SA31A-1597
Temperature and Compositional Responses in the Middle Atmosphere to Solar Irradiance Variation: Comparisons Between TIMED/SABER Measurements and Model Simulations
In this paper we investigate the thermal and photochemical responses of the middle atmosphere to solar irradiance variation in a two-dimensional model that fully couples radiation, dynamics, and photochemistry in the middle atmosphere. The daily solar irradiance measured by the TIMED/SEE instrument is parameterized and updates the model input in the short wavelength range used to radiatively and photochemically drive the model in a set of seven-year numerical simulations. The model outputs of the temperature and Ox fields are compared with TIMED/SABER measurements in the same period. We find qualitative agreements of the responses between the model and measurements. Statistical and sensitivity analyses to the model outputs differentiate the relative importance of the driving forces that cause the temperature and Ox variations due to radiative heating, chemical heating, and dynamical transport.
SA31A-1598
EUV Emission from Electron Impact of Molecular Oxygen
Electron impact on molecular oxygen has been discussed by McConkey et al.
[doi:10.1026/j.physrep.2008.05.001] in their recent review of dissociation processes in oxygen containing
molecules. On Europa and Ganymede, oxygen emissions have been shown to be primarily due to
dissociative electron impact excitation of O2. Though the majority of the 83.4 nm emission from Earth's
airglow in the lower ionosphere originates from solar EUV photoionization-excitation of atomic oxygen, with
some from electron impact, complete analysis of auroral emissions (at lower altitudes) should involve some
e- + O2 contribution. The magnitudes of the previous O II (83.4 nm) emission cross sections
resulting from e- +O2 were in poor agreement with a factor of about 5 between the largest and
smallest values at 200 eV. This significant disparity in cross sections prompted the present investigation. In
summary, very good overlap was demonstrated between the present O II (83.4 nm) absolute emission cross
section data with those of Ajello and Franklin [1985] and Zipf et al. [1985]. Thus both the shape and the
magnitude of the emission cross section can be considered as well-established in the lower-energy region
below approximately 300 eV.
Acknowledgement: This work was carried out at JPL, Caltech, under contract with NASA with support from
the NASA PATM program.
SA31A-1599
Generation of Emission From the v = 0 levels of the O2 Herzberg States Following the Reaction Between O(1D) and Ozone
The reaction between O(1D) and ozone is highly exothermic, with a variety of possible pathways, and includes all states of O2 lying below the first dissociation limit at 5.1 eV. The three Herzberg states at 4 - 5 eV and even the 5Πg state - bound by only ~0.1 eV - are all accessible. Our interest has been to generate the lowest vibrational level of the lowest Herzberg state, O2(c1Σu-), because O2(c-X) v'=0 emission is a major nightglow feature in the visible spectral region of Venus and presumably of Mars. Although the c1Σu-(v = 0) level is known to be produced by O- atom recombination in a flowing afterglow [Lawrence, et al., 1977; Slanger, 1978; Kenner and Ogryzlo, 1983], it has not previously been generated in a pulsed experiment, where kinetics can be most easily determined. We find that photodissociation of ozone at 248 nm rapidly generates the v=0 levels of all three O2 Herzberg states - c1Σu-, A3Σu+, and A'3Δu - as observed in the Herzberg I/II/III and Chamberlain bands. These levels appear as the O(1D) disappears, each exhibiting different time histories. From earlier work, we established that the O(1D) + O3 reaction generates high vibrational levels of the O2 states [Slanger and Copeland, 2003], but here we see the lowest level. The O2(b - X) Atmospheric bands appear over a yet longer time scale. Emission from the O(1D - 3P) red lines is a prominent feature in these experiments, although the radiating efficiency is only about 10-7. This work has been supported by Grant No. NNG05GO77G from the NASA Planetary Atmospheres program. E. Byler participated as an NSF REU (Research Experience for Undergraduates) student. G.M. Lawrence, et al. Science., 195, 573, 1977. T.G. Slanger, J. Chem. Phys., 69, 4779, 1978. R.D. Kenner and E.A. Ogryzlo, Can. J. Chem., 61, 921, 1983. T.G. Slanger and R.A. Copeland, Chem. Rev. 103, 4731, 2003.
