A Discussion of IGS Solutions and Their Impact on Geodetic and Geophysical Applications   总被引：1，自引：0，他引：1  

Jan Kouba  Yves Mireault  Gerhard Beutler  Tim Springer  Gerd Gendt 《GPS Solutions》1998,2(2):3-15

The International Association of Geodesy officially established the International GPS Service (IGS) on Janaury 1, 1994. Its prime objective is to provide support and a rerefence system for a wide variety of scientific and practical applications involving GPS. To fulfill its role the IGS also generates, in addition to its fundamental products (orbital/staion positions and consistent Earth orientation parameters), additional reference-system products providing the necessary infrastructure, standards, and means of calibrations for timing and various atmospheric applications of GPS. The generation and efficient application of IGS products and their impact on a number of positioning and atmospheric applications, including low earth orbit satellites, is reviewed and discussed. @ 1998 John Wiley & Sons, Inc.  相似文献   

Interplanetary scintillation observations of interaction regions in the solar wind     

A. R. Breen  P. J. Moran  C. A. Varley  W. P. Wilkinson  P. J. S. Williams  W. A. Coles  A. Lecinski  J. Markkanen 《Annales Geophysicae》1998,16(10):1265-1282

Co-rotating interaction regions (CIRs) between fast and slow streams of plasma are a prominent feature of the solar wind. Measurements of interplanetary scintillation (IPS) using the three widely separated antennas of the EISCAT facility have been used to detect the compression regions at the leading edges of interaction regions and to determine the location and velocity of the structure. Observations show that interaction regions have developed as close to the Sun as 25–30 solar radii, a result supported by theoretical modelling which shows that the conditions needed for CIRs to develop exist inside 30 solar radii.  相似文献   

U-Series Chronology of Lacustrine Deposits in Death Valley, California   总被引：1，自引：0，他引：1  

Teh-Lung Ku  Shangde Luo  Tim K. Lowenstein  Jianren Li  Ronald J. Spencer 《Quaternary Research》1998,50(3):261-275

Uranium-series dating on a 186-m core (DV93-1) drilled from Badwater Basin in Death Valley, California, and on calcareous tufas from nearby strandlines shows that Lake Manly, the lake that periodically flooded Death Valley during the late Pleistocene, experienced large fluctuations in depth and chemistry over the last 200,000 yr. Death Valley has been occupied by a long-standing deep lake, perennial shallow saline lakes, and a desiccated salt pan similar to the modern valley floor. The average sedimentation rate of about 1 mm/yr for core DV93-1 was punctuated by episodes of more-rapid accumulation of halite. Arid conditions similar to the modern conditions prevailed during the entire Holocene and between 120,000 and 60,000 yr B.P. From 35,000 yr B.P. to the beginning of the Holocene, a perennial saline lake existed, over 70 m at its deepest. A much deeper and longer lasting perennial Lake Manly existed from about 185,000 to 128,000 yr B.P., with water depths reaching about 175 m, if not 330 m. This lake had two significant “dry” excursions of 10²–10³yr duration about 166,000 and 146,000 yr B.P., and it began to shrink to the point of halite precipitation between 128,000 and 120,000 yr B.P. The two perennial lake periods correspond to marine oxygen isotopic stages (OIS) 2 and 6. Based on the shoreline tufa ages, we do not rule out the possible existence 200,000 yr ago of yet a third perennial lake comparable in size to the OIS 6 lake. The²³⁴U/²³⁸U data suggest that U in tufa owes its origin mainly to Ca-rich springs fed by groundwater that emanated along lake shorelines in southern Death Valley, and that an increase of this spring-water input relative to the river-water input apparently occurred during OIS 6.  相似文献   

Modelling the X-ray cluster dipole and cluster contribution to the soft X-ray background     

V.Kolokotronis  M.Plionis  P.Coles  S.Borgani 《Monthly notices of the Royal Astronomical Society》1998,295(1):19-32

We investigate the sampling and dipole convergence properties of flux-limited samples of mock X-ray clusters in relation to their underlying 'parent' cluster distribution. To this purpose, we resort to numerical simulations of the cluster distribution and extract samples resembling the main observational features of X-ray selected cluster samples. The flux-limited samples, being quite sparse, underestimate the amplitude of the 'parent' cluster dipole by ≈ 15 per cent on average for Local Group-like observers. However, the general shapes of their dipole amplitude profiles are in relatively good agreement. We also calculate the expected contribution of clusters, selected according to the relevant criteria, to the soft (i.e. 0.1–2.4 keV) extragalactic X-ray background (XRB), using the ESO Key Project X-ray luminosity function, assuming a flat universe with vanishing cosmological constant. We obtain a value of about 10 per cent of the observed XRB flux.  相似文献   

Pleistocene climatic change in Southern Australia and its effect on speleothem deposition in some Nullarbor caves     

Albert Coede  Russell S. Harmon  Tim C. Atkinson  Peter J. Rowe 《第四纪科学杂志》1990,5(1):29-38

Activity ratios of ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U, and ²³⁰Th/²³²Th have been determined for calcite, gypsum and halite speleothems from caves of the Nullarbor Plain, mostly in the area N and NW of Mundrabilla Station, for the purpose of U-series dating. All calcite speleothems contain adequate amounts of uranium for dating, but some show an excess of ²³⁰Th. Stratigraphic relationships indicate that there were at least three phases of calcium carbonate deposition in the Nullarbor caves. The calcite samples, with one possible exception, have ages in excess of ca. 400000 yrs BP. This suggests that no significant amounts of calcium carbonate deposition have taken place during the last 400ka. At present, active deposition of speleothems is restricted almost entirely to gypsum and halite. The only gypsum speleothem dated was found to have a finite age of ca. 185 ka. Six dates on a small halite speleothem containing insect and arachnid remains indicate that it formed rapidly during Holocene time.  相似文献   

