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101.
Manh Dung Duong 《Planetary and Space Science》1976,24(10):915-919
Calculations of the properties of the ionospheric duct centered at the F2 layer are carried out with a view to investigating the ducted propagation of Pc1 micropulsations in directions out of the geomagnetic meridian plane. For a horizontally uniform ionosphere, duct properties are found to be essentially the same in all horizontal directions. Propagation characteristics of ducted waves, however, vary according to ionospheric and sunspot conditions. In practice, therefore, it is expected that horizontal propagation over a large recording network is not isotropic because of the diurnal changes in the ionosphere. 相似文献
102.
Loop-like white light coronal transients are generally believed to be nearly planar sheets which are thin compared to the loop extent; however, this picture may be questioned since virtually no observations (of the more than 100 transient events observed during 1973–74 Skylab period) show such loops edge-on. From the group of transient events studied by Munro etal. (1979) for which definite surface associations exist, we find loop transients are strongly correlated with filament regions where the filament axis was oriented north-south. From direct soft X-ray observations of an expanding arch, the possible identification of the soft X-ray signature of footpoints of transient loops, and monochramatic observations of low coronal loops, we infer that loop-like coronal transients have their origin in low-lying coronal loops nearly co-planar with the north-south aligned filament axis. The situation with respect to non-loop events is less clear; such events apparently often arise from more complex filament geometries. Possible reasons for the preference of transients to arise from north-south filament-oriented regions are discussed.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
103.
We present the second part of a complete theory for the plasma and field structure of a cool coronal arch, corresponding to those observed in the EUV from Skylab. The global magneto-hydrodynamic (MHD) stability of a previously described equilibrium-loop model is evaluated, and compared with that of an unmodified ambient force-free field. The influence of the photospheric boundary condition is also evaluated, producing a specification of stability limits which depend on the relative field and plasma pressures and scale widths. The resulting restrictions on the allowable field configuration of a coronal loop are then compared with observed values. The implications of this general method for deducing small-scale coronal magnetic-field structure from the measured plasma profile of an emissive feature are also described. 相似文献
104.
I. Adler J. I. Trombka P. Lowman R. Schmadebeck H. Blodget E. Eller L. Yin R. Lamothe G. Osswald J. Gerard P. Gorenstein P. Bjorkholm H. Gursky B. Harris J. Arnold A. Metzger R. Reedy 《Earth, Moon, and Planets》1973,7(3-4):487-504
A number of experiments carried in orbit on the Apollo 15 and 16 spacecraft were used in the compositional mapping of the lunar surface. The observations involved measurements of secondary (fluorescent) X-rays, gamma rays and alpha particle emissions. A large scale compositional map of over 20% of the lunar surface was obtained for the first time. It was possible to demonstrate significant chemical differences between the mare and the highlands, to find specific areas of high radioactivity and to learn something about the composition of the Moon's hidden side.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April 1973. 相似文献
105.
A Study on Residual Flow in the Gulf of Tongking 总被引:2,自引:0,他引:2
On the basis of observations of water temperature, salinity and wind during winter (December to following February) and summer (June to August) in the Gulf of Tongking, a robust diagnostic three-dimensional model has established that reveals the seasonal variation in residual flow, including wind-driven current, density-driven current and tide-induced residual current. It is shown that in the Gulf of Tongking the wind-driven current plays the most important role in the seasonal variation of residual flow field. Due to strong NE monsoon (9 m/s) during winter the residual flow (reaches 30 cm/s) is stronger than that in summer. At the surface the residual flow direction during winter is basically westward or southwestward, while during summer it is almost reversed. 相似文献
106.
Rachel S. Somerville Gerard Lemson Tsafrir S. Kolatt Avishai Dekel 《Monthly notices of the Royal Astronomical Society》2000,316(3):479-490
We study the merging history of dark matter haloes in N -body simulations and semi-analytical 'merger trees' based on the extended Press–Schechter (EPS) formalism. The main focus of our study is the joint distribution of progenitor number and mass as a function of redshift and parent halo mass. We begin by investigating the mean quantities predicted directly by the Press–Schechter (PS) and EPS formalism, such as the halo mass and conditional mass functions, and compare these predictions with the results of the simulations. The higher moments of this distribution are not predicted by the EPS formalism alone and must be obtained from the merger trees. We find that the Press–Schechter model deviates from the simulations at the level of 30–50 per cent on certain mass scales, and that the sense of the discrepancy changes as a function of redshift. We show that this discrepancy is reflected in the higher moments of the distribution of progenitor mass and number. We investigate some related statistics such as the accretion rate and the mass ratio of the largest two progenitors. For galaxy sized haloes ( M ∼1012 M⊙ ), we find that the merging history of haloes, as represented by these statistics, is well reproduced in the merger trees compared with the simulations. The agreement deteriorates for larger mass haloes. We conclude that merger trees based on the extended Press–Schechter formalism provide a reasonably reliable framework for semi-analytical models of galaxy formation. 相似文献
107.
