Production of vibrationally excited N2 by electron impact

Energy transfer from electrons to neutral gases and ions is one of the dominant electron cooling processes in the ionosphere, and the role of vibrationally excited N₂ in this is particularly significant. We report here the results from a new calculation of electron energy transfer rates (Q) for vibrational excitation of N₂, as a function of the electron temperature T_e. The present study was motivated by the development of a new cross-section compilation for vibrational excitation processes in N₂ which supercedes those used in the earlier calculations of the electron energy transfer rates. We show that the energy dependence and magnitude of these cross sections, particularly in the region of the well-known resonance in N₂, significantly affect the calculated values of Q. A detailed comparison between the current and previous calculated electron energy transfer rates is made and coefficients are provided so that these rates for transitions from level 0 to levels 1-10 can be calculated for electron temperatures less than .     

Venus night airglow: Ground-based detection of OH, observations of O2 emissions, and photochemical model

Venus nightglow was observed at NASA IRTF using a high-resolution long-slit spectrograph CSHELL at LT = 21:30 and 4:00 on Venus. Variations of the O₂ airglow at 1.27 μm and its rotational temperature are extracted from the observed spectra. The mean O₂ nightglow is 0.57 MR at 21:30 at 35°S-35°N, and the temperature increases from 171 K near the equator to ∼200 K at ±35°. We have found a narrow window that covers the OH (1-0) P1(4.5) and (2-1) Q1(1.5) airglow lines. The detected line intensities are converted into the (1-0) and (2-1) band intensities of 7.2 ± 1.8 kR and <1.4 kR at 21:30 and 15.5 ± 2 kR and 4.7 ± 1 kR at 4:00. The f-component of the (1-0) P1(4.5) line has not been detected in either observation, possibly because of resonance quenching in CO₂. The observed Earth’s OH (1-0) and (2-1) bands were 400 and 90 kR at 19:30 and 250 and 65 kR at 9:40, respectively. A photochemical model for the nighttime atmosphere at 80-130 km has been made. The model involves 61 reactions of 24 species, including odd hydrogen and chlorine chemistries, with fluxes of O, N, and H at 130 km as input parameters. To fit the OH vibrational distribution observed by VEX, quenching of OH (v > 3) in CO₂ only to v ? 2 is assumed. According to the model, the nightside-mean O₂ emission of 0.52 MR from the VEX and our observations requires an O flux of 2.9 × 10¹² cm⁻² s⁻¹ which is 45% of the dayside production above 80 km. This makes questionable the nightside-mean O₂ intensities of ∼1 MR from some observations. Bright nightglow patches are not ruled out; however, the mean nightglow is ∼0.5 MR as observed by VEX and supported by the model. The NO nightglow of 425 R needs an N flux of 1.2 × 10⁹ cm⁻² s⁻¹, which is close to that from VTGCM at solar minimum. However, the dayside supply of N at solar maximum is half that required to explain the NO nightglow in the PV observations. The limited data on the OH nightglow variations from the VEX and our observations are in reasonable agreement with the model. The calculated intensities and peak altitudes of the O₂, NO, and OH nightglow agree with the observations. Relationships for the nightglow intensities as functions of the O, N, and H fluxes are derived.     

Dayglow emissions of the O2 Herzberg bands and the Rayleigh backscattered spectrum of the earth

Numerous fluorescent emissions from the Herzberg bands of molecular oxygen lie in the spectral region 242–300 nm. This coincides with the wavelength range used by orbiting spectrometers which observe the Rayleigh backscattered spectrum of the earth for the purpose of monitoring the vertical distribution of stratospheric ozone. Model calculations indicate that Herzberg band emissions in the dayglow could provide significant contamination of the ozone measurements if the quenching rate of O₂(A³Σ) is sufficiently small. This is especially true near 255 nm, where the most intense fluorescent emissions relative to the Rayleigh scattered signal are located and where past satellite measurements show a persistent excess radiance above that expected for a pure ozone absorbing and molecular scattering atmosphere. However, very small quenching rates are adequate to reduce the dayglow emission to negligible levels. Available laboratory data have not definitely established the quenching on the rate of O₂(A³Σ) as a function of vibrational level, and such information is required before the Herzberg band contributions can be evaluated with confidence.     

Radiation chemistry of H2O + O2 ices

The chemistry and spectroscopy of proton-irradiated H₂O + O₂ ices have been investigated in relation to the production of oxidants in icy satellite surfaces. Hydrogen peroxide (H₂O₂), ozone (O₃), and the hydroperoxy (HO₂) and hydrogen trioxide (HO₃) radicals have all been observed, and their temperature and dose dependent production trends have been measured. We find that O₂ aggregates form during the growth of H₂O + O₂ ice films, and the presence of these aggregates greatly affects the HO₂ and H₂O₂ yields. In addition, we have found that the position of the spectral maximum of the ν₃ vibration of O₃ shifts with ice composition, giving an indication of the degree of dispersion of O₃ molecules within the ice. We discuss the relevance of these measurements to icy satellite surfaces.     

