[1] The present study examines the temporal structure of the fast flow in the plasma sheet using both observations and simulations. The data analysis part adopts the strictest criterion ever for the satellite location so that selected flows are mostly convective. From Geotail measurements at X > À31 R E , 818 earthward-flow and 290 tailward-flow events are selected. Superposed epoch analyses are conducted with two different reference times: the start of the fast flow and the time of a sharp change in the B z component. The results are summarized as follows: (1) The magnetic field becomes dipolar in the course of the fast earthward flow; (2) Sharp dipolarization tends to be preceded by a transient decrease in B Z , which starts along with the fast flow and is accompanied by an increase in the plasma density; (3) The corresponding signatures, albeit less clear, can also be found for the tailward flow; (4) Whereas the plasma density decreases in association with the fast flow irrespective of the flow direction (though, more gradually for the tailward flow), the ion temperature increases for the earthward flow and decreases for the tailward flow; (5) The plasma and total pressures decrease in the course of the fast flow, suggesting the reduction of the lobe field strength; (6) In general, magnetic field and plasma parameters change more gradually in time for the tailward flow than for the earthward flow. Those characteristics of the fast flow can be found irrespective of the X distance, even though the ambient magnetic field and plasma vary significantly between X = À5 and À31 R E . The near-Earth reconnection is inferred to be the responsible mechanism for most, if not all, flow events, and the difference between the earthward and tailward flows presumably reflects difference in downstream conditions. On the earthward side of the reconnection site, the flow needs to proceed against the rigid terrestrial magnetic field, whereas on the tailward side the flow does not have any obstruction once reconnection reaches the lobe magnetic field. This idea is consistent with the change of the magnetic inclination, which suggests that the plasma sheet becomes thicker and thinner in the course of the earthward and tailward flows, respectively. These observational results are compared with fast plasma flows modeled by two-fluid simulations of magnetic reconnection. A focus is placed on the reduction of B Z prior to dipolarization for the earthward flow (the precursory B Z increase for the tailward flow) since this is the new finding owing to our strict condition for the convective flow. It is found that the fragmentation of the current sheet and the formation of multiple neutral lines create signatures similar to the satellite observations. After multiple X lines form, one of them dominates and establishes the overall flow pattern associated with reconnection. Magnetic islands formed between the X lines are swept downstream by the reconnection process. The signature of this earthward convection of a magnetic island past a satell...

The spatial variation of the properties of magnetospheric Pi 2 pulsations is studied using magnetic field records acquired simultaneously by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) satellite at radial distances less than 6.6 Earth radii and at geomagnetic latitudes from -16 ø to 16 ø and at the Kakioka ground station located at magnetic shell of L = 1.23. Pi 2 magnetic pulsations are identified from the Kakioka data acquired within 3 hours of midnight, but no restriction is imposed on the local time of CCE. An automated Pi 2 selection procedure resulted in 249 events from the Kakioka data. We have characterized magnetic field variations in the radial Bx, azimuthal By, and compressional B z components at CCE in terms of their spectral density, coherence, and phase relative to those of the Pi 2 pulsation in the horizontal H component of the Kakioka data and then examined how these parameters depend on the location of CCE. It is found that highcoherence events (coherence between CCE and Kakioka > 0.6) are observed primarily when CCE is on the nightside and at L < 4. For these events the magnetic field perturbations at CCE are dominated by the poloidal components Bx and B z, and these components exhibit a ground-to-satellite cross phase of either -0 or -180 ø, depending on the location of the satellite. The spatial phase structure is consistent with the eigenmode structure of a compressional cavity-mode-type resonance excited between two reflecting boundaries. We find no evidence supporting the view that ground Pi 2 are midlatitude toroidal field line resonances excited in response to source waves on auroral zone field lines. Rather, the results imply that midlatitude (2 < L < 5) Pi 2 pulsations observed on the ground originate from a cavity-mode-type resonance excited in the inner magnetosphere bounded below by the ionosphere and at high altitudes by an Alfv6n velocity gradient. The cavity resonance is probably excited by earthward propagating fast mode waves launched at substorm onset by the large-scale magnetic reconfiguration associated with cross-tail current disruption. It is generally accepted that the original source of Pi 2 pulsations is energy and momentum impulsively released as the magnetic fieldCopyright 1995 by the American Geophysical Union. Paper number 95JA01849. 0148-0227/95/95JA-01849505.00 of the near-Earth magnetotail suddenly changes from a stretched configuration to a dipolar configuration at the onset of a substorm [Cummings et al., 1968; Takahashi et al., 1987]. The field reconfiguration occurs as the cross-tail current is disrupted and presumably diverted into the ionosphere via field-aligned currents. The impulsive "dipolarization" or "current disruption" contains magnetic wave energy, both compressional and transverse, in a wide spectral band and can act as a source for a variety of magnetospheric ULF waves. Our goal in this study is to find out how the magnetosphere selects spatially coherent midlatitude Pi 2 signals out of the impulsive...

