Photodissociation of nitric oxide in the middle and upper atmosphere

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

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.     

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

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

On the photodissociation of water vapour in the mesosphere

The photodissociation of water vapour in the mesosphere depends on the absorption of solar radiation in the region (175–200 nm) of the O₂ Schumann-Runge band system and also at H-Lyman alpha. The photodissociation products are OH + H, OH + H, O + 2H and H₂ + O at Lyman alpha; the percentages for these four channels are 70, 8, 12 and 10%, respectively, but OH + H is the only channel between 175 and 200 nm. Such proportions lead to a production of H atoms corresponding to practically the total photodissociation of H₂O, while the production of H₂ molecules is only 10% of the H₂O photodissociation by Lyman alpha.The photodissociation frequency (s^?1) at Lyman alpha can be expressed by a simple formula

$\text{J}{}_{\mathrm{<mtext>Ly</mtext><mtext>\alpha </mtext>}}\text{H}{}_2\text{O=4.5 ×10}{}^{\mathrm{?6}}\text{1+0.2}\text{F}{}_{10.7}\text{?65}\text{100}\text{exp}\text{[?4.4 ×10}{}^{\mathrm{?19}}\text{N}{}^{0.917}\text{]}$

where F_{10.7 cm} is the solar radioflux at 10.7 cm and N the total number of O₂ molecules (cm^?2), and when the following conventional value is accepted for the Lyman alpha solar irradiance at the top of the Earth's atmosphere ($\text{Δλ = 3.5}\text{A}\text{?}$) q_Lyα,∞ = 3 × 10¹¹ photons cm^?2 s^1?.The photodissociation frequency for the Schumann-Runge band region is also given for mesospheric conditions by a simple formula

$\text{J}{}_{\mathrm{<mtext>SRB</mtext>}}\text{(}\text{H}{}_2\text{O}\text{) = J}{}_{\mathrm{<mtext>SRB</mtext><mtext>,\infty </mtext>}}\text{(}\text{H}{}_2\text{O}\text{) exp [?10}{}^{\mathrm{?7}}\text{N}{}^{0.35}\text{]}$

where J_SRB,∞(H₂O) = 1.2 × 10^?6 and 1.4 × 10^?6 s^?1 for quiet and active sun conditions, respectively.The precision of both formulae is good, with an uncertainty less than 10%, but their accuracy depends on the accuracy of observational and experimental parameters such as the absolute solar irradiances, the variable transmittance of O₂ and the H₂O effective absorption cross sections. The various uncertainties are discussed. As an example, the absolute values deduced from the above formulae could be decreased by about 25-20% if the possible minimum values of the solar irradiances were used.     

Atmospheric odd oxygen production due to the photodissociation of ordinary and isotopic molecular oxygen

Through a line by line calculation, the contributions of the Schumann-Runge bands of the ordinary and isotopic oxygen to the photodissociation of these molecules at different altitudes have been calculated. The photodissociation rates are expressed analytically. Contribution of the satellite lines has been taken into account. Due to the broadening of the SR lines, this contribution is insignificant. Similarly, it is shown that the first and higher vibrational states of the initial molecular states contribute insignificantly to the dissociation rates. It is also shown that the main contribution to the odd oxygen production in the important ozone producing altitudes is from the low vibrational and high rotational quantum numbers. The effect of the temperature on dissociation rates has similarly been studied.Due to its selective absorption, the isotopic oxygen ¹⁶O¹⁸O produces at 70 km 10 times as much odd oxygen as would be produced if the isotope did not have selective absorption. At this altitude 6% of the odd oxygen produced is due to this isotope. Also, 1.45% of the odd oxygen produced per second in an atmospheric column is due to ¹⁶O¹⁸O. However, the excess odd oxygen produced is not enough to explain the excess amount of ozone observed in the atmosphere which cannot be accounted for in the photochemical models.The calculated dissociation rates for the isotope are in moderate agreement with similar rates obtained by Blake et al. (1984, J. geophys. Res.89, 7277), but are by an order of magnitude smaller than similar rates given by Cicerone and McCrumb (1980, Geophys. Res. Lett.7, 251).     

