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 reveal ed O-2(a(1) Delta(g)> airglow from both the dayside and nightside of t he 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 oxyg en produced through the photolysis of CO2 on the dayside of Venus even tually recombined to produce O-2 in the excited (a(1) Delta(g)) state. This result was initially surprising because available laboratory mea surements indicated O-2(a(1) Delta(g)) yields from atomic oxygen recom bination reactions that were no larger than a few percent, More recent observations reveal even larger O-2(a(1) Delta(g)) airglow intensitie s as well as dramatic spatial and temporal variations in this airglow, High-resolution (0.3 cm(-1)) spectra of the Venus nightside taken wit h the Canada France Hawaii Telescope/Fourier transform spectrometer in 1991 show spectrally integrated O-2(a(1) Delta(g)) intensities as lar ge as 1.1 mW m(-2) sr(-1). Once these values are corrected for viewing angle and reflection from the underlying clouds, they indicate emissi on rates near 3 MR. These spectra also yield rotational temperatures o f 186 +/- 6 K in the emitting layer (90 to 115 km), Spectral image cub es taken with the Angle-Australian Telescope/infrared imaging spectrom eter and the Canada France Hawaii Telescope/imaging Fourier transform spectrometer during 1991, 1993, and 1994 provide a more complete descr iption of the spatial and temporal variability in this emission, Image s extracted at wavelengths within the O-2(a(1) Delta(g)) Q-branch (1.2 69 mu m) often show contrasts larger than 10 to 1 across the nightside . Even though the disk-averaged intensities are comparable to those se en in 1975, some localized regions have airglow emission rates larger than 5 MR. The brightest emission is often confined to 1000- to 2000-k m- diameter regions, These bright regions have been detected over a br oad range of latitudes and local times, but they are most often seen a t 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 obse rvations are providing improved constraints on atmospheric chemical an d dynamical models of the upper mesosphere and lower thermosphere of V enus.