UVIS observations of the FUV OI and CO 4P Venus dayglow during the Cassini flyby

We analyze FUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus. We use a least-squares fit method to determine the brightness of the OI emissions at 130.4 and OI 135.6 nm, and of the bands of the CO fourth positive system which are dominated by fluorescence scattering. We compare the brightness observed along the UVIS foot track of the two OI multiplets with that deduced from a model of the excitation of these emissions by photoelectron impact on O atoms and resonance scattering of the solar 130.4 nm emission. The large optical thickness 130.4 nm emission is accounted for using a radiative transfer model. The airglow intensities are calculated along the foot track and found to agree with the observed 130.4 nm brightness within ∼10%. The modeled OI 135.6 nm brightness is also well reproduced by the model. The oxygen density profile of the VTS3 model is found to be consistent with the observations. We find that self-absorption of the (0, v″) bands of the fourth positive emission of CO is important and we derive a CO vertical column of about 6.4 × 10¹⁵ cm⁻² in close agreement with the value provided by the VTS3 empirical atmospheric model.     

Venera 11 and Venera 12 observations of e.u.v. emissions from the upper atmosphere of Venus

The results obtained by two extreme ultra violet (e.u.v.) spectrophotometers flown near Venus on VENERA 11 and VENERA 12 in December 1978 are presented. Detectors were placed at discrete wavelength positions to measure e.u.v. emissions from the upper atmosphere of Venus while the spacecraft were drifting on their fly-by orbits. The emissions of HI 121.6 nm (Ly-α), HeI 58.4 nm, and OI 130.4 nm were measured with unprecedented sensitivity and spatial resolution. An OI signal of 500 Rayleigh (R) measured outside the disc suggested the existence of a large bulge of oxygen atoms. The e.u.v. emissions of two ionic species. OII 83.4 nm and HeII 30.4 nm, were measured for the first time in the atmosphere of Venus. The zero order detector of VENERA 12 indicated the presence of a very intense e.u.v. emission (28 kR) lying between the monitored wavelengths. This emission, which was only 3 kR for VENERA 11, is likely to be associated with the solar wind-ionosphere interaction.An attempt to measure ArI and NeI resonance emissions failed.The Lyman alpha (Ly-α) interplanetary background was 4 to 5 times larger than expected, suggestive of a very intense solar flux or an increase of the interplanetary density. The distribution of hydrogen indicates two populations with temperatures of 400 and 700 K.     

Excitation of the oxygen nightglow on the terrestrial planets

A scheme of excitation, quenching, and energy transfer processes in the oxygen nightglow on the Earth, Venus, and Mars has been developed based on the observed nightglow intensities and vertical profiles, measured reaction rate coefficients, and photochemical models of the nighttime atmospheres of the Venus and Mars. The scheme involves improved radiative lifetimes of some band systems, calculated yields of the seven electronic states of O₂ in termolecular association, and rate coefficients of seven processes of electronic quenching of the Herzberg states of O₂, which are evaluated by fitting to the nightglow observations. Electronic quenching of the vibrationally excited Herzberg states by O₂ and N₂ in the Earth's nightglow is a quarter of total collisional removal of the O₂(A, A′) states and a dominant branch for the O₂(c) state. The scheme supports the conclusion by Steadman and Thrush (1994) that the green line is excited by energy transfer from the O₂(A³Σ_u⁺, v≥6) molecules, and the inferred rate coefficient of this transfer is 1.5×10⁻¹¹ cm³ s⁻¹. The O₂ bands at 762 nm and 1.27 μm are excited directly, by quenching of the Herzberg states, and by energy transfer from the O₂(⁵Π_g) state. Quenching of the O₂ band at 762 nm excites the band at 1.27 μm as well. Effective yield of the O₂(a¹Δ_g) state in termolecular association on Venus and Mars is ∼0.7. Quantitative assessments of all these processes have been made. A possible reaction of O₂(c¹Σ_u⁻)+CO is a very minor branch of recombination of CO₂ on Venus and Mars. Night airglow on Mars is calculated for typical conditions of the nighttime atmosphere. The calculated vertical intensity of the O₂ band at 1.27 μm is 13 kR, far below the recently reported detections.     

