Results of neutral temperature measurements in the polar thermosphere between 1968 and 1976 show a large variation as a function of the solar cylce. A very cold thermosphere associated with a high altitude for the mesopause is found at the maximum of solar activity. The amplitude of variation of neutral temperature at 165 km is 500¿ K, and the mesopause is observed to move from 120 to 90 km. Those results are attributed to the variation of both the amplitude and the altitude of turbulent processes as a function of solar activity. Theoretical calculations and experimental facts are in support of that explanation.