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...

Gamma-neutron activation experiments have been performed with relativistic electron beams produced by a laser wakefield accelerator. The electron beams were produced by tightly focusing ͑spot diameter Ϸ6 m) a high power ͑up to 10 TW͒, ultra-short (у50 fs) laser beam from a high repetition rate ͑10 Hz͒ Ti:sapphire ͑0.8 m͒ laser system, onto a high density (Ͼ10 19 cm Ϫ3) pulsed gasjet of length Ϸ1.5 mm. Nuclear activation measurements in lead and copper targets indicate the production of electrons with energy in excess of 25 MeV. This result was confirmed by electron distribution measurements using a bending magnet spectrometer. Measured ␥-ray and neutron yields are also found to be in reasonable agreement with simulations using a Monte Carlo transport code.

Abstract.It is important to appreciate how the morphology of internal charging of spacecraft systems, due to penetrating electrons, differs from that of the more common surface charging, due to electrons with lower energy. A specific and recurrent anomaly on a geostationary communication satellite has been tracked for ten years so that solar cycle and seasonal dependencies can be clearly established. Concurrent measurements of sunspot number, solar wind speed and 2-day >2 MeV electron fluence are presented to highlight pertinent space weather relationships, and the importance of understanding the complex particle interaction processes involved.

We present simultaneous measurements at high temporal resolution of flux transfer events (FTEs) earthward of the magnetopause using four instruments on the Active Magnetospheric Particle Tracer Explorers (AMPTE) UKS spacecraft. The seven events in our study are distributed in local time from early dawn to midafternoon over a GSM latitude range of ∼40°, from 24°N to 17°S. A multilayered structure is revealed with systematic behavior in fields and charged particle populations. The plasma pressure varies in anticorrelation with the field pressure, the total pressure in general exceeding that of the surrounding medium. While it is not the prime concern of this paper to discuss the validity of models proposed for FTEs, the new, detailed experimental results it contains make it clear that the Russell‐Elphic [1978] model needs, at the very least, more detailed development.