Passive remote sensing of aerosols from space now and in the future

Aerosols modify scattered solar radiation leaving the atmosphere and this fact will be exploited to determine the aerosol optical depth. The interaction processes between solar radiation and aerosol particles are outlined. A quasi-linear relationship (‘conversion curves’) between the radiance at the satellite, L_sat, and the aerosol optical depth, a_Dλ, is found from both numerical and empirical studies. Because L_sat is not only controlled by a_Dλ, but also by a series of other atmospheric parameters (perturbing quantities), the concept of ‘favourable viewing conditions’ is presented, where the effects of the perturbing quantities are minimal. The paper ends with some lines of thought on a concept for a turbidity satellite.     

Organic chemistry in Titan's atmosphere: new data from laboratory simulations at low temperature.

Many experiments have already been carried out to simulate organic chemistry on Titan, the largest satellite of Saturn. They can provide fruitful information on the nature of minor organic constituents likely to be present in Titan's atmosphere, both in gas and aerosol phases. Indeed, all the organic compounds but one already detected in Titan's atmosphere have been identified in simulation experiments. The exception, C4N2, as well as other compounds expected in Titan from theoretical modeling, such as other N-organics, and polyynes, first of all C6H2, have never been detected in experimental simulation thus far. All these compounds are thermally unstable, and the temperature conditions used during the simulation experiments were not appropriate. We have recently started a new program of simulation experiments with temperature conditions close to that of Titan's environment. It also uses dedicated analytical techniques and procedures compatible with the analysis of organics only stable at low temperatures, as well solid products of low stability in the presence of O2 and H2O. Spark discharge of N2-CH4 gas mixtures was carried out at low temperature in the range 100-150 K. Products were analysed by FTIR, GC and GC-MS techniques. GC-peaks were identified by their mass spectrum, and, in most cases, by comparison of the retention time and mass spectrum with standard ones. We report here the first detection in Titan simulation experiments of C6H2 and HC5N. Their abundance is a few percent relative to C4H2 and HC3N, respectively. Preliminary data on the solid products indicate an elemental composition corresponding to (H11C11N)n. These results open new prospects in the modeling of Titan's haze making.     

Comparison of ground reflectance measurement with satellite derived atmospherically corrected reflectance: A case study over semi-arid landscape

Atmospheric corrections to satellite data are important for comparing multitemporal data sets over tropical regions with variable aerosol loading. In this study, we evaluated the potential of 6S radiative transfer model for atmospheric corrections of IRS-P6 AWiFS satellite data sets, in a semi-arid landscape. Ground measurements of surface reflectance representing different land use/land cover categories were conducted to relate IRS-P6 AWiFS top of atmospheric reflectance. The 6S radiative transfer model was calibrated for local conditions using ground measurements for aerosol optical depth, water vapor and ozone with a sun photometer. Surface reflectance retrieved from 6S code was compared with top of atmosphere (TOA) reflectance and ground based spectroradiometer measurements. Accurate parameterization of the 6S model using measurements of aerosol optical depth, water vapor and ozone plays an important role while comparing ground and satellite derived reflectance measurements.     

The cold summer mesopause

One of the most characteristic features of the summer mesopause at high latitudes is the very low temperature. Earlier measurements have shown temperatures in the range down to 135 K around 86 km altitude, whereas the most recent $\text{in situ}$ measurements have revealed temperatures still much lower than that in a rather wide altitude region. The reasons for these low temperatures are to be found in the dynamics of the strato- and mesospheres. Upwinds and gravity wave activity over the summer hemisphere cause efficient cooling of the atmosphere.Also other effects are caused by the updrafts. The vertical transport velocity for important minor constituents is increased, which for instance causes the concentration of water vapor around the mesopause to be enhanced by large factors. This situation is of major importance for the possibility of forming noctilucent clouds (NLC).NLC are believed to be composed of small water ice particles, which because of the low temperatures can be formed on existing condensation nuclei. Two of the main questions regarding the formation of NLC concern the water vapor budget of the upper mesosphere and the origin of the condensation nuclei.This paper gives a general introduction to mesospheric physics and composition. Some results from recent satellite and rocket experiments are reviewed and the campaign layout and the performed experiments within the MAP project CAMP are described. The results from the different experiments are presented in four accompanying papers on CAMP results.     

