行星际磁云边界层中的电子分布函数特征
doi: 10.11728/cjss2009.04.359 cstr: 32142.14.cjss2009.04.359
Characteristics of Electron Distribution Functions in Magnetic Cloud Boundary Layers
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摘要: 基于WIND 飞船观测的1995---2006 年间的磁云事件, 研究了磁云边界层中电子的流动图样, 以及电子速度分布函数的特点与电子加热和加速的关系, 得出以下结果. ①磁云边界层中存在的电子流动图样, 包括各向同性、双向流动和单向流动等. ② 相比于背景太阳风和磁云本体, 通常情况下磁云边界层中电子分布函数的核心热电子成分 (E<60eV) 增多, 超热电子成分 (E>60eV) 在沿磁场垂直方向上增多, 而在沿磁场平行或反平行方向以单方向增加为主, 此外, 还在近1/10 的磁云边界层中观测到了高能电子的明显增多. ③对比研究了磁云边界层与磁云驱动的激波对电子速度分布函数的调制作用, 经过激波, 电子分布函数的超热电子成分在各方向上都有增加, 不同于磁云边界层中在沿磁场平行或反平行方向上超热电子成分以单方向增加为主, 表明二者有不同的形成机制. ④考察了磁云边界层中的波活动增强和电子分布函数及离子流量增加的对应关系. 上述观测和对比分析进一步表明了磁云边界层是一种重要的动力学结构, 磁重联是一种可能的形成机制.Abstract: Based on the magnetic cloud events detected by WIND spacecraft from 1995 to 2006, here are studied on the patterns of electron flows, characteristics of the electron distribution functions as well as their relationships with the heating and acceleration of electrons in the Magnetic Clouds Boundary Layers (MCBL). The main results are as follows. Firstly, there are several types of electron flow patterns in the MCBL. Secondly, compared with the background solar wind and the magnetic cloud body, the thermal core parts of the electron velocity distribution functions usually increase, together with increasing of superthermal electrons in directions perpendicular to the magnetic field and either parallel or anti-parallel to the magnetic field. Furthermore, the distributions of energetic electrons increase in all directions in almost ten percent of the MCBL. Thirdly, by comparisons of the modifications to the electron distribution functions by MCBL and interplantery shocks ahead of magnetic clouds, it suggests that the two structures result from different mechanisms. Lastly, investigation on the relationships between the enhancements of wave activities and electron distribution functions as well as ion fluxes was also made. These observations and comparisons indicate further that magnetic clouds are important dynamical structures possibly associated with magnetic reconnection as previous studies on MCBL have shown.
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