Equatorial ion composition, 140‐200 km, based on Atmosphere Explorer E data

We have used in situ measurements of ion composition and horizontal winds, taken from equatorial orbiting Atmosphere Explorer E in eccentric orbit during 1975‐1976 to investigate the bottomside ionosphere at altitudes 140‐200 km. Representative daytime altitude profiles of ionization were stable against wide variations in horizontal wind patterns. On the nightside, ionization profiles displayed much structure, some of which was associated with variations in the horizontal winds. Special features that sometimes appeared in the structured nightside ionization were apparent ion composition waves, intermediate layers of enhanced ionization, and ionization depletions similar to equatorial ionization bubbles. Apparent ion composition waves displayed a horizontal wave length ∼ 650 km. A series of measurements displayed enhanced 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 metallic ions, an indication that metallic ions are not required to produce the layers at altitudes above 140 km. Equatorial ionization depletions were observed at lower altitudes than previously reported and displayed molecular ion depletions as well as O+ depletions.