全文获取类型
收费全文 | 98689篇 |
免费 | 1469篇 |
国内免费 | 814篇 |
专业分类
测绘学 | 2552篇 |
大气科学 | 6879篇 |
地球物理 | 18589篇 |
地质学 | 37294篇 |
海洋学 | 8499篇 |
天文学 | 21787篇 |
综合类 | 375篇 |
自然地理 | 4997篇 |
出版年
2022年 | 527篇 |
2021年 | 902篇 |
2020年 | 999篇 |
2019年 | 1101篇 |
2018年 | 4214篇 |
2017年 | 3943篇 |
2016年 | 3635篇 |
2015年 | 1486篇 |
2014年 | 2519篇 |
2013年 | 4264篇 |
2012年 | 3444篇 |
2011年 | 5417篇 |
2010年 | 4816篇 |
2009年 | 5744篇 |
2008年 | 4878篇 |
2007年 | 5425篇 |
2006年 | 3349篇 |
2005年 | 2746篇 |
2004年 | 2724篇 |
2003年 | 2713篇 |
2002年 | 2443篇 |
2001年 | 1980篇 |
2000年 | 1837篇 |
1999年 | 1562篇 |
1998年 | 1605篇 |
1997年 | 1521篇 |
1996年 | 1300篇 |
1995年 | 1256篇 |
1994年 | 1105篇 |
1993年 | 994篇 |
1992年 | 937篇 |
1991年 | 945篇 |
1990年 | 990篇 |
1989年 | 830篇 |
1988年 | 814篇 |
1987年 | 869篇 |
1986年 | 840篇 |
1985年 | 1059篇 |
1984年 | 1144篇 |
1983年 | 1072篇 |
1982年 | 994篇 |
1981年 | 910篇 |
1980年 | 871篇 |
1979年 | 835篇 |
1978年 | 802篇 |
1977年 | 709篇 |
1976年 | 660篇 |
1975年 | 669篇 |
1974年 | 621篇 |
1973年 | 666篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
Mallory S. E. Roberts Crystal L. Brogan Bryan M. Gaensler Jason W. T. Hessels C.-Y. Ng Roger W. Romani 《Astrophysics and Space Science》2005,297(1-4):93-100
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
13.
14.
15.
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
16.
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
17.
18.
The elm decline of 5000 14C yr ago has been the most widely discussed phenomenon in post‐glacial vegetation history. This pan‐European reduction of elm populations, echoed in the decimation of elmwoods in Europe during the twentieth century, has attracted a series of interrelated hypotheses involving climate change, human activity, disease and soil deterioration. The elm bark beetle (Scolytus scolytus L.) is an essential component of disease explanations. We present evidence for the presence of the beetle over a prolonged period (ca. 7950–4910 yr BP [8800–5660 cal. yr BP]) from a lowland raised mire deposit in northeast Scotland, with its final appearance at this site, and the first and only appearance in another mire of a single scolytid find, around the time of the elm decline. The subfossil S. scolytus finds are not only the first from Scotland, but they also represent the most comprehensive sequence of finds anywhere. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
19.
The growth of Glasgow from a small town into a heavily industrialised conurbation depended greatly on its local geological riches. Extensive mining and quarrying of a range of minerals took place from the 18th century onwards. The early underground stoop and room (pillar and stall) workings, and the backfilled quarries, together with variably consolidated natural superficial deposits, have bequeathed to the city a heritage of unwanted engineering problems which cannot easily be quantified. Recent work by the British Geological Survey (BGS) illustrates the scale of both geological and man-made problems. 相似文献
20.
We consider the particular solutions of the evolutionary system of equations in elements that correspond to planar and spatial circular orbits of the singly averaged Hill problem. We analyze the stability of planar and spatial circular orbits to inclination and eccentricity, respectively. We construct the instability regions of both particular solutions in the plane of parameters of the problem. 相似文献