Atmospheric absorption in the O2 Schumann-Runge band spectral range and photodissociation rates in the stratosphere and mesophere

A general analysis of the absorption of the Schumann-Runge bands of molecular oxygen has been made in order to compare the various experimental and theoretical results which have been obtained for an application to the O₂ atmospheric absorption and its photodissociation in the mesosphere and stratosphere. The different values of the oscillator strengths deduced from the laboratory absorption spectra and of the predissociation linewidths used for the calculation of the absorption have been compared.Calculations based on a Voight profile of the O₂ rotational lines have led to simple formulas for atmospheric applications taking into account that the total photodissociation rate in the stratosphere depends strongly on the absorption of solar radiation in the spectral range of the O₂ Herzberg continuum. Specific examples are given.     

The stratospheric abundance of Hypochlorous Acid (HOCL)

New measurements of the absorption cross-section of HOCl suggest that this molecule may be more stable to photolysis than had been previously thought. These results, combined with recent measurements of the rate of formation, suggest that significant concentrations of HOCl could form in the stratosphere. In the present study, model calculations incorporating HOCl are discussed. The results are compared with available stratospheric measurements for chlorine species, and with calculations not including HOCl. It should be possible to detect HOCl in the stratosphere using i.r. techniques.     

A study of the excitation and radiative decay of the 3s′ 3D0 and 3d 3D0 levels of atomic oxygen

The absolute cross-sections for the excitation of the 989 Å, 1027 Å, 7990 Å, 8446 Å, 1.1287 μm and 1.3164 μm multiplets of atomic oxygen by electron impact dissociation of O₂ are reported. The radiative branching ratios for these transitions are calculated from these results and compared with the NBS compilation of Wiese et al. (1966) and the recent theoretical calculations of Pradhan and Saraph (1977). The cascade models of O⁺ radiative recombination and of electron-impact excitation of the OI(³S) state in the terrestrial airglow are discussed in the light of the laboratory measurements, and the effects of the resonant absorption of components of the λ 989 Å and λ. 1027 Å multiplets by the Birge-Hopfield band system of N₂ are investigated. This process is shown to depend sensitively on the N₂ vibrational temperature and to cause characteristic changes in the OI e.u.v. emission spectrum in auroras and in the sunlit F-region at high exospheric temperatures. It is also suggested that the λ 1027 Å radiation observed in auroral spectra is actually due to molecular nitrogen band emission that has been enhanced by entrapment effects and not to the excitation of the 2p ³P-3d ³D⁰ transition of atomic oxygen as believed previously.     

Absorption and photodissociation in the Schumann-Runge bands of molecular oxygen in the terrestrial atmosphere

The penetration in the terrestrial atmosphere of solar radiation corresponding to the spectral range of the Schumann-Runge bands of molecular oxygen is analyzed between 1750 and 2050 Å. The variation of the absorption cross section with temperature is taken into account and it is shown that average O₂ absorption cross sections cannot lead to correct photodissociation coefficients. Reduction factors are defined in order to simplify the computation of the molecular oxygen photodissociation and to permit a simple determination of the photodissociation coefficients of any minor constituent with smoothly varying absorption cross section. Examples are given for O₂, H₂O, CO₂, N₂O, HNO₃ and H₂O₂. Numerical approximations are developed for three types of spectral subdivisions: Schumann-Runge band intervals, 500 cm^?1 and 10 Å intervals. The approximations are valid from the lower thermosphere down to the stratosphere and they can be applied for a wide range of atmospheric models and solar zenith distances.     

The intensity variation of the atomic oxygen red line during morning and evening twilight on 9–10 April 1969

