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.

Using the observations of LASCO aboard SOHO in the interval from Mar. 1997 to Dec. 2003, 302 earth-directed halo CMEs are selected and the source regions are located in MDI synoptic charts. A statistical analysis has been made with the emphasis on the CMEs' largescale source regions as well as the correlation between CMEs and solar surface activity. The statistics show that CMEs are intrinsically related to surface activity. Four groups of CMEs' large-scale source structures are identied on the photosphere. They are: Ⅰ, Extended bipole regions (EBRs) with long magnetic neutral line; Ⅱ, Closely packed active regions (ARs); Ⅲ, Large-scale magneticux of the same polarity runs through the opposite hemisphere, along the boundary there appears transequatorial laments; Ⅳ, Between two EBRs with long lament. The result shows that CME-associated source activity is closely related to the types of large-scale magnetic structures.

[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.

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.

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.