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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, A12214, doi:10.1029/2005JA011263, 2005

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

Y. Miyashita

Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi, Japan


A. Ieda

Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi, Japan


Y. Kamide

Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi, Japan


S. Machida

Department of Geophysics, Kyoto University, Kyoto, Japan


T. Mukai

Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan


Y. Saito

Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan


K. Liou

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA


C.-I. Meng

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA


G. K. Parks

Space Sciences Laboratory, University of California, Berkeley, Berkeley, California, USA


R. W. McEntire

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA


N. Nishitani

Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi, Japan


M. Lester

Department of Physics and Astronomy, University of Leicester, Leicester, UK


G. J. Sofko

Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Saskatchewan, Canada


J.-P. Villain

Laboratoire de Physique et Chimie de l'Environnement, Centre National de la Recherche Scientifique, Orléans, France


Abstract

Recent studies have statistically shown that the magnetic reconnection site at substorm expansion onset is located in the magnetotail at X ∼ −20 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) ∼ (−7, 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) ∼ (−4, 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.

Received 13 June 2005; accepted 22 September 2005; published 16 December 2005.

Keywords: plasmoid; Geotail.

Index Terms: 2723 Magnetospheric Physics: Magnetic reconnection (7526, 7835); 2744 Magnetospheric Physics: Magnetotail; 2790 Magnetospheric Physics: Substorms; 2704 Magnetospheric Physics: Auroral phenomena (2407); 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions (2431).

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Citation: Miyashita, Y., et al. (2005), Plasmoids observed in the near-Earth magnetotail at X ∼ −7 RE, J. Geophys. Res., 110, A12214, doi:10.1029/2005JA011263.