Ion temperature and ion drift velocity data from Atmosphere Explorer D have verified the existence of interhemispheric plasma transport and ion temperature troughs in the topside equatorial ionosphere. The data were taken during solar minimum conditions at night where the exospheric temperature was typically 700¿K and the O+-H+ transition height was 520 km. Large field-alligned ion velocities were observed above about 700 km, where the H+/O+ number density ratio was about 3. Model calculations have shown that the ion temperature decrease is produced by quasi-adiabatic expansion of the plasma, but the expansion cooling mechanism is less efficient at solar minimum than at solar maximum owing to the lower O+-H+ transition height. Ion temperature troughs greater than 400¿K were not observed even when the field-aligned ion velocity was greater than 700 m s-1. Most of the expansion occurs below the transition height, and during sunspot minimum, thermal coupling to the neutral atmosphere is much more effective in quenching the cooling which the field-aligned transport tends to produce.