三维磁场重联双零点附近的低混杂波的观测

磁场重联是等离子体中一个非常重要的基本物理过程．它能够改变磁场拓扑结构,加热和加速粒子,因而在实验室和自然界等离子体中起着重要的作用．2001年10月1日,Cluster卫星多次穿越磁重联区,Xiao等人首次局地观测到了零点对的存在,并且发现零点振动的功率谱在低混杂波频率附近有最大值．主要研究了双零点附近伴随磁重联过程的位于低混杂波频率附近的电磁波和静电波动增强,并且通过谱分析等确认该波动为准垂直传播的低混杂波．     

无碰撞磁重联现象的电子动力学研究

基于磁流体力学理论,通过双流体MHD方程组,使用专业电磁流体数值仿真模拟软件Usim,研究了无碰撞等离子体磁重联过程中电子的动力学行为,得到了电子的数密度与流速的空间分布.结果表明,重联面内的电子数密度分布区域与速度分布区域相吻合,四极型磁场结构可以表征Hall的电流存在.此外,无碰撞等离子体中Hall电流的存在将使得磁重联现象的重联率大大提高,这对研究快速磁重联的产生机制有着重要的意义.     

中国科学家揭示太空三维磁重联的几何结构——《Nature Physics》再次发表北京大学与国家天文台等最新合作成果

以中国科学院国家天文台肖池阶副研究员、北京大学王晓钢教授、濮祖荫教授等为主的研究小组，继去年首次发现了自然界中存在磁场零点（Nature Physics2（7），478—483，2006）之后，最近又合作完成了三维磁重联完整几何结构的卫星观测研究。     

冕洞边界区磁结构的探讨

摘要用Hell083nm单色像上的极区、低纬冕洞的边界,确定Fe]532.4nm纵向场全日面磁图上对应的极,低纬冕洞的边界.对光球上极、低纬冕洞边界足根连线范围内外的磁场强度,磁极性、磁场几何位形进行了分析讨论.观测结果表明t就极无涧大尺度范围来看,是单极开放场I就48h时标的加强磁结构来说,以开放场为主的情况下,有开放场与闭合场栩混杂的情况.就同时的极冕洞与低纬度冕洞边界足根连线附近磁场位形相比较,差别是明显的.     

磁螺度以及在太阳物理上的应用

磁螺度应用到天体物理尤其是太阳物理上,从而变为一个热门的领域.螺度用以描述磁场的复杂结构,例如磁场的扭曲、缠绕、连结、编辫,作为一个物理量,与能量不同,在完全理想的磁流体力学(MHD)中,它是一个守恒量.在磁重联过程中,螺度近似守恒,应用到太阳大气上,可以计算磁螺度的传输.最近的观测发现在太阳上螺度存在南北半球的不对称性,北半球主要为负螺度,南半球正螺度占优势;阐述了在太阳大气不同层次这种不对称性的表现形式,并介绍了对这种规律的有代表性的解释;介绍了国际国内螺度研究的最新进展,指出了有待于解决的一些关键问题.     

磁旋光成像地球磁场测量方法

提出了磁旋光成像地球磁场测量方法.利用地球表面反射的太阳光的偏振特性以及地球大气层的磁致旋光效应,可在卫星上利用光学系统对地球磁场进行探测.从原理上讲,这一方法具有三维成像能力,可直接获取地球磁场的空间结构图样,其测量速度很高,可在很短时间内完成全球测量任务,同时其测量无盲点,无畸变.文中介绍了这一地磁场测量方法的原理,并对其中的关键问题进行了讨论.     

桩基础钢筋笼长度磁法检测研究

桩基础中的钢筋笼受地磁场磁化从而产生感应磁场,并且与地磁场叠加产生局部磁异常.本文根据这一特点,利用MATLAB平台编写程序建立数值正演模型,对钢筋笼进行剖分,计算桩身附近钢筋笼的磁异常特征,讨论了根据磁异常及其梯度曲线确定钢筋笼接点极其长度的分析方法,并将工程实测曲线与理论曲线进行对比,取得了较好的一致性,从而证明了该方法的良好效果,并且为进一步精细化正演模型,进行更精确的定量计算奠定了基础.     

