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.