Seasonal variations in ratios of main gas components of the thermosphere in the solar activity last minimum (2007-2009)

By means of the authors’ technique on the base of Irkutsk digisonde (52° N, 104° E) data the ratios of the major gas components [O]/[N 2] and [O 2]/[O] were calculated at the altitudes below ionospheric F1-layer maximum in the quiet and disturbed geomagnetic conditions for different seasons during the solar activity last minimum. During all seasons covering three years except for the autumn, the average ratio [O]/[N 2] was found to be maximum in 2007 both in quiet and disturbed days. In autumn period, this ratio is significantly increased (up to 30 %) when moving from 2007 to 2009. When comparing our estimates of [O]/[N 2] with data on GUVI [http://guvi.jhuapl.edu/], the reasonable agreement for individual seasons was obtained.