The Jovian stratosphere in the ultraviolet

The center-of-disk reflectivity of Jupiter in the wavelength range from 1450 to 3150A?has been computed from 30 low-dispersion IUE spectra taken during solar maximum in 1978–1980. A vertically inhomogeneous radiative transfer program is used to compute model reflectivities of various stratospheric compositions for comparison. Ammonia and acetylene are well determined because they show narrow absorption bands in the ultraviolet. Above 1800A?, these two gases provide a good fit to the data, but not below. At shorter wavelengths the fit would be much improved by a small amount (5–15 ppb) of propadiene/allene (C₃H₄). Voyager IRIS spectra show that the IR bands of allene are not strong enough to be detected in such a small amount. Additional absorption around 1600A?can be reproduced best with the presence of cyclopropane (C₃H₆, <15ppb), although other absorbers (e.g., hydrocarbon molecules with more than three carbon atoms, oxygen- or nitrogen-containing molecules, or a high-attitude haze) could also explain the spectrum in this region. The data are too noisy to detect possible CO Cameron band absorption near 2000A?.     

Magnesium ion chemistry in the stratosphere

Laboratory measurements of reaction rate constants of magnesium ions and magnesium containing ions with O₃, NO, HNO₃, and H₂O₂ have been carried out in a flowing afterglow experiment. Mg⁺ ions react with O₃ to produce MgO⁺ ions, which in turn react with O₃ to produce Mg⁺ ions. Mg⁺ ions react with HNO₃ and H₂O₂ to produce MgOH⁺ ions. MgOH⁺ ions react rapidly with HNO₃ to produce NO⁺₂ ions and Mg(HO)₂. One can therefore conclude that Mg⁺, MgO⁺, or MgOH⁺ ions could not have significant concentrations in the stratosphere if gas phase magnesium compounds were present. The failure to observe these ions therefore cannot be used as evidence that the stratospheric magnesium, resulting from meteor ablation at higher altitudes, is in condensed phases. This is in contrast to the case for sodium where the ion chemistry is such that the failure to observe hydrated Na⁺ ions proves that gas phase sodium compounds are not present in the stratosphere.     

Transport of thermospheric NO to the upper stratosphere?

The rate of production of NO in the thermosphere is expected to vary greatly over the course of an 11-year solar cycle because the fluxes of both extreme ultraviolet radiation and auroral particles are known to increase substantially from solar minimum to solar maximum. In the stratosphere, NO participates in a catalytic cycle which constitutes the dominant photochemical destruction mechanism for stratospheric ozone. If appreciable long range transport of NO from the thermosphere to the upper stratosphere occurs, its effects should therefore be manifested in upper atmospheric ozone density variations over the 11-year solar cycle. In this paper, model predictions of the seasonal and latitudinal variations in upper stratospheric O₃ associated with NO transport for different levels of solar activity are compared to satellite observations of upper stratospheric ozone abundances.     

Chemospheric processes of nitric oxide in the mesosphere and stratosphere

The behavior of nitrogen oxides in the stratosphere and mesosphere is discussed with the aid of a model which introduces the photodissociation of nitric oxide and the formation of nitric acid. The profiles of the nitric oxide, nitrogen dioxide and nitric acid concentrations are sensitive to the values of the eddy diffusion coefficients which are adopted. The evaluation of the various reactions which enter the stratosphere shows the role of the formation of nitric acid which is related to the production of OH radicals in the lower stratosphere. An increase of the water vapor in the stratosphere leads to a decrease of nitric oxide and nitrogen dioxide.     