SA31A-1600
Vibrational Relaxation of OH(υ = 1, 2) by O, O2, and CO2
The hydroxyl radical is a key reactant in the energy budget of the terrestrial atmospheres. In the Earth's upper atmosphere the vibrationally excited OH radicals (υ ≤ 9) are formed by the H + O3 reaction. The non-thermal vibrational energy is either emitted as an infrared or visible photon, or converted into translational or internal energy via collisions with ambient gases, particularly O, N2, and O2. Recently OH(υ = 2 and 1) emission has been observed by Piccioni et al. in the atmosphere of Venus,1 and the magnitude of the emission is controlled by the competition between radiative decay and vibrational relaxation by the most abundant constituent, CO2. Considerable disagreement exists among the OH(υ) removal rate constants measured by different laboratories for OH(υ = 2 and 1), and data at lower temperatures of atmospheric relevance are lacking, especially for υ = 2. Given the importance of these rate constants for understanding of behavior in atmospheric OH on both Earth and Venus, we applied a two-laser approach to determine the rate constants for vibrational relaxation of OH(υ = 1, 2) by O-atoms, O2, and CO2. The product pathways for relaxation of OH(υ = 2) were also examined. In the experiments, ozone is almost completely photolysed at 248 nm and most of the resulting O(1D) atoms are quenched to O(3P) by collisions with N2 and CO2. A small fraction of O(1D) reacts with H2O, forming OH with up to two vibrational quanta. The temporal evolutions of OH(υ = 1, 2) are measured using laser induced fluorescence; and kinetic simulations are used to extract the rate constants and the product pathways. Experiments were performed at temperatures between 240 and 295 K. The experimental results and their relevance for current atmospheric modeling calculations will be discussed. This work was supported by the NASA Geospace Science and Planetary Atmospheres Programs. The participation of H. Timmers was supported by an NSF Research Experiences for Undergraduates (REU) Program. Reference List (1.) Piccioni, G.; Drossart, P.; Zasova, L.; Migliorini, A.; Gerard, J.-C.; Mills, F. P.; Shakun, A.; Garcia Munoz, A.; Ignatiev, N.; Grassi, D.; Cottini, V.; Taylor, F. W.; Erard, S.; VIRTIS-Venus Express Technical Team Astronomy and Astrophysics 2008, 483, L29-L33.
SA31A-1601
Vibrational Relaxation of Ground-State Oxygen Molecules With Atomic Oxygen and Carbon Dioxide
Vertical water vapor profiles are key to understanding the composition and energy budget in the mesosphere and lower thermosphere (MLT). The SABER instrument onboard NASA's TIMED satellite measures such profiles by detecting H2O(ν2) emission in the 6.8 μm region. Collisional deactivation of vibrationally excited O2, O2(X3Σ-g, υ = 1) + H2O ↔ O2(X3Σ-g, υ = 0) + H2O(ν2), is an important source of H2O(ν2). A recent study has identified two other processes involving excited O2 that control H2O(ν2) population in the MLT: (1) the vibrational-translational (V-T) relaxation of O2(X3Σ-g, υ = 1) level by atomic oxygen and (2) the V-V exchange between CO2 and excited O2 molecules [1]. Over the past few years SRI researchers have measured the atomic oxygen removal process mentioned above at room temperature [2] and 240 K [3]. These measurements have been incorporated into the models for H2O(ν2) emission [1]. Here we report laboratory studies of the collisional removal of O2(X3Σ-g, υ = 1) by O(3P) at room temperature and below, reaching temperatures relevant to mesopause and polar summer MLT (~150 K). Instead of directly detecting the O2(X3Σ-g, υ = 1) population, a technically simpler approach is used in which the υ = 1 level of the O2(a1Δg) state is monitored. A two-laser method is employed, in which the pulsed output of the first laser near 285 nm photodissociates ozone to produce atomic oxygen and O2(a1Δg, υ = 1), and the pulsed output of the second laser detects O2(a1Δg, υ = 1) via resonance-enhanced multiphoton ionization. With ground-state O2 present, owing to the rapid equilibration of the O2(X3Σ-g, υ = 1) and O2(a1Δg, υ = 1) populations via the processes O2(a1Δg, υ = 1) + O2(X3Σ-g, υ = 0) ↔ O2(a1Δg, υ = 0) + O2(X3Σ-g, υ = 1), the information on the O2(X3Σ-g, υ = 1) kinetics is extracted from the O2(a1Δg, υ = 1) temporal evolution. In addition, measurements of the removal of O2(X3Σ-g, υ = 1) by CO2 at room temperature will also be presented. This work is supported by the Johns Hopkins University, Applied Physics Laboratory, under grant 939991 (under NASA grant NAG5-13002). [1] Feofilov, A., Kutepov, A. A., García-Comas, M., López-Puertas, M., Marshall, B. T., Gordley, L. L., Manuilova, R. O., Yankovsky, V. A., Pesnell, W. D., Goldberg, R. A., Petelina, S. V., and Russell III., J. M. 