Anomalous ion mixing within an MHD scale Kelvin-Helmholtz vortex. 2: Effects of inhomogeneity
Anomalous ion mixing within an MHD scale Kelvin-Helmholtz vortex. 2: Effects of inhomogeneity
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DOI：
10.1029/94ja02219
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发表时间：
1995-07
影响因子：
--
通讯作者：
M. Fujimoto;T. Terasawa
中科院分区：
文献类型：
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作者：
M. Fujimoto;T. Terasawa
已经对 MHD 尺度开尔文-亥姆霍兹 (K-H) 不稳定性进行了混合代码模拟。与均匀背景情况相反，其中跨剪切层的大规模离子混合发生在涡旋内部，遵守流体动力学相似定律，当背景磁场不均匀时，发现离子混合减少。这种稳定效应是双重的：它是一种离子动力学效应，当剪切层宽度和生长模式的波长很小时，它会减小饱和涡流尺寸，从而减小混合长度。即使当剪切层宽度/波长很大并且流体模型中明确的涡旋运动很明显时，减少效果仍然有效。在这种情况下，混合受到抑制，因为较低磁场（磁鞘）侧的离子无法进入涡流岛内：卷起的磁层顶电流片以静电方式将它们阻挡在外。当前结果的地球物理含义是，当磁层顶的磁场强度差异很大时，单独的 K-H 混合可能无法有效地产生低纬度边界层 (LLBL) 的大规模混合斑点。
Hybrid code simulations of the MHD scale Kelvin-Helmholtz (K-H) instability have been conducted. In contrast to the homogeneous background case where the large-scale ion mixing across the shear layer occurs inside the vortex obeying the hydrodynamic similarity law, the ion mixing is found to be reduced when the background magnetic field is inhomogeneous. This stabilization effect is twofold: it is an ion kinetic effect which reduces the saturation vortex size and so the mixing length when the shear layer width and the wavelength of the growing mode are small. The reduction effect still works even when the shear layer width/wavelength are large and a well defined vortical motion as in the fluid model is evident. The mixing is suppressed for this case because the ions on the lower magnetic field (magnetosheath) side cannot enter within the vortex island: The rolled-up magnetopause current sheet block them out electrostatically. The geophysical implication of the present result is that the K-H mixing alone may not be as efficient as to produce the large-scale mixing blob of the low-latitude boundary layer (LLBL) when the magnetic field intensity is highly different across the magnetopause.