Nightglow Emission Research Articles

Singlet oxygen in the environmental sciences

This review of the part played by the singlet states of molecular oxygen in the environment deals with atmospheric aspects. There are five bound excited states of molecular oxygen that correlate with two ground state, 3P, oxygen atoms. Of these, three are singlets, although the other two states (triplets) are closely associated with singlet oxygen processes, especially in the mesosphere. A weakly bound quintet state has been invoked, as well, in explaining some aspects of the physical chemistry of the singlet species. Of the three singlet states, the a1Δg is the most familiar. It has a low excitation energy, a long radiative lifetime, and is rather resistant to collisional deactivation in the gas phase. As a consequence, its chemistry has been susceptible to detailed study in the laboratory. These investigations, coupled with estimates of production rates, suggest that O2(a1Δg) is probably not important in initiating much chemical change in the lower atmosphere, at least in the gas phase; excited molecules dissolved in water droplets may promote chemical change under special circumstances. In the stratosphere and mesosphere, each of the bound excited states gives rise to characteristic emission features of the airglow, both by day and by night. The observational data, obtained from the ground, and from balloons, high-flying aircraft, rockets and satellites is surveyed as a background to examining the chemical and photochemical mechanisms by which the different states become excited. These mechanisms clearly differ by day and by night, and they also depend on the altitude from which the emission comes. The most intense feature of the oxygen dayglow, the Infrared Atmospheric Band, comes from O2(a1Δg) that is produced in the photolysis of ozone. Because dayglow measurements are sometimes used to derive ozone concentrations and altitude profiles in the atmosphere, the efficiency of production of the species in the photolysis of ozone is examined critically, and some unexpected laboratory findings are reported. The b1Σ+g state of oxygen is excited during the day largely by resonance scattering, although some is also populated by energy transfer from O(1D) to O2. At night, recombination of O(3P) atoms is the most likely source of excitation of all the states of oxygen. Laboratory experiments that bear on these processes are reviewed, and theoretical estimates of the partitioning of recombination events between the different states are presented. Direct recombination into the a1Δg and b1Σ+g states is unlikely to be efficient enough to produce the observed concentrations of these species, and some indirect process is thus implicated. Laser excitation experiments show that quenching of the three higher excited (ungerade) states of oxygen by O2 and, especially, N2, can generate O2(b1Σ+g) with high efficiency; similar experiments demonstrate explicitly that the quenching of O2(b1Σ+g) by the atmospheric gases yields O2(a1Δg). A consistent excitation scheme for the nightglow emissions is presented; this scheme also pays attention to the “auroral green” line produced by the 1S state of atomic oxygen, the intensities of which in the atmospheres of Earth and Venus provide some clues about the excitation of the molecular states. Finally, the laboratory studies are shown to indicate that the formation of excited molecular oxygen from vibrationally rich hydroxyl (OH) radicals is unlikely to be of major importance in the atmosphere.

Evidence for non‐local‐thermodynamic‐equilibrium rotation in the OH nightglow

Improved ground‐based Fourier transform infrared spectroscopic measurements of the OH Meinel (M) Δν= 2 and 3 nightglow emissions revealed unexpectedly intense transitions from high rotational levels. The rotational development in the P branches of the OH M (3,1), (6,3), and (7,4) bands has been followed to the P1(N″) transitions with N″ = 10, 12, and 13, respectively. These measurements indicate that ∼10% of the OH(X, ν′ = 7) column rotational population is typically in rotational levels with N′ ≥7, in sharp contrast with the corresponding local thermodynamic equilibrium estimate of ∼1%. The excess high‐N′ population also persists in the lower vibrational states of OH(X), as evidenced by the readily detectable high‐P1(N″) transitions in the (3,1) and (6,3) bands and by the presence of observable R branch heads in the (4,2) and (5,3) bands. The R heads in the (4,2) and (5,3) bands form at moderately high N′ and are typically much more intense than expected on the assumption of complete rotational‐translational equilibration throughout the OH M source region. These new results provide strong evidence for incomplete R‐T equilibration of OH(X²Π,3≤ν′≤7,J′) on a column basis under typical nightglow conditions.

