Long-term variations in the neutral gas composition of the termosphere above Irkutsk

Long-term variations of [O]/[N 2] and [O 2]/[O] ratios of dominant gas components of the thermosphere are analyzed for 2003-2013. The variations are estimated using the technique developed by the authors and Irkutsk (52° N, 104° E) digisonde data for heights lower than the ionospheric layer F1 maximum, quiet and disturbed geomagnetic conditions, and different seasons. For summer quiet and disturbed conditions, the [O 2]/[O] ratio is found to be maximum during long solar activity minimum.