A study of reaction wheel configurations for a 3-axis satellite attitude control

The satellite reaction wheel’s configuration plays also an important role in providing the attitude control torques. Several configurations based on three or four reaction wheels are investigated in order to identify the most suitable orientation that consumes a minimum power. Such information in a coherent form is not summarized in any publication; and therefore, an extensive literature search is required to obtain these results. In addition, most of the available results are from different test conditions; hence, making them difficult for comparison purposes. In this work, the standard reaction wheel control and angular momentum unloading schemes are adopted for all the reaction wheel configurations. The schemes will be presented together with their governing equations, making them fully amenable to numerical treatments. Numerical simulations are then performed for all the possible reaction wheel configurations with respect to an identical reference mission. All the configurations are analyzed in terms of their torques, momentums and attitude control performances. Based on the simulations, the reaction wheel configuration that has a minimum total control torque level is identified, which also corresponds to the configuration with minimum power consumption.     

Hypotheses on the appearance of life on Earth (review).

It is generally accepted within the natural sciences that life emerged on Earth by a kind of proto-Darwinian evolution from molecular assemblies that were predominantly formed from the various constituents of the primitive atmosphere and hydrosphere. Evolutionary stages under discussion are: the self-organization of spontaneously formed biomolecules into early precursors of life (protobionts), their stepwise evolution via (postulated) protocells to (postulated) progenotes and the Darwinian evolution from progenotes to the three kingdoms of contemporary organisms (archaebacteria, eubacteria and eukaryotes). Considerable discrepancies between scientists have arisen because all evolutionary stages from prebiotic molecules to progenotes are entirely hypothetical and so are the postulated environmental conditions. We can only theorize that all those environmental conditions that allow the existence of the various forms of contemporary life might have allowed also the development of their precursors. Because of all these difficulties the hypothesis that life came to our planet from a remote place of our universe (panspermia) has been revived. But experimental evidence only supports the view that spores can--under favorable circumstances--survive a relatively short journey within our solar system (interplanetary transfer of life). It is extremely unlikely that spores can survive a journey of hundreds or thousands of years through interstellar space.     

Ultraviolet observations of Jupiter from Earth-orbiting satellites

The International Ultraviolet Explorer (IUE) has provided both improved spectral resolution and some spatial resolution for UV observations of Jupiter. Previous satellite observations have produced albedo curves for Jupiter showing the influence of Rayleigh scattering, and of some absorber(s) shortward of 2500Å on the UV spectrum. Constraints on the abundance of several minor constituents of the Jovian atmosphere were derived from the OAO-2 data. The IUE low dispersion data has a resolution of 8Å, making it possible to detect individual molecular features. A series of C₂H₂ absorptions in the 1750Å region have been identified, and indications of NH₃ absorptions are present in the 1950Å region.     

Bayesian approach to retrieval of vertical ozone profile from radiometry data

A technique for retrieving vertical distributions (profiles) of atmospheric gas constituents from data of passive remote sensing of the atmosphere is proposed. The goal of the technique based on the statistical (Bayesian) approach to solution of inverse problems is construction of probability distribution for a sought quantity throughout the interval of the studied heights. It is assumed that initial data contain measurement noise, and a priori information about properties of the profile is used. It is proposed to approximate the sought profile by a function in the form of an artificial neural network. This approximation allows optimal inclusion of a priori information into retrieval procedure, thus ensuring the most effective regularization of the problem. Efficiency of the proposed technique is demonstrated on an example of retrieval of vertical ozone profile from data of ground-based sounding of the atmosphere in the millimeter wavelength range. Results of profile retrieval from model data and from spectra of radiation temperature of the atmosphere measured in the Apatity (67° N, 33° E) in the winter of 2002–2003 are presented.     

