Data from the Defense Meteorological Satellite Program are used to examine solar activity variations in ion temperature measured near 800 km altitude across the nighttime equatorial ionosphere. Evidence for ion cooling and heating by adiabatic expansion and compression are seen during high and moderate solar activity conditions. It is found that these effects are strongly dependent on the location of the O+/H+ transition height. During high solar activity a high transition height moves the region of maximum cooling at the dip equator well above 800 km. The region of maximum heating, however, appears in the winter hemisphere near 800 km, where strong longitude variability is seen, owing to the effects of F region neutral winds. During moderate solar activity conditions the transition height is lowered, bringing the region of maximum cooling closer to 800 km altitude. However, the cooling process is not as strong as that seen during high solar activity periods. The heating effects are also lowered during moderate solar activity periods. At 800 km altitude, only relatively small temperature peaks are seen in the winter hemisphere, closer to the dip equator than those seen during high solar activity periods. ¿ 1999 American Geophysical Union