During the evening of 9 April and the morning of 10 April 1969, the twilight zenith intensity of the atomic oxygen red line OI(³P-¹D) at 6300 Å was measured at the Blue Hill Observatory (42°N, 17°W). At the same time incoherent scatter radar data were being obtained at the Millstone Hill radar site 50 km distant. We have used a diurnal model of the mid-latitude F-region to calculate the ionospheric structure over Millstone Hill conditions similar to 9–10 April 1969. The measured electron temperature, ion temperature, and electron density at 800 km are used as boundary conditions for the model calculations. The diurnal variation of neutral composition and temperature were obtained from the OGO-6 empirical model and the neutral winds were derived from a semiempirical three-dimensional dynamic model of the neutral thermosphere. The solar EUV flux was adjusted to yield reasonable agreement between the calculated and observed ionospheric properties.This paper presents the results of these model computations and calculations of the red line intensity. The 6300 Å emission includes contributions from photoelectron excitation, dissociative recombination, Schumann-Runge photodissociation and thermal electron impact. The variations of these four components for morning and evening twilight between 90–120° solar zenith angles, and their relative contributions to the total 6300 Å emission line intensity, are presented and the total is compared to the observations. For this particular day the Schumann-Runge photodissociation component, calculated using the solar fluxes tabulated by Ackermann (1970), is the dominant component of the morning twilight 6300 Å emission. During evening twilight it is necessary to utilize a lower O₂ density than for the morning twilight in order to bring the calculated and observed 6300 Å emission rates into agreement. The implication that there may be a diurnal variation in the O₂ density at the base of the thermosphere is discussed in the light of available experimental data and current theoretical ideas.     

A laboratory discharge lamp source for the O2 atmospheric bands and OI 5577 Å and 6300 Å lines of a known line width

A laboratory discharge lamp is described which strongly emits the forbidden OI 5577 and 6300 Å lines and the O₂ (0?0) atmospheric band. Experimental measurements confirm that these atomic and molecular species are in thermal equilibrium with one another, so that a rotational temperature measurement of the O₂ atmospheric band allows one to deduce the line widths of the 5577 and 6300 Å emissions. This thus provides a useful calibration source for interferometric measurements of these emissions.     

Penetration of solar uv radiation and photodissociation in the Jovian atmosphere

We present computations of the photodissociation coefficients for NH₃, N₂H₄, PH₃, and H₂S in the Jupiter atmosphere. The calculations take into account multiple scattering and absorption using the radiative-transfer method known as δ-Eddington approximation. The atmospheric models include two cloud layers of variable thickness and haze layers above the upper cloud and between the clouds. One of the results of the radiative computations deal with the reflectivity of the Jovian atmosphere as a function of wavelength. A comparison with available data on the albedo of the planet gives some important indications about mixing ratios and distributions of gases and aerosols. The results for the photolysis rates are compared with similar rates obtained by considering either the direct flux or the flux determined by the molecular gas absorption alone. The latter is usually the approximation used in aeronomic models. The results of this comparison show that a considerable difference exists with direct flux photodissociation but significant differences with molecular absorption flux exist only in atmospheric regions where photodissociation is relatively small.     

Molecular photoionisation at 584 Å and 304 Å

The dissociative and non-dissociative photoionisation cross-sections for the HeI and II resonance lines (584.3 and 303.8 Å) have been measured using published data on total cross-sections and experimental results obtained using a new mass spectrometric system essentially free from mass discriminatory effects. Data is presented for hydrogen, nitrogen, oxygen and carbon dioxide, and the overall accuracy of the measurements is assessed.     

The total and relative contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres in the UV and VUV regions

The main aim of this work is to estimate the total contribution of the processes of     molecular ion photodissociation and     collisional absorption charge exchange to the opacity of DB white dwarf atmospheres, and compare this with the contribution of     and other relevant radiative absorption processes included in standard models.
The method for the calculations of the molecular ion     photodissociation cross-sections is based on the dipole approximation and quantum-mechanical treatment of the internuclear motion, while the quasi-classical method for describing absorption processes in     collisions is based on the quasi-static approximation.
Absorption coefficients are calculated in the region  50 nm ≤λ≤ 850 nm  and compared with the corresponding coefficients of other relevant absorption processes; the calculations of the optical depth of the atmosphere layers considered are performed in the far-UV and VUV regions; the contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres is examined.
We examined the spectral ranges in which the total     and     absorption processes dominate in particular layers of DB white dwarf atmospheres. In addition, we show that in the region of  λ≲ 70 nm  the process of     atom photoionization is also important, in spite of the fact that the ratio of hydrogen and helium abundances in the DB white dwarf atmosphere considered is  1:10⁵  .     

