ION ACCELERATION AT THE QUASI-PARALLEL BOW SHOCK: DECODING THE SIGNATURE OF INJECTION
ION ACCELERATION AT THE QUASI-PARALLEL BOW SHOCK: DECODING THE SIGNATURE OF INJECTION
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DOI：
10.3847/0004-637x/820/1/21
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发表时间：
2015-11
期刊：
影响因子：
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通讯作者：
T. Sundberg;C. Haynes;D. Burgess;C. Mazelle
中科院分区：
文献类型：
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作者：
T. Sundberg;C. Haynes;D. Burgess;C. Mazelle
无碰撞激波是高效的粒子加速器。在地球上，当磁几何形状为准平行时，可以在弓激波的上游看到能量超过100keV的离子，而大规模的超新星残余冲击可以将离子加速为宇宙射线能量。这种能化归因于扩散激波加速；然而，为了使这一过程变得活跃，离子必须首先得到足够的能量。这种初始加速度如何以及在哪里发生一直是激波加速理论中尚未解决的关键问题之一。利用星团航天器的观测，我们研究了地球弓激波上游湍流过渡层中离子反射事件的特征，并在激波的混合模拟的支持下，我们证明了这些反射特征是离子注入过程的第一步的特征。这些反射事件特别是在大振幅上游波的后缘与局部激波斜坡相交，上游磁场由准垂直变为准平行的区域内发展起来的。观察到的弥散离子速度特征可以归因于在该波边界上离子反射的快速连续。在离子与激波的初始相互作用后，它们沿着准平行磁场向上流动。上游区域的每个后续波前都会将离子扫向激波，在那里，它们在上游和激波框架之间的每一次转换中都会获得能量。在三到五个回转周期内，一些离子获得了足够的平行速度向上游逃逸，从而完成了注入过程。
Collisionless shocks are efficient particle accelerators. At Earth, ions with energies exceeding 100 keV are seen upstream of the bow shock when the magnetic geometry is quasi-parallel, and large-scale supernova remnant shocks can accelerate ions into cosmic-ray energies. This energization is attributed to diffusive shock acceleration; however, for this process to become active, the ions must first be sufficiently energized. How and where this initial acceleration takes place has been one of the key unresolved issues in shock acceleration theory. Using Cluster spacecraft observations, we study the signatures of ion reflection events in the turbulent transition layer upstream of the terrestrial bow shock, and with the support of a hybrid simulation of the shock, we show that these reflection signatures are characteristic of the first step in the ion injection process. These reflection events develop in particular in the region where the trailing edge of large-amplitude upstream waves intercept the local shock ramp and the upstream magnetic field changes from quasi-perpendicular to quasi-parallel. The dispersed ion velocity signature observed can be attributed to a rapid succession of ion reflections at this wave boundary. After the ions’ initial interaction with the shock, they flow upstream along the quasi-parallel magnetic field. Each subsequent wavefront in the upstream region will sweep the ions back toward the shock, where they gain energy with each transition between the upstream and the shock wave frames. Within three to five gyroperiods, some ions have gained enough parallel velocity to escape upstream, thus completing the injection process.