The high-latitude ionosphere

This paper reviews information on the high-latitude ionosphere derived from the Alouette-ISIS program. It is primarily descriptive and deals mainly with F-region features. Particle effects in the D region and night E ionization are briefly considered. F-region features include the main trough, maxima and minima in electron density in the polar region, and electron-density irregularities revealed by the presence of spread echoes on topside ionograms. The relationship of these features to the plasmapause and the energetic particle boundaries is discussed. Although ionization effects in the D region are not directly detectable by the Alouette satellites, large increases in D-region ionization are produced at high latitudes by the influx of energetic particles which may be detected by some of the energetic particle counters on board. For both polar-cap and auroral events, empirical relations have been derived between particle fluxes and D-region radio-wave absorption measured by riometers.