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The Earth's mesopause region between about 75 and 105 km is characterised by chemiluminescent emission from various lines of different molecules and atoms. This emission was and is important for the study of the chemistry and dynamics in this altitude region at nighttime. However, our understanding is still very limited with respect to molecular emissions with low intensities and high line densities that are challenging to resolve. Based on 10 years of data from the astronomical X-shooter echelle spectrograph at Cerro Paranal in Chile, we have characterised in detail this nightglow (pseudo-)continuum in the wavelength range from 300 to 1800 nm. We studied the spectral features, derived continuum components with similar variability, calculated climatologies, studied the response to solar activity, and even estimated the effective emission heights. The results indicate that the nightglow continuum at Cerro Paranal essentially consists of only two components, which exhibit very different properties. The main structures of these components peak at 595 and 1510 nm. While the former was previously identified as the main peak of the FeO "orange arc" bands, the latter is a new discovery. Laboratory data and theory indicate that this feature and other structures between about 800 and at least 1800 nm are caused by emission from the low-lying A′′ and A′ states of HO2. In order to test this assumption, we performed runs with the Whole Atmosphere Community Climate Model (WACCM) with modified chemistry and found that the total intensity, layer profile, and variability indeed support this interpretation, where the excited HO2 radicals are mostly produced from the termolecular recombination of H and O2. The WACCM results for the continuum component that dominates at visual wavelengths show good agreement for FeO from the reaction of Fe and O3. However, the simulated total emission appears to be too low, which would require additional mechanisms where the variability is dominated by O3. A possible (but nevertheless insufficient) process could be the production of excited OFeOH by the reaction of FeOH and O3. [ABSTRACT FROM AUTHOR]

The Earth's mesopause region between about 75 and 105 km is characterised by chemiluminescent emission from various lines of different molecules and atoms. This emission was and is important for the study of the chemistry and dynamics in this altitude region at nighttime. However, our understanding is still very limited with respect to molecular emissions with low intensities and high line densities that are challenging to resolve. Based on 10 years of data from the astronomical X-shooter echelle spectrograph at Cerro Paranal in Chile, we have characterised in detail this nightglow (pseudo-)continuum in the wavelength range from 300 to 1,800 nm. We studied the spectral features, derived continuum components with similar variability, calculated climatologies, studied the response to solar activity, and even estimated the effective emission heights. The results indicate that the nightglow continuum at Cerro Paranal essentially consists of only two components, which exhibit very different properties. The main structures of these components peak at 595 and 1,510 nm. While the former was previously identified as the main peak of the FeO 'orange arc' bands, the latter is a new discovery. Laboratory data and theory indicate that this feature and other structures between about 800 and at least 1,800 nm are caused by emission from the low-lying A'' and A' states of HO2. In order to test this assumption, we performed runs with the Whole Atmosphere Community Climate Model (WACCM) with modified chemistry and found that the total intensity, layer profile, and variability indeed support this interpretation, where the excited molecules HO2 are mostly produced from the termolecular recombination of H and O2. The WACCM results for the continuum component that dominates at visual wavelengths show good agreement for FeO from the reaction of Fe and O3. However, the simulated total emission appears to be too low, which would require additional mechanisms where the variability is dominated by O3. [ABSTRACT FROM AUTHOR]

This Opinion article begins with a review of important advances in the science of the Mesosphere and Lower Thermosphere (MLT) region of the atmosphere that have occurred over the past two decades since the founding of Atmospheric Chemistry and Physics. The emphasis is on chemistry (although, of course, this cannot be decoupled from discussion of atmospheric physics and dynamics), and the primary focus is on work during the past 10 years. Topics that are covered include: observations (satellite, rocket and ground-based techniques); the variability and connectedness of the MLT on various length- and time-scales; airglow emissions; the cosmic dust input and meteoric metal layers; and noctilucent (or polar mesospheric) ice clouds. The paper then concludes with a discussion of important unanswered questions and likely future directions for the field over the next decade. [ABSTRACT FROM AUTHOR]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.03.001 Byline: Konstantinos S. Kalogerakis (a), Jochen Marschall (a), Anand U. Oza (a), Patricia A. Engel (a), Rhiannon T. Meharchand (a), Michael H. Wong (b) Keywords: Jupiter; atmosphere; Ices; IR spectroscopy; Jovian planets Abstract: We report laboratory experiments and modeling calculations investigating the effect of a hydrocarbon coating on ammonia ice spectral signatures. Observational evidence and thermochemical models indicate an abundance of ammonia ice clouds in Jupiter's atmosphere. However, spectrally identifiable ammonia ice clouds are found covering less than 1% of Jupiter's atmosphere, notably in areas of strong vertical transport, indicating a short lifetime for the signature of ammonia absorption on condensed ammonia particles [Baines, K.H., Carlson, R.W., Kamp, L.W., 2002. Icarus 159, 74-94]. Current literature has suggested coating of ammonia ice particles by a hydrocarbon haze as a possible explanation for this paradox. The work presented here supports the inference of a coating effect that can alter or suppress ammonia absorption features. In the experiments, thin films of ammonia ices are deposited in a cryogenic apparatus, coated with hydrocarbons, and characterized by reflection-absorption infrared spectroscopy. We have observed the effects on the ammonia ice absorption features near 3 and 9 [mu]m with coverage by thin layers of hydrocarbons. Modeling calculations of these multilayer thin films assist in the interpretation of the experimental results and reveal the important role of optical interference in altering the aforementioned ammonia spectral features. Mie and T-matrix scattering calculations demonstrate analogous effects for ammonia ice particles and investigate the relative effects of ammonia ice particle size, shape, and coating layer thickness on the ice particle spectral signatures. Author Affiliation: (a) Molecular Physics Laboratory, SRI International, Menlo Park, CA 94025, USA (b) Astronomy Department, University of California, Berkeley, CA 94720-3411, USA Article History: Received 19 December 2007; Revised 1 March 2008

