排序方式： 共有39条查询结果，搜索用时 15 毫秒
21.
H. R. A. Jones S. Viti J. Tennyson B. Barber G. Harris J. C. Pickering R. Blackwell‐Whitehead J.‐P. Champion F. Allard P. H. Hauschildt U. G. Jorgensen P. Ehrenfreund E. Stachowska H.‐G. Ludwig E. L. MartÍn Ya. Pavlenko Yu. Lyubchik R. L. Kurucz 《Astronomische Nachrichten》2005,326(10):920-924
A full understanding of the properties of substellar objects is one of the major challenges facing astrophysics. Since their discovery in 1995, hundreds of brown dwarfs and extrasolar planets have been discovered. While these discoveries have enabled important comparisons with theory, observational progress has been much more rapid than the theoretical understanding of cool atmospheres. The reliable determination of mass, abundances, gravities and temperatures is not yet possible. The key problem is that substellar objects emit their observable radiation in the infrared region of the spectrum where our knowledge of atomic, molecular and line broadening data is poor. Here we report on the status of our PoSSO (Physics of SubStellar Objects) project. In order to understand brown dwarfs and extrasolar planets increasing more like those in our solar system, we are studying a wide range of processes. Here we give an update on the project and sketch an outline of atoms, molecules and processes requiring study. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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Bo Barker Jorgensen 《Geochimica et cosmochimica acta》1979,43(3):363-374
A diffusion-diagenesis model of the sulfur cycle is developed to calculate theoretical distributions of stable sulfur isotopes in marine sediments. The model describes the depth variation in δ34S of dissolved sulfate and H2S. and of pyrite. The effects of sulfate reduction, sulfate and H2S diffusion. and of sedimentation are considered as well as the bacterial isotope fractionation and the degree of pyrite formation. Under open system conditions of sulfur diagenesis the isotopic difference, ΔSO2?4 — H2S, tends to increase with depth being smaller than the bacterial fractionation factor near the sediment surface and larger in deeper layers. The two isotopes in SO2?4 or in H2S do not diffuse in the same proportion as they occur in the porewater. This explains why sulfur, which is incorporated from seawater sulfate by diffusion and precipitation as pyrite, can be enriched in 32S relative to the seawater sulfate. The model calculations demonstrate the importance of taking the whole dynamic sulfur cycle into account before drawing conclusions about sulfur diagenesis from the stable isotope distribution. 相似文献
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The Alaska Tsunami Warning Center has the responsibility of providing timely tsunami warning services for Alaska and the west coasts of Canada and the United States. Recently, the ATWC implemented a new microcomputer system which is used for both automatic and interactive earthquake processing, and for disseminating critical information to the Tsunami Warning System recipients.Real-time seismic wave form data from 23 short-period and 9 long-period sites in Alaska, the lower 48 States, and Hawaii, are continually computer-monitored for the occurrence of an earthquake. Once detected from the short-period wave form data, pre- and post-earthquake data are displayed on a graphics terminal along with an indicator to identify the time of the onset of theP waves (P-picks). TheP-picks can easily be changed during or after data collection via a mouse. Magnitudes (M
b
,M
l
,M
B
,M
S
) are automatically computed from appropriate short- and long-period wave form data concurrently with the above processing. A second graphics terminal displays cycle-by-cycle long-period wave form data that was used to compute an earthquake'sM
B
andM
S
magnitudes.An earthquake's parametric data and other information are available and printed within tens of seconds after theP wave arrivals are recorded at the first 5 sites, then 7 sites, 9 sites, and a final parametric computation using all collected data. Three video display monitors are used for displaying the parameters, procedural aids, and a map showing the epicenter. Additionally, selected event parameters are immediately transmitted by VHF radio to alphanumeric beepers which are carried by standby duty personnel during those times that the Center is not manned.Using a dedicated video display terminal and printer, the interactive system can use data and parameters resulting from the automatic processes for concurrent parameter recomputations; perform additional computations; disseminate critical information; and generate procedural aids for duty geophysicists to facilitate an earthquake/tsunami investigation. 相似文献
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Dr. D. P. Jorgensen T. Matejka J. D. DuGranrut 《Meteorology and Atmospheric Physics》1996,59(1-2):83-104
Summary Two techniques for deriving horizontal and vertical air motions using vertically scanning airborne Doppler radar data are presented and discussed. These techniques make use of the scanning ability of the NOAA P-3 tail-mounted radar antenna to view a region of space from at least two vantage points during a straight-line flight track. The scanning methodology is termed the Fore/Aft Scanning Technique or FAST because the antenna is alternately scanning forward and then aft of the flight track. The major advantages of FAST over flying two quasi-orthogonal flight tracks with the antenna scanning normal to the flight track are that the data are collected in roughly half the time and the aircraft does not have to execute a right-angle turn. However, accuracy of the resulting wind field is compromised slightly because the beam intersection angle is reduced from 90° to about 50°. The reduction of area covered because of