Standing ULF modes of the magnetosphere: A theory
Standing ULF modes of the magnetosphere: A theory
复制标题
磁层的常设超低频模式：理论
DOI：
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发表时间：
1992
期刊：
影响因子：
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通讯作者：
J. Samson
中科院分区：
文献类型：
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作者：
B. Harrold;J. Samson
JHU/APL高频多普勒雷达最近在静止地磁条件下观测到频率在1 - 4 mHz范围内的磁力线共振。这些结构被观察到具有持续数小时的稳定频率，这使我们得出结论，它们可能是由太阳风驱动的磁层驻波（在径向方向，而不是沿着场线的驻波）。利用这一前提，在理想的线性化磁流体动力学背景下，研究了快模超低频波在磁鞘和近地磁层中的传播。提出了一个模型，其中快波传播在赤道平面之间的侧翼的弓形激波和一个转折点内的磁层深处。由于地球附近的磁场梯度，在转折点和等离子体层顶之间产生磁力线共振。使用一组实际的磁层参数，就有可能重现一组观测到的频率和它们在电离层内的场线共振的各自位置（假设偶极映射）。然而，由于模型腔频率是敏感的磁鞘参数，这个模型不能解释极端稳定的地磁条件下观察到的频率。
Field line resonances with frequencies in the range 1 to 4 mHz have recently been observed by the JHU/APL HF Doppler radar during quiescent geomagnetic conditions. These structures are observed to have stable frequencies for durations of several hours, leading us to the conclusion that they may be standing waves (in the radial direction, as opposed to standing waves along a field line) of the magnetosphere driven by the solar wind. Using this premise, the propagation of fast mode ULF waves in the magnetosheath and the near earth magnetosphere are examined in an ideal, linearized MHD context. A model is presented in which fast waves propagate in the equatorial plane between the flanks of the bow shock and a turning point deep within the magnetosphere. Due to the magnetic field gradient near the earth, a field line resonance develops between the turning point and the plasmapause. Using a realistic set of magnetospheric parameters, it is possible to reproduce the set of observed frequencies and the respective positions of their field line resonances within the ionosphere (assuming a dipole mapping). However, because the model cavity frequencies are sensitive to magnetosheath parameters, this model does not explain the extreme stability with respect to geomagnetic conditions of the observed frequencies.