Recombination of small ions in the troposphere and lower stratosphere

The current practice of treating ionic recombination in the lower atmosphere is in error in two respects: firstly the Thomson formula for the ter-molecular recombination coefficient does not represent the behaviour as accurately as commonly assumed: secondly the ter-molecular recombination coefficient (expressed in binary form) and the rate coefficient for the binary recombination of an isolated ion-pair are not additive. Computer simulated experiments are performed which should give the total recombination coefficient in the region below 40 km with some precision. It is shown that the recombination rate is not appreciably affected by inhomogeneity in the cosmic ray ionization. The equilibrium small ion number density profile is calculated and found to be in good agreement with the observed profile. It is inferred that the mixing ratio of the trace gas x which reacts with proton hydrates to form non-proton hydrates has an approximately constant value of 5 × 10⁻¹³ from 20 to 42 km.     

The photodissociation of nitric oxide in the mesosphere and stratosphere

A detailed calculation of the nitric oxide photodissoeiation rate has been made for application to mesospheric and stratospheric photochemistry. It takes into account a new determination of the oscillator strengths of the NO bands and is based on a critical analysis of the solar flux. Moreover it entails a complete determination of the molecular oxygen attenuation allowing for the rotational fine structure including the Voight profile of the Schumann-Runge bands.     

Key reactions in the photochemistry of hydrocarbons in Neptune's stratosphere

We studied the propagation of uncertainties carried by the reaction rate coefficients in the photochemistry of Neptune's stratosphere. We showed that the uncertainties on the mole fractions of main hydrocarbons are equal to or larger than the estimated uncertainties on abundances gathered from observations. From a global sensitivity analysis study, we determined a list of 26 key reactions and discussed the 7 main key reactions that should be studied in priority to lower the uncertainties in the mole fractions computed from a photochemical model. This methodology is essential to improve the predictivity of photochemical models and, consequently, to better understand the physical and chemical processes that govern the composition of giant planet atmospheres.     

Radiation chemistry in the Jovian stratosphere: laboratory simulations

Low-pressure continuous-flow laboratory simulations of plasma induced chemistry in H2/He/CH4/NH3 atmospheres show radiation yields of hydrocarbons and nitrogen-containing organic compounds that increase with decreasing pressure in the range 2-200 mbar. Major products of these experiments that have been observed in the Jovian atmosphere are acetylene (C2H2), ethylene (C2H4), ethane (C2H6), hydrogen cyanide (HCN), propane (C3H8), and propyne (C3H4). Major products that have not yet been observed on Jupiter include acetonitrile (CH3CN), methylamine (CH3NH2), propene (C3H6), butane (C4H10), and butene (C4H8). Various other saturated and unsaturated hydrocarbons, as well as other amines and nitriles, are present in these experiments as minor products. We place upper limits of 10(6)-10(9) molecules cm-2 sec-1 on production rates of the major species from auroral chemistry in the Jovian stratosphere, and calculate stratospheric mole fraction contributions. This work shows that auroral processes may account for 10-100% of the total abundances of most observed organic species in the polar regions. Our experiments are consistent with models of Jovian polar stratospheric aerosol haze formation from polymerization of acetylene by secondary ultraviolet processing.     

The first height measurements of the negative ion composition of the stratosphere

For the first time, height profiles of the stratospheric negative ion composition are presented. The results are from two nights of balloon borne mass spectrometers and cover an altitude range from 23.8 to 38.9 km. Below approx. 30km, NO^?₃ · mHNO₃ ions are dominant. These are replaced by HSO₄^? · nH₂SO₄ · oHNO₃ ions above this height. There are indications that the most abundant ions above 32 km have masses greater than 280 atomic mass units (amu), the instruments' mass range. The fractional ion count rates as a function of altitude are presented and their significance for neutral trace gas analysis and ion sampling is discussed.     

