Tangent Height Research Articles

CO2 non‐local thermodynamic equilibrium radiative excitation and infrared dayglow at 4.3 μm: Application to Spectral Infrared Rocket Experiment data

Infrared radiative excitation in non‐local thermodynamic equilibrium (non‐LTE) regions of the Earth's atmosphere for the v3‐mode vibrationally excited states of CO2 under sunlit conditions and the resulting 4.3‐μm limb radiance are calculated using a line‐by‐line (LBL) radiative transfer model. Excited‐state population densities and the corresponding vibrational temperature profiles are calculated for the important emitting states using a model which includes radiative absorption and emission as well as various collisional processes. The quenching of O(1D) by N2 has a greater impact on these population densities than has been previously reported in the literature. Integrated radiance in a limb view for the 4.3‐μm bands is calculated from the model and compared with sunlit earthlimb measurements obtained by the Spectral Infrared Rocket Experiment (SPIRE). Solar pumping is the dominant excitation process for the 4.3‐μm emitting states in the daytime. The major contribution to the total limb radiance for tangent heights of 55–95 km is made by the fluorescent states at approximately 3600 cm−1 which absorb sunlight at 2.7 μm and then emit preferentially at 4.3 μm. The predicted radiance is in good agreement with the SPIRE measurements for all tangent heights in the 50‐ to 130‐km range. This is the first detailed comparison of results of a full line‐by‐line non‐LTE radiative transfer calculation with 4.3‐μm earthlimb radiance data.

Analysis of hydroxyl earthlimb airglow emissions: Kinetic model for state‐to‐state dynamics of OH (υ,N)

Detailed spectroscopic analysis of hydroxyl fundamental vibration‐rotation and pure rotation emission lines has yielded OH(υ,N) absolute column densities for nighttime earthlimb spectra in the 20 to 110‐km tangent height region. High‐resolution spectra were obtained in the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A) experiment. Rotationally thermalized populations in υ = 1–9 have been derived from the fundamental bands between 2000 and 4000 cm−1. Highly rotationally excited populations with N ≤ 33 ( ≤ 2.3 eV rotational energy) have been inferred from the pure rotation spectra between 400 and 1000 cm−1. These emissions originate in the airglow region near 85–90 km altitude. Spectral fits of the pure rotation lines imply equal populations in the spinrotation states F1 and F2 but a ratio Π(A′):Π(A″) = 1.8±0.3 for the Λ‐doublet populations. A forward predicting, first‐principles kinetic model has been developed for the resultant OH(υ,N) limb column densities. The kinetic model incorporates a necessary and sufficient number of processes known to generate and quench OH(υ,N) in the mesopause region and includes recently calculated vibration‐rotation Einstein coefficients for the high‐N levels. The model reproduces both the thermal and the highly rotationally excited OH(υ,N) column densities. The tangent height dependence of the rotationally excited OH(υ,N) column densities is consistent with two possible formation mechanisms: (1) transfer of vibrational to rotational energy induced by collisions with O atoms or (2) direct chemical production via H + O3 → OH(υ,N) + O2.

Special Sensor Ultraviolet Limb Imager: an ionospheric and neutral density profiler for the Defense Meteorological Satellite Program satellites

The Naval Research Laboratory is developing a series of far- and extreme-ultraviolet spectrographs (800 to 1700 Å) to measure altitude profiles of the ionospheric and thermospheric airglow from the U.S. Air Force Defense Meteorological Satellite Program's Block 5D3 satellites. These spectrographs, which comprise the Special Sensor Ultraviolet Limb Imager (SSULI), use a near-Wadsworth optical configuration with a mechanical grid collimator, concave grating, and linear array detector. To image the limb, SSULI employs a rotating planar SiC mirror that sweeps the field of view perpendicular to the limb of the Earth. In the primary operating mode, the mirror sweeps the instrument field of view through 17 deg to view tangent heights from about 50 to 750 km. The SSULI detectors use microchannel plate intensification and wedge-and-strip decoding anodes to resolve 256 pixels in wavelength dispersion. The detector is windowless and uses an o-ring sealed door to protect the Csl photocathode from exposure prior to insertion in orbit. The altitude distributions of the airglow measured by the SSULI sensors will be used to infer the altitude distributions of electrons and neutral species. At night, electron densities will be determined by measurement of ion recombination nightglow. Daytime electron densities will be obtained from measurements of multiple resonant scattering of O⁺ 834-Å radiation produced primarily by photoionization excitation of atomic oxygen. Dayside neutral densities and temperatures will be inferred from the measurement of dayglow emissions from N₂ and O produced by photoelectron impact excitation.