Determination of the quenching rate of the O(¹D) by O(³P) from rocket‐borne optical (630 nm) and electron density data

The quenching rate k7 of the O(¹D) by O(³P) in the nocturnal F region is evaluated on the basis of rocket‐borne photometric and electron density measurements that have been carried out aboard a Brazilian Sonda III rocket launched from Natal (geography 5.8°S, 35.2°W) on October 31, 1986, at 2259 Local Standard Time (LST). The nightglow emission and electron density vertical profiles were obtained respectively from a 630‐nm photometer and a high‐frequency capacitance probe. The quenching rate k7 was evaluated from model calculations of the OI 630‐nm emission rate, utilizing as input data the measured electron density. Assuming the value of 1.1 for the quantum yield factor f(1;D) a mean value estimated for k7 turned out to be 9.2×10−13 cm3; s−1, the upper and lower limits being 2.8×10−12 cm3 s−1 and zero, respectively. On the other hand, assuming Bates' [1990] height profile of Franck‐Condon f(1;D) for this same experiment, k7 resulted in 2.55×10−12 cm3 s−1. These results are discussed and compared with previous measurements from other authors.

Evidence of preferential directions for gravity wave propagation due to wind filtering in the middle atmosphere

All‐sky TV images of wave structure in the near‐infrared hydroxyl (OH) nightglow emission were recorded over a 3‐month period during May, June, and July 1988 from a high‐altitude site at the Mountain Research Station (40.0° N, 105.6° W, 3050 m), near Nederland, Colorado. Well‐defined, coherent wave patterns associated with the passage of short period (<1 hour) gravity waves were observed on a total of 22 occasions. The wave motions exhibited similar spatial and temporal properties during each month but a distinct tendency for northward propagation (68% of the wave azimuths within ± 40.0° N), with some eastward motion in May and June, was observed throughout the campaign. Although it is theoretically well known that upward propagating gravity waves can be blocked at a critical layer produced by the interaction of the waves with the horizontal background wind, observational evidence of this phenomenon is rare. To investigate the possibility that the asymmetry in the wave propagation directions was caused by the critical layer, a model based on mean climatological background winds and numerical tidal wave modes valid for any mid‐latitude site and time of the year was constructed to show the regions forbidden to upward gravity wave propagation from critical layer theory. These “blocking diagrams” which vary with height and time were constructed for the OH altitude (∼87 km) for the present paper. Comparison of the predicted (i.e., least restricted) and the observed directions of the wave motion show almost complete agreement. This suggests that middle atmospheric winds can play an important role in determining the flux and the azimuthal distribution of short‐period waves reaching the upper atmosphere.

Near-ultraviolet limb imaging spectrograph for sounding rockets

As part of an ongoing investigation of airglow emissions of the upper atmosphere, an intensified CCD imaging spectrograph has been developed for a sounding rocket project called GEMINI (general excitation mechanisms in nightglow). The instrument, known as LISA (limb-imaging spectrograph for airglow), will be used to measure the limb profiles of some important nighttime airglow emission features. The observed limb profiles will be analyzed to provide atmospheric temperatures and density profiles of excited atomic and molecular species of interest to specific modelling problems in the mesopause and lower thermosphere. The GEMINI rocket is to be launched from White Sands Missile Range, New Mexico, in late 1993 or early 1994. The payload will be three-axis stabilized and absolute pointing will be derived from a star video camera. We describe the design capabilities of the LISA instrument, which include a spectral range of 310 to 390 nm, a wavelength resolution of ~0.3 nm, a height resolution of 1 km, and a theoretical count rate of 0.04 count R^-1 s^-1, where R represents rayleighs. The imager design is discussed and we present the results of some laboratory tests performed by means of an artificial source of the oxygen nightglow emission.

