Solar cycle variability of exospheric temperature at Millstone Hill between 1970 and 1980

Neutral exospheric temperatures from Thomson scatter measurements made between 1970 and 1980 at Millstone Hill (42.6°N) are examined for seasonal and solar cycle variations during geomagnetically undisturbed times. Analysis reveals unexpectedly large diurnal temperature amplitudes during solar cycle 21 at moderate to high levels of solar activity (150<=F_10.7<=230) pointing to the increased role of ion drag in the adiabatic compression of the thermosphere. A heuristic model assuming momentum balance between the pressure gradient and ion drag, and thermal balance between EUV heating, its vertical conduction and adiabatic compression serves to illustrate modulation of adiabatic cooling efficiency by enhanced ion drag effect.