X. Hernandez David Valls-Gabaud Gerard Gilmore 《Monthly notices of the Royal Astronomical Society》2000,316(3):605-612
We use data from the Hipparcos catalogue to construct colour–magnitude diagrams for the solar neighbourhood, which are then treated using advanced Bayesian analysis techniques to derive the star formation rate history, SFR ( t ), of this region over the last 3 Gyr. The method we use allows the recovery of the underlying SFR ( t ) without the need of assuming any a priori structure or condition on SFR ( t ), and hence yields a highly objective result. The remarkable accuracy of the data permits the reconstruction of the local SFR ( t ) with an unprecedented time resolution of ≈50 Myr. An SFR ( t ) that has an oscillatory component of period ≈0.5 Gyr is found, superimposed on a small level of constant star formation activity. Problems arising from the non-uniform selection function of the Hipparcos satellite are discussed and treated. Detailed statistical tests are then performed on the results, which confirm the inferred SFR ( t ) to be compatible with the observed distribution of stars. 相似文献
108.
Vera van Zoest Frank B. Osei Gerard Hoek Alfred Stein 《International journal of geographical information science》2020,34(5):851-865
ABSTRACT Recently developed urban air quality sensor networks are used to monitor air pollutant concentrations at a fine spatial and temporal resolution. The measurements are however limited to point support. To obtain areal coverage in space and time, interpolation is required. A spatio-temporal regression kriging approach was applied to predict nitrogen dioxide (NO2) concentrations at unobserved space-time locations in the city of Eindhoven, the Netherlands. Prediction maps were created at 25 m spatial resolution and hourly temporal resolution. In regression kriging, the trend is separately modelled from autocorrelation in the residuals. The trend part of the model, consisting of a set of spatial and temporal covariates, was able to explain 49.2% of the spatio-temporal variability in NO2 concentrations in Eindhoven in November 2016. Spatio-temporal autocorrelation in the residuals was modelled by fitting a sum-metric spatio-temporal variogram model, adding smoothness to the prediction maps. The accuracy of the predictions was assessed using leave-one-out cross-validation, resulting in a Root Mean Square Error of 9.91 μg m?3, a Mean Error of ?0.03 μg m?3 and a Mean Absolute Error of 7.29 μg m?3. The method allows for easy prediction and visualization of air pollutant concentrations and can be extended to a near real-time procedure. 相似文献
109.
110.
Rawad Massoud Alan.L Shihadeh Mohamed Roumié Myriam Youness Jocelyne Gerard Nada Saliba Rita Zaarour Maher Abboud Wehbeh Farah Najat Aoun Saliba 《Atmospheric Research》2011,101(4):893-901
The results of the first large scale chemical characterization of PM10 and PM2.5 at three different sites in the urban city of Beirut, Lebanon, are presented. Between May 2009 and April 2010 a total of 304 PM10 and PM2.5 samples were collected by sampling every sixth day at three different sites in Beirut. Observed mass concentrations varied between 19.7 and 521.2 μg m? 3 for PM10 and between 8.4 and 72.2 μg m? 3 for PM2.5, respectively. Inorganic concentrations accounted for 29.7–35.6 μg m? 3 and 46.0–53.5 μg m? 3 of the total mass of PM10 and PM2.5, respectively. Intra-city temporal and spatial variations were assessed based on the study of three factors: correlation coefficients (R) for PM and chemical components, coefficient of divergence (CODs), and source apportionment using positive matrix factorization (PMF). Based on R and COD of PM concentrations, the three sites appear homogeneous. However, when individual elements were compared, heterogeneity among sites was found. This latter was attributed to the variability in the percent contribution of biogenic and local anthropogenic source factors such as traffic related sources and dust resuspension. Other factors included the proximity to the Mediterranean sea, the population density and the topographical structure of the city. Hence, despite its small size (20.8 km2), one PM monitoring site does not reflect an accurate PM level in Beirut. 相似文献