On the occurrence of near syzygies in the solar system     

Alastair J. C. McDonald 《Celestial Mechanics and Dynamical Astronomy》1986,38(2):139-153

This paper derives the average period of occurrence of syzygies (conjunctions and oppositions) ofp planets in unperturbed, coplanar, circular orbits about the sun. Numerical results indicate the reliability of the theory and applications are suggested. For example, the theory is relevant to the search for resonances and near mirror configurations.  相似文献   

Fast Object Detection for Use Onboard Satellites     

Martin Bange  Stefan Jordan  Michael Biermann  Thomas Kämpke  Ralf-Dieter Scholz 《Experimental Astronomy》2002,13(2):101-117

We propose an object detection algorithm which is efficient and fast enough to be used in (almost) real time with the limited computer capacities onboard satellites. For stars below the saturation limit of the CCD detectors it is based on a four neighbourhood local maximum criterion in order to find the centre of a stellar image. For saturated stars it is based on the assumption that the image is increasing monotonically towards the centre in the unsaturated part of the image. The algorithm also calculates approximate stellar magnitudes and efficiently rejects most of the cosmics which would otherwise lead to a large number of false detections. The quality of the algorithm was evaluated with the help of a large set of simulated data for the DIVA satellite mission; different assumptions were made for the noise level, and the presence of cosmics or for a variable sky background.We could show that our algorithm fulfills the requirements for DIVA; only in the case of simulated images which included the bright galaxy M31 some fainter stars could not be detected in the galaxy's vicinity. Since stellar images contain large areas without any stars, we propose an additional block-skipping algorithm which can be coded on special-purpose hardware. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Comparison of observed Ca xix and Ca xviii relative line intensities with current theory     

C. Jordan  N. J. Veck 《Solar physics》1982,78(1):125-135

A comparison is made between Ca xix and Ca xviii line ratios observed in solar flares with the Bent Crystal Spectrometer (BCS) on the Solar Maximum Mission (SMM) satellite and currently available atomic data. Close agreement is found with the excitation rates recently published by Pradhan et al. (1981). The observations show little dependence of line ratios on electron temperature, supporting a further conclusion that cascade contributions to the 2³ P and 2³ S levels are not significant.  相似文献   

Causes of spatially variable tectonic subsidence in the Miocene Bermejo Foreland Basin, Argentina   总被引：1，自引：0，他引：1  

N. Cardozo  T. Jordan 《Basin Research》2001,13(3):335-357

ABSTRACT Tectonic subsidence in the 20–9 Ma Bermejo basin resulted from spatially variable crustal loading on a lithosphere of spatially variable strength (e.g. elastic thickness). Reconstruction of the crustal loads added between 20 and 9 Ma, and assessment of the effects of these loads on an elastic, isotropic lithosphere confirm this hypothesis. Elastic models effectively explain tectonic subsidence east of the Iglesia–Calingasta basin, but west of it crustal loads were locally compensated. Elastic models also prove that the 20–9 Ma Frontal Cordillera loading is of no importance in the mechanical system of the Bermejo basin. 2D and 3D elastic models of a uniformly strong lithosphere under 20–9 Ma crustal loads corrected for post‐9 Ma erosion successfully replicate the 9 Ma Bermejo basin's proximal palaeotopography. However, they fail to replicate the 9 Ma basin's medial and distal palaeotopography. A 3D finite element model of a lithosphere with bimodal strength (weak below the Bermejo basin and west of the Precordillera, and strong below the Precordillera and east of the Valle Fértil lineament) successfully replicates the 9 Ma basin's palaeotopography. That variable strength model introduces a southward decrease in the wavelength of flexural deformation, which results in a basin that narrows southward, consistent with the 9 Ma Bermejo basin. The preferred 9 Ma lithospheric strength distribution is similar to the present lithospheric strength field derived from gravity data, suggesting that the bimodal strength signature was retained throughout the entire basin's evolution. Late Miocene flattening of the subducting slab, tectonic change to a broken foreland, or deposition of a thick (～8–10 km) sedimentary cover did not affect the strength of the lithosphere underlying the Bermejo basin. The long‐term bimodal strength field does not correlate with the documented thickness of the seismogenic crust.  相似文献   

Teleseismic inversion for the second‐degree moments of earthquake space–time distributions     

Jeffrey J. McGuire  Li Zhao  Thomas H. Jordan 《Geophysical Journal International》2001,145(3):661-678

