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   139741篇
  免费   2230篇
  国内免费   1861篇
测绘学   4247篇
大气科学   10487篇
地球物理   27571篇
地质学   51912篇
海洋学   11473篇
天文学   27737篇
综合类   2427篇
自然地理   7978篇
  2021年   914篇
  2020年   1058篇
  2019年   1126篇
  2018年   7803篇
  2017年   7051篇
  2016年   5934篇
  2015年   2028篇
  2014年   2966篇
  2013年   5612篇
  2012年   4383篇
  2011年   7725篇
  2010年   6724篇
  2009年   8184篇
  2008年   6965篇
  2007年   7584篇
  2006年   4183篇
  2005年   3760篇
  2004年   3954篇
  2003年   3748篇
  2002年   3345篇
  2001年   2717篇
  2000年   2674篇
  1999年   2220篇
  1998年   2256篇
  1997年   2127篇
  1996年   1901篇
  1995年   1770篇
  1994年   1587篇
  1993年   1464篇
  1992年   1400篇
  1991年   1283篇
  1990年   1474篇
  1989年   1238篇
  1988年   1162篇
  1987年   1354篇
  1986年   1197篇
  1985年   1490篇
  1984年   1705篇
  1983年   1564篇
  1982年   1459篇
  1981年   1377篇
  1980年   1233篇
  1979年   1191篇
  1978年   1239篇
  1977年   1080篇
  1976年   1043篇
  1975年   988篇
  1974年   948篇
  1973年   986篇
  1971年   610篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
This work was sponsored at Sandia National Laboratories (contract DE-AC04-76DP00789) by the US Department of Energy, Office of Basic Energy Sciences, as part of the Continental Scientific Drilling Program  相似文献   
22.
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM.  相似文献   
23.
24.
The intrablock deformation of Meshkenet Tessera on Venus is mostly due to responses of the uppermost surface bedrock to tensional stresses. It is found that complex deformation structures within the highland blocks resemble those of formed in chocolate tablet boudinaging which has taken place after original parallel faulting and bar-like crustal block formation. The high-angle tessera structures with varying cross-cutting relations define styles and locations of multiphase deformation most evidently related to local relaxation of tessera topography. Series of progressive or superposed fracturing events with alternating fault directions took place at high angles during this relaxational deformation. Compressional ridges often surround these tesserae.  相似文献   
25.
Book reviews     
Howard  R. F.  van den Oord  G. H. J.  Švestka  Z. 《Solar physics》1996,169(1):225-227
  相似文献   
26.
27.
The stability and evolution of cold, shock-bounded slabs is studied using numerical hydrodynamic simulations. We confirm the analysis of Vishniac (1994) [ApJ, 428, 186], who showed that such slabs are unstable if they are perturbed by a displacement larger than their width. The growth rate of this nonlinear thin shell instability (NTSI) is found to increase with decreasing wavelength, in qualitative agreement with Vishniac's analysis. The NTSI saturates when the bending angle becomes large and the growth in the width of the slab pinches off the perturbation. After saturation, the slab remains greatly extended with an average density much less than the original slab density, supported primarily by supersonic turbulence within the slab. Linear perturbations are also found to be unstable in that they can lead to turbulent flow within the slab, although this response to linear perturbations is distinct from, and much less violent than the NTSI.Richard McCray  相似文献   
28.
The observed times of minimum light derived from the photometry of the Wolf-Rayet eclipsing binary stars CQ Cep and V444 Cyg are used to estimate the mass-loss rate of the Wolf-Rayet components in several modes of mass-loss and mass-exchange.  相似文献   
29.
In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques.  相似文献   
30.
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