Long-term variations of thermospheric temperature and composition

Abstract

Long-term variations of the thermospheric structure in the altitude range 100 to 400 km have been deduced from incoherent scatter observations performed above St Santin (France, 45°N) in 1969–1970. This technique provides the following relevant information: the thermal structure from 100 to 400 km; a parameter proportional to the molecular nitrogen concentration around 100 km; a determination of the atomic oxygen concentration between 200 and 400 km; and the ratio of the atomic oxygen concentration to a weighted sum of the molecular (N2, O2) species around 200 km. The last three parameters are determined during daytime. Combining these data, we find that (1) a seasonal change in the composition at 200 km results from a seasonal change in the thermal structure between 100 and 200 km and from a superimposed lower boundary equivalent to seasonal changes in the atomic oxygen concentration (winter to summer ratio of 1.5) and in the molecular oxygen concentration (summer to winter ratio of about 4); (2) these later variations are linked to the F2 winter anomaly and support theoretical predictions and previous experimental work; (3) annual and semiannual variations of the thermal structure, combined with annual changes in the neutral composition, result in a semiannual variation of the air density that is already well known from satellite drag data.