International co-operation in the use of satellites for the world climate programme

The World Climate Programme (WCP), in dealing with the complex topic of climate, is highly dependent on observations and measurements of many parameters and phenomena occurring from the surface of the Earth to the top of the atmosphere, and global in extent. Satellite observations and measurements are therefore critical to the success of many different components in the WCP. The present network of polar-orbiting and geostationary satellites represents nearly 25 years of international co-operation and now constitutes a part of the Global Observing System of the World Weather Watch. The WCP can satisfy a number of its observation and measurement requirements by making use of this existing satellite network. This can be done either through use of the operational products produced for near-real time applications or through use of the satellite data stored in the archives. An awareness of how to interact with the sources, combined with knowledge about the limitations and deficiencies of satellite data and products, are critical for scientists working in climate research and applications. Among the most important characteristics of satellite observations and measurements for the WCP are the global coverage, consistency and continuity of the data sets.     

Ozone variability related to several SEP events occurring during solar cycle no. 23

Data from geostationary operational environmental satellite (GOES) series were used to identify intense solar energetic particle (SEP) events occurred during the solar activity cycle no. 23. We retrieved O₃, NO, NO₂, HNO₃, OH, HCl and OHCl profiles coming from different satellite sensors (solar occultation and limb emission) and we looked for the mesospheric/stratospheric response to SEPs at high terrestrial latitudes. The chemistry of the minor atmospheric components is analysed to evaluate the associated odd nitrogen (NO_x) and odd hydrogen (HO_x) production, able to cause short (h) and medium (days) term ozone variations. We investigated the effects of SEPs on the polar atmosphere in three different seasons, i.e., January 2005, April 2002 and July 2000. The inter-hemispheric variability of the ozone, induced by the SEP series of January 2005, has been compared with the effects connected both to larger and quite similar events. We found that during SEP events: (i) solar illumination is the key factor driving SEP-induced effects on the chemistry of the polar atmosphere; (ii) even events with limited particle flux in the range 15–40 MeV are able to change the abundance of the minor constituents in the mesosphere and upper stratosphere.     

Results from the LIMS experiment for the PMP-1 winter 1978/79

The Limb Infrared Monitor of the Stratosphere (LIMS) experiment utilized a 6-channel limb scanning infrared radiometer on the Nimbus 7 spacecraft, for the purpose of determining global distributions of temperature and trace constituents in the middle atmosphere (15–65 km) with high vertical resolution.LIMS observations provide a detailed picture of atmospheric variability during the disturbed winter of 1978/79. During January and February, three large disturbances caused simultaneous temperature changes through most of the global middle atmosphere. The amplitudes and phases of the planetary temperature and height waves show large variations during this time. Their values and derived quantities are larger than those resulting from data with lower vertical resolution. The Eliassen-Palm fluxes calculated for late January are in qualitative agreement with the observed acceleration of the zonal winds.     

On the influence of rough water surfaces on polarimetric investigations of aerosols from space

The equation of radiative transfer is solved for the complete Stokes' vector in order to study the polarization of solar radiation on top of a turbid atmosphere.The surface characteristics (bidirectional anisotropy and polarization properties of randomly oriented water surface elements) are contained in the lower boundary condition of the integro-differential equation. Results of surface model calculations are compared with polarimeter measurements of a natural scenario.On top of the atmosphere the degree of polarization is found to depend generally on the turbidity of the atmosphere. The degree of polarization reaches its maximum in the vicinity of the angle of specular reflection on the water surface, if the sun's zenith distance is about the Brewster's angle.According to the model calculations observation angles can be found where the degree of polarization does not depend on the surface roughness. Apparently these angles have to be preferred for determination of the atmospheric turbidity.     

