Burch J L, Goldstein J, Lewis W S, Young D T, Coates A J, Dougherty M K, André N
Southwest Research Institute, PO Drawer 28510, San Antonio, Texas 78228-0510, USA.
Nature. 2007 Jun 14;447(7146):833-5. doi: 10.1038/nature05906.
Rotating at over twice the angular speed of Earth, Saturn imposes a rapid spin on its magnetosphere. As a result, cold, dense plasma is believed to be flung outward from the inner magnetosphere by centrifugal force and replaced by hotter, more tenuous plasma from the outer magnetosphere. The centrifugal interchange of plasmas in rotating magnetospheres was predicted many years ago and was conclusively demonstrated by observations in Jupiter's magnetosphere, which--like that of Saturn (but unlike that of Earth)--is rotationally dominated. Recent observations in Saturn's magnetosphere have revealed narrow injections of hot, tenuous plasma believed to be the inward-moving portion of the centrifugal interchange cycle. Here we report observations of the distribution of the angle between the electron velocity vector and the magnetic field vector ('pitch angle') obtained in the cold, dense plasma adjacent to these inward injection regions. The observed pitch-angle distributions are indicative of outward plasma flow and consistent with centrifugal interchange in Saturn's magnetosphere. Further, we conclude that the observed double-peaked ('butterfly') pitch-angle distributions result from the transport of plasma from regions near the orbits of Dione and Tethys, supporting the idea of distinct plasma tori associated with these moons.
土星的自转速度超过地球两倍，这使其磁层快速旋转。结果，据信冷而密集的等离子体因离心力从内磁层被甩向外层，取而代之的是来自外磁层更热、更稀薄的等离子体。旋转磁层中等离子体的离心交换早在多年前就已被预测，并且在木星磁层的观测中得到了确凿证实，木星磁层（与土星磁层类似，但与地球磁层不同）由自转主导。最近对土星磁层的观测揭示了热而稀薄等离子体的狭窄注入，据信这是离心交换循环中向内移动的部分。在此，我们报告了在这些向内注入区域附近的冷而密集等离子体中获得的电子速度矢量与磁场矢量之间夹角（“俯仰角”）分布的观测结果。观测到的俯仰角分布表明等离子体向外流动，并且与土星磁层中的离心交换一致。此外，我们得出结论，观测到的双峰（“蝴蝶”）俯仰角分布是由来自土卫四和土卫五轨道附近区域的等离子体传输造成的，这支持了与这些卫星相关的不同等离子体环面的观点。
