全文获取类型
收费全文 | 1838篇 |
免费 | 118篇 |
国内免费 | 30篇 |
专业分类
测绘学 | 74篇 |
大气科学 | 146篇 |
地球物理 | 383篇 |
地质学 | 669篇 |
海洋学 | 172篇 |
天文学 | 330篇 |
综合类 | 7篇 |
自然地理 | 205篇 |
出版年
2023年 | 12篇 |
2022年 | 7篇 |
2021年 | 38篇 |
2020年 | 47篇 |
2019年 | 65篇 |
2018年 | 70篇 |
2017年 | 73篇 |
2016年 | 93篇 |
2015年 | 75篇 |
2014年 | 74篇 |
2013年 | 145篇 |
2012年 | 82篇 |
2011年 | 113篇 |
2010年 | 95篇 |
2009年 | 118篇 |
2008年 | 97篇 |
2007年 | 95篇 |
2006年 | 86篇 |
2005年 | 60篇 |
2004年 | 66篇 |
2003年 | 54篇 |
2002年 | 40篇 |
2001年 | 32篇 |
2000年 | 38篇 |
1999年 | 27篇 |
1998年 | 27篇 |
1997年 | 15篇 |
1996年 | 22篇 |
1995年 | 14篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 12篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 13篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 13篇 |
1984年 | 15篇 |
1983年 | 13篇 |
1982年 | 13篇 |
1981年 | 9篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1972年 | 3篇 |
排序方式: 共有1986条查询结果,搜索用时 203 毫秒
231.
Brian A. Ebel Benjamin B. Mirus Christopher S. Heppner Joel E. VanderKwaak Keith Loague 《水文研究》2009,23(13):1949-1959
Distributed hydrologic models capable of simulating fully‐coupled surface water and groundwater flow are increasingly used to examine problems in the hydrologic sciences. Several techniques are currently available to couple the surface and subsurface; the two most frequently employed approaches are first‐order exchange coefficients (a.k.a., the surface conductance method) and enforced continuity of pressure and flux at the surface‐subsurface boundary condition. The effort reported here examines the parameter sensitivity of simulated hydrologic response for the first‐order exchange coefficients at a well‐characterized field site using the fully coupled Integrated Hydrology Model (InHM). This investigation demonstrates that the first‐order exchange coefficients can be selected such that the simulated hydrologic response is insensitive to the parameter choice, while simulation time is considerably reduced. Alternatively, the ability to choose a first‐order exchange coefficient that intentionally decouples the surface and subsurface facilitates concept‐development simulations to examine real‐world situations where the surface‐subsurface exchange is impaired. While the parameters comprising the first‐order exchange coefficient cannot be directly estimated or measured, the insensitivity of the simulated flow system to these parameters (when chosen appropriately) combined with the ability to mimic actual physical processes suggests that the first‐order exchange coefficient approach can be consistent with a physics‐based framework. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
232.
233.
We present measurements of magnetic field strength and geometry on the surfaces of T Tauri stars (TTS) with and without circumstellar disks. We use these measurements to argue that magnetospheric accretion models should not assume that a fixed fraction of the stellar surface contains magnetic field lines that couple with the disk. We predict the fractional area of accretion footpoints, using magnetospheric accretion models and assuming field strength is roughly constant for all TTS. Analysis of Zeeman broadened infrared line profiles shows that individual TTS each have a distribution of surface magnetic field strengths extending up to 6 kG. Averaging over this distribution yields mean magnetic field strengths of 1-3 kG for all TTS, regardless of whether the star is surrounded by a disk. These strong magnetic fields suggest that magnetic pressure dominates gas pressure in TTS photospheres, indicating the need for new model atmospheres. The He I 5876 Å emission line in TTS can be strongly polarized, so that magnetic field lines at the footpoints of accretion have uniform polarity. The circular polarization signal appears to be rotationally modulated, implying that accretion and perhaps the magnetosphere are not axisymmetric. Time series spectropolarimetry is fitted reasonably well by a simple model with one magnetic spot on the surface of a rotating star. On the other hand, spectropolarimetry of photospheric absorption lines rules out a global dipolar field at the stellar surface for at least some TTS. 相似文献
234.
235.
Christopher J. Duffin 《Geology Today》2007,23(4):132-142
This year marks the bicentenary of the birth of Louis Agassiz, a Swiss whose aim was to be the premier naturalist of his time. His father's assessment of him was that he had 'a mania for rushing full gallop into the future'. His life was marked by the production of grandiose schemes, many of which he had the drive to complete. In addition to his massive ambition, he had a brilliant intellect, charming manner and boundless energy. These qualities often lead to his schemes coming to fruition, occasionally against tremendous odds, although sometimes with great cost to his health and personal relationships. 相似文献
236.