基于航天器解体事件的空间碎片寿命算法

研究了针对航天器解体事件所生成的空间碎片的寿命计算方法.给出了基于NASA标准航天器解体模型的航天器解体算法.该算法生成的一系列碎片参数,将作为寿命计算的初始条件.总结了现有求解碎片寿命的算法,并提出了一种半分析算法.该算法运用平均根数法的思路,计算了在J2摄动项的影响下,碎片的半长轴和偏心率的变化率;并采用微分积分法预报半长轴和偏心率随时间的变化.为了适应时变大气模型,该算法限制了计算步长.通过与数值法的比较分析了算法的计算速度和精度.选用了3种大气模型:SA76、GOST和MSIS-00,分析了不同大气模型在计算碎片寿命之间的差异.通过与P-78卫星解体事件的实测数据对比验证了整个算法的正确性.      

Thermospheric storm effects

A brief review is given of our current understanding of the atmospheric perturbations in the thermosphere and exosphere that are related to geomagnetic disturbances and of current efforts to represent these in empirical models of the upper atmosphere. A particular model, based on ESRO4 mass spectrometer observations of neutral composition and density, is presented in detail. This model gives the effects on the principal constituents of the upper atmosphere as a function of the geomagnetic coordinates and the K_p geomagnetic index. It is a modification of an earlier model, the most important difference being the inclusion of the variation with magnetic local time.     

Proposed ozone reference models for the middle atmosphere

Since the publication of the last COSPAR International Reference Atmosphere (CIRA 72), large amounts of ozone data acquired from satellites have become available in addition to increasing quantities of rocketsonde, balloonsonde, Dobson, M83, and Umkehr measurements. From the available archived satellite data, models are developed for the new CIRA using 5 satellite experiments (Nimbus 7 SBUV and LIMS, AEM-2 SAGE, and SME IR and UVS) of the monthly latitudinal and altitudinal variations in the ozone mixing ratio in the middle atmosphere. Standard deviations and interannual variations are also quantified. The satellite models are shown to agree well with a previous reference model based on rocket and balloon measurements.     

Interaction of a mixed yeast culture in an "autotroph-heterotroph" system with a closed atmosphere cycle and spatially separated components.

The study considers an experimental model of the "autotroph-heterotroph" system with a closed atmosphere cycle, in which the heterotrophic link is a mixed yeast population. The autotrophic link is represented by the algae Chlorella vulgaris and the heterotrophic link by the yeasts Candida utilis and Candida guilliermondii. The controls are populations of Chlorella and the same yeasts isolated from the atmosphere. It has been shown that the outcome of competition in the heterotrophic link depends on the strategy of the yeast population towards the substrate and oxygen. The C. utilis population quickly utilizes the substrate as it is an r-strategist and is less sensitive to oxygen deficiency. The C. guilliermondii population consumes low concentrations of the substrate because it is a K-strategist, but it is more sensitive to oxygen deficiency. That is why, in the "autotroph-heterotroph" system with a closed gas cycle, after a considerable amount of the substrate has been consumed, the C. guilliermondii population becomes more competitive that the C. utilis population. In the culture of yeasts, isolated from the atmosphere, the C. utilis population finds itself in more favorable conditions due to oxygen deficiency. The system with a complex heterotrophic component survive longer than a system whose heterotrophic component is represented by only one yeast species. This is explained for by the positive metabolite interaction of yeasts and a more complete utilization of the substrate by a mixed culture of yeasts featuring different strategies towards the substrate.     

