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1.
G. Kockarts 《Planetary and Space Science》1976,24(6):589-604
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. 相似文献
2.
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. 相似文献
3.
The electromagnetic and particle cascade resulting from the absorption of galactic cosmic rays in the atmosphere of Titan is shown to be an important mechanism for driving the photochemistry at pressures of 1 to 50 mbar in the atmosphere. In particular, the cosmic ray cascade dissociates N2, a process necessary for the synthesis of nitrogen organics such as HCN. The important interactions of the cosmic ray cascade with the atmosphere are discussed. The N2 excitation and dissociation rates and the ionization rates of the principal atmospheric constituents are computed for a Titan model atmosphere that is consistent with Voyager 1 observations. It is suggested that HCN may be formed efficiently in the lower atmosphere through the photodissociation of methylamine. It is also argued that models of nitrogen and hydrocarbon photochemistry in the lower atmosphere of Titan should include the absorption of galactic cosmic rays as an important energy source. 相似文献
4.
The available solar flux at a given altitude in the atmospheres of Mars and Venus is attenuated mainly by CO2 (molecular absorption and Rayleigh scattering) with an extra contribution due to SO2 on Venus. The dissociation cross section of CO2 depends on temperature. At temperatures appropriate for these atmospheres (～250°K), the cross sections are about 15% lower than those at room conditions (Y.L. Yung and W.B. De More, 1982, Icarus, 51, 199). It is shown that this temperature effect cannot be neglected in the evaluation of photolysis rates. Calculations of the photodissociation coefficients of CO2, SO2, HCl, and H2O are presented. For example, at the surface of Mars, the coefficient of H2O is nearly multiplied by a factor of 10! 相似文献
5.
A model of the atmospheric structure of Uranus is presented which differs from previous types of models in two important respects: (1) The CH4/H2 ratio is sufficiently large that CH4 is saturated to large depths in the Uranian atmosphere. (2) The internal energy flux is small compared with that due to solar heating. Because of the small internal flux, the thermal flux decreases rapidly with depth and the atmosphere is radiative to large optical depths. A CH4 droplet cloud forms where the atmosphere finally becomes convective due to the internal flux. The model is shown to be in reasonable agreement with published observations of the H2 quadrupole 3-0 and 4-0 bands, the visible (4000–6000 Å) CH4 bands, and the infrared emission spectrum. 相似文献
6.
We examine the effects of NH3 ice particle clouds in the atmosphere of Jupiter on outgoing thermal radiances. The cloud models are characterized by a number density at the cloud base, by the ratio of the scale height of the vertical distribution of particles (Hp) to the gas scale height (Hg), and by an effective particle radius. NH3 ice particle-scattering properties are scaled from laboratory measurements. The number density for the various particle radius and scale height models is inferred from the observed disk average radiance at 246 cm?1, and preliminary lower limits on particle sizes are inferred from the lack of apparent NH3 absorption features in the observed spectral radiances as well as the observed minimum flux near 2100 cm?1. We find lower limits on the particle size of 3 μm if Hp/Hg = 0.15, or 10μmif Hp/Hg = 0.50 or 0.05. NH3 ice particles are relatively dark near the far-infrared and 8.5-μm atmospheric windows, and the outgoing thermal radiances are not very sensitive to various assumptions about the particle-scattering function as opposed to radiances at 5 μm, where particles are relatively brighter. We examined observations in these three different spectral window regions which provide, in principle, complementary constraints on cloud parameters. Characterization of the cloud scale height is difficult, but a promising approach is the examination of radiances and their center-to-limb variation in spectral regions where there is significant opacity provided by gases of known vertical distribution. A blackbody cloud top model can reduce systematic errors due to clouds in temperature sounding to the level of 1K or less. The NH3 clouds provide a substantial influence on the internal infrared flux field near the 600-mbar level. 相似文献
7.
