NON-THERMAL ELECTRON ACCELERATION IN LOW MACH NUMBER COLLISIONLESS SHOCKS. I. PARTICLE ENERGY SPECTRA AND ACCELERATION MECHANISM
NON-THERMAL ELECTRON ACCELERATION IN LOW MACH NUMBER COLLISIONLESS SHOCKS. I. PARTICLE ENERGY SPECTRA AND ACCELERATION MECHANISM
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DOI：
10.1088/0004-637x/794/2/153
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发表时间：
2014-06
期刊：
影响因子：
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通讯作者：
Xinyi Guo;L. Sironi;R. Narayan
中科院分区：
文献类型：
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作者：
Xinyi Guo;L. Sironi;R. Narayan
对星系团和太阳耀斑的射电和X射线观测揭示了电子在低马赫数(ms≲5)激波中加速为非热能的现象，但电子加速的机制尚不清楚。扩散激波加速，也称为一阶费米加速，不能直接用来解释电子的加速。相反，需要一种额外的机制来预先将电子从热能加速到超热能量，这样它们才能参与费米过程。在这项工作中，我们使用二维和三维粒子胞内等离子体模拟来研究低马赫数激波中的电子加速。我们主要研究了在准垂直激波前磁场作用下，M_s=3的参考行程中的粒子能谱和加速机制。我们发现，大约15%的电子可以被有效地加速，在能谱上形成一个非热幂函数尾巴，斜率为p≃2.4。最初，热电子通过激波漂移加速(SDA)在激波前沿被激发。然后，加速的电子被反射回上游，在那里它们与到来的气流相互作用产生磁波。反过来，这些波将上游传播的电子散射回激波，通过SDA进一步激发能量。总而言之，自产生的波允许SDA的重复循环，类似于持续的费米过程。这一机制自然地解决了从星系团外围观测到的明亮的射电同步辐射与低马赫数激波中通常预期的低电子加速效率之间的冲突。
Electron acceleration to non-thermal energies in low Mach number (Ms ≲ 5) shocks is revealed by radio and X-ray observations of galaxy clusters and solar flares, but the electron acceleration mechanism remains poorly understood. Diffusive shock acceleration, also known as first-order Fermi acceleration, cannot be directly invoked to explain the acceleration of electrons. Rather, an additional mechanism is required to pre-accelerate the electrons from thermal to supra-thermal energies, so they can then participate in the Fermi process. In this work, we use two- and three-dimensional particle-in-cell plasma simulations to study electron acceleration in low Mach number shocks. We focus on the particle energy spectra and the acceleration mechanism in a reference run with Ms = 3 and a quasi-perpendicular pre-shock magnetic field. We find that about 15% of the electrons can be efficiently accelerated, forming a non-thermal power-law tail in the energy spectrum with a slope of p ≃ 2.4. Initially, thermal electrons are energized at the shock front via shock drift acceleration (SDA). The accelerated electrons are then reflected back upstream where their interaction with the incoming flow generates magnetic waves. In turn, the waves scatter the electrons propagating upstream back toward the shock for further energization via SDA. In summary, the self-generated waves allow for repeated cycles of SDA, similarly to a sustained Fermi-like process. This mechanism offers a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low electron acceleration efficiency usually expected in low Mach number shocks.