Detection of a CH4 atmosphere on Pluto

Using a low-resolution spectrograph and a CCD array, a spectrum of Pluto from 0.58 to 1.06 μm was obtained. The spectrum had a resolution of $\text{～25}\text{A}\text{?}$ and a signal-to-noise ratio of ～300. It showed CH₄ absorption bands at 6200, 7200, 7900, 8400, 8600, 8900 and 10,000 Å. The strongest of these bands was at 8900 Å with an absorption depth of 0.23. This band was heavily saturated, compared to the weaker bands, providing proof for the gaseous origin of the observed absorptions. By applying CH₄ band model parameters to our data, a total CH₄ abundance of 80 ± 20 m-am was derived. This translates into a one-way abundance of 27 ± 7 m-am and a CH₄ surface pressure of 1.5 × 10^?4 atm. An upper limit to the total pressure of ～0.05 atm could be set. First-order calculations on atmospheric escape showed that this methane atmosphere would be stable if the mass of Pluto is increased 50% over its current value and its radius is 1400 km. Alternatively a heavier gas mixed with the CH₄ atmosphere would aid its stability. The relatively large amount of gaseous CH₄ observed implies that the absorption bands recently reported at 1.7 and 2.3 μm are likely due to atmospheric CH₄ absorptions rather than surface frost as interpreted earlier.     

Photometry of Venus below 2200 Å from the orbiting astronomical observatory-2

Spectrophotometry of Venus from 2170 to about 1950 Å has been obtained by OAO-2 at 10 Å resolution. The new data confirm and extend previous indications that the geometric albedo decreases continuously below 2500 Å. Secular changes in either the amount or distribution, or both, of absorbing constituents in the upper atmosphere are strongly suggested. A narrow absorption feature is found near 2145 Å, confirming an earlier report by Anderson et al. [J. Atmos. Sci.26 (1969), 874–888]. Absorption by trace amounts of nitrogen-bearing molecules, including N₂O, HNO₃ in aqeous solution, and possibly also NO, together with Rayleigh scattering from CO₂, can account for the variation in albedo below 3200 Å, but other explanations are not excluded. For example, H₂S may contribute to or be responsible for the decrease in albedo below 2500 Å.     

The ultraviolet radiation environment in the habitable zones around low-mass exoplanet host stars

The EUV (200–911 Å), FUV (912–1750 Å), and NUV (1750–3200 Å) spectral energy distribution of exoplanet host stars has a profound influence on the atmospheres of Earth-like planets in the habitable zone. The stellar EUV radiation drives atmospheric heating, while the FUV (in particular, Lyα) and NUV radiation fields regulate the atmospheric chemistry: the dissociation of H₂O and CO₂, the production of O₂ and O₃, and may determine the ultimate habitability of these worlds. Despite the importance of this information for atmospheric modeling of exoplanetary systems, the EUV/FUV/NUV radiation fields of cool (K and M dwarf) exoplanet host stars are almost completely unconstrained by observation or theory. We present observational results from a Hubble Space Telescope survey of M dwarf exoplanet host stars, highlighting the importance of realistic UV radiation fields for the formation of potential biomarker molecules, O₂ and O₃. We conclude by describing preliminary results on the characterization of the UV time variability of these sources.     

Photometric properties of powdered sulfur

The spectrophotometric (0.39 < λ < 0.7 λm) properties of three particle-size fractions (diameters <10 λm, <150 λm, and 420–850 λm) of sulfur have been investigated in the laboratory. Particle size, temperature, thermal history, and scattering geometry are all shown to influence the spectral reflectance of the normal (S₈) sulfur samples and an “orange-colored” S₈ sample produced by quenching molten sulfur. A scattering law consisting of a linear combination of lunar-like and Lambertian terms adequately describes the data for all particle sizes. Where sulfur is darkest (λ < 0.45 λm), the reflectance decreases with increasing particle size, whereas where sulfur is brightest (λ > 0.45 λm) the reflectance increases with decreasing particle size. In reflected light, the long wavelength edge of the strong ultraviolet absorption retreats smoothly to shorter wavelengths with decreasing temperature at ～1.6Å/°K, a value lower than the 2.2Å/°K value previously reported for transmitted light. Near opposition, sulfur powders are found to follow closely a Minnaert limb darkening law except where the reflectance is low, i.e., in the strong ultraviolet absorption band of the larger particle size fractions. It is clear from our data that quantitative comparisons between disk-integrated observations of Io and laboratory measurements of flat samples of sulfur are not adequate unless temperature effects and changes in scattering geometry are included.     

