Particle acceleration in a reconnecting current sheet: PIC simulation
Particle acceleration in a reconnecting current sheet: PIC simulation
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DOI：
10.1017/s0022377809008009
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发表时间：
2009-05
影响因子：
2.5
通讯作者：
T. Siversky;V. Zharkova
中科院分区：
文献类型：
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作者：
T. Siversky;V. Zharkova
摘要采用粒子池（PIC） 2D3V（二维空间和三维速度空间）编码，在质子电子质量比为100的条件下，模拟了重连电流片（RCS）中质子和电子的加速过程。形成RCS的电磁结构包含磁场（包括引导场）和漂移电场的所有三个组成部分。PIC模拟表明，在加速过程中，由于电子与质子向RCS的中间层分离而产生极化电场。当等离子体密度较低时，极化场较弱，PIC模拟的粒子轨迹与测试粒子（TP）方法相似。等离子体密度越高，极化场越强，在加速过程中，极化场通过增加质子的轨道来影响质子的运动轨迹。与TP方法相比，该场还导致向中间层喷射的质子的不对称丰度减少。对于给定的磁性拓扑，PIC模拟中的电子被喷射到与质子相同的半空间，与TP结果相反。这是因为极化场的范围远远超出了电流片的厚度。这个场使电子减速，这些电子最初被弹射到与质子相反的半空间中，并将它们返回到RCS，最终导致电子弹射到与质子相同的半空间中。与TP法得到的双峰窄能量分布不同，该方法的发射电子能量分布是单峰宽的。在强引导场的情况下，发现喷射电子的平均能量比解析和TP模拟预测的要小。加速电子束也被发现以朗缪尔波的形式产生紊流电场。
Abstract The acceleration of protons and electrons in a reconnecting current sheet (RCS) is simulated with a particle-in-cell (PIC) 2D3V (two-dimensional in space and three-dimensional in velocity space) code for the proton-to-electron mass ratio of 100. The electromagnetic configuration forming the RCS incorporates all three components of the magnetic field (including the guiding field) and a drifted electric field. PIC simulations reveal that there is a polarization electric field that appears during acceleration owing to a separation of electrons from protons towards the midplane of the RCS. If the plasma density is low, the polarization field is weak and the particle trajectories in the PIC simulations are similar to those in the test particle (TP) approach. For the higher plasma density the polarization field is stronger and it affects the trajectories of protons by increasing their orbits during acceleration. This field also leads to a less asymmetrical abundance of ejected protons towards the midplane in comparison with the TP approach. For a given magnetic topology electrons in PIC simulations are ejected to the same semispace as protons, in contrast to the TP results. This happens because the polarization field extends far beyond the thickness of a current sheet. This field decelerates the electrons, which are initially ejected into the semispace opposite to the protons, returns them back to the RCS, and, eventually, leads to the electron ejection into the same semispace as protons. The energy distribution of the ejected electrons is rather wide and single-peaked, in contrast to the two-peak narrow-energy distribution obtained in the TP approach. In the case of a strong guiding field, the mean energy of the ejected electrons is found to be smaller than it is predicted analytically and by the TP simulations. The beam of accelerated electrons is also found to generate turbulent electric field in the form of Langmuir waves.