The two-body dissociation reactions of the dication C2H2+2, initiated via double ionization of acetylene molecules by photons in the energy range 31.9-50.0 eV, have been studied by coupling photoelectron-photoion-photoion coincidence and ion imaging techniques. The angular distributions and kinetic energy of product ions, measured in the 31.9-50.0 eV energy range, exhibit significant differences for the three leading dissociation reactions with respect to a previous investigation carried out at a fixed energy of 39.0 eV, providing thus new information on the dynamical evolution of the system. The analysis of the results indicates that such dissociation reactions occur with a different mechanism. In particular, the symmetric dissociation in two CH+ ions is characterized by different dynamics, and the anisotropy of the angular distribution of ionic products increases with photon energy in a more pronounced way than the other two reactions. Moreover, the kinetic energy distribution of the symmetric dissociation reaction exhibits several components that change with photon energy. The new experimental findings cast light on the microscopic evolution of the system and can provide a laboratory reference for new theoretical calculations on specific features of the multidimensional potential energy surface, namely, the structure, energy and symmetry of dication states, the electronic state of dissociation products, energy barriers and their dependence on the geometry of the intermediate state. [ABSTRACT FROM AUTHOR]

Energy transfer from O(1D) to O2 is the main source of O2(b1∑g+) in vibrational levels v = 0 and 1 in the Earth's thermosphere. Knowledge of the relative yields for O2(b1∑g+) production in v = 0 and 1 is essential for a reliable interpretation and modeling of the O2 atmospheric band emissions (b1∑g+-X3∑g- ) from these two vibrational levels. We report laboratory measurements of the relative yields at room temperature. In the experiments, O2(b1∑g+,v=0,1) is generated by O(1D) + O2 collisions following partial photodissociation of O2 at 157.6 nm. O2(b1∑g+,v=0,1) emission detection is used to monitor the temporal evolution of the vibrational level populations. The measured fractional yield for v = 1 is 0.8±0.1, in contrast with the results of previous studies that indicated dominant O2(b1∑g+,v=0,1)production. A revision is warranted of the values used for these relative yields in atmospheric models. [ABSTRACT FROM AUTHOR]

Retrieving the total number density [OH], which is used in the chemical balance equations and is the sum of both ground and excited vibrational state populations, is a challenging problem to such a degree there exist no such estimates from recent space observations. We present a novel retrieval approach to simultaneously and self‐consistently derive both [O(3P)] and total [OH] in the nighttime mesosphere and lower thermosphere which operates with the Meinel band volume emission rates and their ratios. Its application to the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) and Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) observations shows a good agreement of the retrieved [O(3P)] taking into account the measurement uncertainties and variation of model inputs used. However, retrieved [OH] show a large discrepancy, mainly above 90 km. SCIAMACHY [OH] is generally in good agreement with WACCM [OH] after accounting for uncertainties, while SABER OH is larger than WACCM [OH] by up to a factor of 2. Plain Language Summary: The hydroxyl molecule plays an important role in the physics, chemistry, and dynamics of the middle and upper atmosphere region (60–120 km). Obtaining total hydroxyl density from its molecular emissions is a challenging problem to such a degree there exist no such estimates from the recent space observations. We present a novel approach to obtain both atomic oxygen and total hydroxyl densities in the middle and upper atmosphere from nighttime observations of the hydroxyl molecular emissions. We apply the new approach to two different space‐based instrument observations of hydroxyl emissions at multiple infrared wavelengths. The atomic oxygen densities obtained from the two instruments using our new approach are in good agreement after taking into account uncertainties, while the hydroxyl densities show a large discrepancy. Key Points: A novel approach to the simultaneous retrieval of nighttime O(3P) and OH densities from space‐based satellite observations is presentedTotal OH densities are retrieved for the first time in the mesosphere and lower thermosphere (MLT)Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY retrieved OH densities are comparable to chemical models in the MLT while Sounding of the Atmosphere using Broadband Emission Radiometry retrieved OH densities are higher [ABSTRACT FROM AUTHOR]

