The excitation of the infrared atmospheric oxygen bands in the nightglow

Simultaneous observations of the nightgiow emission profiles of $\text{O}{}_2\text{(}{}^1\text{Δ)}$ and the OH Meinel bands have been used to show that the excitation mechanism for $\text{O}{}_2\text{(}{}^1\text{Δ)}$ in the night-time is through the reaction between OH¹ and atomic oxygen and the recombination of atomic oxygen. These reactions, and the proposed rate constants, have been used to derive the atomic oxygen profile appropriate to the observations. It is suggested that the atomic oxygen profile may exhibit significant structure near the mesopause at high latitude. It is also suggested that the extent of this structure may be influenced by transport effects related to stratospheric warming events.     

Excitation of O2 Herzberg Bands in the nightglow

The excitation mechanism for O₂ Herzberg Bands as given by Young and Black (1966) is examined. It is found that O₂ Herzberg Bands are heavily quenched by N atoms, while (0,0) and (0,1) Atmospheric Bands are quenched mainly by CO, NO, O₂ and N₂, NO, O₂ respectively. The emission of Herzberg Bands is found to arise from two layers centred at 80 and 100 km. The rate coefficients of a number of quenching reactions involving atmospheric gases are obtained theoretically.     

Excitation rate of sodium D line in the nightglow

The processes responsible for the emission of Na-D line in the Earth's atmosphere and laboratory are briefly reviewed. From the laboratory results of Ghoshet al. (1970), the rate coefficient of reactions exciting sodium D line is estimated to be 4.73×10^–25 cm⁶/sec², and its intensity in the nightglow is found to be about 114R in summer and 302R in winter.     

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.     

The excitation of ionic lines and molecular bands in meteor wakes

The appearance of multiplets arising from the 4²P state of CaII (the H and K lines and an infrared triplet), the 1st positive band of N₂ and possibly certain multiplets of FeII in meteor wake spectra is explained semi-quantitatively in terms of the two-step sequence: collisional ionization of major atmospheric species O₂ and N₂ followed by resonant charge exchange with ablated meteoric atoms. Many other features which could arise through this mechanism (multiplets of MgII, SiII and FeII, and bands of O₂ and N₂) are likely to be weak or to have escaped detection owing to observational selection.     

Dayglow emissions of the O2 Herzberg bands and the Rayleigh backscattered spectrum of the earth

Numerous fluorescent emissions from the Herzberg bands of molecular oxygen lie in the spectral region 242–300 nm. This coincides with the wavelength range used by orbiting spectrometers which observe the Rayleigh backscattered spectrum of the earth for the purpose of monitoring the vertical distribution of stratospheric ozone. Model calculations indicate that Herzberg band emissions in the dayglow could provide significant contamination of the ozone measurements if the quenching rate of O₂(A³Σ) is sufficiently small. This is especially true near 255 nm, where the most intense fluorescent emissions relative to the Rayleigh scattered signal are located and where past satellite measurements show a persistent excess radiance above that expected for a pure ozone absorbing and molecular scattering atmosphere. However, very small quenching rates are adequate to reduce the dayglow emission to negligible levels. Available laboratory data have not definitely established the quenching on the rate of O₂(A³Σ) as a function of vibrational level, and such information is required before the Herzberg band contributions can be evaluated with confidence.     

The excitation of the coronal line 5303 Å

The relation between the intensities of the line 5303 Å and that of the nearby continuum has been investigated at the solar eclipse of March 7, 1970. As this relation depends on the local structure of the corona, spectrograms have been taken at 16 different position angles. For the present investigation, 4 spectrograms have been selected that show the line 5303 Å up to r = 2. The results are presented in Figure 2. For distances r >1.8 the ratio of line-to continuum-intensity, Q, becomes constant. This is the region where the excitation by radiation dominates. Towards smaller distances Q increases as the excitation by collisions becomes more and more important. At r = 1.3 the contributions from both mechanisms are equal. For r <1.3 Q increases stronger than expected for an isothermal and homogeneous model. In the innermost corona the observed values are twice as high as the theoretical ones. This difference can be accounted for by inhomogeneities of the corona.     

Improved absorption cross-sections of oxygen in the wavelength region 205–240 nm of the Herzberg continuum

The laboratory values of the Herzberg continuum absorption cross-section of oxygen at room temperature from Cheung et al. (1986, Planet. Space Sci. 34, 1007), Jenouvrier et al. (1986a, Planet. Space Sci. 34, 253) and Jenouvrier et al. (1986, J. quant. Spectrosc. radiat. Transfer 36, 349) have been compared and re-analyzed. There is no discrepancy between the absolute values of these two sets of independent measurements. These values have been combined together in a linear least-squares fit to obtain improved values of the Herzberg continuum cross-section of oxygen at room temperature throughout the wavelength region 205–240 nm. Agreement with in situ and other laboratory measurements is discussed.     

