Light pollution and night sky brightness at the site of Kottamia Observatory     

S. Nawar  A. B. Morcos  J. S. Mikhail 《Earth, Moon, and Planets》1995,70(1-3):133-141

Photoelectric measurements of the night sky brightness and the light pollution of Kottamia Observatory have been carried out and the deduced results are expressed in mag/sec². The maximum brightness of the sky in the direction of Cairo city at zenith distance 45° and azimuth 70° when the sun is almost 60° below the horizon are 22.68; 21.54 and 19.82 mag/sec² for blue, yellow and red colours respectively. The corresponding values of night sky background are 22.94; 21.85 and 20.14 mag/sec² respectively.The isophotes of the sky brightness at Kottamia Observatory have been drawn for blue, yellow and red colours. The variations of the night sky brightness and the (B-V) colour index with altitude of the observed point have been studied.The light pollution and the night sky brightness at the site of Kottamia Observatory is compared with that deduced by different investigators at other sites. It has been shown that the sky brightness at zenith distance 45° at Kottamia Observatory site is similar to Kitt Peak and Palomar Observatory sites. Kottamia Observatory site is slightly brighter than Junipero Serra while it is darker than Mount Hamilton and San Jose sites. The comparative results have been carried out at blue and yellow colours. No comparison is obtained at red as there is no data published for the red colour.  相似文献   

Propagation of waves in a medium with high radiation pressure. I. Basic equations and the case of a homogeneous medium     

G. S. Bisnovatyi-Kogan  S. I. Blinnikov 《Astrophysics》1978,14(4):316-325

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Mineralogy and petrography of amoeboid olivine aggregates from the reduced CV3 chondrites Efremovka,Leoville and Vigarano: Products of nebular condensation,accretion and annealing     

Mutsumi KOMATSU  Alexander N. KROT  Mikhail I. PETAEV  Alexander A. ULYANOV  Klaus KEIL  Masamichi MIYAMOTO 《Meteoritics & planetary science》2001,36(5):629-641

Abstract— Amoeboid olivine aggregates (AOAs) from the reduced CV chondrites Efremovka, Leoville and Vigarano are irregularly‐shaped objects, up to 5 mm in size, composed of forsteritic olivine (Fa_<10) and a refractory, Ca, Al‐rich component. The AOAs are depleted in moderately volatile elements (Mn, Cr, Na, K), Fe, Ni‐metal and sulfides and contain no low‐Ca pyroxene. The refractory component consists of fine‐grained calcium‐aluminum‐rich inclusions (CAIs) composed of Al‐diopside, anorthite (An₁₀₀), and magnesium‐rich spinel (～1 wt% FeO) or fine‐grained intergrowths of these minerals; secondary nepheline and sodalite are very minor. This indicates that AOAs from the reduced CV chondrites are more pristine than those from the oxidized CV chondrites Allende and Mokoia. Although AOAs from the reduced CV chondrites show evidence for high‐temperature nebular annealing (e.g., forsterite grain boundaries form 120° triple junctions) and possibly a minor degree of melting of Al‐diopside‐anorthite materials, none of the AOAs studied appear to have experienced extensive (>50%) melting. We infer that AOAs are aggregates of high‐temperature nebular condensates, which formed in CAI‐forming regions, and that they were absent from chondrule‐forming regions at the time of chondrule formation. The absence of low‐Ca pyroxene and depletion in moderately volatile elements (Mn, Cr, Na, K) suggest that AOAs were either removed from CAI‐forming regions prior to condensation of these elements and low‐Ca pyroxene or gas‐solid condensation of low‐Ca‐pyroxene was kinetically inhibited.  相似文献   