The height,spectrum and mechanism of type-B red aurora and its bearing on the excitation of O(1S) in aurora

The height of the lower red border of type-B aurora has been determined by triangulation using TV cameras at two ground stations. A mean height of 91.4 ± 1.1 km was determined from a set of 12 measurements made under ideal conditions. A TV spectrograph was used simultaneously to seek possible spectral changes between 6400 and 6900 Å which would be indicative of changes in the vibrational distribution in the N₂ First Positive bands. No significant difference was found in this distribution between the spectra from 93 and 122 km. The height distribution of contributions to the OI 5577 Å emission relative to the N⁺₂ First Negative emission was modelled from 80 to 160 km. Contributions from electron impact on atomic O, O⁺₂ dissociative recombination and N₂(A)O energy transfer were included. Account was taken of recent laboratory data on O(¹S) quenching. It was concluded that these processes could explain the excitation of O(¹S) in normal aurora and the height distribution of OI 5577 Å in type-B red aurora. It was confirmed that the lifetime ofO(¹S) in type-B red auroral rapid time variations is about 0.5 s and it was found from the model that the observed time variation can be reproduced by the mechanisms considered, provided the concentration of NO in the auroral atmosphere is about 1 × 10⁹ at 95 km. Before reasonable certainty can be attained in the correctness of the interpretation it will however be necessary to have reliable simultaneous observations of neutral atmospheric composition particularly for O and NO as well as unchallengeable measurements of the yields of O(¹S) for the processes considered and for several other processes which have been suggested recently.     

Global atomic oxygen density derived from OGO-6 1304 Å airglow measurements

The OGO-6 UV photometer experiment measured the atomic oxygen OI 1304 Å triplet in the Earth's dayglow between 400 and 1100 km. We have analyzed the data for the period 15 September–25 October 1969 by obtaining best-fit models in which the 1304 Å emission is excited by solar resonance scattering and photoelectron excitation. Provided the excitation processes are specified, we find a unique relationship between the vertical column density of atomic oxygen and the zenith 1304 Å intensity. This is essentially independent of the atmospheric temperature. Because of the large numerical uncertainties, the excitation sources are determined from the 1304 Å data and quiet-time in situ measurements of atomic oxygen density. They are found to be in good agreement with recent solar measurements of the 1304 Å lines and with calculations of the photoelectron excitation source. The deduced variations of atomic oxygen column densities over the daytime atmosphere are found to agree well with the Jacchia 1971 models. During the geomagnetic storm, the column density generally increased above a fixed altitude. However, the latitudinal dependence is complex. Following the strong geomagnetic activity between 15 September and 1 October, depletions in atomic oxygen are observed. At times, there is evidence of high-altitude transport of atomic oxygen from high latitude to low latitude.     

Spectres d'absorption dans le proche ultraviolet de CS2 et SO2 entre 200 et 300K

The near U.V. absorption cross-sections of CS₂ and SO₂ have been measured, at a 0.2 nm resolution, at different temperatures ranging from 200 to 300 K. These data will be used to interpret absorption measurements of the stratosphere of Venus.     

Comparison of absolute photoelectron fluxes measured on AE-C and AE-E with theoretical fluxes and predicted and measured N2 2PG 3371Å volume emission rates

