Ray tracing of ULF waves in a multicomponent magnetospheric plasma: Consequences for the generation mechanism of ion cyclotron waves
Ray tracing of ULF waves in a multicomponent magnetospheric plasma: Consequences for the generation mechanism of ion cyclotron waves
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DOI：
10.1029/ja087ia10p08191
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发表时间：
1982-10
影响因子：
--
通讯作者：
J. Rauch;A. Roux
中科院分区：
文献类型：
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作者：
J. Rauch;A. Roux
强烈的超低频波通常在GEOS 1和2航天器上观测到，在FHe+附近，氦回旋频率。Young等人（1981）将这些波确定为离子回旋波（ICW’s），他们表明它们与磁层等离子体中足够数量的He+密切相关。在这些观察的激励下，我们提出了一种质子回旋频率FH+以下的ULF波的射线追踪研究。在He+存在下，ULF波的色散关系被分成三个分支，我们研究了这三个分支的射线路径。特别令人感兴趣的是离子回旋分支，它位于由He+离子引入的新交叉频率Fcr之上。这种模式在赤道区域被各向异性（T⊥> T∥）高能质子放大。它沿着场线被很好地引导，当局部F = Fcr时遭受极化反转，并继续被引导到F = Fbi，双离子混合频率。然后它被反射并返回赤道，在那里再次被放大。因此，这样的波经过几次反弹，通过放大区域，没有明显的漂移，无论是方位角还是径向。这种镜像效应只能在少量离子（如He+）存在时发生，被认为是ICW扩增的关键。我们还表明，对于每一次这样的赤道交叉，平行波数是守恒的，而垂直波数逐渐增加。因此，ICW变成准静电，这使得它们能够获得一个小而有限的平行电场；这个电场可以反过来加速平行于B的热电子。
Intense ultralow-frequency waves are commonly observed on GEOS 1 and 2 spacecraft, around FHe+, the helium gyrofrequency. These waves were identified as ion cyclotron waves (ICW's) by Young et al. (1981), who showed their close connection with a sufficient amount of He+ in the magnetospheric plasma. Motivated by these observations, we present a ray tracing study of ULF waves below FH+, the proton gyrofrequency. In the presence of He+ the dispersion relation of ULF waves is split into three branches, and we have studied the ray paths for these three branches. Of particular interest is the ion cyclotron branch, which is left handed above the new crossover frequency Fcr introduced by the presence of He+ ions. This mode is amplified, in the equatorial region, by anisotropic (T⊥ > T∥) energetic protons. It is well guided along field lines, suffers a polarization reversal when F = Fcr locally, and continues to be guided up to the point where F = Fbi, the bi-ion hybrid frequency. Then it is reflected and returns to the equator, where it is amplified again. Thus such waves undergo several bounces through the amplifying region without significant drift, either azimuthal or radial. This mirroring effect, which can only occur when minor ions, such as He+, are present, is believed to be crucial for ICW amplification. We also show that for each such equatorial crossing the parallel wave number is conserved while the perpendicular one progressively increases. As a consequence, ICW's become quasi-electrostatic, which enables them to acquire a small but finite parallel electric field; it is suggested that this electric field can in turn accelerate thermal electrons parallel to B.