Molecular oxygen measurements at 200 km from AE‐D near winter solstice, 1975

Utilizing the fly‐through mode, the open source neutral mass spectrometer on Atmosphere Explorer‐D (AE‐D) has measured O 2 densities, as well as N 2 densities and in situ neutral temperatures, at midmorning during winter solstice over the latitude range 90° S to 90° N. We find the expected seasonal variation in N 2 at 200 km but a more complex behavior in molecular oxygen than might be expected from a diffusive equilibrium model with constant lower boundary values. Under geomagnetically quiet conditions the equatorial 200 km value of [O 2 ] was about 1.7 × 10 8 cm −3 . At 120 km and equatorial latitudes a value of [O 2 ] of ∼ 5 × 10 10 cm −3 is calculated if [N 2 ] there is 3.1 × 10 11 cm −3 . A somewhat higher value would be calculated if O 2 is not in diffusive equilibrium above 120 km. A local maximum in the 200 km O 2 densities was found near 70° N, where an average quiet time value was 2.5 × 10 8 cm −3 , implying a 120 km density of 8.3 × 10 10 cm −3 . The in situ temperature measurements confirm the presence of higher temperatures near 70° N latitude, even during geomagnetically quiet conditions.