Velocity shear impoundment of the Io plasma torus
Velocity shear impoundment of the Io plasma torus
复制标题
Io 等离子体环面的速度剪切蓄积
DOI：
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发表时间：
1998
期刊：
影响因子：
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通讯作者：
W. Smyth
中科院分区：
文献类型：
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作者：
D. Pontius;R. Wolf;T. Hill;R. Spiro;Y. S. Yang;W. Smyth
木卫一等离子体环面显然有一个更长的寿命比简单的理论稳定性分析的基础上预期的。这种困惑在环面的外缘，即等离子体斜坡处尤为明显，在那里，等离子体含量的陡峭径向梯度表明存在一些限制影响。以前试图解释这一特征的尝试集中在高能粒子可能会捕获较冷的离子等离子体的可能性上，但这些研究都证明是不确定的。我们考虑另一种机制，即发展的不稳定扰动中断所观察到的剪切旋转速度。一个简化几何的例子演示了径向相邻扰动的相对方位角位移如何消除它们的相干性并施加有限的饱和振幅。完全非线性数值模拟使用赖斯对流模型在木星产生类似的结果，这表明扰动被抑制的剪切是强大的。关键参数是经典的线性增长率与表征速度剪切的速率之比。由更远的不稳定扰动产生的电场被有效地屏蔽在剪切区之外。我们认为，这种效应有助于扣押等离子体环，至少是部分负责产生斜坡。
The Io plasma torus apparently has a much longer lifetime than anticipated on the basis of simple theoretical stability analyses. This quandary is particularly evident at the outer edge of the torus, the plasma ramp, where the steep radial gradient in plasma content indicates the presence of some confining influence. Previous attempts to explain this feature have focused on the possibility that high-energy particles could impound the colder, Iogenic plasma, but these studies have proved inconclusive. We consider an alternative mechanism whereby the development of an unstable perturbation is interrupted by the observed shear in the rotational velocity. An example in simplified geometry demonstrates how the relative azimuthal displacement of radially adjacent perturbations might eliminate their coherency and impose a finite saturation amplitude. Fully nonlinear numerical simulations using the Rice convection model at Jupiter produce analogous results, suggesting that perturbations are suppressed where the shear is strong. The crucial parameter is the ratio of the classic, linear growth rate to a rate that characterizes the velocity shear. The electric fields produced by unstable perturbations farther out are effectively shielded from the shear region. We suggest that this effect helps impound the plasma torus and is at least partly responsible for producing the ramp.