large drift angles is also discussed.A three-dimensional wind field can be constructed using the dual-Doppler equations from FAST data using the two radial velocity estimates and vertical integration of the continuity equation with a boundary condition of no vertical motion at cloud top and the Earth's surface. To keep errors in the calculated winds acceptably small, the elevation angles are typically restricted to ±45° from the horizontal to minimize contamination of the horizontal wind by terminal fallspeeds.A different, and perhaps more believable vertical velocity, can be derived using a second technique that utilizes two (or more) airborne Doppler radar equipped aircraft each using FAST to observe the echo-top vertical velocity at common point (e.g., two aircraft flying parallel flight, paths, or by using an L-shaped flight track with a single aircraft). This technique results in 4 (or more) radial velocity estimates at each point (hence is called the quad-Doppler technique). Horizontal winds can be derived using either an overdetermined three-equation solution or an overdetermined dual-Doppler solution, whichever is more accurate. For the calculation of vertical velocity a new approach is proposed that utilizes the overdetermined triple-Doppler solution for vertical particle motion near cloud top, minus an estimate of terminal fallspeeds, as a top boundary condition for the downward vertical divergence integration to derive vertical air velocity elsewhere in the domain. In addition, this approach allows measurements at steep elevation angles allowing for more depth of coverage for a given range.To show the utility of the method, analyses of data collected using FAST are compared to conventional dual-Doppler-derived wind fields constructed from data collected simultaneously by S-band ground-based Doppler radars. An example of the quad-Doppler technique is also presented from the recently completed Tropical Oceans/Global Atmospheres Coupled Ocean/Atmosphere Response Experiment (TOGA/COARE). Comparisons of quad-Doppler vertical velocity are made with in-situ derived vertical air motions collected by the NASA DC-8 to judge the quality of the approach.With 9 Figures 相似文献
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Jorgensen H. Lipunov V. M. Panchenko I. E. Postnov K. A. Prokhorov M. E. 《Astrophysics and Space Science》1995,231(1-2):389-392
Assuming that gamma-ray bursts (GRB) originate from binary neutron star (NS) or black holes (BH) merging in distant galaxies, theoretical logN-logS distributions for gamma-ray bursts (GRB) are calculated using the compact binaries coalescence rates computed for model galaxies with different star formation histories. A flat cosmological model ( = 1) with different values of the cosmological constant is used. The calculated source evolution predicts a 5–10 times increase of the source statistics at count rates 3–10 times lower than the existing BATSE sensitivity limit. The most important parameter in fitting the 2nd BATSE catalogue is the initial redshift of star formation, which is found to bez
* = 2 — 5 depending on a poorly determined average spectral index of GRB. 相似文献
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Colin R. O'Farrell Arjun M. Heimsath Daniel E. Lawson Laura M. Jorgensen Edward B. Evenson Grahame Larson Jon Denner 《地球表面变化过程与地形》2009,34(15):2008-2022
Glacial erosion rates are estimated to be among the highest in the world. Few studies have attempted, however, to quantify the flux of sediment from the periglacial landscape to a glacier. Here, erosion rates from the nonglacial landscape above the Matanuska Glacier, Alaska are presented and compare with an 8‐yr record of proglacial suspended sediment yield. Non‐glacial lowering rates range from 1·8 ± 0·5 mm yr?1 to 8·5 ± 3·4 mm yr?1 from estimates of rock fall and debris‐flow fan volumes. An average erosion rate of 0·08 ± 0·04 mm yr?1 from eight convex‐up ridge crests was determined using in situ produced cosmogenic 10Be. Extrapolating these rates, based on landscape morphometry, to the Matanuska basin (58% ice‐cover), it was found that nonglacial processes account for an annual sediment flux of 2·3 ± 1·0 × 106 t. Suspended sediment data for 8 years and an assumed bedload to estimate the annual sediment yield at the Matanuska terminus to be 2·9 ± 1·0 × 106 t, corresponding to an erosion rate of 1·8 ± 0·6 mm yr?1: nonglacial sources therefore account for 80 ± 45% of the proglacial yield. A similar set of analyses were used for a small tributary sub‐basin (32% ice‐cover) to determine an erosion rate of 12·1 ± 6·9 mm yr?1, based on proglacial sediment yield, with the nonglacial sediment flux equal to 10 ± 7% of the proglacial yield. It is suggested that erosion rates by nonglacial processes are similar to inferred subglacial rates, such that the ice‐free regions of a glaciated landscape contribute significantly to the glacial sediment budget. The similar magnitude of nonglacial and glacial rates implies that partially glaciated landscapes will respond rapidly to changes in climate and base level through a rapid nonglacial response to glacially driven incision. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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哀牢山北段地区气候特征及变化趋势 总被引：2，自引：0，他引：2
以哀牢山北段地区不同位置的景东（西侧盆地）、徐家坝（山顶）和楚雄（东侧盆地）为研究对象,利用三地1980～2005年同期气候观测资料,通过统计分析,探讨了哀牢山东、西侧盆地及山顶气温和降雨特征及长期变化趋势。结果表明：哀牢山北段地区最热月均出现在6月;最冷月,西侧盆地和山顶为1月,东侧盆地最冷月为12月;降雨的季节差异十分显著,雨季降雨量占全年的85％以上,以7月降雨量最多。从气候的长期变化趋势来看,年、季和月平均气温均呈现显著的升高趋势,气温的显著升高主要发生在干季,增温率为干季〉年〉雨季。最冷月均温增温率最大,最热月均温没有显著增加,气温年较差呈减小趋势。研究表明,哀牢山北段地区山顶及两侧盆地均呈现显著的变暖趋势,与植物生存直接相关的∑t≥0℃有效积温和∑t≥10℃活动积温也显著增加。气温的变化具有显著的空间差异,增温速率为东侧盆地〉山顶〉西侧盆地。山地迎风坡面（西坡）气温垂直递减率显著减小,背风坡面（东坡）气温垂直递减率整体呈增大趋势。降雨整体呈增加趋势,不同季节间降雨的变化差异显著,年降雨量的增加主要由于雨季降雨的增加,干季降雨呈微弱上升或下降趋势,降雨的增加率为东侧盆地〉山顶〉西侧盆地。哀牢山北段地区干季的干热气候特征进一步加剧,这种变化趋势在山地的背风坡面（东坡）表现得更加剧烈。 相似文献