Ion composition in the E- and lower F- region above kiruna during sunset and sunrise

A magnetic type mass spectrometer has been flown on two ESRO sounding rockets from ESRANGE (Kiruna 68°N) on February 25 and 26, 1970. The first launch was at sunset (16:33 UT) and the second the next morning, during sunrise (04:47 UT). For both flights the solar zenith angle was approximately 98°. The instrument was measuring simultaneously the neutral gas and positive ion composition and the total ion density. In this paper the results of the ion composition measurements are presented. For both flights the main ion constituents measured between approximately 110–220 km were O⁺, NO⁺ and O₂⁺. Only at sunset were N⁺ and N₂⁺ detected above 200 km. In spite of the identical solar UV-radiation, pronounced sunset/sunrise variations in the positive ion composition were found. The total ion densities at sunrise were between 5×10³ and 5 × 10⁴ ions cm^?3 and therefore too high to be explained without a night-time ionization by precipitated particles. At sunrise the NO⁺ and O₂⁺ profiles show a correlated wavelike structure with three pronounced almost equally spaced layers in the E-region. Only the highest layer is present in the O⁺ profile. Locally enhanced field aligned ionization originated by particle precipitation and an E × B instability are the most likely source for this structure. In the E- and lower F-regions the $\text{NO}{}^{\mathrm{+}}\text{O}{}_2{}^{\mathrm{+}}$ ration increased overnight from values around 7 at sunset to 15 at sunrise, correlated with an increase of the local magnetic activity index K from 0⁺ to 2°. This could be explained if the NO density and magnetic activity are correlated.     

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.     

Isotope identification in NO as a chemical tracer in the middle atmosphere

The nitrogen isotope ratio of middle atmosphere nitrogen oxide is predicted as a function of altitude. Nitrogen oxides originate photochemically either from stratospheric nitrous oxide reacting with O(¹D) or in the mesosphere and thermosphere from direct dissociation of N₂ and ionization-initiated reactions involving O₂ and N₂. During its formation process, N₂O acquires a nitrogen isotopic composition of N isotopes different than N₂. Photodissociation within the stratosphere also modifies the proportion of isotopes. Reaction of stratospheric NO with O₃ produces NO₂, which when photodissociated yields NO depleted in ¹⁵N relative to NO₂ in laboratory air. The value of δ¹⁵NO in the stratosphere is −100‰. In the altitude region between 50 and 65 km, NO is transformed into NO₂ and then returned to NO by reaction of NO₂ with O and by NO₂ photodissociation. These reactions determine the isotopic makeup of NO. Above 65 km, nitric oxide is produced by local ionization processes and gas phase photochemical reactions involving N₂ and excited O₂. These processes determine the isotopic composition of NO in the upper mesosphere and thermosphere. Here δ¹⁵NO is 0‰. Air transported into the mesosphere above 65 km will reflect the NO isotopic values of the region below, while mesospheric NO transported below 65 km will not be distinguishable from NO originating in the stratosphere.     

Possible new atmospheric sources and sinks of odd nitrogen: 2. A revisit with O2(B) and discussions of other possibilities

Recently, modelers have expressed a concern that the currently known chemistry of atmospheric NO_y is deficient. It is therefore necessary to explore possible sources and sinks of atmospheric NO_x that may have been overlooked. In this context, it is noteworthy that the experimentally observed, four-center, Woodward-Hoffman forbidden, reaction 0₂(B ³Σ) + N₂ → NO(X) + NO (X) is atmospherically significant. In the 20 to 30 km region NO_x production from this reaction may potentially exceed the production from the “classical” N₂0 + O(¹D) reaction, and may provide a new mechanism to couple the solar UV variability and stratospheric ozone. The avoidance of the non-conservation of the orbital symmetry via the production of one NO in the excited electronic state being endothermic, it appears that the interaction of 0₂(B ³Σ) with the adjoining ¹Λ, ³Λ and ³Σ_u⁺ states might be responsible for the reaction. Experimental studies of the reaction as a function of the vibrational levels of the B-state, temperature and pressure are needed for reliable atmospheric applications of this reaction. At altitudes greater than about 50 km the production of NO from 0₂(B) begins to decrease rapidly. The NO production from 0₂ (A ³Σ₊⁺) + N₂ → NO + NO reaction may become important here, if the reaction is confirmed by experiments. These new sources of NOx call for new sinks of this species. In the upper stratosphere and mesosphere the chemical acceleration of NO dissociation via the reactions of electronically and vibrationally excited NO with 0₂ may help. In the lower atmosphere there is a possibility of the annihilation of NO, N0₂pair leading to the recreation of a stable NN bond. This might happen if N₂0₃ from NO and N0₂ recombination may photodissociate as N₂0 + 0₂. Unfortunately the requirements are stringent, and only experiments can tell whether or not this mechanism operates in the atmosphere.     