An analysis of the strong zonal circulation within the stratosphere of Venus

A dynamical model is presented for the observed strong zonal circulation within the stratosphere of Venus. The model neglects rotational effects and considers a compressible and radiating atmosphere. It is shown that diurnal radiative heating is negligible within the lower stratosphere, a region below 85km, while observational evidence for the strong zonal circulation pertains to the lower stratosphere within which a direct thermal driving for the circulation is absent. The analysis, however, suggests that propagating internal gravity waves generated by diurnal solar heating of the upper stratosphere induce mean zonal velocities within the upper and lower stratosphere.Considering the linearized equations of motion and energy, and following Stern's (1971) analysis for an analogous problem, it is shown that the zonal velocity induced by internal gravity waves is retrograde in direction, a result which is in agreement with observation. The nonlinear equations of motion and energy are then solved by an approximate analytical method to determine the magnitude of the zonal velocity. This velocity increases from zero at the tropopause to about 200 msec^?1 at the 85 km level. The velocity near the uv-cloud level compares favorably with the observed value of 100 msec^?1.     

Photochemistry of the stratosphere of Venus: Implications for atmospheric evolution

The photochemistry of the stratosphere of Venus was modeled using an updated and expanded chemical scheme, combined with the results of recent observations and laboratory studies. We examined three models, with H₂ mixing ratio equal to 2 × 10^?5, 5 × 10^?7, and 1 × 10^?13, respectively. All models satisfactorily account for the observations of CO, O₂, O₂(¹Δ), and SO₂ in the stratosphere, but only the last one may be able to account for the diurnal behavior of mesospheric CO and the uv albedo. Oxygen, derived from CO₂ photolysis, is primarily consumed by CO₂ recombination and oxidation of SO₂ to H₂SO₄. Photolysis of HCl in the upper stratosphere provides a major source of odd hydrogen and free chlorine radicals, essential for the catalytic oxidation of CO. Oxidation of SO₂ by O occurs in the lower stratosphere. In the high-H₂ model (model A) the OO bond is broken mainly by S + O₂ and SO + HO₂. In the low-H₂ models additional reactions for breaking the OO bond must be invoked: NO + HO₂ in model B and ClCO + O₂ in model C. It is shown that lightning in the lower atmosphere could provide as much as 30 ppb of NO_x in the stratosphere. Our modeling reveals a number of intriguing similarities, previously unsuspected, between the chemistry of the stratosphere of Venus and that of the Earth. Photochemistry may have played a major role in the evolution of the atmosphere. The current atmosphere, as described by our preferred model, is characterized by an extreme deficiency of hydrogen species, having probably lost the equivalent of 10²–10³ times the present hydrogen content.     

A determination of the composition of the Saturnian stratosphere using the IUE

We reduced ultraviolet spectra of Saturn from the IUE satellite to produce a geometric albedo of the planet from 1500 to 3000 Å. By matching computer models to the albedo we determined a chemical composition consistent with the data. This model includes C₂H₂ and C₂H₆ with mixing ratios and distributions of (9 ± 3) × 10^?8 in the top 20 mbar of the atmosphere with none below for C₂H₂ and (6 ± 1) × 10^?6 also in the top 20 mbar with none below for C₂H₆. The C₂H₂ and C₂H₆ distributions and the C₂H₆ mixing ratio are taken directly from the Voyager IRIS model [R. Courtin et al., Bull. Amer. Astron. Soc.13, 722 (1981), and private communication]. The Voyager IRIS model also includes PH₃, which is not consistent with the uv albedo from 1800 to 2400 Å. Our model requires a previously unidentified absorber to explain the albedo near 1600 Å. After considering several candidates, we find that the best fit to the data is obtained with H₂O, having a column density of (6 ± 1) × 10^?3 cm-am.     

