Department of Electronic Engineering, University of Electro-Communications, Tokyo, Japan

The timing of the near-Earth neutral line (NENL) formation relative to substorm onset signatures has been a central debate in substorm research for some time. Several recent studies have suggested that the direct identification of the reconnection site, at least in its initial stage, is difficult because it may be a very limited region spatially. Our approach to this timing problem is the examination of the midtail lobe traveling compression regions (TCRs) relative to onset during substorm events. Fifty-nine midtail TCRs in the IMP 8 magnetotail observations are analyzed. These midtail TCRs differ from the distant tail TCRs in that they lack the first half of the north and south tilting of the field during the compression. The flat B_z phase preceding the south tilting interval in these midtail compression regions has been interpreted as being due to the close proximity to the growing plasmoid bulge just prior to plasmoid ejection down the tail. The results of our analysis show that these TCRs start a few minutes before or simultaneously (accuracy of 1 min) with Pi2 onset at low-latitude or midlatitude ground stations and the particle injection at the geosynchronous orbit. Before the plasmoid bulge formation starts, the NENL must start to form in the plasma sheet; that is, the formation of NENL must initially occur on closed field lines to form the plasmoids which compress the lobes to cause the TCRs. Our results support this view and allow us to conclude that NENL forms before ground and geosynchronous substorm onset signatures, i.e., in the late growth phase. The implications of these results for magnetospheric substorm models are discussed.

J. Geophys. Res., 103, No. A11, 26607-26612, Nov. 1998