Characteristics of plasma density irregularities in the equatorial ionosphere

We analyze high-resolution measurements of ion number density in the equatorial ionosphere made from the AE-E satellite during the years 1977-1981, and investigate the global distribution of density irregularities and the evolution of irregularity structure. In the high-altitude regions (above 350 km) the measurements show large density depletions of bubble-like structures which are confined to narrow local time, longitude, and magnetic latitude ranges while those in the low-altitude regions (below 300 km) show relatively small-depletions that are more broadly distributed in space. Seasonal variations of irregularity occurrence probability are significant in the Pacific regions while the occurrence probability is always high in the Atlantic-African regions and is always low in the Indian regions. We find that the high occurrence probability in the Pacific regions is associated with isolated bubble-structures while that near 0{dollar}spcirc{dollar} longitude is produced by large depletions with bubble-structures that are superimposed on a large-scale wavelike background. Seeding effects are most obvious near 0{dollar}spcirc{dollar} longitude while the most easily observed effect of the {dollar}F{dollar} region is the suppression of irregularity growth by interhemispheric neutral winds. By performing spectral analysis of the ion density measurements in the spatial range 18 km-200 m we investigate the irregularity structure and its temporal evolution. The local time evolution of irregularity structure is investigated by using average statistics for the low- and high-intensity structures in the altitude regions above 350 km and below 300 km. Above 350 km, a spectral break near 1 km scale size is more pronounced for low-intensity structure than for high-intensity structure. The spectral break for the first case is produced by the enhancement of power near 1 km while that for the second case is produced by the maintenance of power near 1 km. The temporal evolution of spectral parameters in the low-altitude region is relatively slow because the local time variations of power at middle-scales are not significant. The small amplitude of several-kilometer scale density modulations is often observed in the absence of irregularities at meter-scales and the power spectrum of these density structures exhibit a steep spectral slope at kilometer-scales.