首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   29589篇
  免费   478篇
  国内免费   319篇
测绘学   627篇
大气科学   1852篇
地球物理   5334篇
地质学   11358篇
海洋学   3017篇
天文学   6900篇
综合类   72篇
自然地理   1226篇
  2022年   298篇
  2021年   461篇
  2020年   516篇
  2019年   563篇
  2018年   1098篇
  2017年   1048篇
  2016年   1159篇
  2015年   554篇
  2014年   1060篇
  2013年   1720篇
  2012年   1169篇
  2011年   1453篇
  2010年   1300篇
  2009年   1580篇
  2008年   1341篇
  2007年   1416篇
  2006年   1312篇
  2005年   767篇
  2004年   718篇
  2003年   673篇
  2002年   689篇
  2001年   627篇
  2000年   572篇
  1999年   467篇
  1998年   480篇
  1997年   458篇
  1996年   395篇
  1995年   351篇
  1994年   384篇
  1993年   295篇
  1992年   298篇
  1991年   290篇
  1990年   323篇
  1989年   224篇
  1988年   227篇
  1987年   269篇
  1986年   209篇
  1985年   315篇
  1984年   279篇
  1983年   263篇
  1982年   277篇
  1981年   205篇
  1980年   245篇
  1979年   198篇
  1978年   212篇
  1977年   167篇
  1976年   163篇
  1975年   174篇
  1974年   169篇
  1973年   169篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
An experiment aboard the Scripps Institution of Oceanography's RV Thomas Washington has demonstrated the seafloor mapping advantages to be derived from combining the high-resolution bathymetry of a multibeam echo-sounder with the sidescan acoustic imaging plus wide-swath bathymetry of a shallow-towed bathymetric sidescan sonar. To a void acoustic interference between the ship's 12-kHz Sea Beam multibeam echo-sounder and the 11-12-kHz SeaMARC II bathymetric sidescan sonar system during simultaneous operations, Sea Beam transmit cycles were scheduled around SeaMARC II timing events with a sound source synchronization unit originally developed for concurrent single-channel seismic, Sea Beam, and 3.5-kHz profile operations. The scheduling algorithm implemented for Sea Beam plus SeaMARC II operations is discussed, and the initial results showing their combined seafloor mapping capabilities are presented  相似文献   
92.
93.
Type II radio bursts are produced by material moving outwards in the solar atmosphere. Their drift in frequency allows the calculation of the radial speed with which the shock is moving- very basic information in assessing the likelihood that the shock will reach the Earth and its time of arrival. This paper compares the shock speeds derived from radio bursts observed by the Swept Frequency Interferometric Radiometer (SFIR) equipment at the US Air Force Radio Solar Telescope Network (RSTN) of observatories with those measured with the Culgoora radiospectrograph operated by IPS Radio and Space Services. The SFIR shock speeds are found to be 1.5–3.0 times larger than the Culgoora values which are consistent with earlier results. This difference appears to originate from the incorrect interpretation of events as a result of the smaller frequency range of the SFIR equipment.  相似文献   
94.
95.
We create mock pencil-beam redshift surveys from very large cosmological N -body simulations of two cold dark matter (CDM) cosmogonies, an Einstein–de Sitter model ( τ CDM) and a flat model with Ω0=0.3 and a cosmological constant (ΛCDM). We use these to assess the significance of the apparent periodicity discovered by Broadhurst et al. Simulation particles are tagged as 'galaxies' so as to reproduce observed present-day correlations. They are then identified along the past light-cones of hypothetical observers to create mock catalogues with the geometry and the distance distribution of the Broadhurst et al. data. We produce 1936 (2625) quasi-independent catalogues from our τ CDM (ΛCDM) simulation. A couple of large clumps in a catalogue can produce a high peak at low wavenumbers in the corresponding one-dimensional power spectrum, without any apparent large-scale periodicity in the original redshift histogram. Although the simulated redshift histograms frequently display regularly spaced clumps, the spacing of these clumps varies between catalogues and there is no 'preferred' period over our many realizations. We find only a 0.72 (0.49) per cent chance that the highest peak in the power spectrum of a τ CDM (ΛCDM) catalogue has a peak-to-noise ratio higher than that in the Broadhurst et al. data. None of the simulated catalogues with such high peaks shows coherently spaced clumps with a significance as high as that of the real data. We conclude that in CDM universes, the regularity on a scale of ∼130  h −1 Mpc observed by Broadhurst et al. has a priori probability well below 10−3.  相似文献   
96.
97.
Eight-year-old Loblolly pine (Pinus taeda) plots located in South Carolina were evaluated using high resolution aircraft multispectral scanner data to determine if biomass could be accurately measured. Sixteen pine plots located on both sandy and clay soils were treated with 0, 180, or 360 kg. (approximately 0, 400, or 800 lb.) of nitrogen per plot. Indices of biomass from remote sensing data were significantly correlated with in situ biomass measurements made in each plot. A ratio of infrared (.9–1.1 μm) and red (.65–.70 μm) channels yielded the best correlation. The indices were not sensitive to differences in soil type (sandy or clay).  相似文献   
98.
The treatment followed by Coxet al. (1973) for the estimation of the effects of thermal imbalance on the pulsational stability of stars is applied to Roche and point source (Chandrika Prashad) model. The calculations suggest that the eigenvalues obtained for the above model are correct to 0(t ff/ts). The expressions for stability coefficients and corresponding eigenfunctions for the stars in thermal imbalance for the above models have also been derived.  相似文献   
99.
Two high resolution spectra of the hot RCrB star DY Cen in the red region are compared. The photospheric absorption lines show a radial velocity variation of 12 kms-1 between 1989 July and 1992 May. Emission components to some CII lines present in 1989 are almost entirely absent in 1992. Nebular forbidden lines of [OI], [NII] and [SII] appear unchanged from 1989 to 1992  相似文献   
100.
The main analytical properties of the generalized Shkarofsky function and a numerical code for its computation are discussed. The results of a numerical analysis are compared with the results of an asymptotic analysis for parameter values relevant to the problem of whistler-mode propagation in the Earth's magnetosphere. This comparison allows us to specify the range of applicability of different approximations to the generalized Shkarofsky function, which have been used for the analysis of relativistic effects on whistler-mode propagation and instability.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号