Observation of nitric oxide rovibrational band head emissions in the quiescent airglow during the Cirris‐1A Space Shuttle Experiment

Band head emissions from highly rotationally excited NO (v,J) (J≈90) have been observed in the quiescent atmosphere at tangent heights between approximately 115 and 190 km for both sunlit and nighttime conditions. The data were obtained by the cryogenic CIRRIS‐1A interferometer which was operated on‐board the space shuttle between 28 and 30 April 1991. Up to ten emission features between 2020 and 1744 cm−1 in the earthlimb spectra have been identified as the R‐branch band heads of the NO Δv=1 sequence for the vibrational states v′ = 1 – 10. The presence of these band heads requires a very high degree of rotational excitation corresponding to rotational energy in excess of 1.4 eV. These are the first observations of emissions from highly rotationally excited NO in the quiescent airglow and they parallel the recent discovery of highly excited pure rotational transitions of OH in the airglow for both nighttime and daytime quiescent conditions.

CIRRIS 1A observation of 13C16O and 12C18O fundamental band radiance in the upper atmosphere

Fundamental band emission from the ν=1 levels of isotopically substituted 13C16O and 12C18O has been observed for the first time in the earth's atmosphere. High‐resolution earthlimb spectra were obtained in the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A) experiment under both nighttime and daytime conditions. Spectral data between 70 and 150 km tangent height were analyzed in the 4.7 µm wavelength region, providing absolute 12C16O, 13C16O, and 12C18O column radiances. The minor isotope column radiances are up to 30 times more intense than predicted, based solely on the natural isotopic abundances of 13C and 18O. The derived line‐of‐sight radiances were accurately modeled using the Phillips Laboratory Atmospheric Radiance Code, and provide a strong test of radiative transport and excitation algorithms, as well as model climatologies.

Non‐local thermodynamic equilibrium studies of the 15‐μm bands of CO2 for atmospheric remote sensing

A new line‐by‐line non‐local thermodynamic equilibrium (non‐LTE) radiance model based on the GENLN2 radiative transfer code is presented. This is capable of high resolution spectral radiance calculations for the upper atmosphere. We describe the non‐LTE model implementation and discuss the molecular state vibrational temperature input requirements for studies of the 15‐μm ν2 bands of CO2. Monochromatic and band‐integrated radiance calculations have been performed for atmospheric limb view tangent heights between 50 and 120 km for non‐LTE nighttime and daytime conditions. Two model atmospheres are considered, the U.S. 1976 Standard and a subarctic summer, which show, respectively, mean and large radiance differences from a reference LTE radiance calculation. Non‐LTE radiance considerations are shown to be important for the 15‐μm CO2 bands for tangent heights greater than 70 km, the magnitude of the divergence from LTE values and diurnal variation being dependent on the band and kinetic temperature profile. We show that the use of the Voigt line shape, rather than the Doppler, is important for tangent heights below 85 km, and that the inclusion of the effect of overlapping lines is a consideration for tangent heights below 75 km. We present calculations of synthetic radiance spectra showing the non‐LTE effect for two CO2 temperature sounding channels of instruments aboard the Upper Atmosphere Research Satellite as a demonstration of the model capability.