Some astrophysical observations of molecular hydrogen point to a broadening of the velocity distribution for molecules at excited rotational levels. This effect is observed in both Galactic and high-redshift clouds. Analysis of H2, HD, and CI absorption lines has revealed the broadening effect in the absorption system of QSO 1232+082 (z abs = 2.33771). We analyze line broadening mechanisms by considering in detail the transfer of ultraviolet radiation (in the resonance lines of the Lyman and Werner H2 molecular bands) for various velocity distributions at excited rotational levels. The mechanism we suggest includes the saturation of the lines that populate excited rotational levels (radiative pumping) and manifests itself most clearly in the case of directional radiation in the medium. Based on the calculated structure of a molecular hydrogen cloud in rotational level populations, we have considered an additional mechanism that takes into account the presence of a photodissociation region. Note that disregarding the broadening effects we investigated can lead to a significant systematic error when the data are processed. 相似文献
8.
Lj. M. Ignjatovi&#; A. A. Mihajlov N. M. Sakan M. S. Dimitrijevi&#; A. Metropoulos 《Monthly notices of the Royal Astronomical Society》2009,396(4):2201-2210
The main aim of this work is to estimate the total contribution of the processes of molecular ion photodissociation and collisional absorption charge exchange to the opacity of DB white dwarf atmospheres, and compare this with the contribution of and other relevant radiative absorption processes included in standard models.
The method for the calculations of the molecular ion photodissociation cross-sections is based on the dipole approximation and quantum-mechanical treatment of the internuclear motion, while the quasi-classical method for describing absorption processes in collisions is based on the quasi-static approximation.
Absorption coefficients are calculated in the region 50 nm ≤λ≤ 850 nm and compared with the corresponding coefficients of other relevant absorption processes; the calculations of the optical depth of the atmosphere layers considered are performed in the far-UV and VUV regions; the contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres is examined.
We examined the spectral ranges in which the total and absorption processes dominate in particular layers of DB white dwarf atmospheres. In addition, we show that in the region of λ≲ 70 nm the process of atom photoionization is also important, in spite of the fact that the ratio of hydrogen and helium abundances in the DB white dwarf atmosphere considered is 1:105 . 相似文献
The method for the calculations of the molecular ion photodissociation cross-sections is based on the dipole approximation and quantum-mechanical treatment of the internuclear motion, while the quasi-classical method for describing absorption processes in collisions is based on the quasi-static approximation.
Absorption coefficients are calculated in the region 50 nm ≤λ≤ 850 nm and compared with the corresponding coefficients of other relevant absorption processes; the calculations of the optical depth of the atmosphere layers considered are performed in the far-UV and VUV regions; the contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres is examined.
We examined the spectral ranges in which the total and absorption processes dominate in particular layers of DB white dwarf atmospheres. In addition, we show that in the region of λ≲ 70 nm the process of atom photoionization is also important, in spite of the fact that the ratio of hydrogen and helium abundances in the DB white dwarf atmosphere considered is 1:10
9.
We have used the measurements of the solar flux obtained by the Venera 8 spacecraft inside the atmosphere of Venus and the values of the Venus spherical albedo to deduce the characteristics of the clouds and of the ground. The method used is the exponential kernel approximation and the results have been tested by exact computations with the spherical harmonics method.A cloud layer with an optical thickness , an albedo for single scattering in the rear infrared, above a Rayleigh layer between 0 and 32 km and a ground of reflectivity ? = 0.4, gives a good agreement with the experimental results. A model with two cloud layers is also discussed. 相似文献
10.
It is shown that photoionization of vibrationally excited H2 and photodissociation of the H 2 + ions produced thereby constitute a significant electron production route in high UV flux situations. A significant fraction of the electron density in the direction of ζ Oph (?15 km s?1 cloud) deduced from observations may be expected to arise in this way. 相似文献
11.
Marcel Nicolet 《Planetary and Space Science》1985,33(1):69-80
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. 相似文献
12.