'SABER/TIMED Observations of Water Vapor in the Mesosphere: Retrieval Methodology and First Results'. Submitted to J. of Atmos. and Terrest. Phys., (2008). [2] Kalogerakis, K. S., Copeland, R. A., and Slanger, T. G., J. of Chem. Phys., 123, 194303, (2005). [3] Pejakovic, D. A., Campbell, Z., Kalogerakis, K. S., Copeland, R. A., and Slanger, T. G., Eos. Trans. AGU 85(47), Fall Meet. Suppl., Abstract SA41A-1032, (2004).
SA31A-1602
Sound Absorption in Molecular Gas Mixtures: Master Equation for Rotational and Vibrational Excitation, Relaxation, and Energy Transfer
Laboratory sound absorption measurements provide much of what we know about the vibrational kinetics of air mixtures, forming the core basis for retrieving the altitude profile of water in the mesosphere from infrared emissions between 6.3 and 6.9 μm. Here we show that sound-absorption and laser-excitation experiments follow exactly the same kinetics, reflect the same underlying reaction rates, and can be vulnerable to similar ambiguities. This has not been obvious because the literature lacks a consistent prescription for calculating the sound absorption frequency spectrum from the reaction rate coefficients. We have developed the first general theoretical formalism for calculating the absolute magnitude of sound absorption per-unit-length, versus sound frequency, for any number of collisional excitation, relaxation, and energy transfer processes, for any mixture of atomic and molecular gases. This new formalism, and the computer code that implements it, provide the first systematic means for inferring collisional rate coefficients from sound absorption measurements in which more than one rotational or vibrational mode is active, such as N2/O2/H2O/CO2 gas mixtures in the laboratory and the atmosphere. When a sound wave travels through a gas, the alternating compression and expansion cycles heat and cool the gas. If the acoustic frequency roughly matches the rate of vibrational relaxation, then the effective vibrational temperature lags behind the translational temperature and the energy in the sound wave is attenuated. The measured frequency of maximum absorption is proportional to the vibrational relaxation rate. In the simplest laser-based experiment, we excite a single molecular level and record its exponential time decay, with the vibrational relaxation rate being proportional to the decay frequency. In both experiments we derive the relaxation rate coefficient from the linear graph versus gas pressure. The technical problem is that any mixture of molecular gases will have more than one relaxation time constant. Thus we write the chemical kinetics master equation as (1) (d/dt) Nm = Σnpq [ - kmn→pq Nm Nn + kpq→mn Np Nq ] which has the well-known time-dependent solution given by (2) Nm(t) = Σn Cmn exp(-λn t) where the λn values are the decay frequencies and the Cmn coefficients depend on how the gas was initially excited. What we have contributed is the frequency-dependent sound absorption solution to Equation (1): (3) cvint(ω) = Σnk Wn / (1 + i ω/λn) where cvint(ω) is the complex heat capacity (per molecule), ω is the circular sound frequency, 2πf, the λn are the calculated decay frequencies [as in Equation (2)] and k Wn is the real effective heat capacity for decay mode n. As pointed out by Landau and Teller [Phys. Z. Sowjet. 10, 34-43 (1936)], for a simple case when the decay modes correspond to vibrational modes, Wn is the ordinary heat capacity of the vibrational mode. In the more complicated case involving one or more reversible energy-transfer steps, e.g., water and oxygen, the vibrational modes and the decay modes do not correspond to each other, and we need to use the rate coefficients in Equation (1) to calculate both λn and Wn.
