Abstract
Several possible mechanisms are investigated which could be invoked to explain the observed semiannual density variation in the thermosphere and exosphere. A variation of the height of the mixtopause leads to a large density variation for heights above 700 km. Below that height, however, the density is essentially invariant to this process. This invariance is to some degree caused by the neglect of downward heat transport by eddy diffusion at the bottom of the thermosphere. The limitations of using the simple mixtopause scheme in this context are discussed.
Another mechanism can be ruled out on the grounds that it fails to explain the observed amplitude at a height of 200 km. This mechanism is a small permanent heat flux conducted into the lower exosphere from above. A variation of this flux by 3×10−2 erg cm−2 sec−1 would yield a sufficiently large density variation only for heights above 300 km. The recent observations at heights below 200 km indicate that the temperature and density at the bottom of the thermosphere (90–120 km) vary with a semiannual period.
