Ab initio calculated electron densities for tetrahedral sheet [H2Si2O5]∞ in phyllosilicates

Periodic ab initio Hartree-Fock LCAO calculations have been carried out on the two dimensional sheet of SiO₄ tetrahedra, representing one of the basic constituting units of layer silicates, using Huzinaga's DZP basis sets. The influence of the basis set on the chemical bonding picture is characterized by Mulliken atomic charges and by electron density maps. Silicon atomic charges +1.6 ¦e¦ are more realistic than those +2.4 ¦e¦ reported for smaller basis sets. The silicon d orbital population is found to be 0.6 in close agreement with molecular data. Electron density maps indicate the absence of charge density in the center of the ditrigonal cavity. The charge buildup of nonbonding basal oxygen orbitals is directed mainly downwards perpendicular to the sheet plane.     

Geomagnetic activity as a possible cause of changes in the middle atmosphere and troposphere; observations made in Prague in the years 1932–1987

Summary A possible relation between the 10-day, 1-month, and 2-month geomagnetic as well as solar activity, characterized by Kp and Wolf's number R, and temperatures measured at the Prague-Klementinum meteorological station in January–February of the years 1932–1987 was sought. It was found that:1) Kp correlates with the temperature in Prague better than Wolf's number R.2) If the QBO phase is taken into account, the correlations improve.3) Even if the correlations improves with increasing length of the subintervals into which the individual winter periods (January-February) were derived, their statistical significance declines.     

Eta model forecasts of tropical cyclones from Australian Monsoon Experiment: Dynamical adjustment of initial conditions

Summary The University of Belgrade/National Meteorological Centre, Washington (UB/NMC) limited area Eta Model predicted the development, structure, associated precipitation and tracks of the Australian Monsoon Experiment (AMEX) (10 January through 15 February 1987) tropical cyclonesConnie, Irma, Damien andJason. The initial positions and intensities of the tropical cyclone vortices from the global European Centre for Medium-Range Weather Forecasts (ECMWF) analyses, which are used as initial data, do not quite agree with the observations. These disagreements produce additional erros in predicting the tropical cyclone tracks.To improve the initial position of the vortex, the flow is split into the small and the large scale motions, and during the first two hours of the integration, the small-scale part is forced in small steps towards its observed position. The adjustment is performaed with the reduced model dynamics (adjustment processes only) and no physics.With the adjustment during the first two hours of the integration, the model successfully adjusts to the new position of the initial vortex. After the completion of the adjustment stage, the model runs normally, i.e., without any modification. The tracks of the 48-h forecasts with the adjusted initial vortices are parallel to the tracks obtained in the control forecasts without the adjustment. However, e.g., the mean absolute error of the positions during 48-h forecast of the tropical cycloneConnie was reduced from 174 km in the control case to 129 km in the case with adjustment of the position.The latent heat, the thermal energy, the kinetic energy and the total energy of the extracted small scale vortices are calculated every three hours of the integration time. These small-scale energies obtained in the 48-h control forecast are compared to those of the rund with the initial vortex adjustment to monitor the spin up of the model.With 7 Figures     

Some results of biogeographic research on meadows in the Dyje-Svratka basin in the Czech Republic

Some results of biogeographic research on meadows in the Dyje-Svratka basin in the Czech Republic. The paper shows one example of biogeographic inventory of natural ecosystems in an unflooded meadow in the Dyje-Svratka basin. It is part of the complex physio-geographic research of the Geographic Department of the Masaryk University in Brno aimed at gaining a basis for territorial interpretation of ecological regularities and territorial application of general ecological optimizing principles. A segment of biocenosis of Ulmi-fraxineta carpini has been analyzed (Zlatník 1956).The endeavour of Czech geographers to study geosystems of various dimensions completely gets in the limelight. In question is the understanding of mutual relationship of components of geosystems based on the exchange of mass and energy. There are two ways of studying them: spatial relations or approaches concentrated on processes. In contrast to the above unilateral concept, the endeavour to make the approaches compatible appears rarely. This complex view is made possible by geobiocenologically taken biogeography in papers of Zlatník and Rauer, their students and successors. This paper shows only one example of the results of biogeographic stock-taking of meadows in the Dyje-Svratka basin, indicating relations of eco- and geosystems which confirm the necessity of ecological orientation of the present biogeography and its potential contribution to ecology.     

