Ground-based near-infrared observations of the Venus nightside: 1.27-mu m O-2(a(1)Delta(g)) airglow from the upper atmosphere

Near-infrared spectroscopic observations of Venus taken in 1975 revealed O-2(a(1) Delta(g)> airglow from both the dayside and nightside of the planet with emission rates 10(12) photons cm(-2) s(-1) into 4 pi sr). These large emiss sion rates indicated that most of the atomic oxygen produced through the photolysis of CO2 on the dayside of Venus eventually recombined to produce O-2 in the excited (a(1) Delta(g)) state. This result was initially surprising because available laboratory measurements indicated O-2(a(1) Delta(g)) yields from atomic oxygen recombination reactions that were no larger than a few percent, More recent observations reveal even larger O-2(a(1) Delta(g)) airglow intensities as well as dramatic spatial and temporal variations in this airglow, High-resolution (0.3 cm(-1)) spectra of the Venus nightside taken with the Canada France Hawaii Telescope/Fourier transform spectrometer in 1991 show spectrally integrated O-2(a(1) Delta(g)) intensities as large as 1.1 mW m(-2) sr(-1). Once these values are corrected for viewing angle and reflection from the underlying clouds, they indicate emission rates near 3 MR. These spectra also yield rotational temperatures of 186 +/- 6 K in the emitting layer (90 to 115 km), Spectral image cubes taken with the Angle-Australian Telescope/infrared imaging spectrometer and the Canada France Hawaii Telescope/imaging Fourier transform spectrometer during 1991, 1993, and 1994 provide a more complete description of the spatial and temporal variability in this emission, Images extracted at wavelengths within the O-2(a(1) Delta(g)) Q-branch (1.269 mu m) often show contrasts larger than 10 to 1 across the nightside. Even though the disk-averaged intensities are comparable to those seen in 1975, some localized regions have airglow emission rates larger than 5 MR. The brightest emission is often confined to 1000- to 2000-km- diameter regions, These bright regions have been detected over a broad range of latitudes and local times, but they are most often seen at low latitudes and at local times between midnight and 0300 on Venus, The intensity of the brightest spots can change by 20% in less than 1 hour, and they can vanish entirely in less than 1 day, These new observations are providing improved constraints on atmospheric chemical and dynamical models of the upper mesosphere and lower thermosphere of Venus.