【作者】 周猛；

【导师】 邓晓华； Maha Ashour-Abdalla；

【作者基本信息】 武汉大学 ， 空间物理学， 2009， 博士

【摘要】 地球磁尾是地球磁层的重要组成部分。许多发生在磁尾的爆发性现象对人类的生活有重大影响,比如磁暴产生的高能粒子能损坏近地空间的人造卫星,而电离层性质的剧烈改变能影响无线通讯。另外,磁尾还是人类研究等离子物理的天然实验室。磁场重联是天体物理中的普遍现象。磁场重联可以在短时间内将磁能转化为粒子动能和热能,并且改变磁场的大尺度拓扑结构。在磁场重联区有丰富的波动现象,但到底是哪种波动在重联层中占主导,这些波动仅仅是重联的副产物,还是能够触发或者调制重联?这些都是空间和试验等离子体物理学家需要解答的问题。磁零点是重联中至关重要的一个区域,在零点磁力线断开并重新连接。揭开磁零点的详细结构,波动和粒子特征,对于理解三维磁场重联是很有意义的。高能粒子注入以及偶极化是亚暴过程的两个重要组成部分,对它们的研究可以让我们了解亚暴过程的能量释放机制,以及亚暴的触发机制。在本博士论文里,我们结合卫星观测和数值模拟,对发生在地球磁尾的多尺度动力学过程进行了研究,主要讨论了波粒相互作用,离子和电子加速过程。以下是本文的主要成果：1,研究了Cluster卫星观测到的重联扩散区内,不同区域内不同波动的特征。我们在重联扩散区内薄电流片附近确认了静电和电磁模式的低混杂漂移波的存在。在卫星穿越磁分界线时,观测到了等离子体流和Hall磁场的反向,同时观测到了强的静电模式的低混杂漂移波。在扩散区的中心电流片内观测到了强的电磁扰动,使用多卫星干涉法对波的色散关系进行分析,发现波的模式与低混杂漂移波一致。这是首次报道在重联扩散区内观测到电磁模式的低混杂漂移波。我们估算了由该电磁波动提供的反常电阻,发现不足以提供卫星实测的电场。我们研究了在X线地向侧有小导向场存在的高β区域观测到的低频波动模式。通过K滤波法得到了低频部分的波矢,发现波动是高斜向传播的。将实测的色散关系与理论色散关系做了比较,证实了Alfven-Whistler波在重联层的存在。另外,在扩散区内的磁分界线上找到了一个密度耗空区,并且研究了耗空区内的波粒相互作用过程。在耗空区内存在强的反平行电流,并且里面有介于离子回旋频率和低混杂频率间的静电波动。极化分析表明该波动是线性准垂直极化的,同低混杂波的性质一致。同时还观测到了斜向传播的哨声波。电子分布有平行方向的电子束。我们讨论了耗空区内可能的波粒相互作用,以及耗空区对电子加速的作用。2,报道了Cluster卫星对扩散区内磁零点结构的局地观测,以及对应的电子动力学和波动特征。我们在扩散区内找到了可能的螺旋零点对以及由三个零点组成的零点簇结构。发现零点及磁场Bz分量的双极化结构和能量至100keV的高能电子通量增强之间存在着紧密联系。Cluster4颗卫星处于3维磁场重联的不同拓扑区域内,并且其中一颗卫星离电子尺度的零点磁分离线仅19km。在穿越磁分离线的过程中,观测到了半宽为4-6个电子惯性长度的薄电流片,反平行方向电流的峰值,以及能量电子通量增强,此时电子能谱最硬,幂指数约为-3.4。在磁分离线附近还观测到了静电孤立波,哨声波和低混杂波,说明电子动力学和波粒相互作用在无碰撞磁场重联中起很重要的作用。还发现零点对之间的fan平面的夹角同理论预测的由哨声波调制的重联层中,哨声波的最大群速度张角一致。3,开展了一系列的2维粒子模拟来研究在磁层中经常观测到的电子等离子体幅度调制波的产生机制。发现弱电子束不稳定性可以激发调制的Langmuir波,而当背景磁场比较大时,垂直方向上的电场没有调制现象。观测到的垂直极化的调制波可以由弱的损失锥不稳定性激发形成。当弱电子束带有损失锥分布时,调制的波动呈现出很快的在平行极化和垂直极化之间的转换,这也解释了在重联层中观测到的这种波形的产生机制。波形短时间内极化的转变可能是由于不同的调制波动的捕获相位不同导致的。4,通过THEMIS和LANL卫星观测,以及大尺度动力学模拟,研究了一次亚暴注入事件。我们通过跟踪大量粒子在全球磁流体力学模拟得到的电磁场中的运动,模拟了高能离子注入。我们的模拟可以重构出THEMIS和LANL卫星观测到的注入事件的主要现象,包括通量增加的时序和色散特性。亚暴期间粒子主要通过两个区域获得能量。一个区域是在近尾x线附近(X--20 RE),粒子在该区域在强感应电场的作用下非绝热的获得能量。另一个区域是在X=-18 RE至X=-7RE之间的若干个狭长或分立的区域,这些区域磁场较强,在该区域粒子在强的势电场的作用下非绝热的获得能量。我们的研究表明,磁场重联和非绝热加速对亚暴注入事件中离子能量的获得起着重要的作用,这对传统的认为粒子仅仅在偶极化区域加速,或者认为绝热加速起主导作用的观点是个重要的补充或修正。5,通过THEMIS卫星观测研究了若干个偶极化锋面对应的微观物理过程。2008年2月15日,THEMIS卫星在亚暴期间近尾探测到了多个偶极化锋面。偶极化锋面都在地向传播的等离子体泡的前缘。在偶极化锋面处还有高能电子通量的增加,以及大的波动增强,波动频率从低于低混杂频率到高于电子回旋频率。偶极化锋面是尺度为离子惯性长度的薄电流片,并且对应着很强的电场,该电场主要是由Hall电场和低混杂漂移波的电场组成。我们认为低混杂漂移波是密度或温度梯度存在的条件下由退磁化漂移电流激发的。锋面附近还观测到了电子静电回旋波,它很可能是由电子垂直速度分布存在的正梯度激发的。以上观测到的两种波动都有可能加速电子。在偶极化锋面观测到的这些波动,对于理解亚暴期间的电子加速以及电流片中断都有重大意义。2009年2月27日,四颗沿径向从X=-20 RE到X=-10 RE排列的THEMIS卫星观测到了一个地向传播的偶极化锋面。这个锋面也是在等离子体泡的前缘,并且是尺度为离子惯性长度的动力学结构。靠近尾部的两颗卫星(P1/P2)和靠近地球的两颗卫星(P3/P4)观测到的电子和离子分布差异较大。靠近尾部的两颗的卫星都在锋面观测到了哨声波,而靠近地球的两颗卫星没有观测到。2009年3月15日,5颗THEMIS卫星相继观测到了一个偶极化锋面。同2009年2月27日的事件相似,靠近尾部的卫星(P1/P2)观测到了哨声波,而其他的三颗卫星都没有看到。这些现象说明偶极化锋面在传播过程中,在不同区域表现出不同的特征。更多还原

【Abstract】 The Earth’s Magnetotail is a crucial ingredient of our magnetosphere. Many explosive phenomena happened in the magnetotail could greatly influence our living on the earth, such as satellites in the near earth region may be damaged by energetic particles produced during the magnetosphere storm, the dramatic change of the properties of ionosphere may affect the wireless communication. In addition, magnetotail is an excellent natural laboratory for us to study the plasma physics.Magnetic reconnection is a universal process in the astrophysics, which could transfer the magnetic energy to plasma kinetic and thermal energy in a short time period, and also change the large scale topology of magnetic field. There are rich wave activities in the reconnection region. Which wave dominates the reconnection layer? Whether these waves are just the byproduct of reconnection or they could trigger or mediate the reconnection process? These are open questions for space and experimental plasma scientist. Magnetic null point is a crucial region of reconnection, where magnetic field lines break and reconnect. Revealing the detailed structures, waves and particle dynamics around null points could be significantly important to understand the reconnection in three dimensional regime. Energetic particle injection and dipolarization are two important ingredients of substorm. Studying these phenomena could tell us