SA31A-1603
Transfer of Vibrational Energy From O2 to Symmetric Stretch of CO2
Models of 15 micron emission, an important cooling mechanism in the lower thermosphere, require a rate coefficient for the excitation of the bending mode (ν2) of CO2 by atomic oxygen that is 2-3 times larger than that measured in the laboratory. This has motivated us to look for other mechanisms for energy deposition in the ν2 mode. Since the bending mode is strongly coupled to the symmetric stretch mode (ν1) due to Fermi resonance the deposition of energy in the latter mode is equivalent to deposition in the former mode. There are several mechanisms for production of vibrationally excited O¬2 in the lower thermosphere. In this talk we present a calculation of the rate of energy transfer from O2 (band origin at 1556 cm-1) to the symmetric stretch (ν1) mode of CO2 (band origin at 1388 cm-1) and discuss its feasibility as a cooling mechanism.
SA31A-1604
Photolysis and Fluorescence in the /delta and /epsilon Bands of Thermospheric NO
Recent measurements of the oscillator strengths (Yoshino et al, 2006) and predissociation rates (Luque and Crosley, 2000) for the δ and ε band systems of nitric oxide (NO) suggest a reevaluation of the NO photolysis rate. It is well-known that the dominant contribution throughout the atmosphere is due to dissociation in the δ(0,0) and δ (1,0) bands. However, above 90 km, attenuation of the solar VUV irradiance due to the O2 Schumann-Runge system is diminished and the contribution of the δ(2,0), ε(1,0), and ε(2,0) bands to the photolysis rate becomes increasingly significant. In this talk it is shown that the contribution from these bands rises from 16% at 100km to 26% above 120 km. As the ratio of the radiative to predissociation rates for the δ(0,0), ε(0,0), ε(1,0), and ε(2,0) bands is sensitive to the rotational level, expected fluorescence from these bands is also presented using data from the Student Nitric Oxide Explorer (SNOE).
SA31A-1605
Predissociation in the Coupled c4'(0) and b'(1) states of N2 – Rotational-Resolved Fluorescence Spectrometry
An ultrahigh resolution (0.0014 nm) spectrometer has been utilized to study the extreme ultraviolet resonance fluorescence of the (0,v") bands of the c4'-X and the (1,v") bands of the b'-X transitions of N2 in the vicinity of 95.8 nm. The new experimental apparatus allows us to obtain rotationally resolved resonance fluorescence excitation spectra using an ultra-bright undulator synchrotron radiation source. We have found that the c4'(0) state predominantly decays through resonance fluorescence to the ground state while the b'(1) state prefers the b'(1)-a branching transition and the subsequent cascade a-X (LBH) emission. The integrated fluorescence intensities of the c4'-X (0,0) band become saturated at N2 pressures higher than ~0.16 mtorr. Multiple scattering processes apparently cause significant reduction in the c4'-X (0,0) emission rates while significantly enhances the emissions of c4'-X (0,v") with v"=0-2 at ~2 mtorr. Strong perturbation and predissociation effects between the coupled states have been confirmed. The detailed results will be presented. The present results may be useful in the explanation of the important N2 features in dayglow of the Earth observed by the FUSE. We also plan to carry out the experiment at low temperatures to provide data that can be directly applicable to the characterization of the N2 emissions in the atmospheres of Titan and Pluto.