Characteristics of Saturn’s FUV airglow from limb-viewing spectra obtained with Cassini-UVIS

This study reports the analysis of far ultraviolet (FUV) limb spectra of the airglow of Saturn in the 1150-1850 Å spectral window, obtained with the Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini, spanning altitudes from −1200 to 4000 km. The FUV limb emission consists of three main contributions: (1) H Ly-α peaking at 1100 km with a brightness of 0.8 kilo-Rayleighs (kR), (2) reflected sunlight longward of 1550 Å which maximizes at −950 km with 16.5 kR and (3) H₂ bands in the 1150-1650 Å bandwidth, peaking at 1050 km reaching a maximum of 3.9 kR.A vertical profile of the local H₂ volume emission rate has been derived using the hydrocarbon density profiles from a model of the Saturn equatorial atmosphere. It is well matched by a Chapman function, characterized by a maximum value of 3.5 photons cm⁻³ s⁻¹ in the 800-1650 Å UV bandwidth, peaking at 1020 km.Comparisons between the observed spectra and a first-order synthetic airglow H₂ model in the 1150-1650 Å bandwidth show that the spectral shape of the H₂ bands is accounted for by solar fluorescence and photoelectron excitation. The best fits are obtained with a combination of H₂ fluorescence lines and 20 eV electron impact spectra, the latter contributing ∼68% of the total H₂ airglow emission.     

Hydroxyl airglow on Venus in comparison with Earth

Hydroxyl nightglow is intensively studied in the Earth atmosphere, due to its coupling to the ozone cycle. Recently, it was detected for the first time also in the Venus atmosphere, thanks to the VIRTIS-Venus Express observations. The main Δν=1, 2 emissions in the infrared spectral range, centred, respectively, at 2.81 and 1.46 μm (which correspond to the (1-0) and (2-0) transitions, respectively), were observed in limb geometry (Piccioni et al., 2008) with a mean emission rate of 880±90 and 100±40 kR (1R=10⁶ photon cm⁻² s⁻¹ (4πster)⁻¹), respectively, integrated along the line of sight. In this investigation, the Bates-Nicolet chemical reaction is reported to be the most probable mechanism for OH production on Venus, as in the case of Earth, but HO₂ and O may still be not negligible as mechanism of production for OH, differently than Earth. The nightglow emission from OH provides a method to quantify O₃, HO₂, H and O, and to infer the mechanism of transport of the key species involved in the production. Very recently, an ozone layer was detected in the upper atmosphere of Venus by the SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus) instrument onboard Venus Express (Montmessin et al., 2009); this discovery enhances the importance of ozone to the OH production in the upper atmosphere of Venus through the Bates-Nicolet mechanism. On Venus, OH airglow is observed only in the night side and no evidence has been found whether a similar emission exists also in the day side. On Mars it is expected to exist both on the day and night sides of the planet, because of the presence of ozone, though OH airglow has not yet been detected.In this paper, we review and compare the OH nightglow on Venus and Earth. The case of Mars is also briefly discussed for the sake of completeness. Similarities from a chemical and a dynamical point of view are listed, though visible OH emissions on Earth and IR OH emissions on Venus are compared.     