Intensity variations and ratios of (9-4) and (7-3) hydroxyl bands in nightglow at Poona

The observed types of nocturnal intensity variations for the OH (9-4) and OH (7-3) bands during IQSY at this Station are further analysed using the theoretical band intensity distribution of Evans and Llewellyn (1972). A considerable agreement is noticed between observed and theoretical intensity ratios, $\text{I(9-4)}\text{I(7-3)}$, for a major portion of the data (～70%), which has a “continuous decrease” type of noctural intensity variation. This data is thereby satisfactorily explained on the basis of available information.For the remaining portion of the data (～30%), which has “an increase followed by decrease” type of intensity variation and higher intensities, the observed ratios are also systematically higher than the above. A satisfactory explanation is offered, by postulating a second layer of emission, by examining closely several aspects of the other observational results.     

Forbidden oxygen and nitrogen lines in the nightglow

An historical account is given of the development of our knowledge concerning the processes controlling the emission of the λ 5577 and λλ6300, 6364 lines of oxygen and the λλ5198, 5201 lines of nitrogen in the nightglow. Only in the case of the last can the processes be regarded as fully established. It is not yet known whether the emission of the oxygen green line from near the 100 km level follows the three-body process of Chapman (1931) or the two stage mechanism of Barth (1962, 1964). The pattern of the processes germane to the pre-twilight enhancement of the oxygen red doublet is not clear.     

The Venus nightglow: Ground-based observations and chemical mechanisms

In 1999, observations of the Venus nightglow with the Keck I telescope showed that the 5577 Å oxygen green line was a significant feature, comparable in intensity to the terrestrial green line. Subsequent measurements have been carried out at the Apache Point Observatory (APO) and again at Keck I, confirming the presence of the line with substantially varying intensity. The Herzberg II emission intensity, from the O₂(c-X) transition, was found to have an intensity near 3 kR in one APO run, comparable to the value found on all previous measurements. Thus, of the three oxygen features seen at Venus—the green line, the Herzberg II emission system, and the 1.27-μ 0-0 band of the IR atmospheric system—the first is quite variable, the second is relatively constant, while the third also shows large variations. The reaction between O₂(, v=0) and CO is considered as a possible mechanism to explain green line production and its variability, as well as the variability of the 1.27-μ emission and the stability of the CO₂ atmosphere. This reaction may catalyze CO₂ recombination some five orders of magnitude faster than the slow three-body O + CO reaction.     

The O2 nightglow in the martian atmosphere by SPICAM onboard of Mars-Express

We present observations of the O₂(a¹Δ_g) nightglow at 1.27 μm on Mars using the SPICAM IR spectrometer onboard of the Mars Express orbiter. In contrast to the O₂(a¹Δ_g) dayglow that results from the ozone photodissociation, the O₂(a¹Δ_g) nightglow is a product of the recombination of O atoms formed by CO₂ photolysis on the dayside at altitudes higher than 80 km and transported downward above the winter pole by the Hadley circulation. The first detections of the O₂(a¹Δ_g) nightglow in 2010 indicate that it is about two order of magnitude less intense than the dayglow (Bertaux, J.-L., Gondet, B., Bibring, J.-P., Montmessin, F., Lefèvre, F. [2010]. Bull. Am. Astron. Soc. 42, 1040; Clancy et al. [2010]. Bull. Am. Astron. Soc. 42, 1041). SPICAM IR sounds the martian atmosphere in the near-IR range (1–1.7 μm) with the spectral resolution of 3.5 cm^?1 in nadir, limb and solar occultation modes. In 2010 the vertical profiles of the O₂(a¹Δ_g) nightside emission have been obtained near the South Pole at latitudes of 82–83°S for two sequences of observations: L_s = 111–120° and L_s = 152–165°. The altitude of the emission maximum varied from 45 km on L_s = 111–120° to 38–49 km on L_s = 152–165°. Averaged vertically integrated intensity of the emission at these latitudes has shown an increase from 0.22 to 0.35 MR. Those values of total vertical emission rate are consistent with the OMEGA observations on Mars-Express in 2010. The estimated density of oxygen atoms at altitudes from 50 to 65 km varies from 1.5 × 10¹¹ to 2.5 × 10¹¹ cm^?3. Comparison with the LMD general circulation model with photochemistry (Lefèvre, F., Lebonnois, S., Montmessin, F., Forget, F. [2004]. J. Geophys. Res. 109, E07004; Lefèvre et al. [2008]. Nature 454, 971–975) shows that the model reproduces fairly well the O₂(a¹Δ_g) emission layer observed by SPICAM when the large field of view (>20 km on the limb) of the instrument is taken into account.     