Laboratory measurements are reported of the rate coefficient for collisional removal of O2(X3Σg-, υ = 1) by O(3P), and the rate coefficients for removal of O2(a1Δg, υ = 1) by O2, CO2, and O(3P). A two-laser method is employed, in which the pulsed output of the first laser at 285 nm photolyzes ozone to produce oxygen atoms and O2(a1Δg, υ = 1), and the output of the second laser detects O2(a1Δg, υ = 1) via resonance-enhanced multiphoton ionization. The kinetics of O2(X3Σg-, υ = 1) + O(3P) relaxation is inferred from the temporal evolution of O2(a1Δg, υ = 1), an approach enabled by the rapid collision-induced equilibration of the O2(X3Σg-, υ = 1) and O2(a1Δg, υ = 1) populations in the system. The measured O2(X3Σg-, υ = 1) + O(3P) rate coefficient is (2.9 ± 0.6) × 10-12 cm3 s-1 at 295 K and (3.4 ± 0.6) × 10-12 cm3 s-1 at 240 K. These values are consistent with the previously reported result of (3.2 ± 1.0) × 10-12 cm3 s-1, which was obtained at 315 K using a different experimental approach [K. S. Kalogerakis, R. A. Copeland, and T. G. Slanger, J. Chem. Phys. 123, 194303 (2005)]. For removal of O2(a1Δg, υ = 1) by O(3P), the upper limits for the rate coefficient are 4 × 10-13 cm3 s-1 at 295 K and 6 × 10-13 cm3 s-1 at 240 K. The rate coefficient for removal of O2(a1Δg, υ = 1) by O2 is (5.6 ± 0.6) × 10-11 cm3 s-1 at 295 K and (5.9 ± 0.5) × 10-11 cm3 s-1 at 240 K. The O2(a1Δg, υ = 1) + CO2 rate coefficient is (1.5 ± 0.2) × 10-14 cm3 s-1 at 295 K and (1.2 ± 0.1) × 10-14 cm3 s-1 at 240 K. The implications of the measured rate coefficients for modeling of atmospheric emissions are discussed. [ABSTRACT FROM AUTHOR]

We report a laboratory measurement of the rate coefficient for the collisional removal of O2(X3Σg-,υ=1) by O(3P) atoms. In the experiments, 266-nm laser light photodissociates ozone in a mixture of molecular oxygen and ozone. The photolysis step produces vibrationally excited O2(a1Δg) that is rapidly converted to O2(X3Σg-,υ=1–3) in a near-resonant electronic energy-transfer process with ground-state O2. In parallel, a large amount of O(1D) atoms is generated that promptly relaxes to O(3P). Under the conditions of the experiments, only collisions with the photolytically produced O(3P) atoms control the lifetime of O2(X3Σg-,υ=1), because its removal by molecular oxygen at room temperature is extremely slow. Tunable 193-nm laser light monitors the temporal evolution of the O2(X3Σg-,υ=1) population by detection of laser-induced fluorescence near 360 nm. The removal rate coefficient for O2(X3Σg-,υ=1) by O(3P) atoms is (3.2±1.0)×10-12 cm3 s-1 (2σ) at a temperature of 315±15 K (2σ). This result is essential for the analysis and correct interpretation of the 6.3-μm H2O(ν2) band emission in the Earth’s mesosphere and indicates that the deactivation of O2(X 3Σg-,υ=1) by O(3P) atoms is significantly faster than the nominal values recently used in atmospheric models. [ABSTRACT FROM AUTHOR]

