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 O2 densities, as well as N2 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 N2 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 [O2] was about 1.7 × 108 cm−3. At 120 km and equatorial latitudes a value of [O2] of ∼ 5 × 1010 cm−3 is calculated if [N2] there is 3.1 × 1011 cm−3. A somewhat higher value would be calculated if O2 is not in diffusive equilibrium above 120 km. A local maximum in the 200 km O2 densities was found near 70° N, where an average quiet time value was 2.5 × 108 cm−3, implying a 120 km density of 8.3 × 1010 cm−3. The in situ temperature measurements confirm the presence of higher temperatures near 70° N latitude, even during geomagnetically quiet conditions.