Theoretical interpretation of cosmic magnetic fields     

F. I. Mikhail  M. I. Wanas  A. M. Eid 《Astrophysics and Space Science》1995,228(1-2):221-237

The paper discusses the possibilty of interpreting the magnetic fields of astronomical bodies in the framework of a unified field theory.Using one of the solutions of the generalized field theory, a direct relation between the polar magnetic field, the angular velocity and the gravitational potential of the body considered, is obtained. The geometric model used for applications has spherical symmetry and is of the type (FIGI).The predictions of the theoretical formula, obtained from the model, are compared with available observational data, and with the empirical formula of Blackett. The theoretical formula gives a possible interpretation of a seed magnetic field which will develop and produce the large-scale magnetic field observed for celestial objects. The formula shows that the field is generated as a result of rotation of a massive object.  相似文献   

Polarization characteristics of Jensch coelostat     

Mikhail L. Demidov 《Solar physics》1991,135(1):193-197

The polarization characteristics of the Jensch coelostat, used at the Sayan observatory, are presented. Results of calculations of the instrumental polarization (IP) are compared with the observations. It is shown that structural features of the Jensch coelostat lead to a very important dependence of the Stokes's parameters, that characterize the IP, on solar coordinates.  相似文献   

Night Sky Brightness and Colour at Different Zenith Distances at Abu-Simbel Site     

S. Nawar  A.B. Morcos  J.S. Mikhail 《Astrophysics and Space Science》1998,262(4):485-496

Photoelectric observations of night sky brightness at different zenith distances in blue, yellow and red colours have been carried out at Abu-Simbel site. Variations in the night sky brightness and (B-V) colour index with time are detected. These variations have been explained by the change of the galactic latitude of the observed point with time. The deduced results of night sky brightness have been compared with that obtained before at other sites. It has been found for both blue and yellow colours, that Abu-Simbel is the darkest site especially for zenith distances equal or greater than 60°. The present values of night sky brightness and colour have been obtained for the first time at Abu-Simbel site. The average night sky brightness at the galactic plane is 22.58 mag/arc sec² for blue and 21.66 mag/arc sec² for yellow. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

High-Performance Parallel Solver for Integral Equations of Electromagnetics Based on Galerkin Method     

Mikhail Kruglyakov  Lidia Bloshanskaya 《Mathematical Geosciences》2017,49(6):751-776

A new parallel solver for the volumetric integral equations (IE) of electrodynamics is presented. The solver is based on the Galerkin method, which ensures convergent numerical solution. The main features include: (i) memory usage eight times lower compared with analogous IE-based algorithms, without additional restrictions on the background media; (ii) accurate and stable method to compute matrix coefficients corresponding to the IE; and (iii) high degree of parallelism. The solver’s computational efficiency is demonstrated on a problem of magnetotelluric sounding of media with large conductivity contrast, revealing good agreement with results obtained using the second-order finite-element method. Due to the effective approach to parallelization and distributed data storage, the program exhibits perfect scalability on different hardware platforms.  相似文献   

The detailed palaeoecology of a mid‐Wisconsinan interstadial (ca. 32 000 14C a BP) vegetation surface from interior Alaska     

Matthew J. Wooller  Grant D. Zazula  Misha. Blinnikov  Benjamin. V. Gaglioti  Nancy. H. Bigelow  Paul. Sanborn  Svetlana. Kuzmina  Catherine. La Farge 《第四纪科学杂志》2011,26(7):746-756

We present a multi‐proxy reconstruction from a well‐preserved vegetation surface (ca. 32 000 ¹⁴C a BP) from the Fox Permafrost tunnel near Fairbanks, Alaska. A thick litter layer of plant material on the vegetation surface is consistent with the vegetation lacking evidence of disturbance. Plant macrofossils and graminoid cuticle analysis show the presence of a graminoid assemblage consistent with phytolith data. The pollen data indicate that trees were not local to the site and that Artemisia sp. was present in the region. The insect and bryophyte reconstructions are consistent with the vascular plant reconstruction, indicating the site was at least periodically wet. δ¹³C values from the graminoids present show a large range encompassing both the wet and dry range displayed by modern graminoids in Alaska. Sequential δ¹³C analyses conducted along the length of leaves attached to the vegetation surface indicate a seasonal shift towards relatively higher water use efficiency. The lower water use efficiency earlier in the growing season may have stemmed from the use of winter season meltwater by plants at the site – a scenario consistent with the site's cryostratigraphy. Our multi‐proxy reconstruction contributes to the limited palaeoecological data available for graminoid‐dominated vegetation present in Eastern Beringia and particularly the interior of Alaska during the mid‐Wisconsinan interstadial. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