Cassini UVIS observations of Europa's oxygen atmosphere and torus

Observations of the Europa environment using the Cassini UltraViolet Imaging Spectrograph (UVIS) show the presence of an extended atomic oxygen atmosphere in addition to the bound molecular oxygen atmosphere first detected by Hubble Space Telescope in 1994 [D.T. Hall, D.F. Strobel, P.D. Feldman, M.A. McGrath, H.A. Weaver, 1995, Detection of an oxygen atmosphere on Jupiter's moon Europa, Nature 373, 677-679]. The atomic oxygen measurement provides a direct constraint on the sputtering and loss of Europa's water ice surface and the interaction of Europa's atmosphere with Jupiter's magnetosphere. We derive a loss rate for O₂ based on the emission rate of the OI 1356 Å multiplet. UVIS detected substantial variability in the oxygen emission from Europa's oxygen atmosphere that we attribute to the viewing geometry. B.H. Mauk, D.G. Mitchell, S.M. Krimigis, E.C. Roelof, C.P. Paranicas [2003, Energetic neutral atoms from a trans-Europa gas torus at Jupiter, Nature 421, 920-922] inferred the presence of a torus of neutral gas at Europa's orbit based on Cassini's energetic neutral atom (ENA) image of the Jupiter system acquired with the Magnetospheric Imaging Instrument (MIMI), with the most likely torus constituents being hydrogen and oxygen species sputtered from Europa. Cassini UVIS data rule out O and O₂ as the possible torus species reported by Mauk et al. however, unless the torus density is so low that it is undetectable by UVIS (less than 8 atoms / cm³). The UVIS observations indicate the presence of atomic hydrogen and possibly other species, but a full analysis is deferred to a following paper. The hydrogen in the present observations shows a local-time asymmetry and complex spatial distribution.     

Observations of the optical spectrum of the dayside magnetospheric cleft aurora

Optical spectra of the cleft aurora in the region 5000–8500 Å were measured in December, 1977 at Cape Parry, N.W.T. A Michelson interferometer was used at a resolution of 10 cm^?1. The auroral features observed were OI (5577, 6300-64, 7774, 8446 Å), OII (7319-30 Å), NI (5200 Å), Hα, O₂ atm (1,1), some weak N₂1P bands and possibly some Meinel bands of N₂⁺. In addition, nightglow emissions of Na and OH were observed. Theoretical predictions of the OI and NI emission rates using the model of Link et al. (1980) fit the observed rates reasonably well if a 40 eV Maxwellian incident electron spectrum is assumed. The predicted rates for OII exceed the observed value by a factor of 4. It is suggested that the ionization cross-section may be over-estimated.     

Venus upper atmospheric CO, temperature, and winds across the afternoon/evening terminator from June 2007 JCMT sub-millimeter line observations

The Venus mesosphere constitutes a highly variable transition region between the zonal rotation of the lower atmosphere and the diurnal circulation of the upper atmosphere. It further serves as the primary photochemical region of the Venus atmosphere. We obtained James Clerk Maxwell Telescope (JCMT, Mauna Kea Hawaii) sub-millimeter line observations of mesospheric ¹²CO and ¹³CO during coordinated space (MESSENGER and Venus Express) and ground-based observations of Venus in June of 2007. Such CO spectra line measurements support temperature, CO mixing ratio, and wind retrievals over the 80-110 km altitude range, encompassing the upper mesosphere and lower thermosphere of Venus. Five-point beam integrations were obtained across the observed Venus disk, allowing distinction of afternoon (noon to 6 p.m.) versus evening (6 p.m. to midnight) local times and northern (0-60N) versus southern (0-60S) latitudes. Distinctive diurnal variations (noon to midnight) are retrieved for both temperatures above 95 km and CO mixing ratios above 85 km altitudes. Separate CO line maps obtained on (UT) June 2, 3, 6, and 11 indicate moderate daily variability in afternoon and evening CO mixing ratios (20-50%) and temperatures (5-10 K). Average Venus mesospheric temperatures over this period were 10 K warmer than returned from 1978 to 1979 Pioneer Venus or 2000-01 sub-millimeter measurements, without evidence for the very large temperature inversions indicated by Venus Express SPICAV measurements at 90-100 km altitudes (Bertaux, J.L., Vandaele, A.-C., Korablev, O., Villard, E., Fedorova, A., Fussen, D., Quémerais, E., Belyaev, D., Mahieux, A., Montmessin, F., Muller, C., Neefs, E., Nevejans, D., Wilquet, V., Dubois, J.P. Hauchecorne, A., Stepanov, A., Vinogradov, I., Rodin, A., Bertaux, J.-L., Nevejans, D., Korablev, O., Montmessin, F., Vandaele, A.-C., Fedorova, A., Cabane, M., Chassefière, E., Chaufray, J.Y., Dimarellis, E., Dubois, J.P., Hauchecorne, A., Leblanc, F., Lefèvre, F., Rannou, P., Quémerais, E., Villard, E., Fussen, D., Muller, C., Neefs, E., Van Ransbeeck, E., Wilquet, V., Rodin, A., Stepanov, A., Vinogradov, I., Zasova, L., Forget, F., Lebonnois, S., Titov, D., Rafkin, S., Durry, G., Gérard, J.C., Sandel, B., 2007. A warm layer in Venus’ cryosphere and high-altitude measurements of HF, HCl, H₂O and HDO. Nature 450, 646-649). Measured Doppler shifts associated with June 2 and 11 ¹²CO line center absorptions indicate nearly supersonic (200 m/s, Mach 1) afternoon-to-evening (retrograde) circulation; composed of additive subsolar-to-antisolar (SSAS) and zonal retrograde wind components, which are not separable due to the particular observational geometry.     

