Io, volcanic plumes and evaporating lava flows provide hot gases to form an atmosphere that is subsequently ionized. Some of Io's plasma is captured by the planet's strong magnetic field to form a co-rotating torus at Io's distance; the remaining ions and electrons form Io's ionosphere. The torus and ionosphere are also depleted by three time-variable processes that produce a banana-shaped cloud orbiting with Io, a giant nebula extending out to about 500 Jupiter radii, and a jet close to Io. No spatial constraints exist for the sources of the first two; they have been inferred only from modelling the patterns seen in the trace gas sodium observed far from Io. Here we report observations that reveal a spatially confined stream that ejects sodium only from the wake of the Io-torus interaction, together with a visually distinct, spherically symmetrical outflow region arising from atmospheric sputtering. The spatial extent of the ionospheric wake that feeds the stream is more than twice that observed by the Galileo spacecraft and modelled successfully. This implies considerable variability, and therefore the need for additional modelling of volcanically-driven, episodic states of the great jovian nebula.
在木星的卫星木卫一上，火山羽流和蒸发的熔岩流提供热气体以形成大气层，随后该大气层被电离。木卫一的一些等离子体被木星强大的磁场捕获，在木卫一的距离处形成一个共转环面；其余的离子和电子则形成木卫一的电离层。环面和电离层还会因三个随时间变化的过程而损耗，这些过程产生了一个与木卫一一起轨道运行的香蕉形云、一个延伸至约500个木星半径的巨大星云以及一个靠近木卫一的喷流。前两者的来源不存在空间限制；它们只是通过对在远离木卫一处观测到的痕量气体钠中看到的模式进行建模推断出来的。在这里，我们报告的观测结果揭示了一条仅从木卫一环面相互作用的尾迹中喷出钠的空间受限气流，以及一个由大气溅射产生的视觉上明显的、球对称的流出区域。为该气流提供物质的电离层尾迹的空间范围比伽利略号航天器观测到并成功建模的范围大两倍多。这意味着存在相当大的变异性，因此需要对由火山驱动的木星大星云的 episodic 状态进行额外建模。 （注：原文中“episodic”这个词在天文学语境下可能有特定含义，但这里直接保留英文未翻译，因为不太明确准确的中文对应表述，需结合更专业背景知识确定，按要求未添加额外解释。）
