Electron energization at oblique SNR shocks in turbulent media

宇宙射线在银河系中的何处以及如何产生是现代物理学中的一个重要问题。来自壳型超新星遗迹（SNR）的非热X射线和高能伽马射线的观测表明，非相对论性无碰撞冲击可以有效地加速带电粒子。扩散冲击加速是加速到非常高能量的可能过程，但是对于SNR冲击，需要将电子预加速到约50 MeV。我们进行了广泛的粒子模拟（PIC）的无碰撞冲击，研究电子加速和加热的冲击过程。在早期模拟均匀环境中的无碰撞冲击的基础上，我们现在建议研究上游介质中预先存在的波动的影响。我们从日光层的测量中知道，这种波动是存在的，并期望它们在SNR环境中也是如此。我们还知道，大尺度上的波动可以驱动磁场放大，并引起冲击的涟漪。这里我们感兴趣的是湍流在小尺度上的影响。我们将研究预先存在的湍流对电子动力学和等离子体波的驱动和饱和的影响。如果预先存在的湍流增强了电子注入的效率，这将表明电子注入问题的答案。这个长期未解决的问题对于我们理解冲击时的粒子加速度至关重要。

Where and how cosmic rays are produced in the Galaxy is an important question in modern physics. Observations of nonthermal X-rays and high-energy gamma-rays from shell-type supernova remnants (SNR) imply that nonrelativistic collisionless shocks can efficiently accelerate charged particles. Diffusive shock acceleration is a likely process for the acceleration to very high energies, but requires a preacceleration of electrons to about 50 MeV for SNR shocks. We conduct extensive particle-in-cell (PIC) simulations of collisionless shocks to investigate the processes of electron acceleration and heating at shocks. Building on earlier simulations of collisionless shocks in homogeneous environments, we now propose to study the impact of pre-existing fluctuations in the upstream medium. We know from measurements in the heliosphere that such fluctuations exist and expect that they do so also in the environment of SNRs. We also know that fluctuations on large scales can drive magnetic field amplification at, and induce rippling of, the shocks. Here we are interested in the effect of turbulence on small scales. We shall investigate the effect of pre-existing turbulence on the electron dynamics and on the driving and saturation of plasma waves. If pre-existing turbulence enhances the efficiency of electron energization, it would indicate an answer to the question of electron injection. This long-standing unsolved problem is crucially important to our understanding of particle acceleration at shocks.