EQUATORIAL ION COMPOSITION, 140-200 KM, BASED ON ATMOSPHERE EXPLORER-E DATA

We have used in situ measurements of ion composition and horizontal wi nds, taken from equatorial orbiting Atmosphere Explorer E in eccentric orbit during 1975-1976 to investigate the bottomside ionosphere at al titudes 140-200 km. Representative daytime altitude profiles of ioniza tion were stable against wide variations in horizontal wind patterns. On the nightside, ionization profiles displayed much structure, some o f which was associated with variations in the horizontal winds. Specia l features that sometimes appeared in the structured nightside ionizat ion were apparent ion composition waves, intermediate layers of enhanc ed ionization, and ionization depletions similar to equatorial ionizat ion bubbles. Apparent ion composition waves displayed a horizontal wav e length approximately 650 km. A series of measurements displayed enha nced layers of ionization that appeared to be newly separated from the bottomside midnight F layer. Ions detected in the enhanced ionization layers were primarily NO+ and O2+ without significant densities of me tallic ions, an indication that metallic ions are not required to prod uce the layers at altitudes above 140 km. Equatorial ionization deplet ions were observed at lower altitudes than previously reported and dis played molecular ion depletions as well as O+ depletions.