Identification of the UV nightglow from Venus

Abstract

THE presence of atmospheric ultraviolet (UV) emission between 1,350 and 2,200 Å from the nightside of Venus was first detected by instruments on board the Mariner 5 flyby¹. Recently this emission was observed by the UV spectrometer on Pioneer Venus Orbiter at 2,068 Å and identified by Stewart et al.² as the (0, 0) Cameron band (a³Π-X¹Σ) of CO. Their interpretation suggested a significant input of energy into the nightside thermosphere from the interaction of the solar wind with the planet. We report here the preliminary results of observations of the nightside spectrum of Venus made with the International Ultraviolet Explorer (IUE) observatory when Venus was near greatest elongation in January 1979. Three features are clearly seen in the spectra, and the spectral resolution of the instrument is adequate to identify them as bands of the nitric oxide δ(C²Π-->X²Π) system rather than as Cameron bands. The δ-bands, present in the Earth's twilight and night time airglow spectra³, are produced by the radiative association of nitrogen and oxygen atoms and indicate a significant population of atomic nitrogen at or below the Venus turbopause.