Entry of heated solar wind plasma into the magnetosphere is examined using electron distributions measured by AMPTE UKS and HEOS 2. In particular, the angular structure of the electron distributions is studied within the transition region separating the magnetosheath from the inner magnetosphere. The measurements suggest that electrons in the outer part of the transition region originate in the magnetosheath, whilst the population closer to the Earth consists of electrons from the magnetosphere combined with an energized magnetosheath component. This energized component contains “counterstreaming” electrons, which are confined to directions closely parallel and antiparallel to the magnetic field direction. The possibilities, that the energization of the counterstreaming electrons is cumulatively gained from either waves, electric fields perpendicular to the magnetic field, or quasi‐Fermi acceleration, are discussed. It is not possible to identify the topology of the magnetic fields of the outer part of the region, but there is strong evidence that the inner part is on closed magnetic field lines, which map to the day side auroral zone. The outer part of the transition region is a plasma depletion/magnetic field compression layer. The structure of the transition region is similar to that surrounding flux transfer events, which leads to the deduction that the plasma and field signatures of flux transfer events may be the result of displacement of the transition region earthward. Cases where the displacement is such that the field maximum of the depletion/compression region is encountered may well explain “crater” flux transfer event signatures.

Following releases of lithium ions into the solar wind by the Active Magnetospheric Particle Tracer Explorers (AMPTE)‐IRM spacecraft on September 11 and 20, 1984, electron intensities measured at the AMPTE/UKS spacecraft, some 33 km away from the IRM, increased significantly for tens of seconds. The causes of the increases are considered and found to be the result of energization of solar wind electrons. The degree of energy dependence of the energization is established, and possible causative processes are considered. No positive indications of adiabatic compression or acceleration through a potential difference are found. There is, however, evidence for a resonant wave‐particle interaction.

The direct observation of high frequency ∼3 Hz modulation of electron intensities during pulsating aurora is comparatively recent although, from TV and photometric studies, modulation of the visual aurora at these frequencies has been recognized for over a decade. We report here 2.2 ± 0.5 Hz oscillations in the intensities of 4–25 keV electrons producing a pulsating aurora. The electrons were measured from a Petrel sounding rocket launched from Kiruna, Sweden on 25 January 1979. The oscillations like the slower 1–20 s pulsations, exhibit a marked velocity dispersion implying an equatorial origin for both forms of modulation. VLF—hiss observed at about the same time at the Satellite GEOS 2, operating in the same equatorial region, shows remarkable similarities to the modulation in the particle fluxes. A connection between VLF emissions and auroral pulsations has been suggested by Coriniti and Kennel who argue that low frequency, 5–300 s period, micropulsations modulate whistler-mode wave amplitudes leading to variations in the rate of pitch-angle scattering of electrons from the magnetosphere. The isotropic angular distributions reported here suggest that this mechanism acting alone cannot explain the modulation in the present case.

Energy spectra of electrons encountered on a rocket flight across an array of auroral arcs are employed to test three related models of electron acceleration. All three are based on a potential difference existing between the source plasma in the magnetosphere and the observation point in the ionosphere. One of the models provides a satisfactory fit to the observed spectra. Two alternative mechanisms are suggested to explain this model. The first possibility is a time-varying potential difference, which results in the accelerated electrons being observed with a statistical distribution of energy gain. The second possibility, which results in the same energy gain distribution, is a constant potential difference operating in conjunction with plasma instabilities generated by the accelerated beam. The energy gain distribution in the second case is therefore a consequence of a constant potential difference and a variable energy loss. In addition it is suggested that electrostatic waves generated by the instabilities could accelerate ambient plasma to suprathermal energies. Application of the model to the complete data set yields a continuous record of the parameters defining the acceleration and source plasma across the array of arcs. Reference is also made to an acceleration mechanism involving resonance with electrostatic waves.

Two new types of British sounding rocket, a Skylark 12 and a Fulmar, were launched from Andøya in northern Norway, on 21 November 1976, in an experiment to study the particle streams and plasma waves associated with an auroral arc. We report here the first results obtained from channel multipliers and electrostatic analysers carried on the Skylark 12 (designation SL 1422) to detect electrons in the range 0.5–25 keV,and positive ions of 0.5–25 keV per unit charge. We believe that the results are unique and constitute the first detailed comparison of positively- and negatively-charged particles in the region of an auroral arc. They have been made possible by the combined advantages of high altitude (715 km), which enables ions to be studied before absorption in the atmosphere, long range (780 km), which allows a complete traversal of an auroral structure, and new analysers, which extend the dynamic range of measurement.

It has been shown by Hall and Bryant (1974) how the energy spectrum and pitch angle distribution of auroral particles observed on a rocket flight during break-up could be accounted for by a variable acceleration, such as would be produced for example by a fluctuating parallel electric field. The gain in energy was assumed to be normally distributed. The purpose of this paper is to show that acceleration associated with multiple auroral arcs also involves a significant degree of fluctuation, and to distinguish between features in the precipitation that originated in the structure of the acceleration region, and features that are derived from the properties of the plasma source before acceleration.