Seasonal and latitudinal variations of gravity wave‐driven fluctuations in OH nightglow

The seasonal and latitudinal variations of the gravity wave‐driven fluctuations in the OH nightglow are investigated theoretically using a model that accounts for emission from an extended OH layer and includes the effects of eddy diffusivities in the gravity wave dynamics. The mean (unperturbed) state is obtained from a two‐dimensional, nighttime model so that mean‐state number densities, temperatures and eddy diffusivities are all self‐consistent. Seasonal and latitudinal variations in the background OH nightglow emission and in the propagation and dissipation characteristics of the gravity waves influence how the OH nightglow modulations due to gravity waves depend on season and latitude. At intermediate gravity wave periods (i.e., periods between 0.5 and 3 hours for λx = 100 km; between 3 and 10 hours for λx = 500 km; and greater than about 4 hours for λx = 1000 km) possible trends in the behavior of <η> (equal to , where I is the airglow intensity, T is intensity‐weighted temperature, an overbar denotes the mean state, a prime denotes a perturbation about that mean state and the brackets indicate vertical integration over all emission levels) with latitude and season could be masked by interference effects. At long periods (i.e., periods greater than 3 hours and 10 hours for λx = 100 and 500 km, respectively) trends are complicated by the competing effects of eddy diffusivities, which directly modify both local values of and the altitudes of maximum wave amplitude, and the vertical distributions of the minor species. Only at periods of a few hours or less (depending on horizontal wavelength) are any seasonal trends seen in <η>, and these are more noticeable in the magnitude of <η> than in its phase. These seasonal variations in <η> are primarily due to seasonal variations in the undisturbed temperature which affect the temperature‐dependent chemical rate constants involved in the OH nightglow. A specific case showing latitudinal trends is presented, but the result is not representative of all of our results obtained involving latitudinal variations.

Altitude and vibrational distribution of the O 2 ultraviolet nightglow emissions

The three O 2 emission systems that emit in the ultraviolet, Herzberg I ( A 3∑ u +− X 3∑ g −), Chamberlain ( A′ 3 Δ u − a 1 Δ g ) and Herzberg II ( c 1∑ u −− X 3∑ g −), have been studied. Measuremen Herzberg I and Chamberlain emission profiles have been analysed by considering different excitation mechanisms for each state. Photochemical schemes are proposed to explain the measured emission profiles and the relative intensities of the emissions coming from these O 2 excited states. This photochemical scenario is shown to be compatible with the theoretical results and with the rest of the molecular oxygen emissions. The altitude integrated vibrational distributions of the Herzberg I, Herzberg II and Chamberlain bands detected in the nightglow are reproduced. The predicted altitude profiles of the individual vibrational levels can reproduce the experimental results available at present.

Equatorial atomic oxygen profiles derived from rocket observations of OI 557.7 nm airglow emission

Two rocket observations of the OI λ557.7 nm nightglow emission in the lower thermosphere have been carried out from the equatorial region at Natal (5.8°S, 35.2°W), one on 9 December 1985 and the other on 31 October 1986. The volume emission rate obtained from these observations showed a maximum at a height of 97 ± 1 km with a half-width of 6 km for the first experiment and 98 ± 1 km with a half-width of 7 km for the second experiment. The observed emission profiles showed large variability, suggesting dynamical control of the atomic oxygen. The atomic oxygen density profiles deduced from the present data are compared with atmospheric models and currently available experimental data.

Nightglow emissions from oxygen in the lower thermosphere

Several independent trains of reasoning lead to the conclusion that O 2( c 1 Σ − u ) is the precursor for the emission of the 557.7 nm line of atomic oxygen. It is shown that the measured altitude profile of the volume emission rate of the Herzberg I system is reproduced by using the expected rate coefficient for the production of O 2( A 3 Σ u +) by termolecular association and rate coefficients for quenching by atomic and molecular oxygen that accord with laboratory evidence. Termolecular association does not contribute directly to the Atmospheric system because the O 2( b 1 Σ + g ) it generates is mainly in high vibrational levels and vibrational deactivation of these by interchange of electronic energy with normal oxygen molecules is not rapid enough to prevent deactivation to O 2( a 1 Δ g or X 3 Σ − g ) from occurring. The prime source of the O 2( b 1 Σ + g , υ =0) is excitation of the O 2 X 3 Σ − g ν = 0) collision partner involved in O 2( A 3 Σ + u , A′ 3 Δ u and c 1 Σ − u ) deactivation. This source can account for the measured altitude profile of the volume emission rate of the Atmospheric system.