Solar-wind interactions with the moon: Nature and composition of nitrogen compounds

The lunar atmosphere and magnetic field are very tenuous. The solar wind, therefore, interacts directly with the lunar surface material and the dominant nature of interaction is essentially complete absorption of solar-wind particles by the surface material resulting in no upstream bowshock, but a cavity downstream. The solar-wind nitrogen ion species induce and undergo a complex set of reactions with the elements of lunar material and the solar-wind-derived trapped elements. The nitrogen concentration indigeneous to the lunar surface material is practically nil. Therefore any nitrogen and nitrogen compounds found in the lunar surface material are due to the solar-wind implantation of nitrogen ions. The flux of the solar-wind nitrogen ion species is about 6×10³ cm^–2 s^–1. Since there is no evidence for accumulation of nitrogen species in the lunar surface material, the outflux of nitrogen species from the lunar material to the atmosphere is the same as the solar-wind nitrogen ion flux. The species of the outflux are primarily NO and NH₃, and their respective concentrations in the near surface lunar atmosphere are found by calculation to be 327 and 295 cm^–3. The calculated concentration of NH₃ seems to be consistent with the sunrise concentration results of the mass spectrometer implanted on the lunar surface. This is not the case for the concentration of NO. According to the presently calculated concentration value of NO, the mass spectrometer should have detected NO at sunrise, but no report was made for its detection. There is also discrepancy about the concentration of N₂ which is explained in this paper. The concentrations of nitrogen species in the lunar material at the time of sample collection on the Moon remained about the same when the samples were analyzed on the Earth. However, no specific experiment was planned to detect the nitrogen species in the lunar material samples.     

A reanalysis of the Apollo light scattering observations, and implications for lunar exospheric dust

Conspicuous excess brightness, exceeding that expected from coronal and zodiacal light (CZL), was observed above the lunar horizon in the Apollo 15 coronal photographic sequence acquired immediately after orbital sunset (surface sunrise). This excess brightness systematically faded as the Command Module moved farther into shadow, eventually becoming indistinguishable from the CZL background. These observations have previously been attributed to scattering by ultrafine dust grains (radius ∼0.1 microns) in the lunar exosphere, and used to obtain coarse estimates of dust concentration at several altitudes and an order-of-magnitude estimate of ∼10⁻⁹ g cm⁻² for the column mass of dust near the terminator, collectively referred to as model “0”.We have reanalyzed the Apollo 15 orbital sunset sequence by incorporating the known sightline geometries in a Mie-scattering simulation code, and then inverting the measured intensities to retrieve exospheric dust concentration as a function of altitude and distance from the terminator. Results are presented in terms of monodisperse (single grain size) dust distributions. For a grain radius of 0.10 microns, our retrieved dust concentration near the terminator (∼0.010 cm⁻³) is in agreement with model “0” at z=10 km, as is the dust column mass (∼3–6×10⁻¹⁰ g cm⁻²), but the present results indicate generally larger dust scale heights, and much lower concentrations near 1 km (<0.08 cm⁻³ vs. a few times 0.1 cm⁻³ for model “0"). The concentration of dust at high altitudes (z>50 km) is virtually unconstrained by the measurements. The dust exosphere extends into shadow a distance somewhere between 100 and 200 km from the terminator, depending on the uncertain contribution of CZL to the total brightness. These refined estimates of the distribution and concentration of exospheric dust above the lunar sunrise terminator should place new and more rigorous constraints on exospheric dust transport models, as well as provide valuable support for upcoming missions such as the Lunar Atmosphere and Dust Environment Explorer (LADEE).     

