前震瞬变和相关二次震的形成
Formation of foreshock transients and associated secondary shocks
摘要：
在冲击波的上游，前震充满了热离子。当这些离子被集中和热化周围的一个不连续性，一个抗磁腔的压缩边界，称为前震瞬变，形式。有时，上游压缩边界可以进一步变陡成二次激波，这已经被观察到加速粒子并有助于一次激波的加速。然而，二次冲击波的形成条件和过程还没有完全了解。使用粒子模拟，我们揭示了二次冲击是如何形成的。从一维模拟，我们表明，电场在塑造冲击的磁场结构，以及在耦合的热离子的能量的冲击中发挥了关键作用。我们证明了较大的热离子的热速度和浓度比有利于二次冲击的形成。通过更真实的2D模拟，我们研究了不连续性如何与前震离子相互作用，导致前震瞬变和二次冲击的形成。我们的研究结果表明，二次冲击更可能发生在主激波与较高的马赫数。由于二次激波先前已被证明具有与行星弓形激波合作加速粒子的能力，因此在高马赫数天体物理激波的粒子加速中考虑二次激波就更有吸引力了。
英文摘要：
Upstream of shocks, the foreshock is filled with hot ions. When these ions are concentrated and thermalized around a discontinuity, a diamagnetic cavity bounded by compressional boundaries, referred to as a foreshock transient, forms. Sometimes, the upstream compressional boundary can further steepen into a secondary shock, which has been observed to accelerate particles and contribute to the primary shock acceleration. However, secondary shock formation conditions and processes are not fully understood. Using particle-in-cell simulations, we reveal how secondary shocks are formed. From 1D simulations, we show that electric fields play a critical role in shaping the shock's magnetic field structure, as well as in coupling the energy of hot ions to that of the shock. We demonstrate that larger thermal speed and concentration ratio of hot ions favors the formation of a secondary shock. From a more realistic 2D simulation, we examine how a discontinuity interacts with foreshock ions leading to the formation of a foreshock transient and a secondary shock. Our results imply that secondary shocks are more likely to occur at primary shocks with higher Mach number. With the secondary shock's previously proven ability to accelerate particles in cooperation with a planetary bow shock, it is even more appealing to consider them in particle acceleration of high Mach number astrophysical shocks.
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