基于测试粒子模拟的垂直无碰撞激波对离子的加速研究

本文利用测试粒子模拟的方法研究离子在通过垂直无碰撞激波结构时的加速.我们首先探究了在理想激波条件下，离子通过激波结构后的能量增益与其初始参数(包括回旋相位角、初始位置和上游平均能量)的关系; 然后进一步探索了离子通过由自洽的一维混合模拟给出的更接近真实的激波结构时获得的加速，研究了激波内部的电场和磁场结构对离子能量增益的影响.结果显示，当离子能量较低(离子回旋半径小于激波面厚度或相当)时，激波横越电势和磁过冲对离子的能量增益起到促进作用; 而当能量较高时，激波内部结构对于离子能量增益的影响逐渐可以忽略. 此外，与激波横越电势相比，激波磁过冲的强度对离子能量增益的影响占主导，离子的能量增益随着磁过冲的增强而增强.最后，我们发现激波的磁场过冲和横越电势对于离子能量增益的影响之间的关系比较复杂，不能简单叠加，甚至有互相抵消的效应.作为示例，我们给出了一个激波法向电场的双极结构使得离子加速变弱的例证.

Test particle simulations of ion acceleration at perpendicular collisionless shocks

Test particle simulations are performed to investigate ion acceleration at perpendicular collisionless shocks. The dependence of the ion energy gain on some key ion parameters (i.e., initial gyro-phase angle, position, and average upstream kinetic energy) is first explored for an ideal shock profile given by the Rankine-Hugoniot relations. The shock profile is then switched to more realistic ones given by a self-consistent one-dimensional hybrid shock simulation, in order to examine the influence of small scale, internal electric and magnetic shock structures on the ion acceleration. The results show that, when the ion initial energy is relatively low (with the ion gyroradius comparable or shorter than the shock transition thickness), the presence of the electric cross-shock potential and the magnetic shock overshoot enhances the ion energy gain. However, when the ion initial energy becomes higher, the influence of the small scale, internal shock structures diminishes and becomes negligible. In addition, the ion energy gain increases with the strength of the magnetic shock overshoot. The influence of the magnetic overshoot strength on the ion energy gain dominates that of the electric cross-shock potential. Their individual contributions do not simply add up and can cancel each other at times. One such example is presented to demonstrate how the bipolar shock normal electric field structure may reduce the ion energy gain.