As part of the continuing effort to improve the accuracy of the absolute measurements of the ambient photoelectron flux in the thermosphere from the Atmosphere Explorer Satellite Photoelectron Spectrometer experiments (PES), we present a detailed comparison of experimental photoelectron fluxes from AE-C and AE-E together with theoretical calculations of the ambient flux for the same geophysical conditions. As an additional check, the various experimental and theoretical fluxes are used to calculate the expected N₂ 2PG (0, 0) volume emission rate expected at 3371Å and these results are compared to AE-C Visible Airglow Experimental (VAE) experimental results. The comparisons clearly show that because of spacecraft shielding of the sensor on AE-C, the agreement with AE-E spectra for similar geophysical conditions ranges from good when shielding is minimal to poor for severe shielding cases. The calculated fluxes are lower by approx. a factor of 1.5–2.0 in absolute magnitude than the AE-E or unshielded AE-C fluxes. The N₂ 2PG volume emission rates calculated from the measured ambient electron fluxes overestimate the measured VAE volume emission rates by 20–30% while those calculated from the theoretical fluxes underestimate the measured emission rate by typically 30%. These data suggest therefore that the measured AE-E fluxes are 20–30% high.     

The 6300 Å O1D airglow and dissociative recombination

Measurements of night-time 6300 Å airglow intensities at the Arecibo Observatory have been compared with dissociative recombination calculations based on electron densities derived from simultaneous incoherent backscatter measurements. The agreement indicates that the nightglow can be fully accounted for by dissociative recombination. Thecomparisons are examined to determine the importance of quenching, heavy ions, ionization above the F-layer peak, and the temperature parameter of the model atmosphere. Comparable fits between the observed and calculated intensities are found for several available model atmos- pheres (e.g. CIRA, Jacchia). The least-squares fitting process, used to make the comparisons, produces comparable fits over a wide range of combinations of neutral densities and of reaction constants. Yet, the fitting places constraints upon the possible combinations: these constraints indicate that the latest laboratory chemical constants and densities extrapolated to a base altitude are mutually consistent. However, by imposing an additional constraint, an aero- nomically derived preference is given for about one O(^1D) per combination. A preference is also given for the lower base densities of O₂ derived from rockets rather than from models. Altitude profiles for the 6300 Å and 5577 Å emissions are deduced. In the early evening, there are no large discrepancies in the fits that might indicate an effect from elicited states of O₊, vibrational excitation of O₂, or both. The technique of comparing observed and cal- culated 6300 Å intensities has considerable potential as an aeronomical tool for examination of other possible sources of emission and for determination of relative changes in the neutral atmosphere.     

Spatially resolved absolute spectral reflectivity of Jupiter: 3390–8400 Å

Observational determinations of the absolute spectral reflectivities of visually distinct regions of Jupiter are presented. The observations cover the 3390–8400 Å region at 10 Å resolution, and they are compared with observations using 150–200 Å filters in the 3400–6400 Å range. The effective reflectivities for several regions (on the meridian) in the 3400–8400 Å range are: South Tropical Zone, 0.76±0.05; North Tropical Zone, 0.68±0.08; South Equatorial Belt, 0.63±0.08; North Equatorial Belt, 0.62±0.04; and the Great Red Spot, 0.64±0.09. Reflectivities near the limb are also observed. The appropriate blue and red reflectivities are tabulated in support of the Pioneer 10 and 11 imaging photopolarimeter experiments. For the regions listed above, equivalent widths of molecular bands vary as: CH₄ (6190 Å), 14–16 Å; CH₄ (7250 Å), 77–86 Å; and NH₃ (7900 Å), 87–95 Å. Significant differences from the results of C. B. Pilcher, R. G. Prinn, and T. B. McCord (“Spectroscopy of Jupiter: 3200 to 11200 Å,” J. Atmos. Sci.30, 302–307.)     

Photodissociation of nitric oxide in the middle and upper atmosphere

Absorption of solar radiation of wavelengths between 175 to 205 nm plays a fundamental role in the photochemistry of the middle atmosphere. Nitric oxide photodissociates in the δ(0-0) and δ(1-0) bands near 191 and 183 nm, respectively, initiating the primary mechanisms for NO_x removal in the middle atmosphere. The spectrally rich Schumann-Runge (S-R) bands of O₂ are the main source of atmospheric opacity at these wavelengths. A re-evaluation of O₂ absorption has been made based on recent advances in understanding of S-R line shapes, leading to differences with conventional approaches assuming Voigt line profiles in line-by-line calculations of the O₂ cross section. The new results are used to examine the impact of O₂ transmission on the photodissociation of NO in the δ(0,0) and δ(1,0) bands.