The rotational excitation of H2 by H2

We present rate coefficients for rotational transitions induced in collisions between H₂ molecules. Rotational levels J  ≤ 8 and kinetic temperatures T  ≤ 1000 K are considered. The interaction potential computed by Schwenke has been used, together with the quantal coupled channels method of calculating the cross-sections. Comparison is made with the more recent of previous results.     

Airglow on Mars: Some model expectations for the OH Meinel bands and the O2 IR atmospheric band

This work presents model calculations of the diurnal airglow emissions from the OH Meinel bands and the O₂ IR atmospheric band in the neutral atmosphere of Mars. A time-dependent photochemical model of the lower atmosphere below 80 km has been developed for this purpose. Special emphasis is placed on the nightglow emissions because of their potential to characterize the atomic oxygen profile in the 50-80 km region. Unlike on Earth, the OH Meinel emission rates are very sensitive to the details of the vibrational relaxation pathway. In the sudden death and collisional cascade limits, the maximum OH Meinel column intensities for emissions originating from a fixed upper vibrational level are calculated to be about 300 R, for transitions v^′=9→v^′?8, and 15,000 R, for transitions v^′=1→v^′=0, respectively. During the daytime the 1.27 μm emission from O₂(), primarily formed from ozone photodissociation, is of the order of MegaRayleighs (MR). Due to the long radiative lifetime of O₂(), a luminescent remnant of the dayglow extends to the dark side for about two hours. At night, excited molecular oxygen is expected to be produced through the three body reaction O + O + CO₂. The column emission of this nighttime component of the airglow is estimated to amount to 25 kR. Both nightglow emissions, from the OH Meinel bands and the O₂ IR atmospheric band, overlap in the 50-80 km region. Photodissociation of CO₂ in the upper atmosphere and the subsequent transport of the atomic oxygen produced to the emitting layer are revealed as key factors in the nightglow emissions from these systems. The Mars 5 upper constraint for the product [H][O₃] is revised on the basis of more recent values for the emission probabilities and collisional deactivation coefficients.     

The excitation of O2(b1Σg+) in the nightglow

It is proposed that the available measurements of the O₂(b¹Σ_g⁺ ?X³Σ_g^?) atmospheric bands both in the nightglow and in the laboratory indicate that the excitation mechanism is a two-step process rather than the direct three body recombination of atomic oxygen. It is shown that such a two-step mechanism can explain observations of the atmospheric bands both in altitude and intensity.     

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 spatial morphology of Europa's near-surface O2 atmosphere

Results from a three-dimensional ballistic model of Europa's O₂ atmosphere are presented. Hubble Space Telescope (HST) ultraviolet observations show spatially non-uniform O₂ airglow from Europa. One explanation for this is that the O₂ atmosphere is spatially non-uniform. We show that non-uniform ejection of O₂ alone cannot reproduce the required morphology, but that a non-uniform distribution of reactive species in Europa's porous regolith can result in a non-uniform O₂ atmosphere. By allowing O₂ molecules to react with Europa's visibly dark surface material, we produced a spatially non-uniform atmosphere which, assuming uniform electron excitation of O₂ over the trailing hemisphere, compares favorably with the morphology suggested by the HST observations. This model, which requires a larger source of O₂ than has previously been estimated, can in principal be tested by the New Horizons observations of Europa's O₂ atmosphere.     

A measurement of the O2 ultra-violet nightglow emission

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

Collisional excitation of metastable oxygen O(D) atoms through the B∑u channel of O2

Pectroscopic data on the shifts and widths of the energy levels of molecular oxygen have been used in the empirical construction of a diabatic potential matrix that characterizes the interactions of the B³∑⁻_u state with the ⁵Π_u, 2³∑⁺_u, ³Π_u and ¹Π_u states. The diabatic potential matrix is u theory formulation to calculate the cross-sections for the excitation of O(¹D) atoms in collisions of two O(³P) atoms. Total cross-sections are obtained by adding the excitation from the ³Π_g, channel. The rate coefficient for quenching of O(¹D) by O(³P) is evaluated as a function of temperature. The values conflict with a recent analysis of the emission of the oxygen red line in the upper atmosphere.     

