[1] This paper uses the plasma data from Cluster and TC-1 and geomagnetic data to study the geomagnetic signatures of the current wedge produced by fast-flow braking in the plasma sheet. The three fast flows studied here occurred in a very quiet background and were accompanied by no (or weak) particle injections, thus avoiding the influences from other disturbances. All the geomagnetic signatures of a substorm current wedge can be found in the geomagnetic signatures of a current system produced by the braking of fast flows, indicating that the fast flows can produce a complete current wedge which contains postmidnight downward and premidnight upward field-aligned currents, as well as a westward electrojet. The Pi2 precursors exist not only at high latitudes but also at midlatitudes. The starting times of midlatitude Pi2 precursors can be identified more precisely than those of high-latitude Pi2 precursors, providing a possible method to determine the starting time of fast flows in their source regions. The AL drop that a bursty bulk flow produces is proportional to its velocity and duration. In three cases, the AL drops are <100 nT. Because the AE increase of a typical substorm is >200 nT, whether a substorm can be triggered depends mainly on the conditions of the braking regions before fast flows. The observations of solar wind before the three fast flows suggest that it is difficult for the fast flows to trigger a substorm when the interplanetary magnetic field B z of solar wind is weakly southward.Citation: Cao, J. -B., et al. (2010), Geomagnetic signatures of current wedge produced by fast flows in a plasma sheet,

[1] On 26 February 2008, the THEMIS satellites observed two substorms that occurred at about 0405 and 0455 UT. Angelopoulos et al. (2008) made a comprehensive study of the second event. In this paper we display detailed features of the two substorms with emphasis on the first. In both substorms, a distinct auroral intensification occurred during the earliest stage of onset, about 1 to 2 min after midtail reconnection began. This initial intensification was weak and localized and thus had the signatures of a pseudobreakup. In both substorms, a second, major intensification occurred next in the substorm onset sequence, followed by rapid and extensive poleward expansion. This second intensification had the features of the major expansion onset and was nearly coincident with observations of earthward flows and magnetic dipolarization in the near-Earth tail. During the growth phase of the two substorms, open magnetic flux accumulated in the polar cap; in the expansion/recovery phase the polar cap open flux was quickly reduced. These observations are in agreement with the assertion that tail reconnection initiates the initial pseudobreakup and the ensuing major expansion and releases and transports energy to eventually cause near-Earth dipolarization and the expansion phase onset of these two substorms.

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We consider the effect of accelerated magnetopause motions arising from the arrival of a modified-pressure region of the solar wind with a sharp forefront upon the growth rate of the velocity shear layer instability. The range of values of relative pressure variation on the front was taken to lie within 1.25-2, and the front thickness ranged from 200 km to 1RE (6400 km). At different phases of motion of the boundary, the instability growth rate can change in either direction as a consequence of the Rayleigh-Taylor effect, in accordance with a change of sign of acceleration g of the boundary. The typical time during which of the boundary remains inside the new equilibrium position (_• I min) is sufficient for the instability to reach the nonlinear regime. It is possible that plasma flutes on the boundary penetrate the magnetosphere. Generation of disturbances can be modulated by boundary oscillations inside the new equilibrium position. We discuss the interplanetary magnetic field influence upon the growth rate of the MHD instability under consideration. An analysis is made of a number of daytime geophysical conditions which cannot be understood when interpreted in terms of the instability on a stationary boundary. the prime denotes differentiation with respect to the normal to the boundary. If the first factor is absent and the second is present, then the arising instability is said to be the Rayleigh-Taylor (R-T) or flute instability. A relevant classical example is the instability of a heavy fluid above a heated, hghter-weight fluid in the gravity field. In the general case of the presence of both factors the influence of the second mechanism upon the value of the K-H growth rate will be henceforth referred to as the R-T effect. In this case, by the positive and negative R-T effects, we mean an enhancement and an attenuation of the K-H growth rate, respectively. In the vicinity of the subsolar magnetopause the density Copyright 1993 by the American Geophysical Union. Paper number 93JA00417. 0148--0227/93/93 JA-00417505.00 changes abruptly, i.e., plI/pI > 4 (where indices I and II refer to regions inside and outside the boundary), and the flow velocity is minimal [Paschmann et al., 1978]. Since the radius of curvature of the geomagnetic field here is also minimal, R = l0 s km [Guterich and Shcherbakov, 1971], the R-T effect here must manifest itself most conspicuously. Its negative character is attributable to the convexity of the boundary and is associated with two factors' the trapped particles travelling in the magnetosphere, and the flow round

[1] Two case studies are performed to investigate substorm timing and activations based on Double Star TC1, Cluster, Polar, IMAGE, LANL geostationary satellites and ground-based geomagnetic field measurements. In both events, an earthward flow associated with plasma sheet thinning is measured by Cluster 8-10 min ahead of the auroral breakup. A couple of minutes after the breakup, either TC1 at $X-10 R E first detects plasma sheet expansion and then the LANL satellites near the midnight measure energetic electron injections at geostationary orbit or the LANL satellites first measure the electron injections and then TC1 detects the plasma sheet expansion. More than about 20 min later, Cluster at X$16 R E and Polar (at higher latitude) successively observe plasma sheet expansion. The open magnetic flux of the polar cap, Y, is found to continually increase during the early substorm phase and then to rapidly fall when the IMF turns northward. When Y reaches its minimum value, bright and broad auroral activities start to decrease. Tailward progression of the magnetic dipolarization and a poleward expansion of auroral bulges are shown to closely map to one another. These results suggest that substorm activations start in the midtail before ground onset and then move earthward, which leads to an expansion onset in the near-Earth tail around X$ -(8-9) R E . After onset, the activations progress both earthward and tailward. Substorm onset is possibly related to plasma sheet reconnection of close field lines, while tail lobe reconnection of open field lines release more energy to support the full expansion of the substorm. In a fully developed expansion phase, an initial dipolarization in the near-Earth may eventually evolve to enable disruption of the cross-tail current over a wide region of the magnetotail. Citation: Cao, X., et al. (2008), Multispacecraft and ground-based observations of substorm timing and activations: Two case studies,