SA31A-1606
Photodissociation rates and middle atmosphere photochemistry modeling
Several different parameterizations of photodissociation based on different datasets of solar irradiance have been used in modeling the photochemistry of the mesosphere and lower thermosphere and its response to solar activity. TIMED/SEE provides irradiance data critical for our understanding of middle atmosphere photochemistry over the entire declining phase of a solar cycle. In this paper we use the JHU/APL two- dimensional photochemistry and transport model to examine the sensitivity of modeled MLT composition (e.g., Ox and HOx) to the rates of O2 and H2O photodissociation by Lyman-alpha and the Schumann--Runge bands and continuum commonly used today and those based on TIMED/SEE measurements. The effects of solar cycle variability are also reported.
SA31A-1607
Calibration of Direct O Measurements by Simultaneous O2 Airglow Measurements
Accurate knowledge about the distribution of atomic oxygen is crucial for many studies of the mesosphere and lower thermosphere. Direct measurements of atomic oxygen by the resonance fluorescence technique at 130 nm have been made from several sounding rocket payloads in the past. This measurement technique yields atomic oxygen profiles with good sensitivity and altitude resolution. However, accuracy is a problem as calibration and aerodynamics make the quantitative analysis challenging. In general, accuracies better than a factor 2 are not to be expected from direct atomic oxygen measurements. As an example, we present results from the NLTE sounding rocket campaign at Esrange, Sweden, in 1998, with simultaneous O2 airglow and O resonance fluorescence measurements. O number densities are found to be consistent with the nightglow analysis, but only within the uncertainty limits of the resonance fluorescence technique. Based on these results, we here describe how better atomic oxygen number densities can be obtained by calibrating direct techniques with complementary airglow photometer measurements and detailed aerodynamic analysis. Night-time direct O measurements can be complemented by photometric detection of the O2 (1Σ) Atmospheric Band at 762 nm, while during daytime the O2 (1Δ) Infrared Atmospheric Band at 1.27 μm can be used. The combination of a photometer and a rather simple resonance fluorescence probe can provide atomic oxygen profiles with both good accuracy and good height resolution.
SA31A-1608
Theory and Applications of a Faraday Filter-Based Spectrometer to Measure Sodium Nightglow D2/D1 Intensity Ratios
The Chapman mechanism (1939) offers the accepted chemical pathway for the production of excited states of mesospheric sodium, leading to nightglow at two wavelengths: D2 (589.158 nm) and D1 (589.756 nm). While the Chapman mechanism leaves open the possibility that the intensity ratio of the two transitions may vary due to the chemical reaction involving atomic oxygen, early observations by Sipler and Biondi (1978) yielded the value of two within experimental error. Recent work by Slanger et al. (2005), however, showed that not only does the intensity ratio vary, but its value is related to the concentration ratio of atomic oxygen [O] to molecular oxygen [O2]. They proposed a modification of the Chapman mechanism involving two competing chemical pathways for sodium production to account for the observed variation. This paper will describe our compact, Faraday filter-based spectrometer to measure the D2/D1 intensity ratio of the sodium nightglow from the upper mesosphere. The novelty of this method also permits determination of the fractional contributions of the two chemical pathways to test the validity of the modified Chapman mechanism for Na chemistry, as well as to infer information about [O]/[O2]. Since the delineation between the two chemical pathways requires a spectral resolution of 0.0002 nm, this is not possible with any other existing instrument. With this spectrometer deployed at the Colorado State University sodium lidar facility (41°N, 105°W), we expect to be able to measure short-term variations of the sodium nightglow intensity ratio and the chemical pathway fraction, from which [O]/[O2] can be inferred. These observations may yield new insights into mesospheric chemistry, especially for atomic and molecular oxygen, which play a key role in upper atmospheric chemistry and dynamics.