Preliminary measurements of mesospheric OH X2II by ISO on ATLAS 1

Resonance fluorescence of the OH radical was observed in the mesosphere by the Imaging Spectrometric Observatory (ISO) on ATLAS 1. A preliminary determination of the OH density profile from 70 to 80 km has been made from these observations. This marks the first measurement of ground state OH in the mesosphere since Anderson's [1971 a,b] sounding rocket measurements, and the first from space. ISO imaged resonance scattered sunlight in the OH A2Σ–X2II(0, 0) band during limb scans at tangent heights between 60 and 85 km, at 1.6 km spatial resolution, using an f/3.5 diffraction grating spectrometer with spectral resolution of 0.5 Å. OH observations were conducted throughout most of the dayside passes during the mission, covering much of the northern hemisphere to 57°N latitude. Here we report results from an observation at 39°N, local solar time 13:15, on March 30, 1992; we find OH densities on the order of 8 × 106 cm−3 from 70 to 80 km, decreasing rapidly above 80 km.

Error analysis of ClO, O3, and H2O abundance profiles retrieved from millimeter wave limb sounding measurements

A rigorous error analysis is applied to the retrieval of chlorine monoxide (ClO), ozone (O3), and water vapor (H2O) profiles from millimeter wave spectra measured with a limb sounding instrument at 204, 184, and 183 GHz. The effect of different height resolutions on the accuracy and precision is studied. The nature of the different error sources and their contributions to the total error are discussed. A quantitative analysis for a specific limb sounding experiment, the MAS on the ATLAS spacelab shuttle program, is presented. The accuracy of the profiles to be retrieved is 30% for ClO around 35 km (for an altitude resolution of 5 km and an integration time of 200 s per tangent height). O3 can be retrieved at an accuracy of 5 to 9% between 18 and 60 km (3 km; 1–100 s, depending on height). H2O can be retrieved at an accuracy of 4 to 9% between 20 and 75 km (3 km; 1–100 s). For O3, measurements at a resolution of 1 km are possible between 18 and 50 km, for H2O between 20 and 60 km.

Middle and upper atmosphere pressure‐temperature profiles and the abundances of CO2 and CO in the upper atmosphere from ATMOS/Spacelab 3 observations

Based on a new approach, profiles of kinetic temperature and atmospheric pressure between 20 and 116 km altitude and CO2 and CO volume mixing ratios between 70 and 116 km have been derived from the ∼0.01‐cm−1 resolution infrared solar occultation spectra recorded by the atmospheric trace molecular spectroscopy (ATMOS) Fourier transform spectrometer during the Spacelab 3 shuttle mission (April 29 to May 6, 1985). The physical model and CO2 profile results have been obtained from simultaneous multiscan least squares fitting of microwindows containing CO2 lines with temperature‐dependent and temperature‐independent intensities and N2 lines with temperature‐independent intensities. The analysis included the retrieval of tangent height separations between spectra in the lower stratosphere, where independent information shows drifts of the sun tracker field of view on the solar disk. The physical model results are compared with climatological profiles, lower thermospheric temperatures computed with the MSISE‐90 model, and other data. The CO2 retrievals indicate a nearly constant volume mixing ratio of 320 ± 35 ppmv (parts per million, 10−6, by volume) in the 70‐to 90‐km altitude region with a rapid decline in the CO2 volume mixing ratio beginning between 90 and 100 km; at 116 km the CO2 volume mixing ratio has declined to about 70 ppmv and is equal to the CO volume mixing ratio, which has been derived from fits to the (1‐0) CO vibration‐rotation band. The absorption by the v2 + v3 ‐ v2 band of CO2 has been analyzed to quantify nonlocal thermodynamic equilibrium (non‐LTE) effects in the upper mesosphere and lower thermosphere. Up to an altitude of 100 km, the results indicate that the vibrational temperature of the v2 state of CO2 is within 5 K of the kinetic temperature. At higher altitudes, non‐LTE effects are observable with the vibrational temperature of the v2 state cooler than the kinetic temperature by 40 K (48°S) and 70 K (30°N) at 112 km. Above 112 km, lines of the v2 + v3 ‐ v2 band are too weak to be observed in the ATMOS spectra.