Photochemistry of minor constituents in the troposphere

Altitude profiles for the number densities of NO, NO₂, NO₃, N₂O₅, HNO₂, CH₃O, CH₃O₂, H₂CO, OH, and HO₂ are calculated as a function of time of day with a steady-state photochemical model in which the altitude profiles for the number densities of H₂O, CH₄, H₂, CO, O₃, and the sum of NO and NO₂ are fixed at values appropriate to a summer latitude of 34°. Average daily profiles are calculated for the long-lived species, HNO₃, H₂O₂, and CH₃O₂H.The major nitrogen compound HNO₃ may have a number density approaching 5 × 10¹¹ molecules cm^?3 at the surface, although an effective loss path due to collisions with particulates could greatly reduce this value.The number density of OH remains relatively unchanged in the first 6 km and reaches 1 × 10⁷ molecules cm^?3 at noon, while the number density of HO₂ decreases throughout the lower troposphere from its noontime value of 8 × 10⁸ molecules cm^?3 at the surface.H₂O₂ and H₂CO both have number densities in the ppb range in the lower troposphere.Owing to decreasing temperature and water concentration, the production of radicals and their steady-state number densities decrease with altitude, reaching a noontime minimum of 1 × 10⁸ molecules cm^?3 for OH and 3 × 10⁷ molecules cm^?3 for HO₂ at the tropopause. The related minor species show even sharper decreases with increasing altitude.The primary path for interconverting OH and HO₂ serves as the major sink for CO and leads to a tropospheric lifetime for CO of ~0.1 yr.Another reaction cycle, the oxidation of CH₄, is quite important in the lower troposphere and leads to the production of H₂CO along with the destruction of CH₄ for which a tropospheric lifetime of ~2 yr is estimated.The destruction of H₂CO that was produced in the CH₄ oxidation cycle provides the major source of CO and H₂ in the atmosphere.     

On the temperature dependence of the reaction O + NO → NO12

The published data on the temperature dependence of the radiative combination of atomic oxygen with nitric oxide at pressures near 1 torr is examined. Arguments are advanced to suggest that radiation near the cut-off wavelength (～ 3875Å) is coming from the unstabilized activated complex, No¹₂. At 4000Å a positive activation energy of 1 kcal mole^?1 is deduced. Application of this temperature dependence with the rate coefficient at 5200Å is made to airglow measurements in aurora. The deduced NO concentration is about 10⁹ cm^?3, in general agreement with that deduced from the measured NO⁺/O⁺₂ ratio as well as an auroral model prediction.     

A sensitive search for nitric oxide in the lower atmospheres of Venus and Mars: Detection on Venus and upper limit for Mars

High-resolution spectra of Venus and Mars at the NO fundamental band at 5.3 μm with resolving power ν/δν=76,000 were acquired using the TEXES spectrograph at NASA IRTF on Mauna Kea, Hawaii. The observed spectrum of Venus covered three NO lines of the P-branch. One of the lines is strongly contaminated, and the other two lines reveal NO in the lower atmosphere at a detection level of 9 sigma. A simple photochemical model for NO and N at 50-112 km was coupled with a radiative transfer code to simulate the observed equivalent widths of the NO and some CO₂ lines. The derived NO mixing ratio is 5.5±1.5 ppb below 60 km and its flux is . Predissociation of NO at the (0-0) 191 nm and (1-0) 183 nm bands of the δ-system and the reaction with N are the only important loss processes for NO in the lower atmosphere of Venus. The photochemical impact of the measured NO abundance is significant and should be taken into account in photochemical modeling of the Venus atmosphere. Lightning is the only known source of NO in the lower atmosphere of Venus, and the detection of NO is a convincing and independent proof of lightning on Venus. The required flux of NO is corrected for the production of NO and N by the cosmic ray ionization and corresponds to the lightning energy deposition of . For a flash energy on Venus similar to that on the Earth (∼¹⁰⁹ J), the global flashing rate is ∼90 s⁻¹ and ∼6 km⁻² y⁻¹ which is in reasonable agreement with the existing optical observations. The observed spectrum of Mars covered three NO lines of the R-branch. Two of these lines are contaminated by CO₂ lines, and the line at 1900.076 cm⁻¹ is clean and shows some excess over the continuum. Some photochemical reactions may result in a significant excitation of NO (v=1) in the lowest 20 km on Mars. However, quenching of NO (v=1) by CO₂ is very effective below 40 km. Excitation of NO (v=1) in the collisions with atomic oxygen is weak because of the low temperature in the martian atmosphere, and we do not see any explanation of a possible emission of NO at 5.3 μm. Therefore the data are treated as the lack of absorption with a 2 sigma upper limit of 1.7 ppb to the NO abundance in the lower atmosphere of Mars. This limit is above the predictions of photochemical models by a factor of 3.     