Temporal variability of ultraviolet cloud features in the Venus stratosphere

Statistics on the temporal variability of uv cloud features on Venus during 66 days of nominal mission imaging by the Pioneer Venus Orbiter Cloud Photopolarimeter reveal at least five types of systematic variability on large scales: (1) a low-latitude global-scale wave of period 3.94 ± 0.1 days corresponding to longitudinal motion of the dark equatorial band and propagating westward relative to the mean flow; (2) a midlatitude wave of period 5.20 ± 0.2 days corresponding to wavenumber 1 oscillations of the latitude of the bright polar bands and propagating eastward relative to the mean flow; (3) ～2- to 3-week fluctuations in the slope of longitudinal cloud brightness power spectra at intermediate wavenumbers manifested by variations in the intensity of large bow-shaped features; (4) ～2-month variations in polar region brightness consistent with polar brightening episodes observed from Earth; and (5) a monotonic decrease in the disk-integrated brightness of Venus during the nominal mission which may be either a true time variation or a solar-locked longitudinal dependence of brightness. Small-scale features appear to correlate with large-scale albedo patterns. Specifically, cellular features exist primarily where large-scale dark material is present, while the orientation of streak features with respect to latitude circles oscillates with the same ～4-day period as the large-scale features at low latitudes. The wide range of time scales present in the data suggests the complexity of Venus stratospheric dynamics. Extended observations over many years may be becessary to define the general circulation.     

The relative abundance of ethane to acetylene in the Jovian stratosphere

The observed ratio of C2H6 to C2H2 in the Jovian stratosphere increases from approximately 55 at 2 mbar to approximately 277 at 12 mbar. In current photochemical models this ratio typically increases between 2 and 12 mbar by a factor of < or = 3. Recent laboratory kinetics studies on the reaction between C2H3 and H2 to form C2H4 suggest an efficient chemical mechanism for hydrogenation of C2H2 to C2H6. Inclusion of this scheme as part of a comprehensive updated model for hydrocarbon photochemistry in the atmosphere of Jupiter provides an explanation of the altitude variation of the C2H6/C2H2 ratio. The sensitivity of these results to uncertainties in the key rate constants at low temperatures is illustrated, identifying needs for additional laboratory measurements. Since the key reaction rate constants decrease with decreasing temperature, the hydrogenation of C2H2 as proposed predicts a qualitatively decreasing trend in the C2H6/C2H2 value with decreasing distance from the Sun. The observed variation between Jupiter and Saturn is consistent with this prediction.     

Condensation in Titan's stratosphere during polar winter

In Titan's north polar region stratospheric clouds are expected to form due to a combination of low temperatures and downward motion of volatile-enriched air. Here we investigate possible sources of stratospheric clouds at Titan's pole using data from the Cassini Composite Infrared Spectrometer and a simple condensation model. An upper limit for C₄N₂ gas was determined to be 9×¹⁰⁻⁹, which is less than required to make the C₄N₂ cloud at the Voyager epoch. Hence, the presence of this cloud after equinox remains a mystery. The largest cloud seen in far-infrared spectra has a feature around 220 cm⁻¹ and is located around an altitude of 140 km. The upper limit for propionitrile (C₂H₅CN) gas shows that the feature around 220 cm⁻¹ is probably not due to pure propionitrile ice. Instead, our model calculations show that HCN should cause by far the largest cloud around 140 km. We therefore propose that HCN ice plays an important role in the formation of the massive polar cloud, because of the unavailability of sufficient condensable gas other than HCN to produce a strong enough condensate feature. However, the signature at 220 cm⁻¹ is not consistent with that of pure HCN ice at 172 cm⁻¹ and mixing of HCN ice with other ices, or chemical alteration of HCN ice might mask the HCN ice signature.     

Temperature trends in the stratosphere and lower mesosphere of the Northern Hemisphere

Rocketsonde-derived temperature fluctuations within Northern Hemisphere are examined for the stratosphere and lower mesosphere in seasonal basis for the years 1969–78, inclusive. The rocketsonde records presentd here are homogeneous because of are mostly based on the Datasone system. It is suggested that Stratospheric-lower mesospheric temperature variations are about one order of magnitude larger than recorded in the literature before. The main feature in all seasons is that the cooling trend has maximum values at low latitudes in the lower Mesosphere.     