how energy releases during the substorm and how the substorm be triggered.In this thesis, by combining satellite observation and numerical simulation, we primarily studied multi-scale kinetic processes in the magnetotail, in particular the wave-particle interaction and acceleration of ions and electrons. Following are our main results:1, We studied different wave characteristics at different regions inside one reconnection diffusion region observed by Cluster spacecraft.We identified both electrostatic and electromagnetic modes of lower hybrid drift (LHD) wave in the reconnection region around a thin current sheet. During the crossing of the separatrix with the reversal of plasma flow and Hall magnetic fields, strong electrostatic LHD mode was observed. Strong electromagnetic fluctuations were observed in the center of the current sheet in the diffusion region. The dispersion properties of the electromagnetic wave were studied by using the interferometer method and are consistent with the properties of LHD wave. This is the first observation evidence of electromagnetic mode of LHD wave inside reconnection diffusion region. We estimated the anomalous resistivity provided by the electromagnetic mode of LHD wave, and found that it could not balance the measured electric field in the reconnection region.We also studied low frequency wave characteristics at the earthward region of the X-line, which was a highβregion with small guide field. We obtained wave vectors in low frequency range using the k-filtering method and found that waves in the diffusion region are highly oblique propagating mode. We compared the measured dispersion relation with the theoretical dispersion relation and confirm the existence of Alfven-Whistler waves in the reconnection region.In addition, we identified a density depletion layer inside the diffusion region and examined the wave-particle interaction associated with the layer. Strong anti-parallel electric current was observed through the layer. Electrostatic wave enhancements between the ion cyclotron frequency and the lower hybrid frequency were observed. The polarization analysis shows the wave is mainly linearly and quasi perpendicularly polarized, which is consistent with the lower hybrid wave. Moreover, oblique propagating whistler wave were observed at the same time. The electron distribution shows there were strong parallel beams. The possible mechanism of wave-particle interaction and the role of density cavities in electron acceleration are discussed.2, We show the Cluster in situ observation of magnetic null structures in the diffusion region, as well as the electron dynamics and associated waves. Possible spiral null pair and null clusters formed by three nulls have been identified in the diffusion region. There is a close relation among the null points, the bipolar signature of the Z-component of magnetic field and enhancement of the flux of energetic electrons up to 100 keV. The four satellites were located in different topological domains of the 3D reconnection structure, with one being located just 19 km apart from the separator line of magnetic null structures in electron scale. On crossing of the separator line, a very thin current sheet with half-width of 4-6 electron initial scale lengths and a peak of anti-parallel current density were found, and high energetic electron enhancement was observed with the hardest energy spectrum (-3.4). Electrostatic solitary waves, whistler-mode waves and lower hybrid waves were identified near the separator line, indicating that electron dynamics and wave-particle interactions play an important role in collisionless reconnection. It is found that the angle between the fans of the nulls is quite close to the theoretically estimated maximum value of the group-velocity cone angle for whistler wave regime of reconnection.3, We performed a series two dimensional Particle-In-Cell (PIC) simulations to study the possible generation mechanism of modulated electron plasma waves often observed in the magnetosphere. It is shown that weak beam instability