Jovian ionospheric models

This paper reports results obtained on ionosphere formation in the Jovian upper atmosphere with special reference to some of the recently available reaction rates, and to recent models of the Jovian neutral atmosphere based on the possibility of a warmer mesopause. We find that the role of the hypothetical radiative association of H⁺ to H₂ to form H₃⁺, as brought to light in our earlier study, is still important, even with a reaction rate as low as 10^?15 cm³sec^?1. In the lower regions of the ionosphere three-body processes leading to the formation of H₃⁺ and H₅⁺ ions, which have very fast dissociative recombination rates, produce a dramatic reduction in the electron density. When no radiative association takes place, and the H⁺ ions are lost by radiative recombination alone, we confirm that the photochemical equilibrium profile is also the diffusive equilibrium profile. However, with collisional-radiative recombination, whose rate becomes altitude-dependent, diffusion tends to bring about some redistribution of the ionization. Inclusion of radiative association enhances the role of diffusion. In this case, diffusion brings about all the expected changes. In particular, the differences in the electron density profile, originated in the lower-middle ionosphere by radiative association, are propagated up to all higher altitudes by diffusion. The rate constant of radiative association is, however, unknown. It is hoped that the critical importance of this reaction for the Jovian ionosphere will be an incentive towards a careful laboratory determination of its rate coefficient. In the older models of the Jovian ionosphere the major ions were H⁺ which were lost only by pure radiative recombination. This led to high electron densities and practically no diurnal change. In contrast, our new models have relatively much smaller electron densities, especially in lower regions, and may be susceptible to significant diurnal variation.     

Theoretical ion densities in the lower ionosphere

We have solved the coupled momentum and continuity equations for NO⁺, O₂⁺, and O⁺ions in the E- and F-regions of the ionosphere. This theoretical model has enabled us to examine the relative importance of various processes that affect molecular ion densities. We find that transport processes are not important during the day; the molecular ions are in chemical equilibrium at all altitudes. At night, however, both diffusion and vertical drifts induced by winds or electric fields are important in determining molecular ion densities below about 200 km. Molecular ion densities are insensitive to the O⁺ density distribution and so are little affected by decay of the nocturnal F-region or by processes, such as a protonospheric flux, that retard this decay. The O⁺ density profile, on the other hand, is insensitive to molecular ion densities, although the O⁺ diffusion equation is formally coupled to molecular ion densities by the polarization electrostatic field. Nitric oxide plays an important role in determining the NO⁺ to O₂⁺ ratio in the E-region, particularly at night. Nocturnal sources of ionization are required to maintain the E-region through the night. Vertical velocities induced by expansion and contraction of the neutral atmosphere are too small to affect ion densities at any altitude.     