Magnetospheric substorms represent key explosive processes in the interaction of the Earth's magnetosphere with the solar wind, and their understanding and modeling are critical for space weather forecasting. During substorms, the magnetic field on the nightside is first stretched in the antisunward direction and then it rapidly contracts earthward bringing hot plasmas from the distant space regions into the inner magnetosphere, where they contribute to geomagnetic storms and Joule dissipation in the polar ionosphere, causing impressive splashes of aurora. Here we show for the first time that mining millions of spaceborne magnetometer data records from multiple missions allows one to reconstruct the global 3‐D picture of these stretching and dipolarization processes. Stretching results in the formation of a thin (less than the Earth's radius) and strong current sheet, which is diverted into the ionosphere during dipolarization. In the meantime, the dipolarization signal propagates further into the inner magnetosphere resulting in the accumulation of a longer lived current there, giving rise to a protogeomagnetic storm. The global 3‐D structure of the corresponding substorm currents including the substorm current wedge is reconstructed from data.

[1] The local time distribution of the ring current in the 27-119 keV range during several geomagnetic storm main phases have been investigated. The data was obtained by the high energy neutral atom (HENA) imager onboard IMAGE. Global proton distributions are derived from the observed energetic neutral atom (ENA) images using a linear inversion technique. During storms with low IMF B y the peak of the proton distribution is around 01 MLT. For storms with high IMF B y the peak can rotate to dawn. The rotation angle depends on solar wind velocity and interplanetary magnetic field (IMF) B y , but less on IMF B z . We discuss how this morphology implies the existence of strong and skewed equatorial electric fields in the inner magnetosphere. Our results are consistent with in-situ ring current measurements, radar observations and with kinetic models that self-consistently calculate the electric field produced by the closure of the partial ring current.

The substorm onset region and the radial development of the tail current disruption are examined from a new viewpoint. The reconfiguration of the magnetotail field at substorm onset can be understood in terms of a sudden decrease (disruption) in tail current intensity. The north‐south component (BZ) is very sensitive to whether the spacecraft position is earthward or tailward of the disruption region, while the change in the Sun‐Earth component (BX) is most sensitive to the change in tail current intensity near the spacecraft. If the current disruption starts in a localized range of radial distance and expands radially, a distinctive phase relationship between the changes in BX and BZ is expected to be observed. This phase relationship depends on whether the current disruption starts on the earthward side or the tailward side of the spacecraft. Thus it is possible to infer the direction of the radial expansion of the current disruption from magnetic field data of a single spacecraft. This method is applied to ISEE observations of a tail reconfiguration event that occurred on March 6, 1979. The phase relationship indicates that the disruption region expanded tailward from the earthward side of the spacecraft during the event. This model prediction is consistent with the time lag of magnetic signatures observed by the two ISEE spacecraft. The expansion velocity is estimated at 2 RE/min (∼200 km/s) for this event. Furthermore, it is found that the observed magnetic signatures can be reproduced to a good approximation by a simple geometrical model of the current disruption. The method is used statistically for 13 events selected from the ISEE magnetometer data. It is found that the current disruption usually starts in the near‐Earth magnetotail (|X| < 20RE and often within 15 RE of the Earth. The phase relationship between BX and BZ is discussed in terms of the near‐Earth neutral line model, and the result is compared with previous studies on the substorm onset region.