Plasmoids observed in the near‐Earth magnetotail at X ∼ −7 R_E

Abstract

[1] Recent studies have statistically shown that the magnetic reconnection site at substorm expansion onset is located in the magnetotail at X $ À20 R E on average. For a substorm event that occurred at $0153 UT on 2 July 1996, however, Geotail observed a series of tailward but slow flows with southward magnetic fields fairly close to the Earth at (X, Y) $ (À7, 9) R E . The flows had enhancements of the total pressure and the total magnetic field as well as bidirectional field-aligned low-energy electrons in their central part. We interpret these as signatures for tailward moving small plasmoids with scales of $0.5-3 R E . Considering that GOES-8 observed a dipolarization at (X, Y) $ (À4, 5) R E after the expansion onset, we estimate that the magnetic reconnection occurred between the Geotail and GOES-8 positions. UVI auroral images from Polar and ground magnetic field data show that this substorm, initiated at $20 hours MLT and $64°magnetic latitude, was not very intense, and the period examined was not during an intense storm. The southward interplanetary magnetic field (IMF) was not very large, while the large duskward IMF persisted for more than 12 hours before the onset as well as the somewhat large solar wind dynamic pressure. It seems likely that the global ionospheric convection was not very strong. Locally enhanced convection and auroral oval expansion due to the large IMF B y and the solar wind dynamic pressure might lead to the initiation of the magnetic reconnection much closer to the Earth than usual.

Structure of the near-Earth plasma sheet during tailward flows

“…In contrast with previously discussed observations of tailward fast flows in the near-Earth magnetotail (Nagai et al, 1998a;Eriksson et al, 2004;Miyashita et al, 2005), these three flow bursts were observed during northward IMF. IMF B z was northward (of 5-10 nT) during ∼2.5 h before the first tailward flow was observed.…”

“…Their appearance implies either enhanced magnetic activity (e.g. Miyashita et al, 2005;Nagai et al, 2005) or a mechanism that differs from acceleration at a NENL (e.g. Schödel et al, 2001).…”

“…Sergeev et al, 1995;Nagai et al, 1998a;Miyashita et al, 2005). Alternative hypotheses of a near-Earth flow onset are based on the current disruption (CD) concept (e.g.…”

Remote estimation of reconnection parameters in the Earth's magnetotail: model and observations

“…On the other hand, the presence of the X-line location closer to the Earth has been shown in different studies; e.g., Petrukovich et al (2009) estimated the x GSM ∼ −17 R E from the Cluster survey in the 11-20 R E region in the tail. From case studies using multi-spacecraft conjunctions, the near-Earth X-line position is obtained at x GSM ∼ −7 R E (Miyashita et al, 2005), in the range between ∼9-10 to ∼13-14 R E and ∼10-12 R E . One of the probable causes of the discrepancy in the X-line location is suggested to be in the various solar wind conditions before substorm onset .…”

Reconstruction of time-varying reconnection rate and X-line location

“…Requiring no reconnection, these waves produce the same twin signature in the plasma sheet/PSBL (plasma sheet boundary layer) or in the lobes: the bipolar B z -variation plus the B x -compression (Lee et al, 1988;Miyashita et al, 2005;Takada et al, 2005).…”