[en] The importance of auroral-zone X-ray measurements with balloon-borne instruments during the GEOS project is discussed. Emphasis is given to special investigations using simultaneous X-ray measurements and particle and field data from a geostationary satellite. Problems involved in balloon programmes during the GEOS mission are also discussed

[en] The dynamic processes occurring in the magnetosphere during substorms affect the distribution of high energy electrons (E>30 keV) in a manifold manner. Considerable temporal and spatial variations of the distribution parameters are observed that are caused by magnetic field disturbances, electric fields, and different kinds of interactions between particles and electromagnetic waves. Although the high energy electrons play a more passive role, the observed variations reflect the characteristic features of the dynamic processes sometimes very closely. High energy electron observations can therefore be used to analyze the dynamic processes themselves. In this paper results are presented, that were obtained with predominant use of a special observational method: the measurement of auroral X-rays with balloon-borne instruments. (Auth.)

[en] Interaction of energetic electrons (16 - 300 keV) and ELF waves (100 Hz - 3 kHz) has been studied by using data of the GEOS-1 and -2 satellites. In steady-state, it is demonstrated that this interaction enters quite well into the framework of the quasi-linear theory of pitch-angle diffusion. By calculating the wave growth rate at different magnetic latitudes, it is shown that inside the plasmasphere the path integrated gain is large enough for explaining generation of hiss, in agreement with theoretical and experimental results on wave propagation. Outside the plasmasphere hiss generation deserves further study. At the time of storm sudden commencements, non-steady state behavior is observed that results in antiphase oscillations of the maximum of the wave spectrum and of the corresponding growth rate and anisotropy. 53 refs

[en] The relationship between particle precipitation and the plasma sheet has been studied with the use of simultaneous measurements from balloons and a geostationary satellite during a substorm on July 3, 1979. At least two of the three precipitation mechanisms observed were directly related to the plasma sheet dynamics. The impulsive precipitation of electrons near earthward moving injection fronts and the interaction of hot unstably distributed plasma sheet electrons with whistler mode electromagnetic waves. The impulsive precipitation was related to field aligned electron beams that were probably generated by local acceleration by magnetic field aligned electric fields. The electron distribution during the wave particle interaction events can be described with a bi-Maxwellian distribution function at energies between 3 and 70 keV and with a power law function at higher energies. Both components were anisotropically distributed. Numerical calculations of the anisotropy and the wave growth rate provided results well in agreement with the wave observations. The wave particle interaction generated strong pitch angle diffusion that was responsible for the precipitation during these events

[en] The measurement of auroral X-rays with balloon-borne instruments is an efficient way to study electrons with energies exceeding about 30 keV in the magnetosphere during disturbed periods. This paper discusses the perspectives for such measurements in the 1980's. Recently developed payloads are described that can be used as a basis for further technical developments and satellite projects scheduled for the 1980's are presented that are suited for coordinated X-ray measurements (these projects include EISCAT-SAT and OPEN)

[en] The cusp/cleft region provides an entrance for magnetosheath particles into the magnetosphere and a sink for magnetospheric particles. In addition, strong acceleration and/or scattering of particles takes place. The Swedish satellite Viking crossed this region at midaltitudes. Measurements from this spacecraft were used to determine the average spatial distributions of H⁺ and He⁺⁺ ions in the energy/charge range 2 keV/e ≤ E/Q ≤ 60 keV/e and of electrons with 7 keV ≤ E ≤ 97 keV. The data supply information on the structure of the midaltitude cusp/cleft region, the particle sources, and dynamical processes. Four different parts can be distinguished: (1) The cusp extends from about 76 degree to 82 degree invariant latitude (INL) and from 0800 to 1400 MLT. It is characterized by the presence of magnetosheath origin particles and important electron acceleration signatures. (2) A smaller region inside the cusp (77 degree-82 degree INL, 1000-1330 MLT) contains magnetosheath origin ions without electron acceleration. This is regarded as the cusp proper connected to the exterior cusp. (3) Poleward of the cusp magnetosheath origin ions are still present, but no magnetosheath electrons. This region is related to the plasma mantle. (4) Another region without magnetosheath origin ions but with strong electron acceleration extends equatorward of the cusp and probably constitutes part of the cleft, likely to be connected to the low-altitude boundary layer

[en] Measurements made with the charged particle spectrometer of the Max-Planck-Institut fuer Aeronomie onboard GEOS-1 were used to investigate the behaviour of energetic electrons and ions in the dusk sector of the magnetosphere. During substorms the integral ion flux (> approximately 24 keV) increased whereas the integral electron flux (> approximately 20 keV) first decreased and later on also increased. The dependences of these flux variations on pitch-angle and particle energy are described and discussed in terms of particle drift in the geomagnetic and geoelectric fields and adiabatic energy variations. The results also provide some information on the source region of the drifting particles. (Auth.)