A comparative study of low-latitude ionospheric and OI 630nm nightglow observations with the SLIM and IRI models

Measurements of ionospheric F-region peak electron densities, peak heights and atomic oxygen 630 nm airglow emission column intensities, carried out at a low-latitude station located at Cachoeira Paulista (22.7° S, 45.0° W; dip latitude 11.9° S), in Brazil, are analyzed and compared with the ionospheric parameters predicted by the SLIM and by the IRI ionospheric models. The MSIS neutral atmosphere model is used with the IRI model to calculate the 630 nm nightglow emission intensities. This comparative analysis considers data representative of the summer, winter and equinox seasons, obtained during the high solar activity period around 1980. Reasonably good agreement is obtained between the local time variations of the ionospheric and airglow data, and the IRI model predicted parameter variations, but the SLIM model overestimates the F-region peak heights especially during the late afternoon and nighttime hours. These height discrepancies are explained in terms of the strong longitudinal dependence of the electromagnetic E × B vertical plasma drifts and thermospheric neutral winds, which greatly influences the dynamics of the low-latitude ionospheric plasma and airglow emissions. Due to the importance of the relative alignement condition between terminator and magnetic meridian, with its longitudinal and seasonal dependence, on the vertical plasma drifts around sunset, it is suggested that the SLIM ionospheric model should separately consider the longitudinal zones of positive, negative or zero magnetic declination.

Altitude Distribution of O2 1.27.MU.m Nightglow Emission Observed by a Rocket-Borne Radiometer.

The O2 1.27μm nightglow emission was observed by using a rocket-borne radiometer equipped with a high sensitive Ge sensor cooled by liquid nitrogen. The radiometer was flown on board the S-310-20 rocket launched at 04:30 JST, approximately 11 hours after local ground sunset, on January 28, 1990 from Kagoshima Space Center (31°N, 131°E), Japan. As the noise equivalent radiance at 1.27μm was only ±1.2kR when the time constant was 10ms, very fine data were obtained. The total zenith intensity was 207.1±2.4kR, which was 2-3 times as strong as those previously observed. The volume emission rate as a function of altitude showed two-layer structure, and the integrated intensity of a major layer centered at 92km was about 4 times as strong as that of the minor layer at 83km. The direct excitation by the three-body recombination of atomic oxygen could explain the emission of the major layer, when the excitation efficiency for O2(1Δg) is 0.75. The minor layer was found to be explained by an energy transfer from vibrationally excited OH to O2.

Simultaneous intensity, temperature and imaging measurements of short period wave structure in the OH nightglow emission

A low light TV camera co-aligned with a narrow field Michelson interferometer have been used to investigate how variations in the intensity and temperature of the near infrared hydroxyl nightglow emission are related to the passage of short period mesospheric gravity waves. The observations were made from Sacramento Peak, NM (32.8°N, 105.8°W) on the night of 14/15 June 1983, during a spectacular nightglow display that also exhibited wave structure visible to the dark adapted eye. A coherent wave-like pattern of horizontal wavelength 23 ± 1 km moving uniformly with a velocity of 28 ± 2 m s −1 (apparent period 13.7 ± 1.2 min), was measured at several azimuths over an interval of 2.5 h. The induced intensity and temperature perturbations were wave-like in nature and exhibited a high degree of correlation. The intensity variations were large, up to 30%, yet the brightest wave forms were ⩽ 10 K hotter than the adjacent dark forms. The amplitude of the intensity fluctuation was found to be ∼ 8 times larger than the temperature variation. In all cases the intensity wave led the thermal wave, but only by a small amount. High contrast wave patterns of this type are uncommon and are a principal feature of “bright night” displays.