NO emissions as observed by SPICAV during stellar occultations

Ultraviolet (UV) nightglow data from the SPICAV instrument (SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus) onboard the Venus Express spacecraft, currently in orbit around Venus, are presented. In its extended source mode, SPICAV has shown that the Venus nightglow in the UV contains essentially Lyman-α and Nitric Oxide (NO) emissions. In the stellar mode, when the slit of the spectrometer is removed, an emission is also observed at the limb in addition to the stellar spectrum. A forward model allows us to identify this feature as being an NO emission. Due to radiative recombination of N and O atoms produced on the dayside of Venus, and transported to the nightside, NO nightglow provides important constraints to the Solar-to-Anti Solar thermospheric circulation prevailing above 90 km. The forward model presented here allows us to derive the altitude of the peak of emission of the NO layer, found at 113.5±6 km, as well as its scale height, of 3.4±1 km and its brightness. The latter is found to be very variable with emissions between 19 Kilo-Rayleigh (kR) and 540 kR. In addition, the NO nightglow is sometimes very patchy, as we are able to observe two distinct emission zones in the field of view. Finally, systematic extraction of this emission from stellar occultations extends the database of the NO emission already reported elsewhere using limb observations.     

Spatially-resolved high-resolution spectroscopy of Venus 1. Variations of CO2, CO, HF, and HCl at the cloud tops

Variations of the upper cloud boundary and the CO, HF, and HCl mixing ratios were observed using the CSHELL spectrograph at NASA IRTF. The observations were made in three sessions (October 2007, January 2009, and June 2009) at early morning and late afternoon on Venus in the latitude range of ±60°. CO₂ lines at 2.25 μm reveal variations of the cloud aerosol density (∼25%) and scale height near 65 km. The measured reflectivity of Venus at low latitudes is 0.7 at 2.25 μm and 0.028 at 3.66 μm, and the effective CO₂ column density is smaller at 3.66 μm than those at 2.25 μm by a factor of 4. This agrees with the almost conservative multiple scattering at 2.25 μm and single scattering in the almost black aerosol at 3.66 μm. The expected difference is just a factor of (1 − g)⁻¹ = 4, where g = 0.75 is the scattering asymmetry factor for Venus’ clouds. The observed CO mixing ratio is 52 ± 4 ppm near 08:00 and 40 ± 4 ppm near 16:30 at 68 km, and the higher ratio in the morning may be caused by extension of the CO morningside bulge to the cloud tops. The observed weak limb brightening in CO indicates an increase of the CO mixing ratio with altitude. HF is constant at 3.5 ± 0.2 ppb at 68 km in both morningside and afternoon observations and in the latitude range ±60°. Therefore the observations do not favor a bulge of HF, though HF is lighter than CO. Probably a source in the upper atmosphere facilitates the bulge formation. The recent measurements of HCl near 70 km are controversial (0.1 and 0.74 ppm) and require either a strong sink or a strong source of HCl in the clouds. The HCl lines of the (2-0) band are blended by the solar and telluric lines. Therefore we observed the P8 lines of the (1-0) band at 3.44 μm. These lines are spectrally clean and result in the HCl mixing ratio of 0.40 ± 0.03 ppm at 74 km. HCl does not vary with latitude within ±60°. Our observations support a uniformly mixed HCl throughout the Venus atmosphere.     