could generate the modulated Langmuir wave, and when the ambient magnetic field is strong, there is no modulation on the perpendicular electric field. The observed perpendicular polarized modulated waves could be generated by weak loss cone instability. When the weak beam has loss cone distribution, the modulated waves show quick transition between parallel and perpendicular polarization, which explains the observed waveform around the reconnection layer. The quick change of polarization might be the result of trapping phase difference of different modulated waves.4, We studied one substorm injection event by THEMIS and LANL observation, as well as large scale kinetic simulation. We followed millions of particles in the magnetic and electric field obtained from a global MHD simulation to model the energetic ion injection. It is found that our simulation could capture the main feature of ion injection observed by both THEMIS and LANL spacecraft, including the timing and dispersion properties of energetic flux increase. It is found that there were primarily two energization regions for particles to gain energy during this substorm. One is around the near-earth X-line (Ⅹ～-20 RE), where particles were mostly accelerated in non-adiabatic motion under strong inductive electric field. The other were several stretched or localized regions between X=-18 RE and X=-7 RE, where particles were also accelerated in non-adiabatic motion but under potential electric field. Our results imply the importance of reconnection and non-adiabatic motion in the energization of ions during substorm. This is a significant supplement and revision to the previous models which either believes particles only gain energy in the dipolarization region, or adiabatic motion dominates the energization process during substorm.5, We investigated the micro-physics associated with several dipolarization fronts observed by THEMIS observation.On Feb 15,2008, multiple dipolarization fronts were observed by THEMIS spacecraft in the near Earth magnetotail during a substorm. The dipolarization fronts were located at the leading edge of earthward propagating plasma bubbles. Major energetic electron flux enhancements were observed at the dipolarization fronts, which were also associated with large wave fluctuations extending from below the lower hybrid frequency to above the electron cyclotron frequency. Intense electric field wave packets, primarily contributed by the Hall electric field and LHD wave, were observed right at the front, which was a thin current layer with size of the order of the ion inertial length. The LHD wave was believed to be generated by a diamagnetic current in the presence of density and temperature gradients. Electrostatic electron cyclotron waves were detected slightly after the front. The electrostatic electron cyclotron waves were probably generated by the positive slope of the electron perpendicular velocity distribution. Both of these waves are suggested to be able to heat electrons. The observation of these waves at the dipolarization front could be important for the understanding of electron energization during substorm injection, as well as the mechanism of current disruption.On Feb 27,2009, four THEMIS spacecraft, located between X=-20 RE and X=-10 RE, captured one earthward propagating dipolarization front. The dipolarization front was also located at the leading edge of plasma bubble and a kinetic structure with width on the order of ion inertial length. The ion and electron distribution varies significantly between outer (P1/P2) and inner (P3/P4) probes. Two outer spacecraft (P1/P2) detected whistler waves around the front; however, two inner spacecraft (P3/P4) did not. On Mar 15,2009, Five THEMIS spacecraft observed one dipolarization front one by one. Similar as the event of Feb 27,2009, two outer spacecraft (P1/P2) detected whistler waves around the front, while the other three spacecraft did not. Above evidence implies that dipolarization front has different characteristics in different regions during its propagation.更多还原