We investigate effects of upstream density fluctuations on the diffusive shock acceleration by Monte Carlo simulations. The simulations show that particles are reaccelerated in the shock downstream region by a sound wave generated at the shock front. The time scale of turbulent acceleration by the sound wave is estimated. We propose a new back reaction of particles accelerated around the shock front. The accelerated particles generate the upstream density fluctuations by the Drury instability, which are converted to the downstream sound waves by the shock. The downstream sound waves modify the momentum spectrum of particles accelerated around the shock front. This new back reaction affects emission from the accelerated particles, which gives a new constraint on the acceleration efficiency of the diffusive shock acceleration.

中文翻译：

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传播到非均匀介质的冲击波中的粒子加速度

我们通过蒙特卡罗模拟研究上游密度波动对扩散冲击加速度的影响。模拟表明，粒子在激波下游区域被激波前沿产生的声波重新加速。估计声波湍流加速度的时间尺度。我们提出了一种新的粒子在激波前沿周围加速的反向反应。加速粒子通过德鲁不稳定性产生上游密度波动，通过激波将其转化为下游声波。下游的声波改变了在激波前沿周围加速的粒子的动量谱。这种新的反向反应影响了加速粒子的发射，这对扩散冲击加速度的加速效率提出了新的限制。