Detection of a separator line that connects magnetic nulls and the determination of the dynamics and plasma environment of such a structure can improve our understanding of the three-dimensional (3D) magnetic reconnection process 1-9. However, this type of field and particle configuration has not been directly observed in space plasmas. Here we report the identification of a pair of nulls, the null-null line that connects them, and associated fans and spines in the magnetotail of Earth using data from the four Cluster spacecraft. With di and de designating the ion and electron inertial lengths, respectively, the separation between the nulls is found to be ～0.7±0.3 di and an associated oscillation is identified as a lower hybrid wave with wavelength ～de. This in situ evidence of the full 3D reconnection geometry and associated dynamics provides an important step toward to establishing an observational framework of 3D reconnection.

2004-03-18 23:10～23:50 ur期间，“双星（Double Star）”探测一号卫星（TC-1）在向阳面磁层顶高纬晨侧由内向外穿越磁层顶，其时TC-1的GSM坐标为（7.5RE，-5.5RE，-5.4Re），RE为地球半径。穿越过程中TC-1观测到了8个通量管和1个磁通量传输事件（FTEs）。在此期间Cluster星簇位于向阳面太阳风内，其GSM坐标为（18.0RE，-3.1RE，-6.2RE），其4颗卫星监测到行星际磁场（IMF）的Bz分量持续南向，BY有较大的负值．本文的研究表明：TC-1观测到的前7个通量管具有准周期重现性，周期大约是1～4min，明显小于以前所观测到的FrEs的平均周期（8～11min）；所有的通量管都具有较强的核心场．本文分别使用最小方差分析法（MVA）和Grad-Shafranov反演方法（GSR）对通量管的轴向进行了分析和对比，发现所有的通量管主轴基本沿晨昏向，结果显示GSR方法在轴向分析上比MVA优越。本文使用GSR方法对通量管的磁场结构进行了分析，恢复出了通量管的磁场在卫星穿越面的结构图；此外，本文还对这次多重通量管事件进行了deHoffmann-Teller（HT）分析，结果表明，所有通量管大致朝南极方向运动，均来源于向日面低纬区域．这说明它们可能起源于向日面低纬区，由该区的磁场分量重联产生。

Magnetic reconnection is one of the most important processes in astrophysical, space and laboratory plasmas. Identifying the structure around the point at which the magnetic field lines break and subsequently reform, known as the magnetic null point, is crucial to improving our understanding reconnection. But owing to the inherently three-dimensional nature of this process, magnetic nulls are only detectable through measurements obtained simultaneously from at least four points in space. Using data collected by the four spacecraft of the Cluster constellation as they traversed a diffusion region in the Earth's magnetotail on 15 September, 2001, we report here the first in situ evidence for the structure of an isolated magnetic null. The results indicate that it has a positive-spiral structure whose spatial extent is of the same order as the local ion inertial length scale, suggesting that the Hall effect could play an important role in 3D reconnection dynamics.

Magnetic reconnection is a main process converting the magnetic energy into thermal and kinetic energy in plasmas. It is one of the fundamental problems of crucial importance not only to space plasmas physics and space weather studies, such as the solar flare, coronal mass ejections and magnetospheric substorms, but also to the stability analysis in magnetically confined fusion. In general, except for cases with periodical boundary conditions, three-dimensional (3D) magnetic reconnection occurs on magnetic separatrices generated by magnetic nulls. Here we briefly introduce/review the theories and some recent satellite observations of 3D magnetic reconnection. Topics to be further studied are also discussed.

