Recent progresses in theoretical studies and satellite observations for collisionless magnetic reconnection

As an essential mechanism in large scale fast magnetic energy releases and field reconfigurations processes in space,astrophysical, and laboratory plasmas,magnetic reconnection,particularly collisionless magnetic reconnection,has been studied for more than 65 years.Many progresses have been achieved in recent years and basic features of the process have been well understood,largely due to more and more satellite observation data available in the last decade.However,a few outstanding issues are still remained unresolved.We in the paper review the development of collisionless magnetic reconnection studies and major achievements in recent years,and also briefly discuss the open questions remained to be answered in studies of collisionless magnetic reconnection.     

1999年8月16日EUV增亮事件数值研究的初步结果

我们基于TRACE卫星上EUV观测,采用三阶迎风紧致格式模拟了1999年8月16日EUV增亮事件。数值模拟结果可以给出这次EUV增亮事件的形成和演化过程的一种可能的解释。重联X点处最初的重联流可能与EUV增亮相对应。合并磁岛产生的高度叠加的抛射流,可能与最初表现为吸收特征随后表现为EUV发射结构的提升物质相对应。双向重联喷流可能与继续沿着似乎开放磁力线向上喷发,或流向太阳表面的提升物质相对应。     

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.     

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.     

Double Star TC-1 observation of the earthward flowing plasmoids in the near magnetotail

We analyze Double Star TC-1 magnetic field data from July to September in 2004 and find that plas-moids exist in the very near-Earth magnetotail. It is the first time that TC-1 observes the plasmoids inthe magnetotail at X > ?13 RE. According to the difference of the magnetic field structure in plasmoids,we choose two typical cases for our study: the magnetic flux rope on August 6 with the open magneticfield and the magnetic loop on September 14 with the closed magnetic field. Both of the cases are as-sociated with the high speed earthward flow and the magnetic loop is related to a strong substorm. Theions can escape from the magnetic flux rope along its open field line, but the case of the closed mag-netic loop can trap the ions. The earthward flowing plasmoids observed by TC-1 indicate that the mul-tiple X-line magnetic reconnection occurs beyond the distance of X=?10 RE from the earth.     

Unsteady magnetic reconnection in non-periodic multiple current sheets

The process of magnetic reconnection in non-periodic three-layer current sheets is studied numerically by using two-dimensional magnetohydrodynamic simulation. The results show that unlike periodic current sheets, it is complex unsteady magnetic reconnection. It may be important for solar flare and corona heating.     

Nonlinear dependence of anomalous resistivity on the reconnecting electric field in the Earth’s magnetotail

Based upon the observational data of the fast magnetic reconnection in the nearly collisionless magnetotail and the particle in cell (PIC)simulations on the electron acceleration in the reconnecting current sheet with guide magnetic field,we self consistently solved one dimension Vlasov equation with the magnetotail parameters and realistic mass ratio to explore the relationship between the anomalous resistivity and the induced electric field.As compared with theoretic formula for the current driven ion-acoustic and Buneman anomalous resistivity,the anomalous resistivity may result from the ion acoustic instability for small reconnecting electric field and the Buneman instability for large reconnecting electric field.The discrepancy between the theoretic results and numerical simulations may be caused by the high frequency instability that results from the deviation of electron distribution from Maxwellian one.These results are consistent with the early experimental results and favorable for the fast reconnection to take place.     

Pseudo-reconnection in MHD numerical simulation

A class of pseudo-reconnections caused by a shifted mesh in magnetohydrodynamics (MHD) simulations is reported. In terms of this mesh system, some non-physical results may be obtained in certain circumstances, e.g. magnetic reconnection occurs without resistivity. After comparison, another kind of mesh is strongly recommended.     

Cluster observations of high-altitude cusp during multiple fast-turning IMF

Using coordinated observations from the Cluster,DMSP and ESR radar,high-altitude cusp was investigated for a time period characterized by four fast reversals of the IMF BZ.On February 12,2003,Cluster was flying sunward at duskside over the Northern Hemisphere,across the high-altitude northern cusp twice.Both the two crossings are during steady northward IMF interval,whereas ion fluxes detected during the time period are quite different.The cusp in the first crossing is a typical northward-IMF one,with short...     

