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

Abstract

Recent studies have statistically shown that the magnetic reconnection site at substorm expansion onset is located in the magnetotail at X ∼ −20 RE 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) ∼ (−7, 9) RE. 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 RE. Considering that GOES‐8 observed a dipolarization at (X, Y) ∼ (−4, 5) RE 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 By and the solar wind dynamic pressure might lead to the initiation of the magnetic reconnection much closer to the Earth than usual.