Magnetic properties,self-reversal remanence and thermal alteration products of smythite

Summary This study is a follow up of the investigation of some magnetic properties and metastability of greigite in samples obtained from Miocene claystones in the Kruné hory (Erzgebirge) Piedmont basins (Bohemia). Three different methods of upgrading the smythite were applied; the magnetic properties of the concentrates are compared. The thermal conversion of smythite sets in at 200°C while greigite converts at 250°C. The first intermediate products to be formed are iron sulphides, marcasite clearly dominating over pyrite and pyrrhotite. Apart from a Fe³⁺ sulphate with a composition of Fe₂(SO₄)₃, oxidation of these sulphides gives rise to -Fe₂O₃. The result of the subsequent decomposition of the mentioned sulphate is the formation of -Fe₂O₃, which retains the sulphate structure. The final product of the thermal decomposition at 800°C is -Fe₂O₃. In the smythite concentrate the conversion to Fe³⁺ sulphate and -Fe₂O₃ is about twice as intensive as in greigite. No direct conversion to -Fe₂O₃ was found. During the thermal process self-reversals of remanence were observed, in various samples as many as four reversals in the temperature interval from 340 to 590°C. The occurrences of self-reversals of remanence were only observed at high degrees of thermal demagnetization, of the order of 10^–2 down to 10^–3 in the temperature interval of sulphide origin (below 400°C), and of the order of 10^–4 down to 10^–6 in the temperature interval of Fe-oxides origin (above 400°C).Presented at the 3rd Conference on New Trends in Geomagnetism, Castle of Smolenice, Czechoslovakia, June 22–29, 1992     

Dust particle and solar radiation

Influence of the solar radiation (electromagnetic and corpuscular - solar wind) on the motion of the interplanetary dust particle is investigated. The ratio time of inspiralling toward the Sun: time of inspiralling neglecting the change of mass of the particle is presented as a function of initial eccentricities.     

An analysis of close binaries (CB) based on photometric measurements (VII)

The paper is devoted to the actual problematics in the determination of orbital and physical parameters of active CB on the basis of the interpretation of photometric observations. One solves the problem in two stages: by obtaining a synthetic light curve in the case when the parameters of the corresponding CB model (Djuraevi, 1992a) are given a priori (direct problem) and by determining the parameters of the given model for which the best fit between the synthetic light curve and the observations is achieved (inverse problem) (Djuraevi, 1992b). In this paper, the above procedure is applied to a particular case of an interpretation of CB light curves AU Ser and RV Corvi.     

An analysis of close binaries (CB) based on photometric measurements (V)

The author considers the current problematics in the determination of the orbital and physical parameters for active close binaries (CB) of W Ser type based on the interpretation of photometric observations. In the particular case one analyses the light curves of CB RX Cas in the framework of the accretion-disc model explained in the second paper of this series (Djuraevi, 1992a). The change of the light curves with that of the system's physical-activity phase is analysed and the orbital and physical parameters of the system are determined for the maximum, minimum and the transition regime of the physical activity by applying the inverse-problem method described in the third paper of the series (Djuraevi, 1992b). In the paper a graphical illustration of the solutions obtained is also given.     

Equation de Szebehely et principes variationnels

Resume On montre l'équivalence entre l'équation de Szebehely du problème inverse et un problème variationnel déduit du principe de Maupertuis.

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Szebehely's equation and principle variations

We show that the inverse problem Szebehely's equations is equivalent to a multiple variation problem deduced from the principle of Maupertuis.