Shadowing of Electron Azimuthal-Drift Motions near the Noon Magnetopause
Shadowing of Electron Azimuthal-Drift Motions near the Noon Magnetopause
复制标题
正午磁层顶附近电子方位角漂移运动的阴影
DOI：
--
复制
发表时间：
1972
期刊：
影响因子：
--
通讯作者：
J. Walton
中科院分区：
文献类型：
--
作者：
H. I. West;R. Buck;J. Walton
地磁捕获的电子在镜像点之间来回弹跳时，会在大约半小时内绕地球缓慢向东漂移。 Roederer1 发展了这些粒子在地球磁层扭曲（外部）区域中的方位漂移的理论。我们可以通过检查两种极端情况下的粒子运动来简要描述他的结果：（1）磁赤道上的粒子镜像和（2）远离赤道的粒子镜像。出于我们的目的，我们只需要考虑前两个绝热不变量：(1) 磁矩 µ=E⊥/B，其中 B 是磁场，E⊥ 是粒子的垂直能量；(2) 作用积分 ，其中 Bm 是镜像场，as 是沿弹跳路径的弧增量。图 1 中 Fairfield2 的一些结果举例说明了我们感兴趣的磁层畸变方面，其中显示了平均磁层的恒定赤道 B 的轮廓。请注意，在 10.5 RE 处触及正午磁层顶的等值线映射回大约 6.5 RE 处的午夜。这些等值线代表了以常数 B 漂移的粒子的漂移路径，情况 (1)。对于情况 (2)，我们必须沿着反弹路径评估 Open image in new window 并发现（因为 B/Bm 在大部分反弹路径上都很小）积分大约是沿着反弹路径的距离。因此，从当地时间 0900 点 9 RE 开始检查一组粒子，我们发现情况 (1) 的粒子必定漂移到磁层顶。由于磁场配置在地球的白天侧被压缩，情况（2）粒子将移动，使得它们的赤道交叉点移近地球（因为镜像点以常数 B 漂移，并且反弹路径的长度几乎恒定），因此漂移路径保持在磁层内部。我们现在检查当地时间 1500 点 9 RE 处预期的俯仰角分布，假设磁层畸变关于正午对称。此时，在空间中，粒子的初始漂移路径（在所有俯仰角）已返回到一起。然而，现在，漂移路径的“磁层顶阴影”的影响应该是显而易见的，也就是说，我们应该观察到相对于开始时盛行的俯仰角分布在 90° 俯仰角处粒子通量的消耗。
GEOMAGNETICALLY trapped electrons, as they bounce back and forth from mirror point to mirror point, drift slowly eastward around the Earth in about half an hour. Roederer1 has developed the theory for the azimuthal drift of these particles in the distorted (outer) regions of the Earth's mag-netosphere. We can describe his results briefly by examining the particle motion in two extreme cases: (1) particles mirroring on the magnetic equator and (2) particles mirroring well off the equator. For our purposes, we need consider only the first two adiabatic invariants: (1) the magnetic moment µ=E⊥/B, where B is the magnetic field and E⊥ is the particle's perpendicular energy and (2) the action integral Open image in new window , where Bm is the mirror field and as is an increment of arc along the bounce path. The aspects of magnetospheric distortions of interest to us are exemplified by some results of Fairfield2 in Fig. 1, showing contours of constant equatorial B for an average magnetosphere. Note that the contour that touches the noon magnetopause at 10.5 RE maps back to midnight at about 6.5 RE. These contours, then, represent the drift paths of particles drifting at constant B, case (1). For case (2) we must evaluate Open image in new window along the bounce path and find (because B/Bm is small along most of the bounce path) that the integral is approximately the distance along the bounce path. Hence, examining a group of particles starting at, say, 9 RE at 0900 local time, we find that case (1) particles must drift to the magnetopause. Because the magnetic field configuration is compressed on the day side of the Earth, case (2) particles will move so that their equatorial crossing has moved closer to the Earth (because the mirror points drift at constant B and the length of the bounce path is almost constant) and hence the drift paths stay inside the magnetosphere. We now examine the pitch-angle distributions to be expected at 9 RE at 1500 local time, assuming symmetry about noon in the magnetospheric distortions. At this point in space, the initial drift paths of the particles (at all pitch angles) have come back together. Now, however, the effects of the “magnetopause shadowing” of the drift paths should be apparent, that is, we should observe a depletion of the particle fluxes at 90° pitch angles with respect to the pitch-angle distribution prevailing at the start.