High resolution absorption cross section measurements and band oscillator strengths of the (1, 0)−(12, 0) Schumann-Runge bands of O2

Cross sections of O₂ at 300 K have been obtained from photoabsorption measurements at various pressures throughout the wavelength region 179.3–201.5 nm with a 6.65 m photoelectric scanning spectrometer equipped with a 2400 lines mm^?1 grating and having an instrumental width (FWHM) of 0.0013 nm. The measured absorption cross sections of the Schumann-Runge bands (12, 0) through (1, 0) in this wavelength region are absolute, i.e., independent of the instrumental width, a result not achieved previously. The measured cross sections are presented graphically and are available at wavenumber intervals of > sim; 0.1 cm^?1 as numerical complications stored on magnetic tape from the National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. Band oscillator strengths of the (12, 0) through (1, 0) bands have been determined by direct numerical integration of the measured cross sections.     

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

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

A numerical response of the middle atmosphere to the 11-year solar cycle

A two-dimensional numerical model with coupled photochemistry and dynamics has been used to investigate the response of the middle atmosphere (16–116 km) to changes in solar activity over the 11-year solar cycle. Model inputs that vary with solar cycle include solar radiation, cosmic ray and auroral ionization rates and the flux of NO_x at the model's upper boundary.In this study, the results of model runs for solar cycle minimum and maximum conditions are compared. In the stratosphere, using currently accepted estimates of changes in solar radiation at wavelengths longer than 180 nm, only small responses in ozone, temperature and zonal winds are obtained. On the other hand, changes at shorter wavelengths, and the effects of particle precipitation, lead to large variations in the abundances of trace species in the thermosphere and upper mesosphere. In particular, very large abundances of NO_x are produced above 90 km by auroral particle precipitation. Considerable amounts of NO_x are transported subsequently to the stratosphere by the global mean meridional circulation. It is shown that this excess NO_x can lead to significant decreases in ozone concentrations at high latitudes and that it may explain observations of nitrate deposition in Antarctic snow.     

Aeronomical aspects of mesospheric photodissociation: Processes resulting from the solar H Lyman-alpha line

An analysis is made of the photodissociation and photoionization processes in the mesosphere due to the solar H Lyman-alpha line. The irradiance of the line and its variation with solar activity are considered in the determination of the photodissociation of CH₄, CO₂, H₂O and O₂, and of the photoionization of NO. Lyman-alpha contributes directly to these processes in the mesosphere after its absorption, which depends on wavelength and temperature, by molecular oxygen. The H Lyman-alpha radiation considered for mesospheric processes is characterized by a profile of an emission line with a central reversal, and wings extending to about ± 1.75 A where the intensity reaches about 1% of that of the peak. Simple formulae are deduced for the photodissociation optical depths and frequencies and these take into account the various solar activity conditions and the different spectral characteristics of each molecule.     

Effects of solar XUV variations on the thermosphere and the variability of the photoionization efficiency parameter during the solar cycle 21

Making use of the latest available semi-empirical atmospheric models, solar XUV radiations rates of photoionization and absorbed energy profiles have been graphically presented showing the latitudinal, seasonal and solar cycle variations. The photoionization limits of the major neutral constitutents of the terrestrial atmosphere O₂, O, and N₂ that occur at wavelengths 102.7, 91.2, and 79.6 nm, respectively have been quantified by showing the photoionization rates of O ₂ ⁺ , O⁺, and N ₂ ⁺ for different spectral groups both under quiet and different solar flare conditions. The variability of the photoionization efficiency parameter which is height-dependent, from winter to summer, for solar minimum to solar maximum for four significantly different latitudes under local noon conditions have been investigated during the solar cycle 21. More energy is required to produce an electron-ion pair in a denser atmosphere than in a thinner atmosphere and hence more energy is being deposited in the height range between 100–120 km which itself manifests in raising the electron gas temperatures higher than the neutral gas temperatures.     

