Identification of low‐frequency fluctuations in the terrestrial magnetosheath
Identification of low‐frequency fluctuations in the terrestrial magnetosheath
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DOI：
10.1029/93ja03300
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发表时间：
1994-04
影响因子：
--
通讯作者：
P. Song;C. Russell;S. Gary
中科院分区：
文献类型：
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作者：
P. Song;C. Russell;S. Gary
在磁流体理论的基础上，我们发展了一种方案来区分在高β各向异性等离子体中传播的四种低频模：快磁声模、慢磁声模、阿尔芬模和镜面模。我们使用四个参数：磁场中横向功率与压缩功率之比、热压与磁场中波功率之比、热压与磁场中微扰之比、速度与磁场中波功率之比。以准垂直激波下游日地线附近的主动磁层粒子示踪探测器/离子释放模块(AMPTE/IRM)磁鞘为例，可以在空间和光谱上清楚地区分这四种模式。在弓激波附近，波在1到100 MHz的大频率范围内是Alfvenic波。在中层和内层磁鞘中，10 MHz以下的波是Alfvenic波。快模波出现在增强频谱的高频端，中磁鞘为80 MHz，内鞘为55 MHz。中频的波增强是内鞘的慢模和中鞘的镜面模。这证实了早先关于在磁层顶附近存在慢模波的报道。这些慢波提供了证据，表明磁层顶是鞘中波的活跃来源。我们还表明，当磁鞘流动处于稳态时，测得的波的频率接近一个不变量。因此，增强波的频率变化表示波的出现、衰减或模式转换。
On the basis of MHD theory we develop a scheme for distinguishing among the four low-frequency modes which may propagate in a high-β anisotropic plasma such as the magnetosheath: the fast and slow magnetosonic, the Alfven, and mirror modes. We use four parameters: the ratio of transverse to compressional powers in the magnetic field, the ratio of the wave powers in the thermal pressure and in the magnetic field, the ratio of the perturbations in the thermal and magnetic pressures, and the ratio of the wave powers in the velocity and in the magnetic field. In the test case of an Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) magnetosheath pass near the Sun-Earth line downstream of a quasi-perpendicular shock, the four modes can be clearly distinguished both spatially and spectrally. Near the bow shock, the waves are Alfvenic in a large frequency range, 1 to 100 mHz. In the middle and inner magnetosheath, the waves below 10 mHz are Alfvenic. The fast mode waves occur in the higher-frequency end of the enhanced spectrum, 80 mHz for the middle magnetosheath and 55 mHz for the inner sheath. The wave enhancement in the intermediate frequencies is slow modes in the inner sheath and mirror modes in the middle sheath. This confirms the earlier report of the existence of the slow mode waves near the magnetopause. These slow waves provide evidence that the magnetopause is an active source of the waves in the sheath. We also show that the measured frequency of a wave is close to an invariant if the magnetosheath flow is in a steady state. Therefore changes in the frequencies of enhanced waves indicate emergence, or damping, or mode conversion of the waves.