【作者】 王荣生；

【导师】 陆全明； 李中元；

【作者基本信息】 中国科学技术大学 ， 空间物理， 2010， 博士

【摘要】 磁场重联是空间物理和实验室等离子体物理中重要的研究课题,也是研究热点和难点。卫星观测是研究磁场重联的重要手段之一。本文利用Cluster卫星磁尾观测数据,研究了磁场重联扩散区结构、电子密度空穴、高能电子加速、电子投掷角分布和亚暴等问题,得到了一些重要结果，具体如下：1.无碰撞磁场重联扩散区结构及次级磁岛利用Cluster卫星2003年10月4日的观测数据,分析了单个重联事例。观测发现，离子扩散区中心区域出现了次级磁岛。次级磁岛带有很强的核心磁场，磁岛长宽比约为2：1.即磁岛是扁平结构，在z方向被压缩(GSM坐标系)。磁岛内部,能量达200keV的高能电子通量出现峰值,电子平行温度大于垂直温度，对磁岛内部结构进一步分析发现,磁岛的“外部区”电子密度出现峰值.而磁岛“核心区”电子密度降低到很低水平。同时发现磁岛“外部区”.沿磁力线方向存在很强的电子束流，从而形成了由磁场数据计算得到的逆着磁力线的电流。此电流产生了核心区很强的核心磁场,挤压电子，使得电子堆积于磁岛的“外部区”，分析扩散区内电子谱线发现，扩散区内电子分布呈平顶分布,在磁岛的“外部区”,这种平顶分布依然存在,但是在“核心区”.平顶分布消失,出现了电子的指数分布。由于低能电子被加速,低能电子的相空间密度必然降低,而较高能量电子的相空间密度增加,使得在一定能量范围内，电子的相空间密度保持定值,及所谓的平顶分布。重联电场和扩散区及磁岛“外部区”观测到的低混杂波可能同时加速电子，形成了观测上的高能电子分布。2.无碰撞磁场重联扩散区内电子投掷角分布利用2001年9月10日的观测数据,发现Cluster卫星分成两组同时穿越了磁尾重联扩散区的南北部分。分析发现,X线区域附近分界线外侧区域,低能电子呈双向分布、而高能电子呈各向同性分布；分界线以内的区域,低能电子依旧出现双向分布.而高能电子出现逆着磁力线方向的电子束流。对于出流区域,分界线附近,低能电子沿磁力线进入X线区域、高能电子逆着磁力线流出X线区域,靠近电流片区域,低能电子出现双向分布、而较高能量的电子各向同性分布。利用粒子模拟,给出了这些分布的形成原因。X线区域,低能电子由于磁镜效应,将在X线区域多次弹跳,故低能电子出现双向分布。在X线区域被加速的电子，沿磁力线离开,形成高能尾,所以,出流区分离线附近,低能电子流入X线区域、高能电子流出。当卫星进入出流区电流片中心区域,低能电子出现双向分布而高能电子各向同性。由于磁镜效应,低能电子沿着磁力线多次来回弹跳,形成了这种双向分布。对于高能电子.电子的回旋半径和磁场曲率相当.电子随机运动、形成了各向同性的分布。3.无碰撞磁场重联扩散区内高能电子空间分布利用2001年9月10日的重联事例,分析了扩散区内部高能电子的空间分布。分析发现,高能电子堆积于重联堆积区,而在X线区域高能电子很少。等离子体温度也出现了类似的变化：X线附近温度降低,而在堆积区温度升高。此外,高能电子分布在X线地侧和尾侧不对称,在地侧的高能电子通量高于尾侧的通量。依据观测推断电子只在扩散区被加速,被加速电子堆积在重联堆积区。观测否定了二步加速机制,因为如果电子在堆积区被进一步加速,我们将会在堆积区之外观测到更多高能电子。X线地侧和尾侧高能电子分布的不对称性可能是由于地球偶极磁场所致。4.磁层亚暴单事例分析磁尾磁场重联和磁层亚暴关系密切,利用位于中磁尾的CLUSTER卫星,同步轨道附近LANL-01、LANL-97卫星,近磁尾POLAR和极区IMAGE卫星的观测,分析了单个亚暴事例。在此事件中,中磁尾磁场重联起始比近尾电流片中断早3分钟发生,电流片中断发生4分钟后,IMAGE卫星观测到极光增亮,同时AE指数突然增大,亚暴膨胀相起始。观测结果与亚暴中性线模型较为吻合。更多还原