Line‐by‐line radiative excitation model for the non‐equilibrium atmosphere: Application to CO2 15‐μm emission

We describe a new line‐by‐line (LBL) algorithm for radiative excitation in infrared bands in a non local thermodynamic equilibrium (non‐LTE) planetary atmosphere. As a specific application, we present a predictive model for the terrestrial CO2 15‐μm emission that incorporates this generic algorithm, and validate the model by comparing its results with emission spectra obtained in a limb‐scanning rocket experiment. The radiative‐excitation algorithm has certain unique features. Being a completely monochromatic calculation, it includes the detailed layer‐by‐layer variation of the shapes of the individual lines in its evaluation of atmospheric transmittivity; and, being an iterative algorithm, it avoids the need to construct and invert large matrices, so that a fine layering scheme can be implemented. It also includes a simple correction procedure to minimize the most serious error due to having discrete layers. These features contribute to accurate radiative transfer results and reliable atmospheric cooling rates. For altitudes above 40 km, we present results of model calculations of CO2(v2) vibrational temperatures, 15‐μm limb spectral radiances, and cooling rates, for the main band as well as for weaker hot and isotopic bands. We calculate the excitation and deexcitation rates due to different processes, including the radiative pumping due to individual atmospheric layers. We compare the predicted limb radiance with earthlimb spectral scans obtained in the SPIRE rocket experiment over Poker Flat, Alaska, and get excellent agreement as a function of both wavelength and tangent height. This constitutes the first validation of a long‐wavelength CO2 non‐LTE emission model using an actual atmospheric data set, and it verifies the existence of certain aeronomic features that have only been predicted by models. It also constrains the previously unknown value of the very important rate constant for deactivation of the CO2 bending mode by atomic oxygen to the range of 5–6 × 10−12 cm3/(mol s) at mesospheric and lower thermospheric temperatures. We discuss the significance of this large value for the terrestrial and Venusian thermospheres. We also discuss the convergence rate of the iterative scheme, the model's sensitivity to the background atmosphere, the importance of the lower boundary surface contribution, and the effects of the choice of the layer thickness and the neglect of line overlap.

Infrared absorption-coefficient data on SF 6 applicable to atmospheric remote sensing

Spectral absorption coefficients k v (cm -1 atm -1) of SF 6 have been measured in the central Q-branches of the v 3-fundamental at 947 cm -1 at various temperature-pressure combinations representing tangent heights in solar-occultation experiments or layers in the atmosphere. The data obtained with the Doppler-limited spectral resolution (∼ 10 -4 cm -1) of a tunable-diode laser spectrometer are useful in the atmospheric remote sensing of this trace gas.

Absorption coefficients of CFC-11 and CFC-12 needed for atmospheric remote sensing and global warming studies

Spectral absorption coefficients (or absorption cross-sections) k v (cm -1 atm -1) of CFC-11 and CFC-12 have been measured in the spectral region (8–12 μm) known as the atmospheric window. Data obtained with a grating spectrometer, which has wide spectral coverage and adequate spectral resolution (0.4 cm -1), are compared with the so-called NCAR cross-sections that have recently been introduced into the HITRAN Tape. We have also performed measurements, at various temperature-pressure combinations which are chosen to represent tangent heights (as in solar-occultation experiments) or layers in the atmosphere, of the k v of CFC-12 in the 921 and 923 cm -1 Q-branches and at 924.1, 927.6, 931.6 935.0 and 1106 cm -1 with the Doppler-limited spectral resolution (∼10 -4cm -1) of a tunable-diode laser spectrometer. The latter are especially well suited for atmospheric remote sensing, while the grating spectra are useful in global warming studies. The effects of temperature and pressure on the spectral absorption coefficients are discussed.

Absorption spectra of HCFC-22 around 829 cm -1 at atmospheric conditions

Spectral absorption coefficients, which are also known as absorption cross-sections k v (cm -1 atm -1), of HCFC-22 have been measured around 829 cm -1 in the laboratory at various temperature-pressure combinations chosen to represent tangent heights (as in solar- occulation experiments) or layers in the atmosphere. The k v data measured employing the Doppler-limited spectral resolution (∼10 -4 cm -1) of a tunable diode laser spectrometer are free of instrumental distortion and are more practical in this case than the spectral line parameters adapted in conventional line-by-line procedures for analysing atmospheric spectra. The present data obtained with N 2 as the broadening gas are shown to be directly applicable to the real atmosphere.