Temporal Behavior of Stratospheric Ammonia Abundance and Temperature Following the SL9 Impacts

Infrared emission lines of stratospheric ammonia (NH₃) were observed following the collisions of the fragments of Comet Shoemaker-Levy 9 with Jupiter in July of 1994 at the impact sites of fragments G and K. Infrared heterodyne spectra near 10.7 μm were obtained by A. Betz et al. (in Abstracts for Special Sessions on Comet Shoemaker-Levy 9, The 26th Meeting of the Division for Planetary Sciences, Washington DC, 31 Oct.-4 Nov. 1994, p. 25) using one of the Infrared Spatial Interferometer telescope systems on Mount Wilson. Lineshapes of up to three different NH₃ emission lines were measured at a resolving power of ∼10⁷ at multiple times following the impacts. We present here our radiative transfer analysis of the fully resolved spectral lineshapes of the multiple rovibrational lines. This analysis provides information on temperature structure and NH₃ abundance distributions and their temporal changes up to 18 days after impact. These results are compared to photochemical models to determine the role of photochemistry and other mechanisms in the destruction and dilution of NH₃ in the jovian stratosphere after the SL9 impacts.One day following the G impact, the inferred temperature above 0.001 mbar altitude is 283±13 K, consistent with a recent plume splashback model. Cooling of the upper stratosphere to 204 K by the fourth day and to quiescence after a week is consistent with a simple gray atmosphere radiative flux calculation and mixing with cold jovian air. During the first 4 days after impact, NH₃ was present primarily at altitudes above 1 mbar with a column density of (7.7±1.6)×10¹⁷ cm⁻² after 1 day and (3.7±0.8)×10¹⁷ cm⁻² after 4 days. (Errors represent precision.) We obtained >2.5 times more NH₃ than can be supplied by nitrogen from a large cometary fragment, suggesting a primarily jovian source for the NH₃. By 18 days postimpact, a return to quiescent upper stratospheric temperature is retrieved for the G region, with an NH₃ column density of 7.3×10¹⁷ cm⁻² or more in the lower stratosphere, possibly supplied by NH₃ upwelling across an impact-heated and turbulent tropopause, which may have been masked by initial dust and haze. Above the 1-mbar level, the maximum retrieved column density decreased to 6.5×10¹⁶ cm⁻². Comparison to photochemical models indicates that photolysis alone is not sufficient to account for the loss of NH₃ above 1 mbar by that time, even when chemical reformation of NH₃ is ignored. We speculate that the dispersion of plume material at high altitudes (above 1 mbar) is responsible for the change in the spectra observed a few days postimpact. Data on the K impact region provide qualitatively consistent results.     

Stratospheric observations of NO3 and its experimental and theoretical distribution between 20 and 40 km

A simultaneous night-time observation of NO₃ and 0₃ has been made by means of a balloon-borne spectrophotometer pointing at the rising planet Venus. The spectrum recorded between 642 and 672 nm makes it possible to determine the NO₃ and O₃ absorptions in the 662 nm band and the Chappuis bands, respectively. The NO₃ vertical distribution is deduced, and is found to reach a peak of (3.4 ± 0.4) 10⁷ molecules cm^?3at 35 km. Such an observational result can be interpreted in terms of a theoretical profile deduced from a one-dimension time-dependant photochemical model which takes account of the night-time stratospheric NO₂, NO₃ and N₂O₅ constituents and the latest kinetic and photochemical data for the rate constants.     

The production of water-cluster positive ions in the quiet daytime D region

In the quiet daytime D region, the primary positive-ion species is thought to be NO⁺, produced by solar Lyman-alpha ionization of NO. Below the altitude of the mesopause, however, the dominant ambient species observed are water-cluster ions of the general type H⁺(H₂O)_n. No satisfactory reaction scheme for producing these cluster ions from NO⁺ has yet been proposed. Following earlier suggestions, a model calculation has been carried out in which successive hydrations of NO⁺ take place through clustering with N₂ and CO₂, followed by “switching” reactions with H₂O. The third hydrate of NO⁺ is then converted into the water-cluster species H⁺(H₂O)₃, and the other water-cluster species are produced by successive clustering and thermal breakup reactions. Many of the reactions involved have not been measured in the laboratory, but reasonable estimates of their rates can be made on the basis of existing measurements of other species. Since both temperature and water-vapor content are of major importance in the model, calculations were carried out for two temperature profiles and two water-vapor profiles. It is shown that the results are in reasonably good agreement with observations as far as the water-cluster species are concerned. Under low-temperature conditions, the model predicts relatively large concentrations of various clusters of NO⁺, in agreement with some observations but in disagreement with others. The importance of sampling breakup of these weakly bound clusters, and their relevance to the free electron concentrations are discussed.     