Effects of scattered solar radiation and absorption by SO2 on the photodissociation processes in the stratosphere of Venus

In the stratosphere of Venus, the available luminous flux which locally produces the photodissociation processes at a given altitude may be divided into three parts: direct incoming downward flux, flux resulting from the reflection on the surface of the clouds, and flux due to molecular scattering. A relatively simple computation method has been used to evaluate the relative importance of these three parts at altitudes between 65 and 100 km. It is shown that the extra contribution of the reflected and scattered fluxes to photodissociation processes cannot be neglected in the uv and visible regions. In the case of SO₂, for instance, which presents an absorption band in the uv, the photodissociation coefficient is increased 30% due to these effects. Calculations of the photodissociation coefficients of CO₂, O₃, H₂S, and SO₂ are presented. As a result of the increase by 60% in the ozone photolysis rate, the calculated O₂ infrared band at 1.27 μm is larger by a factor of nearly 2 than is expected from a calculation without taking albedo or scattering into account.     

Further study of changes in thermospheric molecular oxygen abundance inferred from twilight 6300 Å airglow

Measurements of the [OI] 6300 Å twilight airglow during 1973 near Boulder, Colorado, show a strong dependence upon geomagnetic activity for the morning enhancement at solar depression angles where production of O¹D) is due primarily to photodissociation of O₂ and local photoelectron excitation. Analysis indicates that photodissociation is the dominant source; hence we infer a well defined magnetic dependence for the O₂/N₂ concentration ratio in the thermosphere. A seasonal variation in the twilight enhancement intensity is barely evident, in contrast with earlier observations made near solar maximum; the smaller variation is associated with a corresponding reduction in the seasonal variation of geomagnetic activity.     

Photodissociation lifetime of 32S2 molecule in comets

Photodissociation lifetime of ³²s₂in comets is calculated by absorption of solar photons into the B^3– state and velocity distributions of sulphur atoms are determined. Absorption of solar photons of wavelength ~ 280 nm leads to a photodissociation lifetime of about 250 s for ³²S₂ molecule in comets when sun-comet distance is 1 AU. Forbidden lines corresponding to ¹D-³P transitions of neutral sulphur atom may be detectable at about 11 306 and 10 821 Å in comets. The production rate of ³²S₂ dimer in comet IRAS-Araki-Alcock 1983d compares well with the production rate of CS, observed in comet Bradfield, when compared at the same heliocentric distance. The chemistry of ³²S₂ dimer formation in the inner coma of a comet is discussed in the framework of some gas phase reactions.Work partially supported by the CNPq, Brasilia, Brasil under contract No. 30.4076/77.     

The aeronomic dissociation of nitric oxide

A detailed study is made of the atmospheric attenuation of the dissociation of nitric oxide in the mesosphere and stratosphere. The nitric oxide dissociation profile depends on the absorption of the discrete Schumann-Runge bands of O₂. The major contribution to the dissociation rate of NO is the predissociation of the δ(0-0) and δ(1-0) bands which can reach the stratosphere.     

An SPE-disturbed D-region model

For the first time, a model of the daytime disturbed D-region is presented which is consistent with experimental solar proton event (SPE) data, that of the 2–5 November, 1969 event in particular. Sunset electron concentration profiles also are shown to be quite compatible with the experimental results, but computed sunrise electron concentrations are found to rise faster with solar elevation than do the measurements. In the daytime, O₂^?, O^?, CO₄^? and CO₃^? ions apparently do not retain electrons in contrast to NO₂^? and NO₃^? ions. Hydration of the latter two species is probably unimportant since photodetachment and/or photodissociation of these ions are insignificant processes even when they are unattached to water molecules. Difficulties at sunrise are thought to arise most likely from our omission of hydration processes for negative ions, the pre-sunrise negative ion populations undoubtedly having the highest diurnal hydration level. Sunset ozone computations using the latest chemistry are shown to match the data except for some problem at the highest altitude, near 70 km, for the earlier, more disturbed, of the two experimental profiles.     

Observation of mesospheric ozone at low latitudes

Stellar ultraviolet light near 2500 Å is attenuated in the Earth's upper atmosphere due to strong absorption in the Hartley continuum of ozone. The intensity of stars in the Hartley continuum region has been monitored by the University of Wisconsin stellar photometers aboard the OAO-2 satellite during occultation of the star by the Earth's atmosphere. These data have been used to determine the ozone number density profile at the occultation tangent point. The results of approximately 12 stellar occultations, obtained in low latitudes, are presented, giving the nighttime vertical number density profile of ozone in the 60- to 100-km region. The nighttime ozone number density has a bulge in its vertical profile with a peak of 1 to 2×10⁸ cm^?3 at approximately 83 km and a minimum near 75 km. The shape of the bulge in the ozone number density profile shows considerable variability with no apparent seasonal or solar cycle change. The ozone profiles obtained during a geomagnetic storm showed little variation at low latitudes.     