磁阻尼器的研制及其在动导数实验中的应用

磁阻尼器由均匀空间磁场和正弦波电流发生器组成.通过磁阻尼器在动导数实验中的应用,将测量求出的阻尼与施加的已知阻尼相比较就可确定该套测量系统的可行性.磁阻尼器的研制,实现了在风洞外对动导数测量装置的模拟加载调试,大大降低了风洞的占用时间.     

复印机粘结NdFeB磁辊的研究

环氧树脂粘结NdEeB这种新型磁性材料用于复印机磁辊，以替代以往的铁氧体磁辊，可以极大地提高磁能积，更好地满足复印机对磁场的要求．针对标准辊的制备过程、磁场的测量、磁场的数学模型的推导以及这种磁辊的磁场分布进行了理论分析，并给出了一个较精确的解．此解经试验验证证实，与实际测量的结果基本相符，而且使磁能积提高了4至5倍。     

超磁致伸缩力传感器及其实验研究

基于磁致伸缩逆效应原理,以超磁致伸缩棒为敏感元件研究了一种具有高灵敏度的新型超磁致伸缩力传感器,通过集成在结构内部的霍尔传感器测量磁通密度来实现静态力的测量.同时,为了提高传感器的测量灵敏度,提出了一种安装在霍尔传感器周围的不锈钢钢环的特殊结构.给出了超磁致伸缩力传感器的测量原理和设计过程,并通过实验研究确定了偏置磁场、预紧力和超磁致伸缩棒的尺寸等因素对传感器输出特性的影响规律,分别得到了传感器工作的最佳偏置磁场和预紧力,为超磁致伸缩力传感器的深入研究和精确控制提供了一种技术途径.     

Structures of magnetic null points in reconnection diffusion region: Cluster observations

Magnetic reconnection is a very important and fundamental plasma process in transferring energy from magnetic field into plasma. Previous theory, numerical simulations and observations mostly concentrate on 2-dimensional （2D） model; however, magnetic reconnection is a 3-dimensional （3D） nonlinear process in nature. The properties of reconnection in 3D and its associated singular structure have not been resolved completely. Here we investigate the structures and charactedstics of null points inside the reconnection diffusion region by introducing the discretized Poincar6 index through Gauss integral and using magnetic field data with high resolution from the four satellites of Cluster mission. We estimate the velocity and trajectory of null points by calculating its position in different times, and compare and discuss the observations with different reconnection models with null points based on characteristics of electric current around null points.     

Extended magnetic reconnection at the Earth's magnetopause from detection of bi-directional jets

Magnetic reconnection is a process that converts magnetic energy into bi-directional plasma jets; it is believed to be the dominant process by which solar-wind energy enters the Earth's magnetosphere. This energy is subsequently dissipated by magnetic storms and aurorae. Previous single-spacecraft observations revealed only single jets at the magnetopause--while the existence of a counter-streaming jet was implicitly assumed, no experimental confirmation was available. Here we report in situ two-spacecraft observations of bi-directional jets at the magnetopause, finding evidence for a stable and extended reconnection line; the latter implies substantial entry of the solar wind into the magnetosphere. We conclude that reconnection is determined by large-scale interactions between the solar wind and the magnetosphere, rather than by local conditions at the magnetopause.     

