Nonthermal electron acceleration at collisionless quasi-perpendicular shocks
Nonthermal electron acceleration at collisionless quasi-perpendicular shocks
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无碰撞准垂直冲击下的非热电子加速
DOI：
10.1007/s41614-022-00093-1
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发表时间：
2022
期刊：
影响因子：
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通讯作者：
Hoshino Masahiro
中科院分区：
文献类型：
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作者：
Amano Takanobu;Matsumoto Yosuke;Bohdan Artem;Kobzar Oleh;Matsukiyo Shuichi;Oka Mitsuo;Niemiec Jacek;Pohl Martin;Hoshino Masahiro
几十年来，人们对在无碰撞日光层和天体物理等离子体中传播的冲击波进行了广泛的研究。一个主要动机是了解冲击时的非热粒子加速度。尽管扩散冲击加速（DSA）理论长期以来一直是冲击时宇宙射线加速的标准，但对粒子加速的等离子体物理理解仍然难以捉摸。在这篇综述中，我们讨论了准垂直激波下的非热电子加速机制，近年来在这方面取得了实质性进展。本综述中的讨论仅限于以下三个具体主题：第一个是随机冲击漂移加速（SSDA），这是一种相对较新的电子注入 DSA 的机制。讨论其基本机制、相关的原位观测和动力学模拟结果以及如何与DSA联系。其次，我们讨论与年轻超新星遗迹（SNR）相关的极高马赫数冲击下的冲击冲浪加速度（SSA）。虽然一维假设下的最初提议是不现实的，但 SSA 现已通过全三维动力学模拟证明是有效的。我们讨论了 SSA 的多维性质及其在电子注入中的作用。最后，我们讨论了目前对磁化韦贝尔主导激波的理解。它本质上是一种磁化激波，其中反射旋动离子主导激波结构的形成，但由于离子韦贝尔不稳定性而具有显着的磁场放大。激波结构内自生电流片的自发磁重联是韦贝尔产生的强磁湍流的一个有趣的结果。尽管主动磁重联的确切条件尚未阐明，但我们认为，阿尔文马赫数和声音马赫数都超过 20-40 的高马赫数激波很可能表现为韦贝尔主导激波。尽管最近有许多有趣的发现，但 SSDA、SSA 和磁重联对于无碰撞冲击下电子加速的相对作用以及主要粒子加速机制如何根据冲击参数而变化仍有待解答。
Shock waves propagating in collisionless heliospheric and astrophysical plasmas have been studied extensively over the decades. One prime motivation is to understand the nonthermal particle acceleration at shocks. Although the theory of diffusive shock acceleration (DSA) has long been the standard for cosmic-ray acceleration at shocks, plasma physical understanding of particle acceleration remains elusive. In this review, we discuss nonthermal electron acceleration mechanisms at quasi-perpendicular shocks, for which substantial progress has been made in recent years. The discussion presented in this review is restricted to the following three specific topics: The first is stochastic shock drift acceleration (SSDA), which is a relatively new mechanism for electron injection into DSA. The basic mechanism, related in-situ observations and kinetic simulations results, and how it is connected with DSA will be discussed. Second, we discuss shock surfing acceleration (SSA) at very high Mach number shocks relevant to young supernova remnants (SNRs). While the original proposal under the one-dimensional assumption is unrealistic, SSA has now been proven efficient by a fully three-dimensional kinetic simulation. We discuss the multidimensional nature of SSA and its role in electron injection. Finally, we discuss the current understanding of the magnetized Weibel-dominated shock. It is essentially a magnetized shock in which the reflected-gyrating ions dominate the formation of the shock structure but with a substantial magnetic field amplification by the ion-Weibel instability. Spontaneous magnetic reconnection of self-generated current sheets within the shock structure is an interesting consequence of Weibel-generated strong magnetic turbulence. Although the exact condition for active magnetic reconnection has not been clarified, we argue that high Mach number shocks with both Alfvén and sound Mach numbers exceeding 20–40 will likely behave as a Weibel-dominated shock. Despite a number of interesting recent findings, the relative roles of SSDA, SSA, and magnetic reconnection for electron acceleration at collisionless shocks and how the dominant particle acceleration mechanisms change depending on shock parameters remain to be answered.