
Contents
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13.1 Development of a Tail-Like Field during the Growth Phase 13.1 Development of a Tail-Like Field during the Growth Phase
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13.2 How Thin Does the Current Sheet Get? 13.2 How Thin Does the Current Sheet Get?
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13.3 Dipolarization 13.3 Dipolarization
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13.4 Dispersionless Injections 13.4 Dispersionless Injections
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13.5 The Substorm Current Wedge 13.5 The Substorm Current Wedge
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13.6 Pi 1 and Pi 2 Pulsations and the Timing and Location of Substorm Onset 13.6 Pi 1 and Pi 2 Pulsations and the Timing and Location of Substorm Onset
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13.7 Generation of Midlatitude Pi 2 13.7 Generation of Midlatitude Pi 2
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13.8 Azimuthal Propagation of Dipolarization near Geostationary Orbit 13.8 Azimuthal Propagation of Dipolarization near Geostationary Orbit
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13.9 Size of Initial Dipolarization Region 13.9 Size of Initial Dipolarization Region
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26313.10 Radial Propagation of Dipolarization 26313.10 Radial Propagation of Dipolarization
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13.11 Plasmoids Escaping from the Geostationary Region 13.11 Plasmoids Escaping from the Geostationary Region
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13.12 Plasma Sheet Dropouts in the Post-Geosynchronous Region 13.12 Plasma Sheet Dropouts in the Post-Geosynchronous Region
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13.13 Plasma Sheet Recoveries in the Postgeosynchronous Region 13.13 Plasma Sheet Recoveries in the Postgeosynchronous Region
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13.14 Summary 13.14 Summary
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24813 The Geosynchronous Substorm
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Published:April 1996
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Abstract
The reconnection model of substorms deals with the large-scale changes in the structure of the magnetosphere and tail as convection intensifies following a sudden increase in the dayside reconnection rate. The model has difficulty making statements relevant to the small scales that characterize auroral onset. However, there has been considerable progress in assembling high-resolution observations of the events in space that now appear to be tightly coupled to the dramatic auroral events that first defined the term substorm. We will call this clear and consistent ensemble the geosynchronous model of substorms, since most of it was first conceived from observations made on geostationary spacecraft. We will also include in this ensemble the recent observations made using the quasigeostationary spacecraft, AMPTE/CCE, and so, by the geosynchronous substorm, we really mean the substorm as it appears on the earth's nightside typically between 6 and, say, 10 RE downtail. The earth’s magnetic field at geosynchronous orbit is about 100 nT, some three times larger than in the tail lobes. Study of quiet field intervals singles out the dependence of the geosynchronous field on solar wind dynamic pressure, since the modulation due to changes in the direction of the interplanetary field is presumably negligible during quiet conditions. The periodic variations in the quiet field depend on local time, season, and orientation of the earth’s dipole axis relative to spacecraft location (McPherron and Barfield, 1980; Rufenach et al., 1992). Superposed on the quiet field are perturbations up to about 50 nT due to several magnetospheric current systems, including the magnetopause current, the ring current, and the cross-tail current; the most striking are due to changes in the cross-tail current system. Observations from geosynchronous orbit were the first to indicate that the nightside magnetic field becomes more “tail-like” during substorm growth phase, and more dipolar during the expansion phase. This simple observation is the foundation on which today’s elaborate geosynchronous substorm model rests. The geosynchronous field becomes progressively more “tail-like” as the cross-tail current system intensifies and/or moves earthward during the substorm growth phase (McPherron et al., 1975; Coleman and McPherron, 1976; McPherron, 1979; Kauffmann, 1987).
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