ETON 6: A rocket measurement of the O2 Infrared Atmospheric (0-0) band in the nightglow

Co-ordinated rocket measurements of the O₂(a¹Δ_g−X³Σ_g⁻) Infrared Atmospheric (0-0) band emission profile and the atomic oxygen densities in an undisturbed night-time atmosphere are used to investigate the processes responsible for the excitation of O₂(a¹Δ_g) in the terrestrial nightglow. It is shown that three-body recombination of atomic oxygen, and subsequent energy transfer processes, can explain only part of the observed emission profile and that at least two other sources of O₂(a¹Δ_g) emission must exist. One of these additional sources, responsible for most of the emission observed below 90km, is identified as arising from the night-time residual of the very large dayglow ¹Δ_g population. The other additional source is required to explain most of the emission observed above 95km. The processes responsible for this high altitude component cannot be identified but the vertical distribution of the required source function strongly resembles the profile of the atomic oxygen density squared and suggests that a two-body radiative recombination process may be involved. However, the measured zenith emission rates can also be explained without the high altitude source of O₂(a¹Δ_g) if optical emission at 1.27 μm was induced by the rocket as it penetrated the nightglow layer.     

Mesospheric O3, H and H2O at high latitudes: A theoretical model

The high latitude thermosphere is characterized by a large heat input which produces a strong wind field. In a previous work, the modification of the vertical transport mechanisms produced by high latitude horizontal winds was studied and the resulting concentration profiles are used here to study the influence at mesospheric levels. We obtain an improved agreement with experimental measurements. High solar zenithal angles could explain other differences between the high and middle latitude's mesosphere, especially below 80 km, approximately.     

Dissociative excitation of SO2 by controlled electron impact

The fragmentation of SO₂ following dissociative electron impact excitation has been studied under single collision conditions for incident electron energies up to 500 eV. The emission spectrum in the far v.u.v. spectral range (450–1100Å) shows many features arising from excited neutral oxygen and ionized oxygen and sulphur fragments. Absolute emission cross sections have been measured for the most intense lines and the maximum values were found to range from 1–12 × 10^?19 cm² with an uncertainty of approx. ± 35%. Dissociation mechanisms are discussed and in some cases the dissociation path could be uniquely identified. The striking differences between the v.u.v. emission spectrum produced by single step dissociation of SO₂ and the spectra emitted by the plasma torus around Jupiter are discussed.     

The H2O and CO2 adsorption properties of phyllosilicate-poor palagonitic dust and smectites under martian environmental conditions

The recent detection of up to ∼10 wt% water-equivalent H heterogeneously distributed in the upper meter of the equatorial regions of the martian surface and the presence of the 3-μm hydrations feature across the entire planet raises the question whether martian surficial dust can account for this water-equivalent H. We have investigated the H₂O and CO₂ adsorption properties of palagonitic dust (<5 μm size fraction of phyllosilicate-poor palagonitic tephra HWMK919) as a martian dust analog and two smectites under simulated martian equatorial surface conditions. Our results show that the palagonitic dust, which contains hydrated and hydroxylated volcanic glass of basaltic composition, accommodates significantly more H₂O under comparable humidity and temperature conditions than do the smectites nontronite and montmorillonite.     

Production, ionization and redistribution of O2 in Saturn's ring atmosphere

Molecular oxygen produced by the decomposition of icy surfaces is ubiquitous in Saturn's magnetosphere. A model is described for the toroidal O₂ atmosphere indicated by the detection of and O⁺ over the main rings. The O₂ ring atmosphere is produced primarily by UV photon-induced decomposition of ice on the sunlit side of the ring. Because O₂ has a long lifetime and interacts frequently with the ring particles, equivalent columns of O₂ exist above and below the ring plane with the scale height determined by the local ring temperature. Energetic particles also decompose ice, but estimates of their contribution over the main rings appear to be very low. In steady state, the O₂ column density over the rings also depends on the relative efficiency of hydrogen to oxygen loss from the ring/atmosphere system with oxygen being recycled on the grain surfaces. Unlike the neutral density, the ion densities can differ on the sunlit and shaded sides due to differences in the ionization rate, the quenching of ions by the interaction with the ring particles, and the northward shift of the magnetic equator relative to the ring plane. Although O⁺ is produced with a significant excess energy, is not. Therefore, should mirror well below those altitudes at which ions were detected. However, scattering by ion-molecule collisions results in much larger mirror altitudes, in ion temperatures that go through a minimum over the B-ring, and in the redistribution of both molecular hydrogen and oxygen throughout the magnetosphere. The proposed model is used to describe the measured oxygen ion densities in Saturn's toroidal ring atmosphere and its hydrogen content. The oxygen ion densities over the B-ring appear to require either significant levels of UV light scattering or ion transmission through the ring plane.     

Brightness of the O2 atmospheric bands in the daytime thermosphere

The Fabry-Perot interferometer on Dynamics Explorer 2 was used as a low sensitivity photometer to study the O₂ Atmospheric A band during the daytime. A study of the brightness of the emission showed that the assumed source of O₂(b¹Σ_g⁺) in the thermosphere, O(¹D), can account for the observed intensity up to about 250 km but with a significantly different scale height. This combined with an enhanced brightness above this altitude suggests an additional source for this emission.