Jarrod R. Hurley † Christopher A. Tout Onno R. Pols ‡ 《Monthly notices of the Royal Astronomical Society》2002,329(4):897-928
We present a rapid binary-evolution algorithm that enables modelling of even the most complex binary systems. In addition to all aspects of single-star evolution, features such as mass transfer, mass accretion, common-envelope evolution, collisions, supernova kicks and angular momentum loss mechanisms are included. In particular, circularization and synchronization of the orbit by tidal interactions are calculated for convective, radiative and degenerate damping mechanisms. We use this algorithm to study the formation and evolution of various binary systems. We also investigate the effect that tidal friction has on the outcome of binary evolution. Using the rapid binary code, we generate a series of large binary populations and evaluate the formation rate of interesting individual species and events. By comparing the results for populations with and without tidal friction, we quantify the hitherto ignored systematic effect of tides and show that modelling of tidal evolution in binary systems is necessary in order to draw accurate conclusions from population synthesis work. Tidal synchronism is important but, because orbits generally circularize before Roche lobe overflow, the outcome of the interactions of systems with the same semilatus rectum is almost independent of eccentricity. It is not necessary to include a distribution of eccentricities in population synthesis of interacting binaries; however, the initial separations should be distributed according to the observed distribution of semilatera recta rather than periods or semimajor axes. 相似文献
237.
Measurements of the north-south (B
z
component of the interplanetary field as compiled by King (1975) when organized into yearly histograms of the values of ¦B
z
¦ reveal the following. (1) The histograms decrease exponentially from a maximum occurrence frequency at the value ¦B
z
¦ = 0. (2) The slope of the exponential on a semi-log plot varies systematically roughly in phase with the sunspot number in such a way that the probability of large values of ¦B
z
¦ is much greater in the years near sunspot maximum than in the years near sunspot minimum. (3) There is a sparsely populated high-value tail, for which the data are too meager to discern any solar cycle variation. The high-value tail is perhaps associated with travelling interplanetary disturbances. (4) The solar cycle variations of B
z
and the ordinary indicators of solar activity are roughly correlated. (5) The solar cycle variation of B
z
is distinctly different than that of the solar wind speed and that of the geomagnetic Ap disturbance index.Now at the Aerospace Corporation, El Segundo, Calif. 90245, U.S.A. 相似文献
238.
Volatile fatty acid (VFA) apparent turnover rates in organic-rich marine sediments were determined by measuring whole sediment VFA concentration and the corresponding first-order reaction rate constants. In order to measure VFA concentrations, bulk wet sediment samples were basified, freeze-dried, extracted with methanol, derivatized to form methyl esters of the VFAs, and analyzed by packed-column gas chromatography using hexanoic acid as an internal standard. The detection limits for acetate, propionate, iso-butyrate and butyrate were 1.0, 0.4, 0.2 and 0.2 μmol l?1s, respectively, for 600 ml samples. Rate constants for acetate and propionate were determined by anaerobically incubating samples at in-situ temperatures with tracer levels of 14C-labelled VFAs. Metabolized label was recovered as CO2, CH4, cellular material, water-soluble material, and VFA (ether-soluble) fractions. Apparent turnover rates measured during summer and winter in anoxic Cape Lookout Bight, North Carolina (U.S.A.) sediments were in the range of 19–330 μmol l?1s h?1 for acetate and 0.7–7.0 μmol l?1s h?1 for propionate. 相似文献
239.
Christopher Packham Keith L. Thompson Almudena Zurita Johan H. Knapen Ian Smail Robert Greimel Daniel F. M. Folha Chris Benn rew Humphrey Rene Rutten David Ciardi Matthieu Bec Richard Bingham Simon Craig Kevin Dee Derek Ives Paul Jolley Peter Moore Marti Pi i Puig Simon Rees Gordon Talbot Sue Worswick 《Monthly notices of the Royal Astronomical Society》2003,345(2):395-405
240.
Although lightning has not yet been observed in Titan's atmosphere, the presence of condensable vapors and the deposition of a significant amount of solar energy at the surface suggest the possibility of lightning activity. Based on an understanding of the relationship of lightning activity to the amount of convective energy available on Titan, a lightning energy dissipation rate of 4 × 10?6, W/m2 can be expected. This value is much lower than that for Earth or Jupiter, and is a result of both the reduced solar flux at Titan and the absorption of sunlight by the aerosols that lie above the convective layer. For this dissipation rate, the amount of HCN and C2N2 produced by lightning should be greater than that by solar UV, but could be less than that produced by electron precipitation and galactic cosmic rays. Equilibrium calculations indicate that large mole fractions of elemental solid phase carbon will also be produced. Using a simplified model of aerosol formation, coagulation, and settling, it is estimated that a lightning-produced aerosol could have a typical optical depth of 10?2, with values as high as 0.1. The accumulation of soot over geological time might reach a meter or more in depth. 相似文献