电荷交换和离解在土星中性云扩散中的作用
The roles of charge exchange and dissociation in spreading Saturn's neutral clouds
摘要：
直接来自土卫二羽流的中性物质最初被限制在土卫二围绕土星的轨道上的一个致密的中性环中。这个中性环通过电荷交换、碰撞/光解离和中性-中性碰撞重新分配，产生土星的中性云。在这里，我们认为前者的过程比以前的研究更详细。在解离的情况下，模型假设OH是以1 km/s的单一速度产生的，而实验室测量表明速度在1和1.6 km/s之间。我们表明，高速的情况下增加解离的影响范围从9到15 Rs。对于电荷交换，我们提出了一种新的建模方法，其中离子在中性背景下进行，而中性云模型通常是从中性粒子的角度构建的。这种方法使我们能够在电荷交换发生的时刻评论离子的回旋相位的意义。考虑回转相位：（1）对H2O云没有影响;（2）使土卫二轨道上的OH局部密度加倍;（3）使土卫二轨道上的氧密度降低不到10%。最后，我们考虑速度依赖，以及物种依赖的截面，并发现从电荷交换产生的氧云是蔓延超过H2O，而OH云是最局限的。
英文摘要：
Neutrals sourced directly from Enceladus's plumes are initially confined to a dense neutral torus in Enceladus's orbit around Saturn. This neutral torus is redistributed by charge exchange, impact/photodissociation, and neutral-neutral collisions to produce Saturn's neutral clouds. Here we consider the former processes in greater detail than in previous studies. In the case of dissociation, models have assumed that OH is produced with a single speed of 1 km/s, whereas laboratory measurements suggest a range of speeds between 1 and 1.6 km/s. We show that the high-speed case increases dissociation's range of influence from 9 to 15 Rs. For charge exchange, we present a new modeling approach, where the ions are followed within a neutral background, whereas neutral cloud models are conventionally constructed from the neutrals' point of view. This approach allows us to comment on the significance of the ions' gyrophase at the moment charge exchange occurs. Accounting for gyrophase: (1) has no consequence on the H2O cloud; (2) doubles the local density of OH at the orbit of Enceladus; and (3) decreases the oxygen densities at Enceladus's orbit by less than 10%. Finally, we consider velocity-dependent, as well as species-dependent cross sections and find that the oxygen cloud produced from charge exchange is spread out more than H2O, whereas the OH cloud is the most confined.
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