Ground images at high latitudes of ULF wave processes in the outer magnetosphere
Ground images at high latitudes of ULF wave processes in the outer magnetosphere
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DOI：
10.1016/s1364-6826(01)00083-9
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发表时间：
2002
影响因子：
1.9
通讯作者：
V. Pilipenko;M. Engebretson
中科院分区：
文献类型：
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作者：
V. Pilipenko;M. Engebretson
空间和实验室等离子体中的磁流体动力学(MHD)波过程使得从整体压缩模到局域Alfven振荡的共振转换成为可能。通过这种方式，由太空中的局部非稳定过程激发的MHD信号可以将信息传递到地面。本文着重讨论了高纬度地区的长周期超低频现象，包括尖点区、夜间椭圆形和极冠上的超低频现象，并着重介绍了与这些纬度的超低频波物理有关的几个悬而未决的问题。重点介绍了几个新的结果：(1)在高空尖端，可以出现没有导电边界的MHD波导/谐振器，从磁鞘穿透的流体湍流可以积累并转化为沿场线逃逸的Alfven波。(2)尖点附近的宽带长周期波活动呈现出一种尚未解释的时间结构，以及频繁的非共轭波功率水平的长间隔。(3)与电离层焦耳耗散或频散泄漏相比，极光加速区出现场向势降可能导致Pc5/Pi2波的严重衰减。(4)原因不明的黑夜极帽存在特定的长周期不规则变化，这可能与磁尾的拍打或湍流的跨极对流有关。最后，我们指出了哪些是未知的，我们目前的知识在多大程度上是不完整的，哪些观察是罕见的，以及未来的研究可以集中在哪些方面。
Magnetohydrodynamic (MHD) wave processes in space and laboratory plasmas make possible the resonant conversion of global compressional modes into localized Alfven oscillations. In this way MHD signals excited by local non-steady processes in space can convey information to the ground. This review focuses on long-period ultra-low-frequency (ULF) phenomena specific to high latitudes, including those in the cusp region, nightside oval, and polar cap, and highlights several unresolved problems related to the physics of ULF waves at these latitudes. Several new results are highlighted: (1) In the high-altitude cusp an MHD waveguide/resonator without conductive boundaries could occur, where hydromagnetic turbulence penetrating from the magnetosheath can accumulate and transform into Alfven waves escaping along the field lines. (2) Broadband long-period wave activity near the cusp exhibits temporal structure that is as yet unexplained, as well as frequent long intervals of non-conjugate levels of wave power. (3) Occurrence of field-aligned potential drops in the auroral acceleration region may cause severe damping of Pc5/Pi2 waves as compared with ionospheric Joule dissipation or dispersive leakage. (4) Specific long-period irregular variations in the nightside polar cap of still unknown origin exist, which might be related to the flapping of the magnetotail or turbulent trans-polar convection. Finally, we point out what remains unknown, to what extent our current knowledge is incomplete, what observations have been rare and what future research could focus on.