The continuous absorption coefficient of a stellar atmosphere

It is pointed out that, when calculating the continuous absorption coefficient in a stellar atmosphere, it is advantageous to use the coefficient per particle of the most abundant element instead of the usual coefficient per gram of matter. The sources of continuous opacity considered are 1) absorption by H^-, HI, H₂^-, H ₂⁺, HeI, HeII, CI, CII, CIII, NI, NII, OI, OII, NaI, MgI, MgII, AlI, AlII, SiI, SiII, ClI, KI, CaII; and 2) Rayleigh scattering by HI, HeI, CI, NI, OI, H₂, and 3) Thomson scattering of free electrons. The calculations are illustrated by the results for a solar-type photosphere.     

Nightglow (557.7 nm of OI) in the central polar cap

The 557.7 nm OI night airglow emission was measured in the central polar cap by ground-based photometric systems at Thule Air Base, Greenland during the winter seasons from 1972–1973 to 1974–1975 and at Thule-Qanaq, Greenland during the winter season of 1973–1974. The behavior of the 557.7 nm night airglow emission in the polar cap was found to be quite different from that observed at mid and low latitudes. No diurnal variation greater than ±5% exist in the data. Large amplitude variations in the 557.7 nm daily average emission intensities can change by up to a factor of approximately 8 over periods ranging from 4 to 19 days. These long-term airglow variations cover at least a 100 km horizontal range as determined by a correlation coefficient of 0.94 between daily average 557.7 nm airglow intensities observed at Thule Air Base and Thule-Qanaq. An interplanetary magnetic field sector related behavior is evident in the daily average intensities which shows an increase of intensity in a positive (+) sector and a decrease of intensity in a negative (?) sector. No significant correlation was found between the 557.7 nm daily average intensities and Zurich sunspot number R_Z, although a season to season positive trend was evident. Correlations between the 557.7 nm daily average intensities and planetary magnetic indices ΣK_p and A_p were found to be inconclusive due to sector related effects. The Barth and Chapman mechanisms are discussed as possible source mechanisms for the 557.7 nm airglow in the central polar cap, and a hypothesis is presented to explain the airglow variations.     

Rocket observations of the extreme ultraviolet dayglow

The ultraviolet dayglow in the wavelength region 750–1050 Å was investigated over the altitude range 100–800 km using a thin film filter photometer. From the airglow spectrum obtained by Carruthers and Page, one of the dominant features in this wavelength range is OII 834 Å. It is pointed out that the major excitation mechanism for this transition is photoionization excitation of atomic oxygen. Solution of the radiative transfer problem for this excitation process shows good agreement with the observed dayglow in the 300–800 km region. At lower altitudes additional components are present and are interpreted as the N₂, OI and possibly HI emissions observed by Carruthers and Page.     

Measurements of the helium 584 Å airglow during the Cassini flyby of Venus

The helium resonance line at 584 Å has been observed with the UltraViolet Imaging Spectrograph (UVIS) Extreme Ultraviolet channel during the flyby of Venus by Cassini at a period of high solar activity. The brightness was measured along the disk from the morning terminator up to the bright limb near local noon. The mean disk intensity was ∼320 R, reaching ∼700 R at the bright limb. These values are slightly higher than those determined from previous observations. The sensitivity of the 584 Å intensity to the helium abundance is analyzed using recent cross-sections and solar irradiance measurements at 584 Å. The intensity distribution along the UVIS footprint on the disk is best reproduced using the EUVAC solar flux model and the helium density distribution from the VTS3 empirical model. It corresponds to a helium density of 8×10⁶ cm⁻³ at the level of where the CO₂ is 2×10¹⁰ cm⁻³.     