Observation of high‐N hydroxyl pure rotation lines in atmospheric emission spectra by the CIRRIS 1A Space Shuttle Experiment

Pure rotation line emissions from highly rotationally excited OH have been observed between 80 and 110 km tangent height under both nighttime and daytime quiescent conditions. Data were obtained using the cryogenic CIRRIS 1A interferometer, operated on the Space Shuttle. Transitions from OH(v=0–2, N′≤33) were identified between 400 and 1000 cm−1, corresponding to states with energies as high as 23000 cm−1. These are the first definitive observations of OH pure rotation transitions in the air glow, and by far the highest N levels observed in any type of OH airglow emission spectrum. The present observations of highly excited rotational states of OH parallel those made during recent field studies of NO and laboratory studies of OH, NO, and CO.

Spectral line shape considerations for limb temperature sounders

The sensitivity of radiative transfer calculations to spectral line shape models is investigated for two CO2 spectral regions used by instruments aboard the Upper Atmosphere Research Satellite for making low‐altitude temperature and pressure retrievals. Line shape models have been tested by comparing calculated spectra with observations made by the Atmospheric Trace Molecule Spectroscopy experiment. Spectral line mixing is discussed and shown to be important at the 617 cm−1 Q branch where its inclusion in the radiative transfer model can lead to differences of more than 2 K in brightness temperature at low tangent heights. The sub‐Lorentzian line wings of the CO2 ν2 band must be correctly modeled in calculations at the 791 cm−1 Q branch otherwise equivalent brightness temperatures for a 15 km limb view will be in error by as much as 10 K.

Upper atmospheric infrared radiance from CO2and NO observed during the SPIRIT 1 Rocket Experiment

Spectral limb radiance data on CO2ν2and NO 5.3‐μm emissions obtained in the Spectral Infrared Interferometric Telescope rocket experiment have been analyzed. The data cover auroral intensities from several kilorayleighs to over 100 kR at 391.4 nm, and tangent heights of ∼70–200 km at high latitudes (60°–65°N). Volumetric emission and kinetic temperature profiles have been obtained using linear least squares inversion procedures. The CO2ν2radiance profile is found to be similar to previous observations. The NO 5.3‐μm emission is somewhat weaker than that typically measured; this is ascribed mainly to below‐average thermospheric temperatures. Auroral enhancement of NO hot bands was not observable with the available sensitivity. An approximate thermospheric temperature profile derived from the NO band shape compares reasonably with the mass spectrometer/incoherent scatter 86 model.

A vibrational analysis of the O2 ( A3Σ+u) Herzberg I system using rocket data

An observation of the u.v. nightglow between 2670 and 3040 Å was conducted over White Sands Missile Range on 22 October 1984. A 1 4 m spectrometer operating at 3.5 Å resolution viewed the Earth's limb at tangent heights between 90 and 110 km for 120 s. A total of 41 spectral scans of the nightglow were obtained with the brightest feature being the O 2 ( A 3 Σ u +− X 3 Σ g −) Herzberg I bands. The data were sorted into two groups, one from the top side of the layer and one containing the emission peak, and compared with synthetic spectra. The deduced O 2 ( A 3 Σ u +) vibrational distributions indicate that at low altitudes, the higher vibrational levels ( v ́ > 6) were relatively depleted; however, the magnitude of the vibrational shift is much less than that predicted from theories of vibrational relaxation. It is shown that increasing the electronic quenching of O 2 ( A 3 Σ g +) with respect to the vibrational quenching can reduce the vibrational shift in the model and qualitatively explain the observations; however, several details of the vibrational distribution are not well reproduced.