Darrell F. Strobel 《Icarus》2010,208(2):878-886
The third most abundant species in Titan’s atmosphere is molecular hydrogen with a tropospheric/lower stratospheric mole fraction of 0.001 derived from Voyager and Cassini infrared measurements. The globally averaged thermospheric H2 mole fraction profile from the Cassini Ion Neutral Mass Spectrometer (INMS) measurements implies a small positive gradient in the H2 mixing ratio from the tropopause region to the lower thermosphere (∼950-1000 km), which drives a downward H2 flux into Titan’s surface comparable to the H2 escape flux out of the atmosphere (∼2 × 1010 cm−2 s−1 referenced to the surface) and requires larger photochemical production rates of H2 than obtained by previous photochemical models. From detailed model calculations based on known photochemistry with eddy, molecular, and thermal diffusion, the tropospheric and thermospheric H2 mole fractions are incompatible by a factor of ∼2. The measurements imply that the downward H2 surface flux is in substantial excess of the speculative threshold value for methanogenic life consumption of H2 (McKay, C.P., Smith, H.D. [2005], Icarus 178, 274-276. doi:10.1016/j.icarus.2005.05.018), but without the extreme reduction in the surface H2 mixing ratio. 相似文献
13.
V.A. Krasnopolsky 《Planetary and Space Science》1985,33(1):109-117
From estimates of drying effect in the cloud layer, data of the Venera 14 X-ray fluorescent spectroscopy, and evaluation of photochemical production of sulfuric acid, it follows that sulfuric acid and/or products of its further conversion should constitute not only the Mode 2 particles but most of the Mode 3 particles as well. The eddy mixing coefficient equals 2 × 104 cm2 s?1 in the cloud layer. The presence of ferric chloride in the cloud layer is indicated by the Venus u.v. absorption spectrum in the range of 3200–5000 Å, by the Venera 12 X-ray fluorescent spectrum, by the coincidence of the calculated FeCl3 condensate density profile and that of the Mode 1 in the middle and lower cloud layer, as well as by the upward flux of FeCl3 from the middle cloud layer which provides the necessary concentration of FeCl3 in H2SO4 solution. FeCl3 as the second absorber explains the localization of absorption in the upper cloud layer due to the FeCl3 conversion to ferric sulfate near the boundary between the upper and middle cloud layers. Other possible absorbers such as sulfur, ammonium pyrosulfite, nitrosylsulfuric acid, etc. are discussed. 相似文献
14.
《Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science》2001,26(7):505-511
There has been a considerable research interest in molecular aggregates, complexes or clusters, relevant to Earth's and other planetary atmospheres. Consequently, a considerable amount of data has been obtained in laboratory observations and also in computations. This report surveys our recent and ongoing computations of several such systems of atmospheric significancy, both systems with relatively weak and relatively strong bonding interactions. Among them, several types of homo- and hetero-dimers with importance to Earth's atmosphere: (H2O)2, (N2)2, N2O2, , (O2)2, or (O3)2. Dimer of carbon dioxide (CO2)2 is computed owing to its supposed significancy in the atmosphere of Venus. Systems with stronger bonding are represented by ClONO2H+ and 2,3,7,8-tetrachlorodibenzo-p-dioxin. The report also discusses computational tools, combining advanced quantum-chemical methods with statistical-mechanical treatments. The structure, energetics, and vibrations of the complexes are evaluated at correlated ab initio levels. The computations typically show several minimum-energy structures and their relative populations are sensitive to temperature. The computed dimerization equilibrium constants are of a special interest as a critical stability measure and an input information for evaluations of the altitude population profiles in the atmosphere. A special attention is paid to the temperature enhancement of clustering degree in saturated vapors. This interesting paradox represents a product of atmospheric studies though it is actually a phenomenon of a more general physico-chemical validity. 相似文献
15.
