The Ogo 4 ultraviolet spectrometer measured the ultraviolet nightglow at tropical latitudes. The spectrum consists of the 1304- and 1356-A lines of atomic oxygen. The Lyman-Birge-Hopfield bands of molecular nitrogen and the ultraviolet lines of atomic nitrogen do not appear. The tropical ultraviolet airglow occurs in two bands north and south of the geomagnetic equator under satellite observation conditions that are best correlated with a local time of 2100 to 2130. When the intensity of this airglow is maximum, the ratio of the intensity of the 1304-A to the 1356-A line is 1.8. The most likely source of excitation of the tropical ultraviolet airglow appears to be an ionospheric recombination mechanism.
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