A facility for simulating Titan’s environment

As a result of measurements acquired by the Cassini–Huygens mission of Titan’s near surface atmospheric composition and temperature, Titan conditions can now be simulated in the laboratory and samples can subsequently be subjected to those conditions. Titan demonstrates an active hydrological-like cycle with its thick atmosphere, dynamic clouds, polar lakes of methane and ethane, moist regolith, and extensive fluvial erosive features. Unlike Earth, Titan’s hydrological-like cycle likely involves several constituents, primarily methane and ethane. Here the properties of a new Titan simulation facility are presented, including conceptual methodology, design, implementation, and performance results. The chamber maintains Titan’s surface temperature and pressure, and the sample cryogenic liquids undergoing experimentation are condensed within the chamber itself. During the experiments, the evaporation rates of the sample liquids are directly determined by continually measuring mass. Constituents are analyzed utilizing a Fourier Transform Infrared Spectroscopy (FTIR), and vapor concentrations are determined using a gas chromatograph fitted with a Flame Ionization Detector (FID). All pertinent data is logged via computer. Under laboratory conditions, the direct measurements of the evaporation rates of methane, ethane, and mixtures thereof can be achieved. Among the processes to be studied are the effects of regolith on transport from the subsurface to the atmosphere, the freezing point depression effects of dissolved nitrogen, and the solubility of various relevant organic compounds.     

The escape of molecular hydrogen and the synthesis of organic nitriles in planetary atmospheres.

What is the influence of hydrogen escape from the atmosphere of small planetary bodies on the synthesis of organic molecules in that atmosphere? To answer this question, laboratory experiments have been performed to study the evolution of different reducing model atmospheres submitted to electrical discharges, with and without the simulation of H₂ escape. A study of mixtures of nitrogen and methane shows a very strong effect of H₂ escape on the formation of organic nitriles, the only nitrogen containing organics detected in the gas phase. These are HCN, CH  CCN, (CN)₂, CH₂CHCN, CH₃ CN and CH₃CH₂CN. The yield of synthesis of most of these compounds is noticeably increased, up to several orders of magnitude, when hydrogen escape is simulated. The escape of H₂ from the atmosphere of the primitive Earth may have played a crucial role in the formation of reactive organic molecules such as CHCCN or (CN)₂, which can be considered as important prebiotic precursors. These experimental results may also explain extant data concerning the nature and relative abundance of organics present in the atmosphere of Titan, a planetary satellite which may be an ideal model within our solar system for the study of organic cosmochemistry and exobiology.     

Properties of the neutral density and composition in the thermosphere

Measurements of the density and composition of the thermosphere between 150 and 500 km, which were obtained by the S3-1 satellite, have been compared with the Jacchia and MSIS models. The measurements of the densities of O, N₂, N and Ar show some differences from the current models which should be considered during the preparation of the next CIRA model. The Ar measurements are particularly useful in examining the response of the neutral atmosphere to geomagnetic heating. These results are useful in establishing the appropriate lower boundary conditions for modeling of the thermosphere.     

Investigation of the atmospheric aerosols by the visible and IR channels of the AVHRR radiometer on NOAA-6

The diffusely reflected radiation in the daytime and diffusely transmitted radiation at night from an inhomogeneous, plane parallel planetary atmosphere bounded by the ocean surface was calculated at the visible and IR region. In the daytime the effect of the solar radiation from top was taken into account. The present study has dealt with the problem of no cloud contamination in data. In the atmosphere, water vapor, molecular nitrogen, nitrous oxide, carbon dioxide and methane as absorbent constituents and aerosols as the scattering constituent were taken into account. The ocean surface as a lower boundary was simulated by many facets whose slopes are according to the isotropic Gaussian distribution [1]. It was found that it is suitable for deriving the total optical thickness in the direction of about 15 degrees away from the specular direction at 3.7μm in the daytime, with the aid of visible region of spectrum [2]. As for sea surface temperature, for a sample analysis, derived SST was within the accuracy of 1–2K at the center of the sun glint and within the accuracy of 0.5K in the rest of the region at 3.7μm in the daytime, Furthermore computational results suggest that the comparison of temperature in the daytime with that at night at 3.7μm make it possible to separate the effect of aerosols from the total optical thickness. Thus it was found from the computations that with the aid of daytime and night time data, it is possible to derive the atmospheric water vapor, aerosols and sea surface temperature simultaneously.