A two-dimensional kinetic model calculation for the water group species (H2O, H2, O2, OH, O, H) in Europa's atmosphere is undertaken to determine its basic compositional structure, gas escape rates, and velocity distribution information to initialize neutral cloud model calculations for the most important gas tori. The dominant atmospheric species is O2 at low altitudes and H2 at higher altitudes with average day-night column densities of 4.5×1014 and 7.7×1013 cm−2, respectively. H2 forms the most important gas torus with an escape rate of ∼2×1027 s−1 followed by O with an escape rate of ∼5×1026 s−1, created primarily as exothermic O products from O2 dissociation by magnetospheric electrons. The circumplanetary distributions of H2 and O are highly peaked about the satellite location and asymmetrically distributed near Europa's orbit about Jupiter, have substantial forward clouds extending radially inward to Io's orbit, and have spatially integrated cloud populations of 4.2×1033 molecules for H2 and 4.0×1032 atoms for O that are larger than their corresponding populations in Europa's local atmosphere by a factor of ∼200 and ∼1000, respectively. The cloud population for H2 is a factor of ∼3 times larger than that for the combined cloud population of Io's O and S neutral clouds and provides the dominant neutral population beyond the so-called ramp region at 7.4-7.8 RJ in the plasma torus. The calculated brightness of Europa's O cloud on the sky plane is very dim at the sub-Rayleigh level. The H2 and O tori provide a new source of europagenic molecular and atomic pickup ions for the thermal plasma and introduce a neutral barrier in which new plasma sinks are created for the cooler iogenic plasma as it is transported radially outward and in which new sinks are created to alter the population and pitch angle distribution of the energetic plasma as it is transported radially inward. The europagenic instantaneous pickup ion rates are peaked at Europa's orbit, dominate the iogenic pickup ion rates beyond the ramp region, and introduce new secondary plasma source peaks in the solution of the plasma transport problem. The H2 torus is identified as the unknown Europa gas torus that creates both the observed loss of energetic H+ ions at Europa's orbit and the corresponding measured ENA production rate for H. 相似文献
16.
17.
Vladimir A. Krasnopolsky 《Icarus》2012,218(1):230-246
The model is intended to respond to the recent findings in the Venus atmosphere from the Venus Express and ground-based submillimeter and infrared observations. It extends down to 47 km for comparison with the kinetic model for the lower atmosphere (Krasnopolsky, V.A. [2007]. Icarus 191, 25–37) and to use its results as the boundary conditions. The model numerical accuracy is significantly improved by reduction of the altitude step from 2 km in the previous models to 0.5 km. Effects of the NUV absorber are approximated using the detailed photometric observations at 365 nm from Venera 14. The H2O profile is not fixed but calculated in the model. The model involves odd nitrogen and OCS chemistries based on the detected NO and OCS abundances. The number of the reactions is significantly reduced by removing of unimportant processes. Column rates for all reactions are given, and balances of production and loss may be analyzed in detail for each species.The calculated vertical profiles of CO, H2O, HCl, SO2, SO, OCS and of the O2 dayglow at 1.27 μm generally agree with the existing observational data; some differences are briefly discussed. The OH dayglow is ～30 kR, brighter than the OH nightglow by a factor of 4. The H + O3 process dominates in the nightglow excitation and O + HO2 in the dayglow, because of the reduction of ozone by photolysis. A key feature of Venus’ photochemistry is the formation of sulfuric acid in a narrow layer near the cloud tops that greatly reduces abundances of SO2 and H2O above the clouds. Delivery of SO2 and H2O through this bottleneck determines the chemistry and its variations above the clouds. Small variations of eddy diffusion near 60 km result in variations of SO2, SO, and OCS at and above 70 km within a factor of ～30. Variations of the SO2/H2O ratio at the lower boundary have similar but weaker effect: the variations within a factor of ～4 are induced by changes of SO2/H2O by ±5%. Therefore the observed variations of the mesospheric composition originate from minor variations of the atmospheric dynamics near the cloud layer and do not require volcanism. NO cycles are responsible for production of a quarter of O2, SO2, and Cl2 in the atmosphere. A net effect of photochemistry in the middle atmosphere is the consumption of CO2, SO2, and HCl from and return of CO, H2SO4, and SO2Cl2 to the lower atmosphere. These processes may be balanced by thermochemistry in the lower atmosphere even without outgassing from the interior, though the latter is not ruled out by our models. Some differences between the model and observations and the previous models are briefly discussed. 相似文献
18.
