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The characteristics of polar cap electron acceleration regions and the relationship of their occurrence to the interplanetary magnetic field (IMF) have been investigated by using data from Atmosphere Explorer D and Imp J. It was found that electron energy spectra and angular distributions within the acceleration regions are generally consistent with models of acceleration of auroral primaries and reflection of atmospheric secondaries by field‐aligned electrostatic potential differences. However, localized strongly field‐aligned fluxes are also observed at energies below the spectral peak. It is suggested that these transient beams of field‐aligned low‐energy electrons result from the acceleration of thermal electrons from within the acceleration regions and that this thermal electron population may be partially replenished by small pitch angle atmospheric secondaries. The occurrence of the polar cap acceleration regions in the northern hemisphere is strongly correlated with IMF vectors which project into the (−X, +Z) sector of the solar magnetospheric X‐Z plane; that is, with northward ‘away’ IMF polarities.

Measurements of electron precipitation and ion drift velocities on the spacecraft Atmosphere Explorer C have revealed that electron acceleration regions (or inverted ‘V's’) in the 1200–1800 MLT quadrant exhibit the following systematic behavior: (1) Electron distribution functions in the acceleration regions are in all cases well described by Maxwellian primary electron beams which have been accelerated through an electrostatic potential V0. (2) The typical inverted V latitudinal structure is always observed in the acceleration regions, with V0 increasing to a maximum and subsequently decreasing to near zero over distances of ∼100 to ∼250 km. (3) In all cases the Maxwellian temperature E0 of the primary electron beam increases systematically with increases in V0. (4) Rather weak acceleration regions, characterized by V0 ≲ 1 keV and E0 values of ∼100 to ∼350 eV, occur in the cusp and in the highest‐latitude portion of the dusk side electron precipitation zone, where the ionospheric convection velocity is beginning to rotate from antisunward to sunward. (5) Values of V0 and E0 and the width of the acceleration regions in the cusp and near dusk are similar, this fact together with observation 4 suggesting that both regions are connected to the magnetosheath. (6) Near dusk, regions of much stronger acceleration, characterized by V0 values of ∼3 to over 10 keV and E0 values in the ∼1‐keV range, typically overlap well into the sunward convection region, at times lying completely within it, this behavior suggesting a link with the plasma sheet. (7) The acceleration regions in the sunward convection zone are associated with significant weakening of ionospheric convection velocities relative to the adjacent regions, resulting perhaps from conductivity enhancements produced by the very high electron fluxes that occur within them.

Recent studies of the low-energy plasma population in the Earth’s space environment have revealed that this plasma population is much more complex than previously supposed and that a simple model of ionospheric evaporation cannot explain the distributions. There was a need to develop an advanced instrument to study this plasma in detail, and this paper describes the scientific background, design, development, and in-flight characteristics of such an instrument, the Light Ion Mass Spectrometer (LIMS). This instrument combines a magnetic mass spectrometer, a planar-grid retarding potential analyzer, and multidirectional sensor heads to measure the mass composition, density, temperature, and flow velocity of low-energy (E〈100 eV) plasma. The studies which were conducted leading to the final design will be discussed in detail and will illustrate certain effects which arose in the combining of energy and mass analysis into a single sensor. The instrument was flown on a high-altitude satellite in February 1979, and selected flight data will be presented to demonstrate the instrument performance.