Observations of the far ultraviolet nightglow

The extreme ultraviolet telescope on the Apollo‐Soyuz mission observed the far UV nightglow in the 1350‐1700 A band from an altitude of 225 km at solar minimum in July 1975. Looking down when the spacecraft was clear of the auroral zones, a faint glow well above instrumental background and varying smoothly with solar zenith angle is recorded. The minimum intensity at solar zenith angles >150° is ≈100 photons cm −2 s −1 sr −1 A −1 (≈ 0.2 Rayleighs) in this passband and is most probably due to N 2 Lyman‐Birge‐Hopfield band emission. Looking up, the airglow, if it exists, is very difficult to disentangle from the astronomical background, except in one case where an obvious equatorial enhancement of ≈ 1000 photons cm −2 s −1 sr −1 A −1 is observed. The measured intensities looking down are in all cases substantially less than those reported by Huffman et al. [1980] and consistent with upper limits established in 1968 by Prinz and Meier [1971]. Thus the enhancements observed by Huffman et al. must be due either to an impulsive short‐lived source or to a secular increase since 1975 of one or more of the parameters affecting the unknown emission mechanism.