Vibrational relaxation of O2(X 3Σg-,υ=2,3) by O2 molecules is studied via a two-laser approach. Laser radiation at 266 nm photodissociates ozone in a mixture of molecular oxygen and ozone. The photolysis step produces vibrationally excited O2(a 1Δg) that is rapidly converted to O2(X 3Σg-,υ=2,3) in a near-resonant adiabatic electronic energy-transfer process involving collisions with ground-state O2. The output of a tunable 193-nm ArF laser monitors the temporal evolution of the O2(X 3Σg-,υ=2,3) population via laser-induced fluorescence detected near 360 nm. The rate coefficients for the vibrational relaxation of O2(X 3Σg-,υ=2,3) in collision with O2 are 2.0-0.4+0.6×10-13 cm3 s-1 and (2.6±0.4)×10-13 cm3 s-1, respectively. These rate coefficients agree well with other experimental work but are significantly larger than those produced by various semiclassical theoretical calculations. [ABSTRACT FROM AUTHOR]

The temperature dependence of the collisional removal of 02 in the v= 3 level of the b&sup1 ∑[sup +][sub g] state by N[sub 2], O[sub 2], and CO[sub 2] was investigated at room temperature and below. Measurements on the v = 2 level with the colliders, O[sub 2], N[sub 2]O, Ar, and He are also reported. For v= 3 removal by O[sub 2], the trend of sharply decreasing loss rate coefficients with increasing v is seen to continue. For example, at 200 K the rate coefficient for collisional removal of O[sub 2] from the v= 3 level by O[sub 2] is some three orders of magnitude smaller than that for the v= 1 level. We argue that the mechanism of the deactivation is electronic-electronic (E-E) energy transfer. Observation of emission from a broad range pf O[sub 2](b¹ ∑[sup +][sub g] vibrational levels in the terrestrial nightglow, recently discovered in astronomical sky spectra, show less than an order of magnitude difference in population between the more highly populated v = 3 level and the less populated v= 1 level. The implications of these two observations on the vibrational-level-specific atmospheric sources of vibrationally excited molecules in the b ¹∑[sup +][sub g] electronic state are explored. [ABSTRACT FROM AUTHOR]

Crossed-beam and beam-gas experiments on the reaction Ba+HI→BaI+H have been performed, in which the most probable collision energy ranges from 3 to 17 kcal/mol. The results, combined with previous experimental studies on this reaction system, show a remarkable collision energy dependence. Between low and high collision energies, a transition occurs in the intensity, width, and peak location of the product vibrational and rotational population distributions. The onset of this transition is estimated to occur at approximately 5 kcal/mol. For collision energies smaller than 5 kcal/mol, the product vibrational distribution is bell shaped and peaks at v=12. For collision energies larger than 5 kcal/mol, a second maximum appears at v=0 in the vibrational distribution. The rotational distributions of the crossed-beam experiments are extremely narrow but broaden at lower collision energies. As the collision energy is increased above 5 kcal/mol, the BaI rotational excitation is very near the energetic limit, and the maximum for the BaI(v=0) rotational population distribution moves from J=415.5 to J=538.5. In contrast, below the transition onset, the maximum remains unchanged around J=420.5. Moreover, the peaks of the BaI(v=1) and BaI(v=2) rotational distributions appear at successively lower J values, as expected from energy conservation arguments. The nature of the kinematic constraints for this reaction allows the determination of the opacity functions for the production of the BaI product in a specific vibrational level v. Detailed analysis of the collision energy dependence of the specific opacity functions offers insight into the role of conservation of energy and angular momentum in influencing this reaction. At low collision energies, the maximum reactive impact parameter, bmax, is determined by an angular momentum (centrifugal) barrier. At collision energies larger than 5 kcal/mol, conservation of energy dictates the value of bmax. These two processes are... [ABSTRACT FROM AUTHOR]

Emission from the OH Meinel bands is routinely used to determine rotational temperatures that are considered proxies for the kinetic temperature near the mesopause region. Previous observations determined OH rotational temperatures that show a dependence on the vibrational level, with the temperature rising overall as the OH vibrational quantum number v increases. The source of this trend is not well understood and has generally been attributed to deviations from thermodynamic equilibrium. This Technical Note demonstrates that the existence of bimodal OH rotational population distributions is an inherent feature of rotational relaxation in gases and can provide an explanation for the previously reported temperature trend. The use of only a few lines from rotational transitions involving low rotational quantum numbers to determine rotational temperatures does not account for the bimodality of the OH rotational population distributions and leads to systematic errors overestimating the OH rotational temperature. This Note presents selected examples, discusses the relevant implications, and considers strategies that could lead to more reliable OH rotational temperature determination. [ABSTRACT FROM AUTHOR]