Observations of short period mesospheric wave patterns: In situ or tropospheric wave generation?

Near infrared images showing wave structure in the hydroxyl (OH) nightglow emission have been obtained from Maui, Hawaii during the ALOHA‐90 campaign. Analysis of two nights during this campaign (25 and 31 March) indicate extensive, highly coherent, linear wave patterns of very short apparent period (∼5 and 10 min respectively). Both displays exhibited several features characteristic of the in situ breakdown of a large scale, long period, upper atmospheric wave disturbance. Data in support of this mechanism was found by other ALOHA instruments which detected concurrent long period (1–2 hour) mesospheric wave disturbances on both occasions. However, a tropospheric source for these waves cannot be ruled out. At least on 25 March a weather front occurred at ∼1400 km range with a favourable orientation and location. Although its range was relatively large, background winds may have substantially increased the path length of the waves through the intervening atmosphere.

Near infrared imaging of hydroxyl wave structure over an ocean site at low latitudes

Coordinated observations of wave structure in the near infrared hydroxyl (OH) nightglow emission have been made from Maui, Hawaii using a suite of narrow angle and all‐sky TV cameras. Two sets of data were obtained, the first in conjunction with the ALOHA‐90 campaign and the second during the subsequent new moon period. Well formed, short period (<20 min) wave patterns of comparable morphology, dynamics and abundance to those regularly imaged from mid‐latitude mountain sites were detected on several occasions. Although the Hawaiian islands comprise several high volcanic peaks, the patterns were not consistent with gravity waves generated by the interaction of strong winds with the local island topography. This suggests that other mid‐latitude wave patterns may also not be of mountain origin. The wave patterns imaged during ALOHA‐90 were of significantly lower contrast than those detected later. This effect may be related to changes in the characteristics of the middle atmosphere that occur shortly after the spring equinox.

Observations of the OH Meinel (6,2) and O2 atmospheric (0,1) Nightglow emissions from Maui during the ALOHA‐90 Campaign

Observations are reported from a CCD nightglow camera deployed on Maui during ALOHA‐90. These data show that there were periods of both strong and weak gravity wave activity, that background winds in the direction of wave propogation can be derived using airglow data alone, and that there may be a connection between sporadic Na and large‐scale wave activity. These data are in fair to good agreement with models of wave induced airglow fluctuations.

Thermally stable field compensated Michelson interferometer for measurement of temperature and wind of the planetary atmospheres

A Michelson interferometer has been built to measure the wind and temperature in the earth's upper atmosphere using nightglow emissions from atomic oxygen and radical OH. The interferometer uses field compensation to give large geometric etendu allowing measurements with emission lines of intensity of approximately 30 R at zenith. For wind measurement, it is thermally stabilized permitting operation without difficulty. The instrument incorporates calibration sources allowing temperature and wind measurements. It is operated at the Observatoire de Haute-Provence (43 degrees 56'N, 5 degrees 43'E, France) in summer and Sodankyla (67 degrees 22'N, 26 degrees 38'E, Finland) in winter. Its performance is illustrated by some examples of measurements and results.

On the origin of ripple-type wave structure in the OH nightglow emission

The origin of small-scale mesospheric wave structure with horizontal wavelengths of only 5–15 km and lifetimes of less than 45 min, termed “ripples”, has been investigated using video observations of the near infra-red OH nightglow emission taken over a period of several years. Previous studies in the literature indicate that ripples are associated with lower lunar transit, lunar high tide and with geomagnetic activity (as measured by K p). Examination of our data indicated a peak in the occurrence of ripples around lower lunar transit. However, the distribution of observing time was also strongly peaked towards lower lunar transit since this occurred at night during New Moon periods, when most of the observations were made. Statistical analysis of these data revealed no significant correlation with lower lunar transit and no evidence was found to support the theory that ripples arise from the breakdown of lunar tides in the upper atmosphere. The comparison of ripple occurrence with K p phase showed a strong correlation, with ripples tending to occur when K p was falling. However, in an analysis using the AE magnetic index, no evidence was found to support the hypothesis that ripples are associated with the breakdown at mid-latitudes of travelling ionospheric disturbances (TID) generated at polar latitudes by auroral processes. It appears more likely that ripples are the result of short-lived velocity shears generated in situ by the chance combination of wind and wave motions.

