Long-term variations in the development of the F1 layer and gas composition of the thermosphere

The results of a study of long-term variations in the gas composition of th e thermosphere, based on the measurements of the degree of F1-layer develop ment in 1980-1997 in Irkutsk, are reported. A situation when these variatio ns are assumed to be dependent on atomic oxygen changes is primarily consid ered. The atomic-oxygen density is found to exhibit a decreasing linear tre nd, which is more evident than the trend due to the corresponding variation s in the levels of solar-activity and geomagnetic-disturbance during the pe riod considered described by the thermospheric model. The ratio of atomic o xygen density to molecular oxygen density at 120 km is found to vary in a s imilar way.