关于磁重联区三维磁零点的卫星观测研究

磁重联是等离子体中磁场能量快速转换为粒子动能和热能的主要途径,是空间物理和等离子体物理中的重要现象.磁重联过程发生在磁场的拓扑分形面上;这种分形面一般是由磁零点附近磁场梯度张量的本征矢量确定的.磁零点的三维特性要求至少空间4点的同时测量.Cluster计划提供了迄今为止唯一的卫星测量手段.基于Poincare指数方法分析Cluster卫星在地球磁尾的探测数据,(1) 找到了磁重联过程中存在4种磁零点的证据;(2) 发现磁零点周围磁场的空间特征尺度大约为离子惯性长度,从而首次揭示霍尔效应可能在三维磁重联中起重要作用;(3) 在重联区找到匹配的磁零点对,并计算出零点连线的长度,确定了所产生的磁场的拓扑分形面;(4) 发现在零点连线附近存在低杂波频率的电磁振荡,这为磁重联过程中可能的电子加速、加热机制提供了观测基础.     

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 ...     

The evidence for the evolution of interplanetary small flux ropes: Boundary layers

We have examined the Wind data in 1996 and identified 21 small interplanetary magnetic flux ropes(SIMFRs),and all the 21 SIMFRs have boundary layer structures.The durations of the boundary layers varied from several minutes to 30 minutes.These boundary layers also have properties of high proton temperature,density,and plasma beta.These boundary layers are formed by magnetic reconnections.In addition,in three events magnetic reconnections were occurring inside the boundary layers.It indicates that the flux rope structures have propagated for some period of time,and their boundaries were still evolving through interaction with the background solar wind.Namely it is very possible that the SIMFRs came from the solar corona.     

Simulation of the dawn-dusk magnetosheath asymmetry under quasi-steady states

The dawn-dusk asymmetry of the magnetosheath under quasi-steady states has been studied by using a newly developed 3D MHD magnetosphere simulation model. The results show that the dawn-dusk asymmetry is substantial because of the Parker spiral IMF. It is found that the dawn-dusk magnetosheath thickness asymmetry is the effect of different shock conditions. The plasma density and flux asymmetry are mainly caused by the different thickness of the dawn-dusk magnetosheath, and the magnetic reconnection on the magnetopause has no significant effects. It is also showed that the Plasma Depletion Layer in front of the dayside magnetopause can cause duskward plasma flow, and the total plasma flux on the dusk side will be higher.     

Low-frequency waves in magnetic reconnection

The characteristics of low-frequency waves in magnetic reconnection are studied using two-dimensional hybrid simulation code. In a coordinate system moving with fluid,the time series of a magnetic field perpendicular to the magnetic reconnection plane,By, is transformed into the power spectrum via fast Fourier transformation,while the wave propagation direction and polarization are determined by minimum variance analysis of the electric field.The results show that low-frequency Alfvén ion-cyclotron waves dominate the reconnection area.These waves have frequencies 0-1Ωp(where Ωp is the local proton gyro frequency)and all are left-handed circularly polarized.Among these waves,large-amplitude turbulence,with frequencies of 0-0.6Ωp and isotropic propagation,dominates the outflow regions.This can cause the reversal of By in the quadrupole structure.In the inflow regions, dominant waves,propagating mainly parallel to the ambient magnetic field,have higher frequencies and smaller amplitudes.The frequency of the main peak of wave energy is usually higher than 0.5Ωp.     

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.     

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 with a PIC simulation

Two-dimensional particle-in-cell (PIC) simulation is used to investigate electron dynamics in collisionless magnetic reconnection, and the proton/electron mass ratio is taken to be m _i /m _e = 256. The results show that the presence of a strong initial guide field will change the direction of the electron flow. The electron density cavities and the parallel electric field can be found in the electron inflow region along the separatrix, and the electron inflow and density cavities only appear in the second and fourth quadrants. What is different from the results with a smaller mass ratio is that new structures appear in the diffusion region near the X line: (1) Narrow regions of density enhancement and density cavities can be found synchronously in this region; and (2) corresponding to the electron density changes near the X line, the strong parallel electric fields are found to occur in the first and third quadrants. These electric fields perhaps play a more important role in acceleration and heating electrons than those fields located in the density cavities. Supported by National Natural Science Foundation of China (Grant No. 40725013) and Open Research Program Foundation of State Key Laboratory for Space Weather, Chinese Academy Sciences