[en] Recordings of Bremsstrahlung X-rays supported by recordings of cosmic noise absorption have been used to study in detail energetic (>= 30 keV) electron precipitation events occurring near local midnight at the onset of the expansion phase of magnetospheric substorms. This type of precipitation occurs during the first 5 to 10 min after bay onset and can usually be distinguished from the subsequent bay-associated precipitation by its characteristic time structure, variation in energy spectrum, and higher intensities. During this same interval, the poleward border of the precipitation region moves rapidly towards higher latitudes with speeds of typically 1 to 2 km/s, whereas the equatorward border seems to move slowly towards lower latitudes. The northward expansion starts just poleward of the lowest latitudes reached during the slow equatorward motion of the preceding growth-phase precipitation. The previous narrow precipitation region may thus expand to as much as 10⁰ of invariant latitude within a few minutes. Within the expanding region there are additional intrinsic temporal variations. As the flux of precipitating electrons tends to be most intense and most energetic near the poleward border, recordings made northward of the latitude where the poleward motion started tend to give the appearance of an impulsive precipiation event. The bay-onset precipiation starts abruptly at the onset of Pi 2 magnetic pulsations. Associated with these pulsations there are modulations of the flux of precipitating electrons. An intensified westward electrojet appears to have its center in the equatorward part of the precipitation region. The results are discussed and a mechanism is proposed. (author)

[en] Eighteen months of data provided by the magnetometer and energetic particle spectrometer on board the geosynchronous satellite GEOS 2 were used to investigate compressional ULF waves with frequencies of a few megahertz associated with large anticorrelated modulations of the ion pressure. The events are classified into two groups according to the behavior of the background magnetic field/plasma configuration at the onset of the oscillations. For one group the oscillation onset is preceded by a sharp increase of the plasma energy density due to the arrival of drifting ion bunches at the satellite and a corresponding diamagnetic decrease of the magnetic field strength. The other group is observed in a steady configuration in the presence of an enhanced ring current. Besides striking differences between the two groups as regards the average event duration and wave frequency, as well as the phase relation between electron and ion flux modulations, the authors present evidence that both groups of events are of antisymmetric structure with respect to the geomagnetic equator and have a small field-aligned wavelength. Furthermore, for both groups the frequency variation with local time and the azimuthal propagation are closely connected with the ion magnetic gradient curvature drifts, and the waves are associated with a plasma distributed anisotropically in velocity space. Therefore the waves most likely represent the excitation of a single wave mode generated via wave-particle interactions. Using plasma parameters measured in situ the authors calculated the growth rate of different hydromagnetic wave modes. It was found that the plasma is able to support a wave mode similar to the drift mirror mode wtih a large parallel wave number, if the effects of a strongly curved magnetic field are taken into account

[en] The morphology of energetic (>= 30 keV) electron precipitation during the growth phase of magnetospheric substorms has been investigated using measurements of auroral-zone Bremsstrahlung X-rays obtained from multiple balloon flights and supplementing riometer recordings. Growth-phase precipitation typically starts about one hour before the onset of a negative magnetic bay and occurs in a limited region parallel to the auroral oval around local midnight. The precipitation is first observed in the northern part of the auroral zone and moves southwards with a speed of 5 to 10 km/min. To the north this precipitation therefore ceases well before bay onset whereas a continuous transition from 'prebay' precipitation to bay-associated precipitation takes place in the south. A decrease in the intensity or at least a levelling off may occur some minutes before bay onset. The southward movement of the precipitation region is associated with a similar movement of a weak ionospheric current system. The events studied were all associated with a southward-pointing interplanetary magnetic field and with growth-phase conditions in the magnetotail. It is suggested that growth-phase precipitation originates from the 'horns' of the plasma sheet. The equatorward motion of the precipitation is then a consequence of an expansion of the polar cap, a thinning of the plasma sheet, and an equatorward motion of its inner edge. It is also suggested that this precipitation provides a stabilization of the outer boundaries of the plasma sheet by restricting the ionospheric mobility of the bordering field lines through enhanced conductivity. (author)