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 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.     

First sodium nightglow results for Natal

The first year of sodium nightglow observations from Natal (6°S, 35°W) are examined. Time variations appear to follow a pattern of their own, different from low latitude results. The major seasonal peak occurs in September-October and the average variation during the night decreases from dusk to dawn. Statistics on cloud coverage show that Natal has roughly only about 3 clear hours per night. The best observing period is April with an average of 5 clear hours per night.     

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.     

The excitation of the 557.7 nm line and Herzberg bands in the nightglow

Measurements of the emission intensities of the 557.7 nm line and Herzberg bands and of O(³P) concentrations carried out on two coordinated rocket flights at South Uist during the night of 8/9 September 1975 are presented. An examination of the 557.7 nm emission and $\text{O}\text{(}{}^3\text{P}\text{)}$ data on the basis of recent data on relevant rate coefficients has shown that the results can be interpreted on the basis of the Barth mechanism for the production of $\text{O}\text{(}{}^1\text{S}\text{)}$ atoms but not the Chapman mechanism. Evidence is provided that the A³Σ⁺_u state of O₂ could be responsible for the $\text{O}\text{(}{}^1\text{S}\text{)}$ production in the Barth mechanism. Values of the rate coefficients involved are deduced from a comparison of the 557.7 nm and Herzberg emission rates.     

Unusual sodium nightglow behaviour and possible association with solar flares

During many nights in October and November 1970 unusual enhancements in sodium nightglow intensity were observed near Belfast. These are discussed in relation to other atmospheric parameters. It is suggested (after Kokin et al., 1971) that instabilities in the atmosphere triggered by a series of solar flares may be the cause, particularly as during this period temperatures in the stratosphere were found to be unstable.     

Diurnal,annual and solar cycle variations of hydroxyl and sodium nightglow intensities in the Europe-Africa sector

Average diurnal variation patterns of the nightglow hydroxyl and sodium intensities are studied using data accumulated over several years by filter photometers at stations in the tropics, and north and south temperate latitudes. These average patterns are shown to depend upon season and latitude. They are compared with the frequencies with which various patterns actually occur, and it is shown that with increasing latitude the type of variation seen on a given night becomes less predictable. The sodium emission co-varies with the OH diurnally. The annual variations of OH and Na are presented and it is shown that the sodium in the southern hemisphere behaves like the sodium in the northern hemisphere displaced by six months. The annual OH variation does not exhibit this property. The OH intensity follows the solar activity cycle, but the sodium does not.     

Average nocturnal and seasonal variations of sodium nightglow at 23°S, 46°W

The seasonal variation of the nocturnal average intensity, and the typical variation of the sodium airglow intensity during the night have been deduced, using a data set that has been accumulated over a period of 5 years. The maximum intensity, which occurs at the equinoxes, is about 2–3 times as large as the intensity during winter, when the intensity of the D₂ line is about 30 R. The nocturnal variation is symmetrical about midnight, where the intensity is some 30% less than just after (before) dusk (dawn) values.     

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 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.     

Comments on the paper: Diurnal,annual and solar cycle variations of hydroxyl and sodium nightglow intensities in the Europe-Africa sector

Night-time variations of the OH nightglow intensity reported by Wiens and Weill are compared with the theoretical predictions of a number of models. The behaviour of this emission agrees better with the theoretical one for locations in the equatorial zone but becomes more variable and less predictable for mid-latitude stations. It is calculated that, as a result of an increase of the eddy diffusion coefficient K, the OH emission can deviate from the typical night-time variation and increase by a factor as high as 2 if K is multiplied by 10. It is suggested that the eddy diffusion coefficient in the upper atmosphere is lower and undergoes lower amplitude variations in the equatorial zone than in the mid-latitude regions.     

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.     

Polarization of the sodium D lines in prominences

The expected polarization of the sodium D lines from solar prominences is computed as a function of the local magnetic field vector. To this aim, the formulation of the Hanle effect in terms of the statistical tensors developed by Landi Degl'Innocenti (1982) is employed, with minor changes connected to hyperfine structure. The sodium atoms are described in the incomplete Paschen-Back regime so that the validity of the results is not limited to weak magnetic fields. The polarization diagrams obtained are discussed and compared with the corresponding diagrams for the helium D₃ line.     

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.     

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.     

A measurement of the O2 ultra-violet nightglow emission

New measurements of the Herzberg I emission height profile in the night airglow are reported and indicate a peak emission height near 96 km in agreement with previous measurements. Using an atomic oxygen concentration profile determined from the oxygen green line profile measured on the same rocket it is concluded that the O₂(A³Σ_u⁺) state is not excited in the direct three body recombination of atomic oxygen. It is suggested that the excitation mechanism is a two step process, similar to the Barth mechanism for the atomic oxygen green lineand that the excited intermediate state is C³Δ_u.     

Modelling hydrogen atom in circumstellar slabs: Excitation,ionization, and line intensities

The statistical study of the individual light curves of magnetic close binary system AM Herculis obtained in 1978–1988 led to the following conclusions: (a) there is a dependence of the amplitude of brightness variations on the luminosity; (b) the shape of the phased light curves is statistically dependent on the luminosity; (c) the photometric period underwent cyclic changes, the phase of minimum is correlated with luminosity. These results confirm the main observational predictions of the swinging dipole model (Astrophys. Space Sci., 1987,131, 557), but the details are to be investigated during the international programme Polar of coordinated observations of AM Her (27 July–8 August, 1989).Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Photometric measurements of the O2 U.V. nightglow

Measurements of the O₂(A³Σ − X³Σ) Herzberg system in the night airglow have been made with the ESRO TD-1 satellite in the wavelength range 2400–3100 A. The slant emission rate varies from 3.5 to 15 kR, indicating an irregular structure of the atomic oxygen near the turbopause. A statistical maximum intensity is found near the tropic in the winter hemisphere. The intensity profile is consistent with excitation by three-body recombination of oxygen atoms. The observed total emission rate can be accounted for by reasonable atomic oxygen densities and an O₂(A³Σ) production efficiency of about 20% if quenching by N₂ occurs at the rate deduced from laboratory and other airglow measurements.