The effects of the guide field on the structures of electron density depletions in collisionless magnetic reconnection

Two-dimensional particle-in-cell simulations are performed to investigate the formation of electron density depletions in collisionless magnetic reconnection.In anti-parallel reconnection,the quadrupole structures of the out-of-plane magnetic field are formed,and four symmetric electron density depletion layers can be found along the separatrices due to the effects of magetic mirror.With the increase of the initial guide field,the symmetry of both the out-of-plane magnetic field and electron density depletion layers is distorted.When the initial guide field is sufficiently large,the electron density depletion layers along the lower left and upper right separatrices disappear.The parallel electric field in guide field reconnection is found to play an important role in forming such structures of the electron density depletion layers.The structures of the out-of-plane magnetic field By and electron depletion layers in anti-parallel and guide field reconnection are found to be related to electron flow or in-plane currents in the separatrix regions.In anti-parallel reconnection,electrons flow towards the X line along the separatrices,and are directed away from the X line along the magnetic field lines just inside the separatrices.In guide field reconnection,electrons can only flow towards the X line along the upper left and lower right separatrices due to the existence of the parallel electric field in these regions.     

Electron dynamics in collisionless magnetic reconnection

Magnetic reconnection provides a physical mechanism for fast energy conversion from magnetic energy to plasma kinetic energy. It is closely associated with many explosive phenomena in space plasma, usually collisionless in character. For this reason, researchers have become more interested in collisionless magnetic reconnection. In this paper, the various roles of electron dynamics in collisionless magnetic reconnection are reviewed. First, at the ion inertial length scale, ions and electrons are decoupled. The resulting Hall effect determines the reconnection electric field. Moreover, electron motions determine the current system inside the reconnection plane and the electron density cavity along the separatrices. The current system in this plane produces an out-of-plane magnetic field. Second, at the electron inertial length scale, the anisotropy of electron pressure determines the magnitude of the reconnection electric field in this region. The production of energetic electrons, which is an important characteristic during magnetic reconnection, is accelerated by the reconnection electric field. In addition, the different topologies, temporal evolution and spatial distribution of the magnetic field affect the accelerating process of electrons and determine the final energy of the accelerated electrons. Third, we discuss results from simulations and spacecraft observations on the secondary magnetic islands produced due to secondary instabilities around the X point, and the associated energetic electrons. Furthermore, progress in laboratory plasma studies is also discussed in regard to electron dynamics during magnetic reconnection. Finally, some unresolved problems are presented.     

Magnetic reconnection from a multiscale instability cascade

Magnetic reconnection, the process whereby magnetic field lines break and then reconnect to form a different topology, underlies critical dynamics of magnetically confined plasmas in both nature and the laboratory. Magnetic reconnection involves localized diffusion of the magnetic field across plasma, yet observed reconnection rates are typically much higher than can be accounted for using classical electrical resistivity. It is generally proposed that the field diffusion underlying fast reconnection results instead from some combination of non-magnetohydrodynamic processes that become important on the 'microscopic' scale of the ion Larmor radius or the ion skin depth. A recent laboratory experiment demonstrated a transition from slow to fast magnetic reconnection when a current channel narrowed to a microscopic scale, but did not address how a macroscopic magnetohydrodynamic system accesses the microscale. Recent theoretical models and numerical simulations suggest that a macroscopic, two-dimensional magnetohydrodynamic current sheet might do this through a sequence of repetitive tearing and thinning into two-dimensional magnetized plasma structures having successively finer scales. Here we report observations demonstrating a cascade of instabilities from a distinct, macroscopic-scale magnetohydrodynamic instability to a distinct, microscopic-scale (ion skin depth) instability associated with fast magnetic reconnection. These observations resolve the full three-dimensional dynamics and give insight into the frequently impulsive nature of reconnection in space and laboratory plasmas.     

In situ detection of collisionless reconnection in the Earth's magnetotail

Magnetic reconnection is the process by which magnetic field lines of opposite polarity reconfigure to a lower-energy state, with the release of magnetic energy to the surroundings. Reconnection at the Earth's dayside magnetopause and in the magnetotail allows the solar wind into the magnetosphere. It begins in a small 'diffusion region', where a kink in the newly reconnected lines produces jets of plasma away from the region. Although plasma jets from reconnection have previously been reported, the physical processes that underlie jet formation have remained poorly understood because of the scarcity of in situ observations of the minuscule diffusion region. Theoretically, both resistive and collisionless processes can initiate reconnection, but which process dominates in the magnetosphere is still debated. Here we report the serendipitous encounter of the Wind spacecraft with an active reconnection diffusion region, in which are detected key processes predicted by models of collisionless reconnection. The data therefore demonstrate that collisionless reconnection occurs in the magnetotail.     