Solar Occultation Measurements of the Martian Atmosphere on the Phobos Spacecraft: Water Vapor Profile,Aerosol Parameters,and Other Results

The Auguste experiment onboard the Phobos spacecraft was devoted to solar occultation spectroscopy of the Martian atmosphere in the ultraviolet through infrared wavelength region. Despite the short duration of the space mission and problems associated largely with a fault in the solar pointing system, data have been obtained on the chemical composition and aerosol content in the atmosphere of Mars at sunset early in the summer at equatorial latitudes (in the northern hemisphere). This paper presents a somewhat detailed review of the experiment performed, the data obtained, and their interpretation, and compares these data with new results. Ozone traces were detected at altitudes of 40–60 km, and, in one case, an ozone profile was obtained. Nine profiles of water vapor content at altitudes between 12 and 50 km were obtained from absorption data in the 1.87-m band. At altitudes of 23–25 km, the mean H₂O concentration profile falls steeply to the value of 3 ppm, but at lower altitudes the relative H₂O content is approximately constant (130 ppm). The overall content of water vapor is estimated as 8.3^+2.5 _-1.5 m of settled water. The temperature profile for the saturated atmosphere yields a cooling rate of 2 ± 1 K/km at altitudes from 25 to 35 km. The atmospheric extinction profiles were measured at altitudes from 10 to 50 km at the wavelengths 1.9 and 3.7 m. The atmosphere is transparent up to 25–33 km; below this level radiation is attenuated by dust; it is also possible that a layer of water ice clouds is present at altitudes of 20–25 km. High-altitude transparent ( 0.03) clouds consisting supposedly of water ice were observed in 5 of 38 cases at altitudes z 50 km. The optical depth ₀ of the atmosphere was estimated to be 0.2 ± 0.1, and constraints on the form of the size distribution of dust particles were established. Spectral features in the 3.7 m range have been previously attributed to formaldehyde; its content is substantially higher than the limits deduced from new ground-based observations. The spectrum in the 3.7 m range is discussed and other unsettled problems are pointed out.     

Influence of peroxyacetyl nitrate (PAN) on odd nitrogen in the troposphere and lower stratosphere

Nonmethane hydrocarbon breakdown in the atmosphere produces aldehydes of which a fraction are transferred into peroxyacetyl nitrates (PAN) in the presence of NO and NO₂. Since ethane is destroyed photochemically primarily above 1 km, PAN can be introduced into the upper troposphere and lower stratosphere without the need to be transported from the boundary layer where most hydrocarbons are destroyed and where PAN may be lost due to thermal decomposition and heterogeneous loss. Mixing ratios of ethane in the lower troposphere increase by a factor of 4–8 from equatorial to northern mid-latitudes. This difference is directly translatable into a PAN latitude gradient. At mid-latitudes the concentration of PAN below 20 km is 0.1 ppb comparable to and in some instances larger than predicted HO₂NO₂ mixing ratios. Like HO₂NO₂ and HNO₃, PAN serves as a reservoir for odd nitrogen.     

Coupling photochemistry with haze formation in Titan's atmosphere, Part II: Results and validation with Cassini/Huygens data

The new one-dimensional radiative-convective/photochemical/microphysical model described in Part I is applied to the study of Titan's atmospheric processes that lead to haze formation. Our model generates the haze structure from the gaseous species photochemistry. Model results are presented for the species vertical concentration profiles, haze formation and its radiative properties, vertical temperature/density profiles and geometric albedo. These are validated against Cassini/Huygens observations and other ground-based and space-borne measurements. The model reproduces well most of the latest measurements from the Cassini/Huygens instruments for the chemical composition of Titan's atmosphere and the vertical profiles of the observed species. For the haze production we have included pathways that are based on pure hydrocarbons, pure nitriles and hydrocarbon/nitrile copolymers. From these, the nitrile and copolymer pathways provide the stronger contribution, in agreement with the results from the ACP instrument, which support the incorporation of nitrogen in the pyrolized haze structures. Our haze model reveals a new second major peak in the vertical profile of haze production rate between 500 and 900 km. This peak is produced by the copolymer family used and has important ramifications for the vertical atmospheric temperature profile and geometric albedo. In particular, the existence of this second peak determines the vertical profile of haze extinction. Our model results have been compared with the DISR retrieved haze extinction profiles and are found to be in very good agreement. We have also incorporated in our model heterogeneous chemistry on the haze particles that converts atomic hydrogen to molecular hydrogen. The resultant H₂ profile is closer to the INMS measurements, while the vertical profile of the diacetylene formed is found to be closer to that of the CIRS profile when this heterogenous chemistry is included.     

