全文获取类型
收费全文 | 6767篇 |
免费 | 1341篇 |
国内免费 | 2072篇 |
专业分类
测绘学 | 1236篇 |
大气科学 | 1054篇 |
地球物理 | 1244篇 |
地质学 | 3671篇 |
海洋学 | 1133篇 |
天文学 | 209篇 |
综合类 | 701篇 |
自然地理 | 932篇 |
出版年
2024年 | 57篇 |
2023年 | 122篇 |
2022年 | 296篇 |
2021年 | 417篇 |
2020年 | 315篇 |
2019年 | 385篇 |
2018年 | 393篇 |
2017年 | 362篇 |
2016年 | 420篇 |
2015年 | 399篇 |
2014年 | 509篇 |
2013年 | 538篇 |
2012年 | 505篇 |
2011年 | 530篇 |
2010年 | 548篇 |
2009年 | 515篇 |
2008年 | 480篇 |
2007年 | 483篇 |
2006年 | 472篇 |
2005年 | 348篇 |
2004年 | 247篇 |
2003年 | 222篇 |
2002年 | 187篇 |
2001年 | 224篇 |
2000年 | 213篇 |
1999年 | 187篇 |
1998年 | 107篇 |
1997年 | 109篇 |
1996年 | 93篇 |
1995年 | 84篇 |
1994年 | 91篇 |
1993年 | 69篇 |
1992年 | 50篇 |
1991年 | 41篇 |
1990年 | 22篇 |
1989年 | 14篇 |
1988年 | 23篇 |
1987年 | 17篇 |
1986年 | 22篇 |
1985年 | 13篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 10篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1976年 | 4篇 |
1974年 | 2篇 |
1957年 | 2篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
811.
812.
813.
Evolution of scouring process downstream of grade‐control structures under steady and unsteady flows
For many incised channels, one of the most common strategies is to install some hard structures, such as grade‐control structures (GCSs), in the riverbed to resist further incision. In this study, a series of experiments, including both steady and unsteady flow conditions, were conducted to investigate the scouring process downstream of a GCS. Three distinct phases, including the initial, developing and equilibrium phases, during the evolution of scour holes were identified. In addition, a semi‐empirical method was proposed to predict the equilibrium scour‐hole profile for the scour countermeasure design. In general, the comparisons between the experimental and simulated results are reasonably consistent. As the studies on temporal variation of the scour depth at GCSs caused by floods are limited, the effect of flood hydrograph shapes on the scour downstream of GCSs without upstream sediment supply was also investigated experimentally in this study. Based on the dimensional analysis and the concept of superposition, a methodology is proposed to simulate the time evolution of the maximum scour depth downstream of a GCS for steady flows. Moreover, the proposed scheme predicts reasonably well the temporal variations of the maximum scour depth for unsteady flows with both single and multiple peak. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
814.
On October 24, 1991, a white-light flare was observed both from space and from the ground. A multi-waveband spectral analysis shows that the peak time of the continuum emission coincides well with that of a radio burst at 2840 MHz and with the hard X-ray emission. Three semi-empirical models, corresponding to the pre-flare condition and to the peak time of continuum emission both with and without non-thermal excitation and ionization of hydrogen by an electron beam, have been obtained. The results indicate that there is fast heating both in the chromosphere and the photosphere. Some evidence is given that this WLF is very likely a result of bombardment by an electron beam. By taking into account non-thermal effects, the chromospheric temperature of the semi-empirical model is significantly reduced. 相似文献
815.
P. Davoodi F. Pozzi S. Oliver M. Polletta A. Afonso-Luis D. Farrah E. Hatziminaoglou G. Rodighiero S. Berta I. Waddington C. Lonsdale M. Rowan-Robinson D. L. Shupe T. Evans F. Fang H. E. Smith J. Surace 《Monthly notices of the Royal Astronomical Society》2006,371(3):1113-1124
We present the rest-frame optical and infrared colours of a complete sample of 1114 z < 0.3 galaxies from the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Legacy Survey and the Sloan Digital Sky Survey (SDSS). We discuss the optical and infrared colours of our sample and analyse in detail the contribution of dusty star-forming galaxies and active galactic nuclei (AGN) to optically selected red sequence galaxies.
We propose that the optical ( g − r ) colour and infrared log( L24 / L 3.6 ) colour of galaxies in our sample are determined primarily by a bulge-to-disc ratio. The ( g − r ) colour is found to be sensitive to the bulge-to-disc ratio for disc-dominated galaxies, whereas the log( L 24 / L 3.6 ) colour is more sensitive for bulge-dominated systems.
We identify ∼18 per cent (195 sources) of our sample as having red optical colours and infrared excess. Typically, the infrared luminosities of these galaxies are found to be at the high end of star-forming galaxies with blue optical colours. Using emission-line diagnostic diagrams, 78 are found to have an AGN contribution and 117 are identified as star-forming systems. The red ( g − r ) colour of the star-forming galaxies could be explained by extinction. However, their high optical luminosities cannot. We conclude that they have a significant bulge component.
The number densities of optically red star-forming galaxies are found to correspond to ∼13 per cent of the total number density of our sample. In addition, these systems contribute ∼13 per cent of the total optical luminosity density, and 28 per cent of the total infrared luminosity density of our SWIRE/SDSS sample. These objects may reduce the need for 'dry mergers'. 相似文献
We propose that the optical ( g − r ) colour and infrared log( L
We identify ∼18 per cent (195 sources) of our sample as having red optical colours and infrared excess. Typically, the infrared luminosities of these galaxies are found to be at the high end of star-forming galaxies with blue optical colours. Using emission-line diagnostic diagrams, 78 are found to have an AGN contribution and 117 are identified as star-forming systems. The red ( g − r ) colour of the star-forming galaxies could be explained by extinction. However, their high optical luminosities cannot. We conclude that they have a significant bulge component.
The number densities of optically red star-forming galaxies are found to correspond to ∼13 per cent of the total number density of our sample. In addition, these systems contribute ∼13 per cent of the total optical luminosity density, and 28 per cent of the total infrared luminosity density of our SWIRE/SDSS sample. These objects may reduce the need for 'dry mergers'. 相似文献
816.
817.
818.
819.
There is observational evidence showing that stellar and solar flares occur with a similar circumstance, although the former are usually much more energetic. It is expected that the bombardment by high-energy electrons is one of the chief heating processes of the flaring atmosphere. In this paper we study how a precipitating electron beam can influence the line profiles of Ly α , H α , Ca ii K and λ 8542. We use a model atmosphere of a dMe star and make non-LTE computations taking into account the non-thermal collisional rates owing to the electron beam. The results show that the four lines can be enhanced to different extents. The relative enhancement increases with increasing formation height of the lines. Varying the energy flux of the electron beam has different effects on the four lines. The wings of Ly α and H α become increasingly broad with the beam flux; change of the Ca ii K and λ 8542 lines, however, is most significant in the line centre. Varying the electron energy (i.e. the low-energy cut-off for a power-law beam) has a great influence on the Ly α line, but little on the H α and Ca ii lines. An electron beam of higher energy precipitates deeper, thus producing less enhancement of the Ly α line. The Ly α /H α flux ratio is thus sensitive to the electron energy. 相似文献
820.