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   256977篇
  免费   5245篇
  国内免费   3289篇
测绘学   7038篇
大气科学   19189篇
地球物理   53881篇
地质学   89605篇
海洋学   21602篇
天文学   55252篇
综合类   1021篇
自然地理   17923篇
  2021年   2165篇
  2020年   2599篇
  2019年   2824篇
  2018年   3854篇
  2017年   3601篇
  2016年   5952篇
  2015年   4236篇
  2014年   6953篇
  2013年   14255篇
  2012年   6630篇
  2011年   8111篇
  2010年   7176篇
  2009年   9752篇
  2008年   8527篇
  2007年   7968篇
  2006年   9625篇
  2005年   7766篇
  2004年   7642篇
  2003年   7151篇
  2002年   6721篇
  2001年   5997篇
  2000年   5950篇
  1999年   5198篇
  1998年   5220篇
  1997年   4977篇
  1996年   4220篇
  1995年   4304篇
  1994年   3998篇
  1993年   3760篇
  1992年   3512篇
  1991年   3527篇
  1990年   3624篇
  1989年   3327篇
  1988年   3160篇
  1987年   3712篇
  1986年   3250篇
  1985年   4119篇
  1984年   4630篇
  1983年   4308篇
  1982年   4225篇
  1981年   3842篇
  1980年   3591篇
  1979年   3433篇
  1978年   3440篇
  1977年   3217篇
  1976年   2964篇
  1975年   2899篇
  1974年   2862篇
  1973年   3059篇
  1972年   1989篇
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
941.
We review the main characteristics of the symbiotic system AG Draconis, with special emphasis on its optical and X-ray variations. We also discuss the X-ray to visual energy distribution during quiescence and outburst and describe our spectroscopic and X-ray observations during the 2003 outburst. Based on X-ray observations collected with the XMM–Newton Observatory, on INES data from the IUE satellite, and on optical spectra collected with the Asiago–Cima Ekar, Bologna–Loiano and La Palma–Galileo Italian telescopes.  相似文献   
942.
943.
We present a study of radiative transfer in dusty, clumpy star-forming regions. A series of self-consistent, 3D, continuum radiative transfer models are constructed for a grid of models parametrized by central luminosity, filling factor, clump radius and face-averaged optical depth. The temperature distribution within the clouds is studied as a function of this parametrization. Among our results, we find that: (i) the effective optical depth in clumpy regions is less than in equivalent homogeneous regions of the same average optical depth, leading to a deeper penetration of heating radiation in clumpy clouds, and temperatures higher by over 60 per cent; (ii) penetration of radiation is driven by the fraction of open sky (FOS) – which is a measure of the fraction of solid angle along which no clumps exist; (iii) FOS increases as clump radius increases and as filling factor decreases; (iv) for values of   FOS >0.6–0.8  the sky is sufficiently 'open' that the temperature distribution is relatively insensitive to FOS; (v) the physical process by which radiation penetrates is preferentially through streaming of radiation between clumps as opposed to diffusion through clumps; (vi) filling factor always dominates the determination of the temperature distribution for large optical depths, and for small clump radii at smaller optical depths; (vii) at lower face-averaged optical depths, the temperature distribution is most sensitive to filling factors of 1–10 per cent, in accordance with many observations; (viii) direct shadowing by clumps can be important for distances approximately one clump radius behind a clump.  相似文献   
944.
Motivated by recent high-resolution observations of the solar surface, we investigate the problem of non-linear magnetoconvection in a three-dimensional compressible layer. We present results from a set of numerical simulations which model the situation in which there is a weak imposed magnetic field. This weak-field regime is characterized by vigorous granular convection and spatially intermittent magnetic field structures. When the imposed field is very weak, magnetic flux tends to accumulate at the edges of the convective cells, where it forms compact, almost 'point-like' structures which are reminiscent of those observed in the quiet Sun. If the imposed field is slightly stronger, there is a tendency for magnetic flux to become concentrated into 'ribbon-like' structures which are comparable to those observed in solar plages. The dependence of these simulations upon the strength of the imposed magnetic field is analysed in detail, and the concept of the fractal dimension is used to make a further, more quantitative comparison between these simulations and photospheric observations.  相似文献   
945.
This paper presents two and three dimensional simulations of the interaction of shocks with media with large numbers of dense inclusions. An approximate model of the interaction of a starburst wind with the surrounding galactic ISM illustrates issues which must be addressed in global models of ISM dynamics. As a step towards developing the sub-grid model of multiphase turbulence, we define and study a form of ‘multiphase Riemann problem’. This allows us to develop macroscopic characteristics of the flows which may be compared to such subgrid models.  相似文献   
946.
947.
948.
Geophysical data from Gerlache Strait, Croker Passage, Bismarck Strait and the adjacent continental shelf reveal streamlined subglacial bedforms that were produced at the bed of the Antarctic Peninsula Ice Sheet (APIS) during the last glaciation. The spatial arrangement and orientation of these bedforms record the former drainage pattern and flow dynamics of an APIS outlet up‐flow, and feeding into, a palaeo‐ice stream in the Western Bransfield Basin. Evidence suggests that together, they represent a single ice‐flow system that drained the APIS during the last glaciation. The ice‐sheet outlet flowed north/northeastwards through Gerlache Strait and Croker Passage and converged with a second, more easterly ice‐flow tributary on the middle shelf to form the main palaeo‐ice stream. The dominance of drumlins with low elongation ratios suggests that ice‐sheet outlet draining through Gerlache Strait was comparatively slower than the main palaeo‐ice stream in the Western Bransfield Basin, although the low elongation ratios may also partly reflect the lack of sediment. Progressive elongation of drumlins further down‐flow indicates that the ice sheet accelerated through Croker Passage and the western tributary trough, and fed into the main zone of streaming flow in the Western Bransfield Basin. Topography would have exerted a strong control on the development of the palaeo‐ice stream system but subglacial geology may also have been significant given the transition from crystalline bedrock to sedimentary strata on the inner–mid‐shelf. In the broader context, the APIS was drained by a number of major fast‐flowing outlets through cross‐shelf troughs to the outer continental shelf during the last glaciation. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
949.
950.
Photoionization by stars cannot be the only cause for the diffuseionized gas component in the distant halos of galaxies.Quasistationary localized magnetic reconnection can efficientlycontribute to the ionization of the extraplanar halo gas. The gas isheated and re-ionized in numerous current filaments fast enough tocompensate recombination. The necessary magnetic energy is provided bythe disk activity.  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