Annual variation of gas composition at the height of the ionospheric E layer

We study results of analysis of seasonal variations in gas composition in the lower thermosphere (ionospheric E region) using ground-based ionospheric sounding data and satellite measurement data on the X-radiation flux obtained during solar X-ray flares. Contrary to previous ideas, peculiarities of annual variation of the E-layer critical frequency and its seasonal anomaly are explained by decrease in molecular oxygen content during the summer solstice rather than during the winter one.