【Abstract】 Magnetic reconnection is one important subject of space physics and laboratory plasma physics. It is a very difficult issue and hot topic also. Observation by spacecrafts is one important method to study magnetic reconnection. Using measurements of the Cluster spacecraft, we study structure of ion diffusion region, electron density cavity, energetic electron acceleration, electron pitch angle distribution, substorrn. and so on. The results are shown as follows.1. Structure of ion diffusion region of collisionless magnetic reconnection and secondary magnetic islandUsing the measurement of Cluster on 4th October 2003. a magnetic reconnection event is reported. One secondary island with a strong core magnetic field is found near the center of the ion diffusion region. The aspect ratio of the island is about 2:1. which means the island is squashed in z axis of GSM coordinates. The fluxes of energetic electron up to 200keV peak within the island and the parallel temperature is stronger than the perpendicular temperature. Furthermore, electron density peaks in the outer region while dips in the core region of the island. At the same time, a powerful electron beam along the magnetic field is found in the outer region of the island, which would form an electron current antiparallel to the magnetic field lines in the region. The electric current produces the strong core magnetic field observed in the core region. This core magnetic field will expel electrons out of the core region. Thus electrons will pile up in the outer region of the island. Electron energy spectrum in the ion diffusion region displays fiat-top distribution which can be observed in the outer region of the island also. However, the distribution shows exponential feature in the core region. Electrons with the lower energy are accelerated to the higher energy. Consequently, the PSD at the lower energy declines whereas that at the higher energy increases, which leads to the constant PSD over certain energy range. So. the flat-top distribution is formed. Both the reconnection electric field and lower hybrid waves are observed in the diffusion region and the outer region of the island. The electric field and lower hybrid waves might accelerate electrons to higher energy.2. Electron pitch angle distribution in ion diffusion region of collisionless magnetic reconnectionOn 10 September 2001. Cluster crossed a diffusion region of magnetic reconnection from tailward to earthward in the magnetotail. In the vicinity of the X line, at lower energies the distributions are field-aligned bidirectional anisotropic. while at higher energies, the electrons are observed to flow away from the X line along the magnetic field lines. The electron distributions change largely in the outflow region. At the edge of the outflow region, at lower energies, the electrons flow toward the X line, while the electrons at higher energies are directed away from the X line. When the spacecraft approaches the center of the current sheet, at lower energies, the electrons have field-aligned bidirectional distributions, while at higher energies, the electron distributions are isotropic. The generation mechanisms of such distributions are explained by following typical electron trajectories in the electric and magnetic fields of magnetic reconnection which are obtained in two-dimensional particle-in-cell simulations. It is shown that the observed high-energy electrons directed away from the X line both in the vicinity of the X line and in the outflow region are due to the acceleration by the reconnection electric field near the X line, and the field-aligned bidirectional distributions at lower energies are caused by the effects of the magnetic mirror in the reconnection site. The isotropic distributions at higher energies in the outflow region are the results of the electron stochastic motions when their gyroradii are comparable to the curvature radii of the magnetic field lines.3. Spatial distribution of energetic electrons in ion diffusion region of collisionless magnetic reconnectionBy the magnetic reconnection event observed by Cluster on 10 September 2001, we analyzed spatial distribution of energetic electrons in the ion diffusion region. Most of energetic electrons reside in the magnetic field line pileup region, and a depletion of energetic electrons can be found near the centre of the diffusion region. The temperature shows a similar distribution. Moreover, the energetic electron fluxes in the earthward of the active X-line are larger than those in the tailward. Therefore, we can conclude that the diffusion region is only region where electrons are accelerated, and the maximum energy reaches in the magnetic field line pileup region. There is no secondary acceleration in the magnetic field line pileup region; otherwise we should find that the peak of the energetic electrons will continue to increase when the satellites leave this region. The asymmetrical distribution of energetic electrons between the earthward and the tailward of the X line might be caused by the dipolar magnetic field of Earth.4. Single substorm eventSubstorm is closely correlated with magnetic reconnection in the magnetotail. With measurements from the Cluster spacecraft in the middle magnetotail, LANL-01 and LANL-97 at the synchronous orbit, POLAR in the near-earth magnetotail and IMAGE at polar region, a single substorm event is presented. The analysis indicates that magnetic reconnection is 3 minutes earlier than the current disruption in the near-earth magnetotail. Aurora brightening is observed by IMAGE 4 minutes after the current disruption. Simultaneously, the sharply increment of the AE index implies the substorm onset. The observational result is consistent with the Near Earth Neutral Line model (NENL).更多还原