A study of day-night variations in the neutral composition of the lower thermosphere

Two Aerobee rocket flights were made from the White Sands Missile Range, New Mexico (32°24′N, 106°20.6′W), at 0445 MST on November 30, 1966, and at 1409 MST on December 2, 1966, for the purpose of investigating day-night effects in the atmosphere. The number densities of N2, O2, O, Ar, and He were determined by using three magnetic deflection mass spectrometers employing ‘open’ and ‘closed’ geometry ion sources. The daytime and nighttime molecular nitrogen, molecular oxygen, and argon densities were found to be very nearly the same. The nighttime atomic oxygen density was a factor of 1.8 higher at 170 km than the daytime value, whereas the helium density was higher during daytime below 200 km; the ratio of the day to night value being 2.5 at 120 km. A comparison of the atomic oxygen density measured in the ‘closed’ and ‘open’ geometries allows an estimate to be made of the wall loss coefficient of both geometries for this species and indicates that the averaged ‘open’ source atomic oxygen densities should be multiplied by about 1.25. Evidence is discussed for the atmosphere being in diffusive equilibrium; the level where the Ar-N2 separation ratio has a value of 1 is indicated to be near 100 and 102 km for day and night, respectively.