Statistical description of the radiation field in a homogeneous medium of finite optical thickness     

A. G. Nikoghossian 《Astrophysics》2006,49(2):228-239

The problem of finding statistical averages which characterize the radiation field in a semi-infinite atmosphere has been discussed by the author in a series of papers. In this paper some of the results obtained there are generalized to media of finite optical thickness. The average number of scattering events undergone by a photon during its random walk in an atmosphere and the average time required for this are determined. Three different types of averages are considered, depending on the photon characteristics over which the averages are taken. Special attention is devoted to the asymptotic behavior of these quantities for media with large optical depths taking the influence of continuum absorption into account. __________ Translated from Astrofizika, Vol. 49, No. 2, pp. 263–275 (May 2006).  相似文献   

    

A. S. Belyakov  V. S. Lavrov  A. V. Nikolaev  L. L. Khudzinskii 《Doklady Earth Sciences》2006,407(2):258

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The grain-size characteristics of Quaternary deposits at Xingshan near Siping in Jilin province, China     

Maxwell A Boateng  Xikui Wang 《世界地质(英文版)》2006,9(1)

This research paper analyses the grain-size characteristics of the Quaternary deposits at Xingshan near Siping, Jilin province in China by employing graphic measures to study the grain size distribution and its mode of transport and deposition. The Quaternary deposits at Xingshan lie unconformable on Cretaceous rocks made of siltstone, mudstone and sandstone. The average grain size is between 8.06 to 8.55Φ (0.002 6 ～0.003 7 mm). The Quaternary deposits at Xingshan mainly compose of very fine silt to clay. The compositions of the grade are clay 63% and silt 37%. The clay size components are weathered debris transported and deposited by flowing water from the SE highlands or hills to the low lying NW Xingshan plains whereas the silty components accumulated by aoelian process. The Quaternary deposits at Xingshan accumulated in the middle and late Pleistocene interglacial periods from (459.12～39.03) ka to (88.92～7.56) ka. The standard deviation ranged from 0.96 to 1.36Φ, indicating that the sediments are moderately to poorly sorted, Coefficient of skewness ranged from 0.16～0.31 with an average skewness of 0.218, (Positively skewed towards fine). Kurtosis values (0.84～1.05) from the grain size distribution and visual inspection of the frequency curves indicate platykurtic to mesokurtic curves and unimodal to bimodal grain-size distribution. The type of deposit formation is sand dune and the source is at a distal from its provenance.  相似文献   

The 2dF QSO Redshift Survey – XII. The spectroscopic catalogue and luminosity function     

S. M. Croom  R. J. Smith  B. J. Boyle  T. Shanks  L. Miller  P. J. Outram  N. S. Loaring 《Monthly notices of the Royal Astronomical Society》2004,349(4):1397-1418

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Radiation exposure and Mission Strategies for Interplanetary Manned Missions (REMSIM)   总被引：1，自引：0，他引：1  

C. Cougnet  N. B. Crosby  S. Eckersley  C. Foullon  V. Guarnieri  S. Guatelli  D. Heynderickx  A. Holmes-Siedle  C. Lobascio  S. Masiello  P. Nieminen  G. Parisi  P. Parodi  M. A. Perino  M. G. Pia  R. Rampini  P. Spillantini  V. Tamburini  E. Tracino 《Earth, Moon, and Planets》2004,94(3-4):279-285

Cosmic radiation is an important problem for human interplanetary missions. The “Radiation Exposure and Mission Strategies for Interplanetary Manned Missions–REMSIM” study is summarised here. They are related to current strategies and countermeasures to ensure the protection of astronauts from radiation during interplanetary missions, with specific reference to: radiation environment and its variability; radiation effects on the crew; transfer trajectories and associated fluences; vehicle and surface habitat concepts; passive and active shielding concepts; space weather monitoring and warning systems.  相似文献   