Catastrophic landslides,glacier behaviour and moraine formation – A view from an active plate margin     

James Shulmeister  Tim R. Davies  David J.A. Evans  Olivia M. Hyatt  Daniel S. Tovar 《Quaternary Science Reviews》2009,28(11-12):1085-1096

The influence of large bedrock landslides (“rock avalanches”) on the behaviour of glaciers is incompletely recognised. Here we present an example from an active tectonic margin in South Island, New Zealand where large earthquakes leave a significant imprint on glacial records. We demonstrate that terminal moraines on the western side of the Southern Alps record both ‘ordinary’ (i.e. climate-driven) and landslide-initiated glacial advances. Following consideration of the processes involved in rock avalanche-initiated moraine construction we suggest ways of determining the nature of the advance that built the terminal moraine. The implications of these observations are important in breaking the conventional linkage of individual terminal moraines with climate forcing.  相似文献   

The effects of rock fragments on hydrologic and hydraulic responses along a slope     

Kou‐Chan Hung  Ken'ichirou Kosugi  Tim Hau Lee  Takahisa Misuyama 《水文研究》2007,21(10):1354-1362

A model describing the three‐dimensional matrix flow along a slope with rock fragments or impermeable blocks was developed. The model was combined with modified Picard's iteration to ensure mass conservation in the unsaturated flow. We found that rock fragments obstruct water flow along the slope. The groundwater table must be raised to provide a sufficient pore water pressure gradient to facilitate water flow, but higher pore water pressure may induce slope failure. We also conducted a bench‐scale laboratory flume experiment to examine the effects of impermeable blocks on downstream seepage flow. In addition, a numerical experiment was conducted to examine how different arrangements of impermeable blocks affect downstream seepage flow and pore water pressure. This research demonstrated that the hydraulic phenomena were affected when impermeable blocks were present, and pore water pressure increased as the position of impermeable blocks was lowered. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Crustal and upper mantle structure of the northwestern North Island, New Zealand, from seismic refraction data   总被引：1，自引：0，他引：1  

Tim Stern  E. G. C. Smith  F. J. Davey  K. J. Muirhead 《Geophysical Journal International》1987,91(3):913-936

The crustal and upper mantle structure of the northwestern North Island of New Zealand is derived from the results of a seismic refraction experiment; shots were fired at the ends and middle of a 575 km-long line extending from Lake Taupo to Cape Reinga. The principal finding from the experiment is that the crust is 25 ± 2 km thick, and is underlain by what is interpreted to be an upper mantle of seismic velocity 7.6 ± 0.1 km s⁻¹, that increases to 7.9 km s⁻¹ at a depth of about 45 km. Crustal seismic velocities vary between 5.3 and 6.36 km s⁻¹ with an average value of 6.04 km s⁻¹. There are close geophysical and geological similarities between the north-western North Island of New Zealand and the Basin and Range province of the western United States. In particular, the conditions of low upper-mantle seismic velocities, thin crust with respect to surface elevation, and high heat-flow (70–100 mW m⁻²) observed in these two areas can be ascribed to their respective positions behind an active convergent margin for about the past 20 Myr.  相似文献   

Dispersion measure variations and their effect on precision pulsar timing     

X. P. You  G. Hobbs  W. A. Coles  R. N. Manchester  R. Edwards  M. Bailes  J. Sarkissian  J. P. W. Verbiest  W. van Straten  A. Hotan  S. Ord  F. Jenet  N. D. R. Bhat  A. Teoh 《Monthly notices of the Royal Astronomical Society》2007,378(2):493-506

We present an analysis of the variations seen in the dispersion measures (DMs) of 20-ms pulsars observed as part of the Parkes Pulsar Timing Array project. We carry out a statistically rigorous structure function analysis for each pulsar and show that the variations seen for most pulsars are consistent with those expected for an interstellar medium characterized by a Kolmogorov turbulence spectrum. The structure functions for PSRs J1045−4509 and J1909−3744 provide the first clear evidence for a large inner scale, possibly due to ion–neutral damping. We also show the effect of the solar wind on the DMs and show that the simple models presently implemented into pulsar timing packages cannot reliably correct for this effect. For the first time we clearly show how DM variations affect pulsar timing residuals and how they can be corrected in order to obtain the highest possible timing precision. Even with our presently limited data span, the residuals (and all parameters derived from the timing) for six of our pulsars have been significantly improved by correcting for the DM variations.  相似文献   

A Geoarchaeological Study of the Middle and Upper Pleistocene Levels at Canteen Kopje,Northern Cape Province,South Africa     

Matt G. Lotter  Ryan J. Gibbon  Kathleen Kuman  George M. Leader  Tim Forssman  Darryl E. Granger 《Geoarchaeology》2016,31(4):304-323

Canteen Kopje, situated in the Northern Cape Province of South Africa, has two main archaeological deposits: alluvial gravels and a mantle of overlying fine sediments known locally as the “Hutton Sands.” This paper focuses on the fine sediments, the three industries contained within them, and the interface with the underlying gravels in an attempt to clarify their formation and transformation. A Fauresmith assemblage is found at this interface; it is thus crucial to understand the processes of deposition and modification at this poorly understood boundary. The methods used in this study involved the analysis of artifact depositional (dip and orientation) and spatial data, artifact condition, raw materials, and assemblage size profiles. Data presented document the mixing between the lowest levels of the fine sediments and the underlying alluvial gravels. This study thus provides important contextual information for the Fauresmith industry at Canteen Kopje.  相似文献