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Observations of gravity waves in the upper and lower stratosphere by lidar and ozonesondes   总被引：1，自引：0，他引：1  

N. J. Mitchell  A. J. McDonald  S. J. Reid  J. D. Price 《Annales Geophysicae》1996,14(3):309-314

Lidar observations of Rayleigh backscatter and temperature profiles measured by ozonesondes have been used to investigate gravity waves in the upper and lower stratosphere, respectively, over Aberystwyth (52.4°N, 4.1°W). Both data sets have been used to investigate the vertical wavenumber spectrum of the wave field at high wavenumbers. Similar analytic techniques applied to each data set enable direct comparison of the spectra. The possibility of laminar structures generated by differential advection contaminating gravity-wave fields deduced from temperature and/or density measurement is discussed and the behaviour of the wave-field mean potential energy reveals a seasonal cycle throughout the stratosphere with a late winter maximum and a summer minimum.  相似文献   

Mariner 9: An instrument of dynamical science     

J.F. Jordan  J. Lorell 《Icarus》1975,25(1):146-165

We review and evaluate the contributions of Mariner 9 in improving our knowledge of the dynamical characteristics of Mars and its two satellites, Phobos and Deimos. Primary results include the discovery of the large gravitational and topographical bulge in the Tharsis region, the development of a detailed gravity model representable as coefficients in a spherical harmonic expansion, the development of a topographic model exhibiting a three kilometer displacement of the center of figure from the center of mass, and the determination of the size, shape and motion of Phobos and Deimos.  相似文献   

Current Trends In French Geography     

James R. McDonald 《The Professional geographer》1975,27(1):15-18

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Correlation of He II lyman alpha with He I 10830 Å, and with chromospheric and EUV coronal emission     

W. T. Thompson  W. M. Neupert  S. D. Jordan  H. Jones  R. J. Thomas  B. Schmieder 《Solar physics》1993,147(1):29-46

This paper describes the results of comparing SERTS-3 images obtained in the transition region line of Heii 304 with chromospheric Hei 10830 absorption, with strong coronal lines of Mgix 368 , Fexv 284 and 417 , and Fexvi 335 and 31 , with H, with Caii 8542 , and with magnetograms in Fei 8688H. All of the images are illustrated, and the image reconstruction techniques used are described and evaluated. The high correlation of the Heii 304 and Hei 10830 images, originally found by Harvey and Sheeley (1977), is confirmed and is put on a quantitative basis. We find that the supergranulation network has greater contrast, and that filaments appear darker, in 10830 than in 304 . In active regions, the 304 line follows more closely the behavior of H and Caii 8542 than the 10830 line.  相似文献   

Using GIS and statistics to study influences of geology on probability features of surface soil geochemistry in Northern Ireland     

Crawford Jordan  Chaosheng Zhang  Alex Higgins 《Journal of Geochemical Exploration》2007

The probability features of non-normality and non-lognormality are widely observed in geochemistry due to the influences of multiple factors that are difficult to quantify and model. In Northern Ireland, the pseudo-total concentrations of 14 elements (Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb and Zn) from 6138 topsoils were measured, and GIS mapping showed that the spatial distribution of these data were in line with the spatial distribution of geology in the area. Investigations into the influences of geology on the concentration data and their probability features were carried out using GIS and statistics in this study. The whole raw data sets for each element were positively skewed and none of them followed either normal or lognormal distributions. Logarithmic transformation was found to have “over-transformed” most of the data sets, changing their skewness from positive to negative values. When soil samples were classified by rock type using a GIS overlay function, obvious differences were observed in the chemical concentrations of soils derived from different rock types. Soils in basalt areas displayed the highest concentrations for most elements under study (Ca, Co, Cr, Cu, Fe, Mg, Mn, Na, Ni, P and Zn) but the lowest concentrations for K, while the highest levels for Cd and Pb occurred in the shale areas. Classifying soils by rock type produced more normally distributed data sets, especially for the igneous rock areas. To restrain the influence of soil type and land cover, samples from both gleys and pastures were extracted via a GIS and it was found the data sets then showed generally greater tendencies towards normality. However, many of the data sets would still not pass a test for normality unless the sample size was small (e.g. of the order of a couple of hundreds). Geology, soil type, land cover and sample size all played important roles in determining soil chemical concentrations and their probability features. However, the influences from other factors were still evident. Attempts made in this study show that it remains a challenging task in geochemistry to separate all the factors and to model their influence at the regional scale.  相似文献