Experimental evidence is provided for the existence of slow-mode magnetosonic solitons in the col-lisionless plasma at the magnetopause boundary layer. The solitons were detected by the fleet of Cluster spacecraft at the dusk flank of the magnetosphere as magnetic field depressions (up to 85%) accom-panied with enhancement of the plasma density and temperature by a factor of 2. The solitons propagate 250 km/s with respect to the satellites and have perpendicular size of 1000-2000 km, which is a few ion inertial scale lengths. The comparison with numerical solutions of a theoretical model shows quantitative agreement between the model and observations.

We present the results of a controlled rocket‐borne experiment designed to study the very early time expansion characteristics (t < 1.0 s) of an artificially created ion cloud in the F region ionosphere. Using an attitude control system which optimized the investigation of parallel and perpendicular to B expansion properties, we have investigated the ionization and multi‐ion expansion features of the cloud constituents Ba+, Li+, and Ti+ and the relative influences of Saha and solar UV ionization mechanisms, ion‐neutral collisionality, and gyrokinetic orbits. Unique “in situ” diagnostic capabilities provided highly resolved measurements of the plasma structure within the cloud, the variations in mean ion composition, and the distributions of electron temperature. Among the findings we report that (1) the observations can be characterized by an expanding photoionized shell of structured cloud ions, with kinetically snowplowed background O+ ions and appreciable amounts of Li+ ions as forerunners on the leading and trailing edges; (2) a 25% depleted ionosphere is left behind the expanding shell; (3) the density structure within the cloud is created by gyrokinetic motion of individual ion components; and (4) electron temperatures are elevated on the average by 80% above the background levels with the hottest region (at twice the background temperature) in the domain of forerunner ions. The results are in agreement with simple analytical models and large‐scale simulations, corroborating and quantifying issues involving ionization, cycloidal dynamics, snowplow effects, depletion levels, and electron heating; while the unexpected discovery of Ti+ suggests an incomplete understanding of the burning process in the artificial cloud source and the temperatures in its ignition system.

Abstract. The results of simultaneous observations of charged particle fluxes within the energy range 50 eV to 20 keV (plasma energy-angle spectrometer, PEAS experiment) and plasma waves within the frequency range 0.1 to 10 MHz (plasma radio spectrometer, PRS 3 experiment) on board the APEX spacecraft are presented. The data were obtained at polar and auroral latitudes in the dawn-dusk and noon-midnight time sectors. The low-frequency (LF) sporadic emission mainly on frequencies lower than the local gyrofrequency was commonly observed within a comparatively narrow latitude interval (-5 ø-6 ø) of the auroral oval. The maximum of spectral wave intensity was revealed at a frequency of-200 kHz. The sporadic character of the observed emissions, the spectra, are very similar to those reported for auroral kilometric radiation (AKR), and the temporal and spatial occurrences imply its relation to the source of AKR emissions measured at the higher altitudes, with regards to our wave measurements as LF AKR-type emissions. Comparative analysis of spatial distributions of charged particle energy spectra and variations of wave spectra at auroral oval latitudes showed that several conditions shotfid be fidfilled for LF AKR generation. These conditions are related both to the intensity and energy of precipitating ion and electron fluxes, and to albedo electrons and the ionospheric plasma parameters. The wideband HF emission was observed together with the LF AKR-type emission during the morning auroral oval crossing. At spacecraft apogee altitudes, in the polar cap zone intense electron precipitations and an increase of plasma noise intensity at frequencies equal or below fm were observed at the period when the IMF was northward. These precipitations and emissions were accompanied by the horizontal currents crossing the polar cap, the so-called them structure.

Abstract.A short account of the physics of electrical discharges in gases is given from the viewpoint of its historical evolution and application to planetary atmospheres. As such it serves as an introduction to the papers on particular aspects of electric discharges contained in this issue, in particular in the chapters on lightning and the discharges which in the last two decades have been observed to take place in Earth's upper atmosphere. In addition to briefly reviewing the early history of gas discharge physics we discuss the main parameters affecting atmospheric discharges like collision frequency, mean free path and critical electric field strength. Any discharge current in the atmosphere is clearly carried only by electrons. Above the lower boundary of the mesosphere the electrons must be considered magnetized with the conductivity becoming a tensor. Moreover, the collisional mean free path in the upper atmosphere becomes relatively large which lowers the critical electric field there and more easily enables discharges than at lower altitudes. Finally we briefly mention the importance of such discharges as sources for wave emission.