[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.

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.

From 23:10 to 23:50 UT on 18 March 2004, the Double Star TC-1 spacecraft detected eight flux ropes at the outbound crossing of the southern dawnside magnetopause. A notable guide field existed inside all ropes. In the mean time the Cluster spacecraft were staying in the magnetosheath and found that the events occurred under the condition of southward IMF Bz and dominant negative IMF By. There are six ropes that appeared quasi-periodically, with a repeated period being approximately 1–4 min. The last flux rope lasts for a longer time interval with a larger peak in the BN variations; it can thus be referred to as a typical FTE. The 18 March 2004 event is quite similar to the multiple flux rope event observed by Cluster on 26 January 2001 at the northern duskside high-latitude magnetopause. A detailed comparison of these two events is made in the paper. Preliminary studies imply that both of these multiple flux ropes events seem to be produced by component reconnection at the dayside low-latitude magnetopause.

More than 40 gradual SEPs have been observed by GOES and SOHO from 2001 to 2003. During 12 SEPs of all these events, energetic electron ux enhancements with energies from 38 keV to 337 keV were observed by RAPID onboard Cluster spacecraft. During these 12 events the variation of the energetic electron ux measured by RAPID/Cluster was closely associated to the variation of the solar energetic proton ux. The observed energetic electron ux was independent on the location of Cluster in the magnetosphere and the background level of magnetospheric electrons (even when the Cluster spacecraft was crossing the magnetopause, the plasma sheet and the low latitude boundary layer). The similar variation of the enhancement of energetic electron ux has also been observed by POLAR and Geotail in some of these events. In some of these events, the electron measurement of RAPID/Cluster will not be contaminated by the SEP protons.

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.