Abstract: The recently discovered fast, multiquantum OH(v)+O(3P) vibrational‐to‐electronic relaxation mechanism provided new insight into the OH(v) Meinel band nighttime emission formation. Using a new detailed OH(v) model and novel retrieval algorithm, we obtained O(3P) densities in the nighttime mesosphere and lower thermosphere (MLT) from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) 2.0‐ and 1.6‐μm radiances. We demonstrate how critical the new OH(v) relaxation mechanism is in the estimation of the abundance of O(3P) in the nighttime MLT. Furthermore, the inclusion of this mechanism enables us to reconcile historically large discrepancies with O(3P) results in the MLT obtained with different physical models and retrieval techniques from WIND Imaging Interferometer, Optical Spectrograph and Infrared Imager System, and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography observations of other airglow emissions. Whereas previous SABER O(3P) densities were up to 60% higher compared to other measurements the new retrievals agree with them within the range (±25%) of retrieval uncertainties. We also elaborate on the implications of this outcome for the aeronomy and energy budget of the MLT region. [ABSTRACT FROM AUTHOR]

In the 1970s, the mechanism of vibrational energy transfer from chemically produced OH(ν) in the nighttime mesosphere to the CO2(ν3) vibration, OH(ν)⇒N2(ν)⇒ CO2(ν3), was proposed. In later studies it was shown that this "direct" mechanism for simulated nighttime 4.3 μm emissions of the mesosphere is not sufficient to explain space observations. In order to better simulate these observations, an additional enhancement is needed that would be equivalent to the production of 2.8-3N2(1) molecules instead of one N2(1) molecule in each quenching reaction of OH(ν)+N2(0). Recently a new "indirect" channel of the OH(ν) energy transfer to N2(ν) vibrations, OH(ν)⇒O(1D)⇒N2(ν), was suggested and then confirmed in a laboratory experiment, where its rate for OH(ν = 9)+O(3P) was measured. We studied in detail the impact of the "direct" and "indirect" mechanisms on CO2(ν3) and OH(ν) vibrational level populations and emissions. We also compared our calculations with (a) the SABER/TIMED nighttime 4.3 μm CO2 and OH 1.6 and 2.0 μm limb radiances of the mesosphere-lower thermosphere (MLT) and (b) with ground- and space-based observations of OH(ν) densities in the nighttime mesosphere. We found that the new "indirect" channel provides a strong enhancement of the 4.3 μm CO2 emission, which is comparable to that obtained with the "direct" mechanism alone but assuming an efficiency that is 3 times higher. The model based on the "indirect" channel also produces OH(ν) density distributions which are in good agreement with both SABER limb OH emission observations and ground and space measurements. This is, however, not true for the model which relies on the "direct" mechanism alone. This discrepancy is caused by the lack of an efficient redistribution of the OH(ν) energy from higher vibrational levels emitting at 2.0 μm to lower levels emitting at 1.6 μm. In contrast, the new "indirect" mechanism efficiently removes at least five quanta in each OH(ν ⩾ 5)+O(3P) collision and provides the OH(ν) distributions which agree with both SABER limb OH emission observations and ground- and space-based OH(ν) density measurements. This analysis suggests that the important mechanism of the OH(ν) vibrational energy relaxation in the nighttime MLT, which was missing in the emission models of this atmospheric layer, has been finally identified. [ABSTRACT FROM AUTHOR]

Since 2002, SABER (Sounding of the Atmosphere using Broadband Emission Radiometry)/TIMED (Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics) has been continuously measuring the day- and nighttime infrared limb radiances of the mesosphere and lower thermoshere (MLT) in ten broadband channels. Recently, the MLT daytime temperature/pressure and CO2 densities have been obtained self-consistently from SABER 15 μm and 4.3 μm emission observations. However, similar nighttime data remain unprocessed due to a lack of understanding of the 4.3 μm emission generating mechanisms. A previous study suggested the "direct" transfer OH(v)) ↑ N2(v)) ↑ CO2(v3)) ↑ 4.3 μm of vibrational excitation from OH(v) to CO2 in the nighttime mesosphere. However, accounting for this excitation mechanism alone leads to significant under-prediction (by up to 80 %) of observed 4.3 μm limb radiances. Recently, theoretical and laboratory studies have suggested an additional "indirect" nighttime channel OH(v)) ↑ O(¹D)) ↑ N2(v)) ↑ CO2(v3)) ↑ 4.3 μm of this energy transfer. We implemented this new channel in our non-LTE (non-Local Thermodynamic Equilibrium) model and show that, for various latitudinal and seasonal scenarios, including this additional channel brings differences between simulated and measured nighttime SABER 4.3 μm limb radiances to (-20, +30) %. These results confirm the important role of the new mechanism as a source of the nighttime 4.3 μm emission. This finding creates new opportunities for the application of CO2 4.3 μm observations in the study of the energetics and dynamics of the nighttime MLT. [ABSTRACT FROM AUTHOR]