Using measurements of electron temperature and density, a model of neutral density, and a theoretical expression for the O+-O collision cross section, we compute the ion temperature and find it to be lower than the measured value. We compute the amount of hot O at 4000 ¿K necessary to provide this heat deficit. At 400 km altitude very little hot O is needed at solar maximum while 1--2% of total O must be hot at solar maximum. Hot O densities at 400 km altitude are lowest during the day, highest at night; lowest at the equinoxes, highest at the solstices; lowest at solar maximum, highest at solar minimum; lowest for magnetically disturbed periods, highest for quiet periods; lowest for a warm, dense thermosphere, highest for a cool, rare thermosphere. All of these variations are consistent with thermal quenching by ambient O, with larger ambient O densities yielding lower hot O densities. This indicates that hot O should form a layer shape in the atmosphere. ¿ 1999 American Geophysical Union