Three-dimensional numerical simulations of particle injection and acceleration at quasi-perpendicular shocks
Three-dimensional numerical simulations of particle injection and acceleration at quasi-perpendicular shocks
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DOI：
10.1029/1999ja000018
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发表时间：
2000-06
影响因子：
--
通讯作者：
J. Giacalone;D. Ellison
中科院分区：
文献类型：
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作者：
J. Giacalone;D. Ellison
我们展示了准垂直无碰撞冲击的三维数值模拟结果，以确定交叉场扩散是否足够大，以有效地加速热粒子和/或拾取离子。交叉场扩散在包含至少一个可忽略的空间坐标的模拟中被人为抑制。我们发现，虽然模拟的下游分布函数相当陡峭，但一小部分粒子被加速到远高于热能的能量。只有当系统包含的波动波长比感兴趣的粒子（高能粒子）的陀螺半径大得多时，才会发生这种情况。我们发现，这是由于与激波垂直的粒子的输运，对下游对流的输运，在大尺度上主要受到场线弯曲的影响。我们还表明，如果系统不包含这些长波，则散射不足以加速热粒子或拾取离子。我们使用两种不同类型的模拟来证明这一点。在第一组模拟中，我们使用了著名的混合模拟，它自一致地处理粒子与场之间的相互作用。为了计算的可跟踪性，这些模拟使用非常小的空间域，并且抑制了大规模场线随机游走的影响。然而，这个近似准确地解决了共振波长散射的物理问题。测试粒子模拟比混合模拟在计算上更容易处理，也用于更大的系统，并说明长波波的影响。
We present results from three-dimensional numerical simulations of quasi-perpendicular collisionless shocks in order to determine whether cross-field diffusion, which is otherwise artificially suppressed in simulations which contain at least one ignorable spatial coordinate, is large enough to efficiently accelerate thermal particles and/or pickup ions. We find that although the simulated downstream distribution functions are quite steep, a fraction of the particles are accelerated to energies well above the thermal energy. This occurs only if the system contains fluctuations with wavelengths that are considerably larger than the gyroradii of the particles of interest (the high-energy ones), We find that this is due to the fact that the transport of the particles normal to the shock, against the downstream convection, is mostly influenced by the meandering of field lines on large scales. We also show that if the system does not contain these long-wavelength waves, the scattering is not sufficient to accelerate thermal particles or pickup ions. We use two different types of simulations to demonstrate this. In the first set of simulations we use the well-known hybrid simulation which treats the interaction between the particles and fields self-consistently. For computational tractability these simulations use very small spatial domains, and the effect of the large-scale field-line random walk is suppressed. However, this approximation accurately addresses the physics of the scattering at resonant wavelengths. Test-particle simulations, which are more computationally tractable than the hybrid simulations, are also performed for larger systems and to illustrate the effect of the long-wavelength waves.