The First Direct Observational Confirmation of Kinematic Collisionless Relaxation in Very Low Mach Number Shocks Near the Earth
The First Direct Observational Confirmation of Kinematic Collisionless Relaxation in Very Low Mach Number Shocks Near the Earth
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DOI：
10.1029/2018ja026223
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发表时间：
2019-03
期刊：
影响因子：
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通讯作者：
S. Pope;M. Gedalin;M. Balikhin
中科院分区：
文献类型：
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作者：
S. Pope;M. Gedalin;M. Balikhin
在已知的宇宙中，无碰撞激波无处不在。它们主要将定向离子流的能量转化为热量。在穿过激波锋后，离子的分布变成非涡旋的。旋翼性的松弛主要通过运动无碰撞的旋相混合和与波的相互作用发生。无碰撞弛豫理论预测，随着离激波过渡层的距离增加，各离子的下游压力呈准周期性变化，变化幅度逐渐减小。每个物种的振动都有自己的空间周期和阻尼尺度。压力平衡要求等离子体总压力的变化应引起磁压力的反相关变化。这一过程在所有马赫数下都应该发生，但在中/高马赫数下很难观察到。相反，这种磁振荡已经在低马赫数的金星弓形激波和行星际激波中被观察到。本文首次利用THEMIS‐B和THEMIS‐C航天器同时进行的原位磁场和等离子体测量，研究了低马赫数冲击下的反相关总离子和磁压空间变化。研究发现，运动无碰撞弛豫是形成下游离子分布和形成观察到的冲击下游磁剖面的主要过程，证实了基本的理论结果。通过与数值模型结果的比较，可以研究不同离子种类的作用，并证实重离子在形成下游磁剖面中的作用。
Collisionless shocks are ubiquitous throughout the known universe. They mainly convert the energy of the directed ion flow into heating. Upon crossing the shock front, the ion distribution becomes nongyrotropic. Relaxation to gyrotropy then occurs mainly via kinematic collisionless gyrophase mixing and interaction with waves. The theory of collisionless relaxation predicts that the downstream pressure of each ion species varies quasi‐periodically with the distance from the shock transition layer and the amplitude of the variations gradually decrease. The oscillations due to each species have their own spatial period and damping scale. Pressure balance requires that the variations in the total plasma pressure should cause anticorrelating variations in the magnetic pressure. This process should occur at all Mach numbers, but its observation is difficult at moderate‐/high‐Mach numbers. In contrast, such magnetic oscillations have been observed at low Mach number cases of the Venusian bow shock and interplanetary shocks. In this paper, simultaneous in situ magnetic field and plasma measurements from the THEMIS‐B and THEMIS‐C spacecraft are used to study, for the first time, the anticorrelated total ion and magnetic pressure spatial variations at low‐Mach number shocks. It is found that kinematic collisionless relaxation is the dominant process in the formation of the downstream ion distribution and in shaping the downstream magnetic profile of the observed shocks, confirming fundamental theoretical results. Comparison with the results from numerical models allows the role of the different ion species to be investigated and confirms the role heavy ions play in forming the downstream magnetic profile.