Interferometric studies of the twilight and nightglow sodium D-line profiles

A 150 mm aperture, pressure scanned Fabry-Perot interferometer has been used to study the midlatitude twilight and nightglow sodium D-line profiles. The line width measurements during evening and morning twilight indicate that the sodium layer temperature rises to a midwinter maximum (～230 K) and then falls to a midsummer minimum (～150 K), in qualitative agreement with the CIRA 1972 model predictions. Nightglow intensity measurements obtained with the interferometer indicate a highly variable behaviour, ranging from near-constant intensities, to monotonically falling, and to rising and falling intensities during the night. Broadening of the nightglow line profiles yields a sodium atom dissociation kinetic energy of (46 ± 4) meV. This suggests that the Chapman NaO + O chemiexcitation process, rather than dissociative recombination of “corkscrewing” ions and electrons, gives rise to the nightglow.     

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.     

Latitudinal profile of UV nightglow and electron precipitations

We studied experimental data on ultra-violet (UV) nightglow in the wavelength range 300-400 nm, and energetic electron fluxes measured by low-altitude polar satellite Universitetskii-Tatiana. From statistical analysis we have found three latitudinal regions of enhanced UV emission at low, middle and high latitudes. Modeling the electron precipitations to the atmosphere gave numerical estimation of the generated UV radiation. We found that the stable and quasi-stable fluxes of electrons precipitating at middle and low latitudes are too weak to explain the observed intensities of UV radiation. The high-latitude UV nightglow with intensity of several kiloRayleighs results from particle precipitation in the regions of aurora and outer radiation belt. The low-latitude UV enhancements of several hundreds Rayleighs can be related to the emission of mesospheric atomic oxygen whose concentration increases substantially at latitudes from 20° to 40°. A mechanism of the mid-latitude UV enhancements is still unknown and requires further investigations.     

O/O2 emissions in the Venus nightglow

The oxygen green line is one of the most characteristic features in the terrestrial visible nightglow; it can be seen in the Venus nightglow, but with much greater intensity variation than in the terrestrial case. Here we synthesize our current understanding of the green line in the Venus nightglow and discuss what might be expected observationally in the rising phase of solar cycle 24.     

Tentative identification of the diffuse interstellar absorption bands and of a diffuse interstellar doublet line

The four diffuse interstellar absorption bands at 4430, 4760, 4890, and 6180 and the two diffuse lines at 5780 and 5797 are interpreted as belonging to pre-ionization transitions in H^– and O^–, respectively. In both cases the identifications are supported by extrapolations of wave numbers of resonance lines along isoelectronic sequences.In the H^– case the hypothesis as to the origin of the bands is supported by quantum-mechanical results byHerzenberg andMandl (1963) as to the positions of resonances in collisions between neutral hydrogen atoms and free electrons. The relatively large intensities of the forbidden transitions indicate that the extent of the ion in its excited states may be very large as compared to ordinary atomic dimensions. In the O^– case the relative doublet separation, as extrapolated along the isoelectronic sequence, is used for the identification of the doublet.     

The excitation mechanism of [Fe XIV] 5303 å line in the inner regions of solar corona

The line intensity of the green coronal line and the continuum intensity are derived from the filter and white light photographs of the solar corona obtained during the 1980 total solar eclipse. Ratio of the line to continuum intensity is plotted against the radial distancer(=R/R₀,R ₀ is the solar radius), in various position angles. A simple model assuming an electron density dependence of the line and continuum intensities suggests a dominant collisional mechanism for the excitation of the line in the innermost regions (～ 1.4R ₀). The measured line to continuum ratio tends to a constant value at different radial distances in different position angles. The constancy of the measured line to continuum ratio indicates significant radiative excitation beyond 1.4 R₀, in some of the position angles.     

The role of a strong far-infrared radiation field in line excitation and cooling of photodissociation regions and molecular clouds

We have theoretically studied the influence of a far-infrared radiation (FIR) field from H_π region on the cooling by C and O atoms, C⁺ ion and CO molecule in a photodissociation region, and a molecular cloud associated with H_π region (hereinafter referred as H_I region) at low temperatures (T _k≤200 K). Comparisons have been made for cooling with and without FIR for two extreme abundances (10⁻⁴ and 10⁻⁷) of the mentioned species for temperatures ranging between 10 and 200K and an hydrogen particle density range 10 cm⁻³≤n _o≤ 10⁷ cm³. The cooling by the species with low line-splitting (C_I, C_π and CO) is significantly influenced by the radiation field for temperaturesT _k < 100 K while the effect of radiation field on cooling by O_I is significant even at higher temperatures (T _k > 100 K). The effect of FIR field on the cooling of CO from low rotational transitions is negligibly small, whereas it is considerable for higher transitions. In general, the cooling terms related to the short-wavelength transitions are more affected by FIR than those related to longer wavelengths. It is also demonstrated here that in the determination of thermal structure of an HI region the dust grains play an important role in the heating of gas only through photoelectron emission following irradiation by far-ultraviolet (FUV) radiation, as the infrared radiation from the dust is too small to have substantial effect on the cooling. It is found that in the H_π /H_I interface the FIR field from grains in the H_π region is not capable of modifying the temperature of the warmest regions but does so in the inner part where the temperature is low enough.