[1] Fast reconnection is crucial to magnetospheric substorms, solar and stellar flares and fusion plasmas. Ultimate confirmation of fast reconnection must be achieved by multi-spacecraft detections of the reconnection rate itself and associated dimensions of the diffusion region. Here we report a multi-spacecraft measurement of fast reconnection rate grec Vin y (0.07-0.15)VA based on directly measurements of the plasma flow into the diffusion region, where Vin is the speed of reconnecting flux and VA the characteristic Alfven speed. It falls in the range of (0.03-0.2)VA predicted in steady state reconnection simulation. The characteristic sizes for the diffusion region of the width Lz 0.9 di(=460km) and the length Lx (3.3-5.1)di (=1680-2597 km) are measured as well. The length of the diffusion region Lx is determined for the first time based on the in situ observations. Furthermore, other features detected during the event also match the previous observation and simulation results. Citation: Xiao, C. J., et al. (2007), A Cluster measurement of fast magnetic reconnection in the magnetotail, Geophys.

Magnetic reconnection at the high-latitude magnetopause is studied using 2.5-dimensional Hall-MHD simulation. Concentric flow vortices and magnetic islands appear when both Hall effect and sheared flow are considered. Plasma mixing across the magnetopause occurs in the presence of the flow vortices. Reconnected structure generated in the vicinity of the subsolar point changes its geometry with increasing flow shear while moving to high latitudes. In the presence of flow shear, with the Hall-MHD reconnection a higher reconnection rate than with the traditional MHD is obtained. The out-of-plane components of flow and magnetic field produced by the Hall current are redistributed under the action of the flow shear, which makes the plasma transport across the boundaries more complicated. The simulation results provide some help in understanding the dynamic processes at the high latitude magnetopause.

Data on plasma sheet crossing meas-ured by Cluster/HIA and Cluster/FGM during the pe-riod from July to October in 2001―2003 are analyzed. Based on previous work on the characteristic fea-tures of continuous lobe reconnection (CLR) de-scribed in reference, two case studies and a statisti-cal analysis were carried out on correlation between CLR in the mid magnetotail and substorm expansion onset for the events occurring during this period. It is found that almost all CLR events are in close con-nection with substorms. The beginning of CLR is al-most always a few minutes ahead of substorm activi-ties seen in the near Earth magnetotail and on the ground-based stations. This provides a clear indica-tion that CLR is the virtual cause of substorm expan-sion onset during the period of continuous southward interplanetary magnetic field.

We present the temporal evolution of magnetic field topology in the magnetotail with a southward IMF in order to identify the magnetic reconnection. The magnetic field topology is uniquely determined by the eigenvalues of the critical points, if they are not degenerated. This is because the critical points, their number, and the rules between them characterize the whole magnetic field pattern. At the critical points, the magnetics become zero. The magnetic vector field curves and surfaces are both integrated out along the principal directions of certain classes of critical points including the Earth’s dipole magnetic field. The skeleton that includes the critical points, characteristic curves, and surfaces provides the three-dimensional topological structure of the reconnection. The change of the skeleton, i.e. the change of the topology, has revealed the occurrence of magnetic reconnection. Namely, threedimensional "X-points" or the more-than-two critical points that are saddle and connected each other are unstable and can move, vanish, and generated.

[1] Fifty-three substorms measured by Double Star/TC-1 in the near-Earth magnetotail from July to October, 2004 are studied. The main features of these events are: (a) Magnetic flux pileup characterized by continuous enhancement of Bz is observed, which starts almost simultaneously with aurora breakup within 1–3 minutes, indicating that substorm onset is in close relation to flux pileup. (b) Sudden plasma sheet expansion with sharp increases in ion temperature and density is seen in all events, which occurs typically (11 minutes after the beginning of pileup. The plasma sheet expansion is shown to be in close relation with the primary substorm dipolarization and, hence, can be referred to as ‘dipolarization-associated expansion’. (c) Evidence indicates that the substorm current wedge first forms earthward of TC-1 position and, hence, inward of the flow braking region, and then propagates tailward with an expansion in the Z-direction. Possible implications of these observations are briefly discussed. Citation: Zhang, H., et al. (2007), TC-1 observations of flux pileup and dipolarizationassociated expansion in the near-Earth magnetotail during substorms, Geophys.