Calculation of infrared limb emission by ozone in the terrestrial middle atmosphere: 2. Emission calculations

The spectrally integrated limb radiance is calculated for ozone and for carbon dioxide in the 9–11 μm spectral interval for tangent heights between 30 and 100 km for noon and midnight conditions using a high‐resolution, line‐by‐line radiative transfer model that incorporates nonlocal thermodynamic equilibrium (NLTE) energy level populations in the transmittance calculations and NLTE source functions. The radiances assuming local thermodynamic equilibrium (LTE) are compared to the NLTE radiances. The NLTE ozone limb radiance is equal to the LTE radiance below 55 km at noon and below 70 km at midnight. The largest difference between the LTE and NLTE spectrally integrated ozone radiances occurs near 96 km in tangent height at noon and near 93 km at midnight. The difference between radiances decreases significantly from noon to midnight and is dependent on the portion of the spectral interval that is observed. Because of the uncertainty in the chemical pumping process, limb radiances utilizing two statistical equilibrium models of ozone are compared, yielding a maximum 32% difference in the spectrally integrated radiance at a tangent height of 84 km. Overlap of ozone spectral lines from bands possessing different source functions is not significant in calculating the infrared limb emission from the mesosphere and lower thermosphere. Finally, carbon dioxide is predicted to be responsible for a significant fraction of the total limb radiance in the 9–11 μm spectral interval in the daytime upper mesosphere and lower thermosphere. The implications of NLTE radiative transfer on the remote sensing of ozone are discussed.

Infrared spectroscopic detection of sulfur hexafluoride (sf6) in the lower stratosphere and upper troposphere

Absorption by sulfur hexafluoride (SF6) has been detected in high‐resolution infrared solar spectra of the lower stratosphere and upper troposphere. The identification is based on measurements of the intense, unresolved ν3 band Q branch at 947.9 cm−1 in solar occultation spectra recorded near 31°N and 47°S latitude by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer on April 30 to May 1, 1985, during the Spacelab 3 shuttle mission. Vertical SF6 volume mixing ratio profiles have been retrieved using an onion‐peeling nonlinear least squares spectral fitting procedure with SF6 line‐by‐line Spectroscopic parameters generated recently by Bobin and coworkers from their analysis of the SF6 ν3 band. Between 12 and 18 km altitude, the region over which the retrievals are most accurate (about ±26% one sigma), the SF6 volume mixing ratio is independent of altitude with an average measured value of 1.42 parts per trillion by volume (pptv) at 31°N latitude. There is evidence for a decline in the SF6 volume mixing ratio with increasing altitude between 18 and 22 km, but the results are not conclusive because of the weakness of the absorption. Absorption by the SF6 Q branch is below the noise level of the spectra at tangent heights above ∼22 km. The measurements are compared with previously reported values and discussed in terms of the atmospheric lifetime of SF6, the long‐term trend of atmospheric SF6, and the possible role of SF6 as an atmospheric greenhouse gas.

Retrieval of atomic oxygen and temperature in the thermosphere. II—feasibility of an experiment based on limb emission in the OI lines

A viable technique for remote sensing of the atomic oxygen density and translational temperature profiles above 80 km altitude, where oxygen atoms are involved in important chemical, collisional and radiative cooling processes, would have enormous value. The two profiles can, in principle, be recovered simultaneously from limb scan data obtained near wavelength 147 μm and/or 63 μm, corresponding to the intermultiplet transitions in the ground electronic state of atomic oxygen. Part I of this paper (1989, Planet. Space Sci. 37, 1333) considered an approach in which the limb radiance data represent just one of the lines observed at high spectral resolution; recovery of the temperature relies mainly on Doppler broadening. Here, in Part II, we evaluate feasibility of the experimental concept when the data are a pair of limb radiance profiles corresponding to the two unresolved OI lines; temperature retrieval relies mostly on the difference in upper state populations. O-atom density and translational temperature profiles were retrieved from noise-contaminated synthetic data by an inversion procedure that uses both the onion peeling and global-fit methods. The latter allows solutions to be obtained despite the occurrence of a singular Jacobian, typically at 200 km tangent height. It is concluded that stable profiles for the altitude range 90–300 km can be recovered when the noise-equvalent radiances in the two channels are no greater than roughly 10 −13 W cm −2 sr −1. We estimate that this level of sensitivity could be achieved in a small space-borne cryogenic sensor based on non-scanning Fabry-Perot etalons. The spectrally-resolved approach described in Part I required a considerably larger, more advanced sensor.