The Arctic Coastal Dynamics Database: A New Classification Scheme and Statistics on Arctic Permafrost Coastlines     

Hugues Lantuit  Pier Paul Overduin  Nicole Couture  Sebastian Wetterich  Felix Aré  David Atkinson  Jerry Brown  Georgy Cherkashov  Dmitry Drozdov  Donald Lawrence Forbes  Allison Graves-Gaylord  Mikhail Grigoriev  Hans-Wolfgang Hubberten  James Jordan  Torre Jorgenson  Rune Strand ?deg?rd  Stanislav Ogorodov  Wayne H. Pollard  Volker Rachold  Sergey Sedenko  Steve Solomon  Frits Steenhuisen  Irina Streletskaya  Alexander Vasiliev 《Estuaries and Coasts》2012,35(2):383-400

Arctic permafrost coasts are sensitive to changing climate. The lengthening open water season and the increasing open water area are likely to induce greater erosion and threaten community and industry infrastructure as well as dramatically change nutrient pathways in the near-shore zone. The shallow, mediterranean Arctic Ocean is likely to be strongly affected by changes in currently poorly observed arctic coastal dynamics. We present a geomorphological classification scheme for the arctic coast, with 101,447?km of coastline in 1,315 segments. The average rate of erosion for the arctic coast is 0.5?m? year^?1 with high local and regional variability. Highest rates are observed in the Laptev, East Siberian, and Beaufort Seas. Strong spatial variability in associated database bluff height, ground carbon and ice content, and coastline movement highlights the need to estimate the relative importance of shifting coastal fluxes to the Arctic Ocean at multiple spatial scales.  相似文献   

On the chemistry of mantle and magmatic volatiles on Mercury     

Mikhail Yu. Zolotov 《Icarus》2011,212(1):24-114

The surface of Mercury contains ancient volcanic features and signs of pyroclastic activity related to unknown magmatic volatiles. Here, the nature of possible magmatic volatiles (H, S, C, Cl, and N) is discussed in the contexts of formation and evolution of the planet, composition and redox state of its mantle, solubility in silicate melts, chemical mechanisms of magma degassing, and thermochemical equilibria in magma and volcanic gases. The low-FeO content in surface materials (<6 wt%) evaluated with remote sensing methods corresponds to less than 2.3 fO₂ log units below the iron-wüstite buffer. These redox conditions imply restricted involvement of hydrous species in nebular and accretion processes, and a limited loss of S, Cl, and N during formation and evolution of the planet. Reduced conditions correspond to high solubilities of these elements in magma and do not favor degassing. Major degassing and pyroclastic activity would require oxidation of melts in near-surface conditions. Low-pressure oxidation of graphite in moderately oxidized magmas causes formation of low-solubility CO. Decompression of reduced N-saturated melts involves oxidation of nitride melt complexes and could cause N₂ degassing. Putative assimilation of oxide (FeO, TiO₂, and SiO₂) rich rocks in magma chambers could have caused major degassing through oxidation of graphite and S-, Cl- and N-bearing melt complexes. However, crustal rocks may never have been oxidized, and sulfides, graphite, chlorides, and nitrides could remain in crystallized rocks. Chemical equilibrium models show that N₂, CO, S₂, CS₂, S₂Cl, Cl, Cl₂, and COS could be among the most abundant volcanic gases on Mercury. Though, these speciation models are restricted by uncertain redox conditions, unknown volatile content in magma, and the adopted chemical degassing mechanism.  相似文献