Possible existence and role of excited ozone precursors in the stratosphere

Laboratory kinetic experiments, quantum theoretical studies and certain recently discovered infrared emissions from the stratosphere together imply the possible existence of a metastable, energetic, and reactive form of ozone in the upper atmosphere. This species can potentially affect the upper atmospheric photochemistry and heat balance. For these reasons there is a need for further quantum theoretical and laboratory chemical kinetic studies addressing some of the relevant properties of the internally excited ozone precursors.     

A steady state model of negative ions in the lower stratosphere

A chemical model of negative ions in the lower stratosphere is presented. Under the steady state condition at each height from 15 to 30 km, the fractional abundances of the individual negatively-charged constituents were calculated by simulating the chemical processes which start from electrons and reach the ion clusters having NO₃⁻and HSO₄⁻ cores. The computed result shows that while the NO₃⁻(HNO₃)₂ ions predominate at lower than 26 km in altitude, the HSO₄⁻(HNO₃)_m type of ions increase slowly with height and the HSO₄⁻(H₂SO₄)_n type of ions increase rapidly with height around 30 km. The result is compared with the experimental result which was obtained by the in situ balloon-borne mass spectrometric measurements (Viggiano et al., 1983). Both results have comparable height profile of respective ions with each other. As the “adjusted” reaction rates for the formation of cluster ions were assumed in the calculations, the result should be considered a preliminary one. Future works to be extended from the present one will be suggested.     

Condensate clouds in Titan's north polar stratosphere

Analysis of the 250-560 cm⁻¹ spectral continuum of Titan's north polar hood just after spring equinox reveals, in addition to the ubiquitous aerosol, a tenuous but relatively uniform cloud of small particles permeating the lower stratosphere at altitudes between 58 and 90 km. Voyager 1 IRIS data suggest the particles are highly scattering, almost certainly condensed organics, with radii between 1 and 5 μm. Mole fractions for the condensed material range between 4×¹⁰⁻⁸ and 4×¹⁰⁻⁶, depending upon particle size. Vapor pressure arguments imply condensed nitriles near 90 km, the most likely being HCN, with condensed hydrocarbons such as C₂H₆ restricted to regions considerably nearer the tropopause. No direct chemical identification is possible. Negligible methane supersaturation in the troposphere at 67.4° N latitude, when compared with degrees of supersaturation at other latitudes, hints at precipitation fluxes of north polar stratospheric condensates during the previous northern winter that were perhaps three orders of magnitude greater than those at low latitudes during that time. A scale height of 1.5 times the density scale height above 160 km is reaffirmed for the photochemical aerosol of the north polar hood. There appears to be a depletion of aerosol somewhere below 160 km. An aerosol mole fraction ∼8×¹⁰⁻⁸ at 160 km is inferred, about 33% greater than the value derived in a previous study. The Cassini CIRS instrument, with its expanded spectral range and higher spectral resolution, should be able to provide highly complementary information for the time period covering most of the northern winter season.     

Characterization of zonal winds in the stratosphere of Titan with UVES

Titan has been observed with UVES, the UV-Visual Echelle Spectrograph at the Very Large Telescope, with the aim of characterizing the zonal wind flow. We use a retrieval scheme originally developed for absolute stellar accelerometry [Connes, P., 1985. Astrophys. Space Sci., 110, 211-255] to extract the velocity signal by simultaneously taking into account all the lines present in the spectrum. The method allows to measure the Doppler shift induced at a given point by the zonal wind flow, with high precision. The short-wavelength channel (4200-5200 Å) probes one scale height higher than the long-wavelength one (5200-6200 Å), and we observe statistically significant evidence for stronger winds at higher altitudes. The results show a high dispersion. Globally, we detect prograde zonal winds, with lower limits of 62 and 50 m s⁻¹ at the regions centered at 200 and 170 km altitude, but approximately a quarter of the measurements indicates null or retrograde winds.