Diurnal and seasonal variations in atomic and molecular oxygen inferred from Atmosphere Explorer‐C

Abstract

Ion mass spectrometer data collected onboard the Atmosphere Explorer‐C satellite during the period December 1974‐January 1975 have been used to infer, via the ion chemistry, the molecular oxygen concentration near 250km. To filter out temperature induced variations the O2 densities and in situ neutral composition measurements of atomic oxygen were extrapolated down to 120km. At this level the latitudinal and diurnal variations of O are found to be consistent with the ones inferred from OGO‐6 during high levels of solar activity. The results also show that at mid to high northern (winter) latitudes the O2 concentration is about a factor of two higher than at low southern (summer) latitudes, thus revealing an apparent reversal of the winter to summer increase in O2 deduced from optical and incoherent scatter measurements during higher levels of solar activity. Considering that the winter oxygen bulge in the lower thermosphere apparently prevails throughout the solar cycle the latitudinal (annual) variations in O2 can play a decisive role in explaining the reversal of the winter anomaly during minimum solar activity. The diurnal variations in O2 reveal a pronounced minimum at 18 LT thus suggesting that photo dissociation of O2 is important.