Analysis of storm-time low-latitude simultaneous ionospheric and nightglow emission measurements

The first simultaneous ground-based observations of the permitted atomic oxygen OI 777.4 nm emission line and the forbidden OI 630.0 and 557.7 nm emission lines have been carried out from Cachoeira Paulista (22.7°S, 45.0°W) and Fortaleza (3.9°S, 38.4°W), Brazil, during the period November 1986–March 1988. In addition, N 2 + 391.4 nm emission observations have been carried out at Cachoeira Paulista during the same period. At both stations ionosonde measurements are carried out on a routine basis. These simultaneous multispectral optical and ionospheric measurements have made possible studies on the response of the equatorial and low-latitude ionospheric F-region to magnetic storms. A comparison of the observations obtained during magnetically quiet and disturbed conditions shows that the nocturnal intensity variations of the atomic oxygen emissions observed are well correlated with the variations of the F-region parameters ( ƒ0F2 and h'F), showing the dominant role played by the enhanced F-region dynamical processes during disturbed periods, possibly induced by substorm-associated electric fields and disturbed neutral winds in the low-latitude F-region. While the N 2 + emission is enhanced in disturbed periods, as expected from energetic neutral particle precipitation, any enhancements in the atomic oxygen emissions were not detectable against the airglow due to recombination processes.

Seasonal variations in the equatorial thermospheric wind measured at Arequipa, Peru

Studies have been carried out at Arequipa, Peru, of the seasonal variations in the thermospheric winds at moderate solar flux levels and low geomagnetic activity. Fabry‐Perot interferometer measurements of the Doppler shifts in the 630.0 nm nightglow emission line from March to August 1983 and from April to October 1988 have yielded monthly‐average meridional winds that are nearly zero (<50 m/s) and possibly fluctuating in direction through much of the night but develop a southward flow at 50–100 m/s in the early and the late night from May onward. The average zonal winds are eastward throughout the night, reaching peak velocities before local midnight and then decreasing. The peak velocities increase to a maximum around the June solstice. The winds are generally stronger in 1988 than in 1983, even though the solar EUV fluxes are comparable for both years. Comparison of the present results with earlier satellite measurements, as embodied in the Horizontal Wind Model of Hedin et al., reveals generally satisfactory agreement at the equinox and June solstice, except for the June 1988 period. Killeen et al.'s vector spherical harmonic form of the NCAR Thermospheric General Circulation Model, calculated for similar solar flux levels, yields meridional and zonal wind variations which exhibit the same temporal behaviors but generally smaller values than the present measurements. The present wind patterns are also compared with those measured during the same years at Arecibo, Puerto Rico; one finds oppositely directed meridional flows and similar, eastward zonal flows at the two locations, as expected from a consideration of solar EUV heating as the principal driving force.

An assessment of proposed O( 1S) and O 2( b1Σg+) nightglow excitation parameters

A database consisting of a number of simultaneously measured O( 1D- 1S) green line and O 2( b 1 Σ g +) − X 3 Σ g )(0,0) atmospheric band nightglow emission profiles is examined to assess the general validity of the airglow excitation parameters recently proposed by McDade et al. (1986, Planet. Space Sci., 34, 789). The measured profiles were obtained under quite diverse atmospheric conditions and should, therefore, allow a critical assessment of the proposed parameters. Model green line emission profiles, calculated from the measured O 2 atmospheric band emission profiles using the proposed parameters, are compared with the green line profiles actually measured on each occasion. The measured and modelled green line profiles are found to be in good agreement under most conditions. The cases for which discrepancies exist are discussed in terms of inadequacies in either the background atmosphere adopted for the analyses or in the parameters themselves.