Rapid magnetic reconnection in the Earth's magnetosphere mediated by whistler waves

Magnetic reconnection has a crucial role in a variety of plasma environments in providing a mechanism for the fast release of stored magnetic energy. During reconnection the plasma forms a 'magnetic nozzle', like the nozzle of a hose, and the rate is controlled by how fast plasma can flow out of the nozzle. But the traditional picture of reconnection has been unable to explain satisfactorily the short timescales associated with the energy release, because the flow is mediated by heavy ions with a slow resultant velocity. Recent theoretical work has suggested that the energy release is instead mediated by electrons in waves called 'whistlers', which move much faster for a given perturbation of the magnetic field because of their smaller mass. Moreover, the whistler velocity and associated plasma velocity both increase as the 'nozzle' becomes narrower. A narrower nozzle therefore no longer reduces the total plasma flow-the outflow is independent of the size of the nozzle. Here we report observations demonstrating that reconnection in the magnetosphere is driven by whistlers, in good agreement with the theoretical predictions.     

Electron density hole and quadruple structure of <Emphasis Type="Italic">B</Emphasis><Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> during collisionless magnetic reconnection

In collisionless reconnection,the magnetic field near the separatrix is stronger than that around the X-line,so an electron-beam can be formed and flows toward the X-line,which leads to a decrease of the electron density near the separatrix.Having been accelerated around the X-line,the electrons flow out along the magnetic field lines in the inner side of the separatrix.A quadruple structure of the Hall magnetic field By is formed by such a current system.A 2D particle-in-cell (PIC) simulation code is used ...     

A magnetic reconnection X-line extending more than 390 Earth radii in the solar wind

Magnetic reconnection in a current sheet converts magnetic energy into particle energy, a process that is important in many laboratory, space and astrophysical contexts. It is not known at present whether reconnection is fundamentally a process that can occur over an extended region in space or whether it is patchy and unpredictable in nature. Frequent reports of small-scale flux ropes and flow channels associated with reconnection in the Earth's magnetosphere raise the possibility that reconnection is intrinsically patchy, with each reconnection X-line (the line along which oppositely directed magnetic field lines reconnect) extending at most a few Earth radii (R(E)), even though the associated current sheets span many tens or hundreds of R(E). Here we report three-spacecraft observations of accelerated flow associated with reconnection in a current sheet embedded in the solar wind flow, where the reconnection X-line extended at least 390R(E) (or 2.5 x 10(6) km). Observations of this and 27 similar events imply that reconnection is fundamentally a large-scale process. Patchy reconnection observed in the Earth's magnetosphere is therefore likely to be a geophysical effect associated with fluctuating boundary conditions, rather than a fundamental property of reconnection. Our observations also reveal, surprisingly, that reconnection can operate in a quasi-steady-state manner even when undriven by the external flow.     

Energetic electrons associated with magnetic reconnection in the sheath of interplanetary coronal mass ejection

Magnetic reconnection is an important universal plasma dissipation process that converts magnetic energy into plasma thermal and kinetic energy,and simultaneously changes the magnetic field topology.In this paper,we report the first observation of energetic electrons associated with asymmetric reconnection in the sheath of an interplanetary coronal mass ejection.The magnetic field shear angle was about 151°,implying guide-field reconnection.The width of the exhaust was about 8×104 km.The reconnection rate was estimated as 0.044-0.08,which is consistent with fast reconnection theory and previous observations.We observed flux enhancements of energetic electrons with energy up to 400 keV in this reconnection exhaust.The region where ener- getic electron fluxes were enhanced is located at one pair of separatrices in the higher density hemisphere.We discuss these observation results,and compare with previous observations and recent kinetic simulations.