We report on our results of modelling the physical processes in the uppermost layers of strongly irradiated atmospheres of
low mass unevolved companions in Precataclysmic Binaries (PCBs). Reprocessing of Lyman continuum L
c
radiation from the hot subdwarf (sdw) primary is studied in detail. We solve explicitly a set of equations of hydrostatic,
ionization and thermal equilibrium to calculate the intensity of the reprocessed emergent radiation in recombinations for
an optically thin plasma. We consider contributions from free-free and bound-free transitions. Diffuse radiation is taken
into account, but the effects of self-absorption of the re-emitted radiation are neglected.
For a purely hydrogenic atmosphere and typical values of incident fluxes, densities, gas pressures for the irradiated upper
atmospheric layers, we find that the L
c
radiation will be absorbed and re-emitted in recombinations within a column of effective thickness (106–108) cm of HII, depending on the electron density, the effective temperature of the sdw and the separation between the components. One of
the non-trivial results of our model computations lies in the overheating of the uppermost layers of the irradiated atmosphere:
the equilibrium temperature of the gas turns out to be considerably higher than the blackbody temperature following from the
diluted incoming radiation of sdw. Energy balance considerations reveal that roughly 25–50% of the L
c
flux is expended on the ionization of H and is re-emitted subsequently in recombinations. The remaining portion of the L
c
flux is spent on readjustment of the uppermost layers of the irradiated atmosphere. One of the consequences is the onset
of thermal instability in these layers. 相似文献
19.
Results of the scattered solar radiation spectrum measurements made deep in the Venus atmosphere by the Venera 11 and 12 descent probes are presented. The instrument had two channels: spectrometric (to measure downward radiation in the range 0.45 < γ < 1.17 μm) and photometric (four filters and circular angle scanning in an almost vertical plane). Spectra and angular scans were made in the height range from 63 km above the planet surface. The integral flux of solar radiation is 90 ± 12 W m?2 measured on the surface at the subsolar point. The mean value of surface absorbed radiation flux per planetary unit area is 17.5 ± 2.3 W m?2. For Venera 11 and 12 landing sites the atmospheric absorbed radiation flux is ～15 W m?2 for H >; 43 km and ～45 W m?2 for H < 48 km in the range 0.45 to 1.55 μm. At the landing sites of the two probes the investigated portion of the cloud layer has almost the same structure: it consists of three parts with boundaries between them at about 51 and 57 km. The base of clouds is near 48 km above the surface. The optical depth of the cloud layer (below 63 km) in the range 0.5 to 1 μm does not depend on the wavelength and is ～29 and ～38 for the Venera 11 and 12 landing sites, respectively. The single-scattering albedo, ω0, in the clouds is very close to 1 outside the absorption bands. Below 58 km the parameter (1 ? ω0) is <10?3 for 0.49 and 0.7 μm. The parameter (1 ? ω0) obviously increases above 60 km. Below 48 km some aerosol is present. The optical depth here is a strong function of wavelength. It varies from 1.5 to 3 at λ = 0.49 μm and from 0.13 to 0.4 at 1.0 μm. The mean size of particles below the cloud deck is about 0.1 μm. Below 35 km true absorption was found at λ < 0.55 μm with the (1 ? ω0) maximum at H ≈ 15 km. The wavelength and height dependence of the absorption coefficient are compatible with the assumption that sulfur with a mixing ratio ～2 × 10?8 normalized to S2 molecules is the absorber. The upper limits of the mixing ratio for Cl2, Br2, and NO2 are 4 × 10?8, 2 × 10?11, and 4 × 10?10, respectively. The CO2 and H2O bands are confidently identified in the observed spectra. The mean value of the H2O mixing ratio is 3 × 10?5 < FH2O < 10?4 in the undercloud atmosphere. The H2O mixing ratio evidently varies with height. The most probable profile is characterized by a gradual increase from FH2O = 2 × 10?5 near the surface to a 10 to 20 times higher value in the clouds. 相似文献
20.
Several models for the atmosphere of Uranus are considered. If the H2 abundance is less than 250 km-am and the internal heat source is only a few percent of the total emitted energy then the cloud at the base of the atmosphere may be composed of solid CH4 particles, while if the H2 abundance is greater than 250 km-am or if the internal heat source is near the current upper limit of 35% of the total emitted energy the cloud at the base of the atmosphere may be composed of either solid NH3 or H2S particles. 相似文献