MASS FLUX OF ASTEROIDAL ORIGIN METEOROIDS ON PERIODIC COMET NUCLEI     

R. L. HAWKES  R. A. EATON 《Earth, Moon, and Planets》2004,95(1-4):187-195

We examine the potential contamination of cometary nuclei through impacts from asteroidal origin meteoroids. The paper uses a simple model and has the goal of determining whether asteroidal contamination is potentially significant. We assume a meteoroid power law mass distribution with index values in the range from s=1.83 to s=2.09. We used maximum and minimum models which we believe will bracket the true meteoroid mass distribution. We identify those comets which are expected to be most significantly contaminated, and find values of up to 3.6 kg of asteroidal meteoroid impact per square meter of the cometary surface per orbital revolution. This is less than the expected mass loss per perihelion passage for most comets. Therefore any remnant effects of the contamination will depend on the penetration depth of the meteoroids in the cometary nucleus, and possibly on the distribution of active and inactive areas on cometary nuclei. We present a simple model which suggests that even small meteoroids will embed relatively deeply into a cometary nucleus.  相似文献   

Radiative Transfer in Inhomogeneous Atmospheres. III     

A. G. Nikoghossian 《Astrophysics》2004,47(3):412-421

The approach proposed in the previous parts of this series of papers is used to solve the radiative transfer problem in scattering and absorbing multicomponent atmospheres. Linear recurrence relations are obtained for both the reflectance and transmittance of these kinds of atmospheres, as well as for the emerging intensities when the atmosphere contains energy sources. Spectral line formation in a one-dimensional inhomogeneous atmosphere is examined as an illustration of the possibility of generalizing our approach to the matrix case. It is shown that, in this case as well, the question reduces to solving an initial value problem for linear differential equations. Some numerical calculations are presented.  相似文献   

Chemical alteration of tephra in the depositional environment: theoretical stability modelling     

A. M. Pollard  S. P. E. Blockley  K. R. Ward 《第四纪科学杂志》2003,18(5):385-394

The study of the chemical stability of vitreous material in aqueous media is well‐established. There has to date been little consideration of the implications of variations in the chemical durability of tephra in Quaternary tephrochronology. Chemical alteration can take the form of cationic leaching from the matrix, or complete destruction of the silica network, either of which could constrain the ability to chemically identify distal tephra. Here we apply established models of vitreous durability to the published chemical analyses of a large number of Icelandic tephras in order to predict their relative durabilities under equivalent conditions. This suggests that some important tephras have relatively poor chemical stability, and that rhyolitic tephras are, in general, more stable than basaltic. We conclude that tephras should be expected to show predictable differential chemical stability in the post‐depositional environment. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Opposition polarimetry and photometry of S- and E-type asteroids     

Irina N Belskaya  Vasilij G Shevchenko  Yurij N Krugly  Yurij S Efimov  Alberto Cellino 《Icarus》2003,166(2):276-284

The first results of the observational program devoted to simultaneous investigation of asteroid polarimetric and photometric opposition phenomena are presented. UBVRI polarimetric and V-band photometric observations of the S-type Asteroid 20 Massalia and the E-type Asteroids 214 Aschera and 620 Drakonia were carried out in 1996-1999 down to phase angles of 0.08°, 0.7°, and 1.2°, correspondingly. The S-type Asteroid 20 Massalia is characterized by the pronounced brightness opposition surge with an amplitude larger than that observed for the E-type asteroids. A sharp peak of negative polarization at small phase angles was not observed for this asteroid. The value of polarization degree at phase angle α<1° is less than 0.5% for both S and E types. The negative polarization branches of S and especially E-asteroids have an asymmetrical shape. The phase angle at which the polarization minimum occurs is close to the angle at which non-linear increase begins in the asteroid magnitude phase curves. A relation of the observed effects to the mechanism of coherent backscattering is discussed.  相似文献   

Galaxy spectral parametrization in the 2dF Galaxy Redshift Survey as a diagnostic of star formation history     

Darren S. Madgwick  Rachel Somerville  Ofer Lahav  Richard Ellis 《Monthly notices of the Royal Astronomical Society》2003,343(3):871-879

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