GLOBAL DISTRIBUTION OF DENSITY IRREGULARITIES IN THE EQUATORIAL IONOSPHERE

We analyzed measurements of ion number density made by the retarding p otential analyzer aboard the Atmosphere Explorer-E (AE-E) satellite, w hich was in an approximately circular orbit at an altitude near 300 km in 1977 and later at an altitude near 400 km. Large-scale (>60 km) de nsity measurements in the high-altitude regions show large depletions of bubble-like structures which are confined to narrow local time, lon gitude, and magnetic latitude ranges, while those in the low-altitude regions show relatively small depletions which are broadly distributed in space. For this reason we considered the altitude regions below 30 0 km and above 350 km and investigated the global distribution of irre gularities using the rms deviation Delta N/N over a path length of 18 km as an indicator of overall irregularity intensity. Seasonal variati ons of irregularity occurrence probability are significant in the Paci fic regions, while the occurrence probability is always high in the At lantic-African regions and is always low in the Indian regions. We fin d that the high occurrence probability in the Pacific regions is assoc iated with isolated bubble structures, while that near 0 degrees longi tude is produced by large depletions with bubble structures which are superimposed on a large-scale wave-like background. Considerations of longitude variations due to seeding mechanisms and due to F region win ds and drifts are necessary to adequately explain the observations at low and high altitudes. Seeding effects are most obvious near 0 degree s longitude, while the most easily observed effect of the F region is the suppression of irregularity growth by interhemispheric neutral win ds.