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The detailed analysis of an isolated dispersionless substorm is performed on the basis of field and particle data collected in situ by the geostationary satellite GEOS 2 and of data from ground-based instruments installed close to the GEOS 2 magnetic footprint. These data give evidence for (1) quasi-periodic variations of the magnetic field configuration, which is alternatively taillike and dipolelike, (2) in-phase oscillations of the flux of energetic electrons, which is high when the configuration is dipolelike and vice versa, (3) a gradient in the flux of energetic ions, which is, on the average, earthward but undergoes large fluctuations around this average direction, and (4) large transient fluctuations of the quasi-dc electric field, which reverses its direction from eastward to westward. It is shown that these results are consistent with the development of an instability which leads to a westward propagating "wave." The source of the instability is the differential drift of energetic electrons and ions in a highly stressed magnetic field configuration (in a high /3 plasma). Evidence is given for a system of localized field-aligned currents flowing alternately earthward and equatorward at the leading and trailing edges of the westward propagating wave. This current system resulting from the temporal development of the instability produces the so-called Pi 2 pulsations, at the ionospheric level. The closure of this current system in the equatorial region leads to a current antiparallel to the tail current, and therefore to its reduction or cancellation. This reduction/ cancellation of the tail current restores the dipole magnetic field (dipolarization) and generates a large westward directed induced electric field (injection). Hence, dipolarization and injection are the consequences of the instability. Finally, it is suggested that the westward traveling surge observed simultaneously by all-sky cameras, close to the magnetic field of GEOS 2, is the image of the instability in the equatorial region transmitted to the upper atmosphere by precipitating electrons. 12 18 06 AFTER EXPANSION 12 Fig. 1. Schematics adapted from Akasofu [1977]. (a) The auroral arcs as they appear at breakup, at the northern boundary of the diffuse auroras, at a magnetic latitude typically between 65 ø and 70 ø. The shaded areas indicate the regions where diffuse auroras are observed. (b) After breakup, the discrete auroral forms expand to the west, to the east, and to the north. At the end of the expansion phase, the discrete arcs cover a region which typically extends from 650-70 ø to 750-80 ø magnetic latitude. diffuse reverse current has been identified west and more clearly east of the surge [e.g., Kozelova and Lyatskiy, 1984]. According to Baumjohann et al. [1981], the ratio between the height-integrated Hall and Pedersen conductivities increases in the westward traveling surge region, due to precipitation of relatively high energy electrons (10-20 keV). This kind of precipitation was directly observed, for example, by Kremser et...

Using an absorption cell, we measured the Doppler shifts of the interstellar hydrogen resonance glow to show the direction of the neutral hydrogen flow as it enters the inner heliosphere. The neutral hydrogen flow is found to be deflected relative to the helium flow by about 4 degrees . The most likely explanation of this deflection is a distortion of the heliosphere under the action of an ambient interstellar magnetic field. In this case, the helium flow vector and the hydrogen flow vector constrain the direction of the magnetic field and act as an interstellar magnetic compass.