The O⁺(²P) emission at 7320 Å in twilight

We describe the measurement of the 7320‐ to 7330‐Å emissions by O + (² P ) in the evening twilight over Arecibo. We show how contamination by scattered sunlight and by OH airglow can be subtracted from the measured spectrum to achieve an unambiguous identification of both doublets. The measured ratio of the brightnesses of the two features is in agreement with theory. The measured wavelength of the doublet at 7320 Å (7319.9 ± 0.1 Å) is different from the value usually quoted (7319.0 Å). We have assumed photoionization to be the major source, and we have compared our surface brightnesses measured as a function of solar depression angle with those derived theoretically from nearly coincident Atmosphere Explorer data on neutral composition and solar ultraviolet flux. Agreement is good. Since quenching of O + (² P ) is negligible under the conditions of our experiment, it appears that ground‐based airglow measurements of the kind we describe can be used to monitor the variation of atomic oxygen density in the F region.