Far ultraviolet spectral properties of Saturn’s rings from Cassini UVIS

Spectra taken by the Cassini Ultraviolet Imaging Spectrograph (UVIS) of Saturn’s C ring, B ring, Cassini Division, and A ring have been analyzed in order to characterize ring particle surface properties and water ice abundance in the rings. UVIS spectra sense the outer few microns of the ring particles. Spectra of the normalized reflectance (I/F) in all four regions show a characteristic water ice absorption feature near 165 nm. Our analysis shows that the fractional abundance of surface water ice is largest in the outer B ring and decreases by over a factor of 2 across the inner C ring. We calculate the mean path length of UV photons through icy ring particle regolith and the scattering asymmetry parameter using a Hapke reflectance model and a Shkuratov reflectance model to match the location of the water ice absorption edge in the data. Both models give similar retrieved values of the photon mean length, however the retrieved asymmetry (g) values are different. The photon mean path lengths are nearly uniform across the B and A rings. Shortward of 165 nm the rings exhibit a slope that turns up towards shorter wavelengths, while the UV slope of 180/150 nm (reflectance outside the water absorption ratioed to that inside the absorption band) tracks I/F with maxima in the outer B ring and in the central A ring. Retrieved values of the scattering asymmetry parameter show the regolith grains to be highly backscattering in the FUV spectral regime.     

An investigation of the SO2 content of the venusian mesosphere using SPICAV-UV in nadir mode

Using the SPICAV-UV spectrometer aboard Venus Express in nadir mode, we were able to derive spectral radiance factors in the middle atmosphere of Venus in the 170-320 nm range at a spectral resolution of R ? 200 during 2006 and 2007 in the northern hemisphere. By comparison with a radiative transfer model of the upper atmosphere of Venus, we could derive column abundance above the visible cloud top for SO₂ using its spectral absorption bands near 280 and 220 nm. SO₂ column densities show large temporal and spatial variations on a horizontal scale of a few hundred kilometers. Typical SO₂ column densities at low latitudes (up to 50°N) were found between 5 and 50 μm-atm, whereas in the northern polar region SO₂ content was usually below 5 μm-atm. The observed latitudinal variations follow closely the cloud top altitude derived by SPICAV-IR and are thought to be of dynamical origin. Also, a sudden increase of SO₂ column density in the whole northern hemisphere has been observed in early 2007, possibly related to a convective episode advecting some deep SO₂ into the upper atmosphere.     

Seasonal variations of OI 6300 Å night airglow emission at Calcutta and other stations and its covariation with OI 5577 Å emission

The paper presents the seasonal variations of OI 6300 Å line night airglow emission at Calcutta and other stations. Their covariations with OI 5577 Å line are also offered. Explanation for the variation is also provided.     

Oxygen airglow emission on Venus and Mars as seen by VIRTIS/VEX and OMEGA/MEX imaging spectrometers

Imaging spectrometers are highly effective instruments for investigation of planetary atmospheres. They present the advantage of coupling the compositional information to the spatial distribution, allowing simultaneous study of chemistry and dynamics in the atmospheres of Venus and Mars. In this work, we summarize recent results about the O₂(a¹Δ_g) night and day glows, respectively obtained by VIRTIS/Venus Express and OMEGA/Mars Express, the imaging spectrometers currently in orbit around Venus and Mars. The case of the O₂(a¹Δ_g - X³Σ_g⁻) IR emission at 1.27 μm on the night side of Venus and the day side of Mars is analyzed, pointing out dynamical aspects of these planets, like the detection of gravity waves in their atmospheres. The monitoring of seasonal and daily airglow variations provides hints about the photochemistry on these planets.     

