Ion Dynamics and Distribution At the Quasiperpendicular Collisionless Shock Front
Ion Dynamics and Distribution At the Quasiperpendicular Collisionless Shock Front
复制标题
准垂直无碰撞激波前沿的离子动力学和分布
DOI：
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发表时间：
1997
期刊：
影响因子：
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通讯作者：
M. Gedalin
中科院分区：
文献类型：
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作者：
M. Gedalin
众所周知，无碰撞冲击是非常有效的离子激发器。在激励的第一步，在离子反射和直接传输期间，在准垂直无碰撞激波锋面附近形成相对低能量的超热离子分布。这些分布是在激波宽度尺度上迅速形成的，主要是由于离子与锋面本身的准稳态电磁结构的相互作用。它们的特征与激波前沿的精细结构密切相关，因为它们不仅取决于马赫数等体激波参数，而且还取决于场分布的细节，特别是激波宽度。因此，对这些分布的研究可以提供有关冲击结构本身的有价值的信息。我们回顾了现场测量（主要是地球弓震）期间收集的观测数据，并将其与数值模拟和理论发展进行比较。静止激波前沿中离子动力学的发展理论将离子反射和加热与交叉激波势引起的斜坡中离子的减速不足联系起来，与朗肯-雨戈尼奥关系所需的预期下游漂移速度相比。结果，它将直流能量转化为回转能量，导致离子分布整体回转并增强在速度空间中的扩散，即有效的无碰撞加热。各向异性和非回旋性是低马赫数激波和高马赫数激波下离子分布的典型特征，这已被观测证实。与时间相关的场在稳态激波模型中没有被考虑，但被认为提供了离子分布的后续平滑和各向同性。这些过程发生的规模远大于激波宽度。
Collisionless shocks are well-known to be very efficient energizers of ions. At the first step of energization relatively low energy suprathermal ion distributions are formed in the vicinity of the quasiperpendicular collisionless shock front during ion reflection and direct transmission. These distributions are formed promptly and at the scale of the shock width mainly due to the ion interaction with the quasistationary electromagnetic structure of the front itself. Their features are intimately related to the fine structure of the shock front in the sense that they depend not only on the bulk shock parameters, such as Mach number, but also on the details of the distribution of the fields, in particular, shock width. Therefore, studies of these distributions may provide valuable information about the shock structure itself. We review the observational data collected during in situ measurements (mainly at the Earth bow shock) and compare it to the numerical simulations and theoretical developments. The developed theory of the ion dynamics in the stationary shock front relates the ion reflection and heating to the insufficient deceleration of the ions in the ramp by the cross-shock potential, as compared to the expected downstream drift velocity, required by the Rankine-Hugoniot relations. As a result, the direct flow energy it transferred into the gyration energy, leading to the gyration of the ion distribution as a whole and enhanced spread in the velocity space, that is, effective collisionless heating. Anisotropy and nongyrotropy are typical features of ion distributions at both low and high-Mach number shocks, which is confirmed by observations. Time-dependent fields, which are not considered in the stationary shock model, are thought to provide subsequent smoothing and isotropization of the ion distributions. These processes occur at scales substantially larger than the shock width.