EUV spectroscopy of the Venus dayglow with UVIS on Cassini

We analyze EUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus on 24 June 1999, a period of high solar activity level. Emissions from OI, OII, NI, CI and CII and CO have been identified and their disc average intensity has been determined. They are generally somewhat brighter than those determined from the observations made with the HUT spectrograph at a lower activity level, We present the brightness distribution along the foot track of the UVIS slit of the OII 83.4 nm, OI 98.9 nm, Lyman-ß + OI 102.5 nm and NI 120.0 nm multiplets, and the CO C-X and B-X Hopfield-Birge bands. We make a detailed comparison of the intensities of the 834 nm, 989 nm, 120.0 nm multiplets and CO B-X band measured along the slit foot track on the disc with those predicted by an airglow model previously used to analyze Venus and Mars ultraviolet spectra. This model includes the treatment of multiple scattering for the optically thick OI, OII and NI multiplets. It is found that the observed intensity of the OII emission at 83.4 nm is higher than predicted by the model. An increase of the O⁺ ion density relative to the densities usually measured by Pioneer Venus brings the observations and the modeled values into better agreement. The calculated intensity variation of the CO B-X emission along the track of the UVIS slit is in fair agreement with the observations. The intensity of the OI 98.9 nm emission is well predicted by the model if resonance scattering of solar radiation by O atoms is included as a source. The calculated brightness of the NI 120 nm multiplet is larger than observed by a factor of ∼2-3 if photons from all sources encounter multiple scattering. The discrepancy reduces to 30-80% if the photon electron impact and photodissociation of N₂ sources of N(⁴S) atoms are considered as optically thin. Overall, we find that the O, N₂ and CO densities from the empirical VTS3 model provide satisfactory agreement between the calculated and the observed EUV airglow emissions.     

On the O2 Schumann-Runge band system in sunspots

The visibility in the umbral ultraviolet spectrum of the O₂ Schumann-Runge absorption band system has been explored. It is found that the band system may be visible in high dispersion sunspot spectrum.     

Stellar occultation measurements of molecular oxygen in the lower thermosphere

Stellar ultraviolet light near 1500 Å is attenuated in the Earth's upper atmosphere due to strong absorption in the Schumann-Runge continuum of molecular oxygen. The intensity of stars in the Schumann-Runge continuum region has been monitored by the University of Wisconsin stellar photometers aboard the OAO-2 satellite during occultation of the star by the Earth's atmosphere. These data have been used to determine the molecular oxygen number density profile at the occultation tangent point. The results of 14 stellar occultations obtained in low and middle latitudes are presented giving the night-time vertical number density profile of molecular oxygen in the 140–200 km region. In general, the measured molecular oxygen number density is about a factor of 2 lower than the number densities predicted by the CIRA 1965 model. Also, the number density at a given height appears to decrease with decreasing solar activity. Measurements taken at low latitudes during the August 1970 geomagnetic storm showed a decrease in the molecular oxygen number density at a given height several days after the peak of the storm followed by a slow recovery to pre-storm densities.     

Intensity variation of atomic oxygen red line in the day airglow with solar activity

Rates of production of O(¹ D) atoms in the upper atmosphere by photodissociation of O₂, dissociative recombination of O₂ ⁺, NO⁺ and electron impact excitation of O(³ P) have been calculated for low, medium and high levels of solar activity. Variations with solar activity, of neutral and ionic composition, electron and neutral temperatures of the upper atmosphere and solar extreme ultraviolet fluxes incident on it have been taken into consideration.Emission rates ofOi red line (6300Å) have been computed taking into account the deactivation both by molecular oxygen and nitrogen. It has been shown that the integrated intensity from low to high activity period varies by approximately an order of magnitude in agreement with the results of experimental observations.