Atmospheric chemistry on Venus, Earth, and Mars: Main features and comparison

This paper deals with two common problems and then considers major aspects of chemistry in the atmospheres of Mars and Venus. (1) The atmospheres of the terrestrial planets have similar origins but different evolutionary pathways because of the different masses and distances to the Sun. Venus lost its water by hydrodynamic escape, Earth lost CO₂ that formed carbonates and is strongly affected by life, Mars lost water in the reaction with iron and then most of the atmosphere by the intense meteorite impacts. (2) In spite of the higher solar radiation on Venus, its thermospheric temperatures are similar to those on Mars because of the greater gravity acceleration and the higher production of O by photolysis of CO₂. O stimulates cooling by the emission at 15 μm in the collisions with CO₂. (3) There is a great progress in the observations of photochemical tracers and minor constituents on Mars in the current decade. This progress is supported by progress in photochemical modeling, especially by photochemical GCMs. Main results in these areas are briefly discussed. The problem of methane presents the controversial aspects of its variations and origin. The reported variations of methane cannot be explained by the existing data on gas-phase and heterogeneous chemistry. The lack of current volcanism, SO₂, and warm spots on Mars favor the biological origin of methane. (4) Venus’ chemistry is rich and covers a wide range of temperatures and pressures and many species. Photochemical models for the middle atmosphere (58-112 km), for the nighttime atmosphere and night airglow at 80-130 km, and the kinetic model for the lower atmosphere are briefly discussed.     

Venus night airglow: Ground-based detection of OH, observations of O2 emissions, and photochemical model

Venus nightglow was observed at NASA IRTF using a high-resolution long-slit spectrograph CSHELL at LT = 21:30 and 4:00 on Venus. Variations of the O₂ airglow at 1.27 μm and its rotational temperature are extracted from the observed spectra. The mean O₂ nightglow is 0.57 MR at 21:30 at 35°S-35°N, and the temperature increases from 171 K near the equator to ∼200 K at ±35°. We have found a narrow window that covers the OH (1-0) P1(4.5) and (2-1) Q1(1.5) airglow lines. The detected line intensities are converted into the (1-0) and (2-1) band intensities of 7.2 ± 1.8 kR and <1.4 kR at 21:30 and 15.5 ± 2 kR and 4.7 ± 1 kR at 4:00. The f-component of the (1-0) P1(4.5) line has not been detected in either observation, possibly because of resonance quenching in CO₂. The observed Earth’s OH (1-0) and (2-1) bands were 400 and 90 kR at 19:30 and 250 and 65 kR at 9:40, respectively. A photochemical model for the nighttime atmosphere at 80-130 km has been made. The model involves 61 reactions of 24 species, including odd hydrogen and chlorine chemistries, with fluxes of O, N, and H at 130 km as input parameters. To fit the OH vibrational distribution observed by VEX, quenching of OH (v > 3) in CO₂ only to v ? 2 is assumed. According to the model, the nightside-mean O₂ emission of 0.52 MR from the VEX and our observations requires an O flux of 2.9 × 10¹² cm⁻² s⁻¹ which is 45% of the dayside production above 80 km. This makes questionable the nightside-mean O₂ intensities of ∼1 MR from some observations. Bright nightglow patches are not ruled out; however, the mean nightglow is ∼0.5 MR as observed by VEX and supported by the model. The NO nightglow of 425 R needs an N flux of 1.2 × 10⁹ cm⁻² s⁻¹, which is close to that from VTGCM at solar minimum. However, the dayside supply of N at solar maximum is half that required to explain the NO nightglow in the PV observations. The limited data on the OH nightglow variations from the VEX and our observations are in reasonable agreement with the model. The calculated intensities and peak altitudes of the O₂, NO, and OH nightglow agree with the observations. Relationships for the nightglow intensities as functions of the O, N, and H fluxes are derived.