Simultaneous measurements of No and No2 in the stratosphere

Simultaneous measurements of NO and NO₂ in the stratosphere leading to an NO_x determination have been performed by means of i.r. absorption spectrometry using the Sun as a source in the 5·2 μm band of NO and in the 6·2 μm band of NO₂. The observed abundance of NO_P peaks at 26 km where it is equal to (4·2 ± 1) × 10⁹ cm^?3. The volume mixing ratio of NO_p was observed to vary from 1·3 × 10^?9 at 20 km to 1·3 × 10^?8 at 34 km.     

Chemical pathway analysis of the Martian atmosphere: CO2-formation pathways

The chemical composition of a planetary atmosphere plays an important role for atmospheric structure, stability, and evolution. Potentially complex interactions between chemical species do not often allow for an easy understanding of the underlying chemical mechanisms governing the atmospheric composition. In particular, trace species can affect the abundance of major species by acting in catalytic cycles. On Mars, such cycles even control the abundance of its main atmospheric constituent CO₂. The identification of catalytic cycles (or more generally chemical pathways) by hand is quite demanding. Hence, the application of computer algorithms is beneficial in order to analyze complex chemical reaction networks. Here, we have performed the first automated quantified chemical pathways analysis of the Martian atmosphere with respect to CO₂-production in a given reaction system. For this, we applied the Pathway Analysis Program (PAP) to output data from the Caltech/JPL photochemical Mars model. All dominant chemical pathways directly related to the global CO₂-production have been quantified as a function of height up to 86 km. We quantitatively show that CO₂-production is dominated by chemical pathways involving HO_x and O_x. In addition, we find that NO_x in combination with HO_x and O_x exhibits a non-negligible contribution to CO₂-production, especially in Mars’ lower atmosphere. This study reveals that only a small number of chemical pathways contribute significantly to the atmospheric abundance of CO₂ on Mars; their contributions to CO₂-production vary considerably with altitude. This analysis also endorses the importance of transport processes in governing CO₂-stability in the Martian atmosphere. Lastly, we identify a previously unknown chemical pathway involving HO_x, O_x, and HO₂-photodissociation, contributing 8% towards global CO₂-production by chemical pathways using recommended up-to-date values for reaction rate coefficients.     

Observed composition of the ionosphere of Venus: Implications for the ionization peak and the maintenance of the nightside ionosphere

Across the nightside of Venus, daily measurements from the PV Orbiter Ion Mass Spectrometer often indicate an ionosphere of relatively abundant concentration, with a composition characteristic of the dayside ionosphere. Such conditions are interspersed by other days on which the ionosphere appears to largely “disappear” down to about 200 km, with ion concentrations at lower heights also much reduced. These characteristics, coupled with observations of strong day to night flows of O⁺ in the upper ionosphere, support arguments that ion transport from the dayside is important for the maintenance of the nightside ionosphere. Also, U.S. and Soviet observations of nightside energetic electron fluxes have prompted consideration of impact ionization as an additional nightside ion source. The details of the ion and neutral composition at low altitudes on the nightside provide an important input for further analysis of the maintenance process. In the range 140–160 km, strong concentrations of O₂⁺ and NO⁺ indicate that the ionization peak is at times composed of at least two prominent ion species. Nightside concentrations of O₂⁺ and NO⁺ as large as 10⁵ and 10⁴/cm³, respectively, appear to require sources in addition to that provided by transport. The most probable sources are considered briefly, and no satisfactory explanation is yet found for the observed NO⁺ concentrations. Further analysis beyond the scope of this paper is required to resolve this issue.