PARTICLE ACCELERATION AND WAVE EXCITATION IN QUASI-PARALLEL HIGH-MACH-NUMBER COLLISIONLESS SHOCKS: PARTICLE-IN-CELL SIMULATION
PARTICLE ACCELERATION AND WAVE EXCITATION IN QUASI-PARALLEL HIGH-MACH-NUMBER COLLISIONLESS SHOCKS: PARTICLE-IN-CELL SIMULATION
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DOI：
10.1088/0004-637x/802/2/115
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发表时间：
2014-07
期刊：
影响因子：
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通讯作者：
T. Kato
中科院分区：
文献类型：
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作者：
T. Kato
本文通过一个长期的、大尺度的、粒子模拟的方法研究了准平行高马赫数无碰撞激波中质子和电子的激波形成和粒子加速过程。我们表明，质子和电子都在冲击波中加速，这些加速粒子在冲击波的上游区域产生大振幅的Alfvénic波。在上游波已经充分增长之后，由于波的大横向磁场，无碰撞激波的局部结构变得基本上类似于准垂直激波的局部结构。一小部分质子在激波中加速，能量分布类似于幂律。质子注入对加速过程的速率近似为常数，在注入过程中，上游波对质子的相位俘获机制起着重要作用。主要的加速过程是通过质子的重复冲击交叉的费米式过程。这个过程是一个“快”的过程，在这个意义上，大多数加速质子完成一个加速过程周期所需的时间比扩散时间短得多。一小部分电子也被相同的机制加速，并具有类似幂律的能量分布。然而，在模拟过程中，喷射没有进入稳定状态，这可能与上游波的间歇性活动有关。在冲击波的上游，一部分电子在到达冲击波之前被预加速，这可能有助于在稍后的时间稳定的电子注入。
We herein investigate shock formation and particle acceleration processes for both protons and electrons in a quasi-parallel high-Mach-number collisionless shock through a long-term, large-scale, particle-in-cell simulation. We show that both protons and electrons are accelerated in the shock and that these accelerated particles generate large-amplitude Alfvénic waves in the upstream region of the shock. After the upstream waves have grown sufficiently, the local structure of the collisionless shock becomes substantially similar to that of a quasi-perpendicular shock due to the large transverse magnetic field of the waves. A fraction of protons are accelerated in the shock with a power-law-like energy distribution. The rate of proton injection to the acceleration process is approximately constant, and in the injection process, the phase-trapping mechanism for the protons by the upstream waves can play an important role. The dominant acceleration process is a Fermi-like process through repeated shock crossings of the protons. This process is a “fast” process in the sense that the time required for most of the accelerated protons to complete one cycle of the acceleration process is much shorter than the diffusion time. A fraction of the electrons are also accelerated by the same mechanism, and have a power-law-like energy distribution. However, the injection does not enter a steady state during the simulation, which may be related to the intermittent activity of the upstream waves. Upstream of the shock, a fraction of the electrons are pre-accelerated before reaching the shock, which may contribute to steady electron injection at a later time.