We analyzed measurements of ion number density made by the retarding potential analyzer aboard the Atmosphere Explorer-E (AE-E) satellite, which 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) density measurements in the high-altitude regions show large depletions of bubble-like structures which are confined to narrow local time, longitude, 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 300 km and above 350 km and investigated the global distribution of irregularities using the rms deviation ΔN/N over a path length of 18 km as an indicator of overall irregularity intensity. 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¿ longitude 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 winds and drifts are necessary to adequately explain the observations at low and high altitudes. Seeding effects are most obvious near 0¿ longitude, while the most easily observed effect of the F region is the suppression of irregularity growth by interhemispheric neutral winds. ¿ 1998 American Geophysical Union