全文获取类型
收费全文 | 29593篇 |
免费 | 3787篇 |
国内免费 | 4822篇 |
专业分类
测绘学 | 3914篇 |
大气科学 | 2856篇 |
地球物理 | 3311篇 |
地质学 | 12835篇 |
海洋学 | 3497篇 |
天文学 | 5439篇 |
综合类 | 2137篇 |
自然地理 | 4213篇 |
出版年
2024年 | 62篇 |
2023年 | 249篇 |
2022年 | 678篇 |
2021年 | 872篇 |
2020年 | 852篇 |
2019年 | 910篇 |
2018年 | 759篇 |
2017年 | 1010篇 |
2016年 | 932篇 |
2015年 | 1103篇 |
2014年 | 1558篇 |
2013年 | 1608篇 |
2012年 | 1745篇 |
2011年 | 1970篇 |
2010年 | 1892篇 |
2009年 | 2112篇 |
2008年 | 2091篇 |
2007年 | 2186篇 |
2006年 | 2121篇 |
2005年 | 1916篇 |
2004年 | 1702篇 |
2003年 | 1489篇 |
2002年 | 1398篇 |
2001年 | 1175篇 |
2000年 | 1075篇 |
1999年 | 895篇 |
1998年 | 774篇 |
1997年 | 548篇 |
1996年 | 458篇 |
1995年 | 410篇 |
1994年 | 375篇 |
1993年 | 305篇 |
1992年 | 208篇 |
1991年 | 151篇 |
1990年 | 119篇 |
1989年 | 107篇 |
1988年 | 79篇 |
1987年 | 53篇 |
1986年 | 43篇 |
1985年 | 50篇 |
1984年 | 39篇 |
1983年 | 27篇 |
1982年 | 33篇 |
1981年 | 7篇 |
1980年 | 15篇 |
1979年 | 5篇 |
1978年 | 9篇 |
1977年 | 18篇 |
1954年 | 3篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 468 毫秒
141.
The sand–loess transition zone in north China is sensitive to climate change, and is an ideal place to investigate past environmental changes. However, past climate change at millennial–centennial timescales in this region has not been well reconstructed because of limited numerical dating. Alternations of sandy loam soils with aeolian sand layers in the Mu Us and Otindag sand fields, which lie along the sand–loess transition zone, indicate multiple intervals of dune activity and stability. This change is probably a response to variations of the East Asian monsoon climate during the late Quaternary. The single aliquot regeneration (SAR) optically stimulated luminescence (OSL) dating protocol, which has been successfully applied to aeolian deposits worldwide, is applied to these two sand fields in this study. The OSL ages provide reliable constraints for reconstruction of past climate changes at suborbital timescale. Sections in both sand fields contain aeolian sand beds recording millennial‐scale episodes of dry climate and widespread dune activation, including episodes at about the same time as Heinrich Event 5 and the Younger Dryas in the North Atlantic region. These results demonstrate the potential of aeolian sediments in semi‐arid north China to record millennial‐scale climatic events, and also suggest that dry–wet climate variation at the desert margin in China may be linked to climatic change elsewhere in the Northern Hemisphere, through atmospheric circulation. This article was published online on 27 November 2008. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected (16 December 2008). Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
142.
143.
144.
145.
146.
147.
J. C. B. Papaloizou 《Celestial Mechanics and Dynamical Astronomy》2003,87(1-2):53-83
We consider orbital resonances in multiplanet systems. These are expected to arise during or just after formation in a gaseous disc. Disc–planet interaction naturally produces orbital migration and circularization through the action of tidal torques which in turn may lead to an orbital resonance. The mass and angular momentum content of the disc is likely to be comparable to that in the planets so that it is essential to fully incorporate the disc in the analysis.We study the orbital evolution of two planets locked in 2:1 commensurability through migration tidally induced by the disc using both analytic methods and numerical hydrodynamic simulations. The planets are assumed to orbit in an inner cavity containing at most only a small amount of disc material. Results are found to be sensitive to initial surface density profile, planet masses and disc parameters. The evolution may range between attaining and subsequently maintaining a resonance lock with two angles librating to divergent migration with no commensurability formed. In the former case eccentricities increase monotonically with time while the system undergoes inward migration. If the migration is halted by loss of the disc leaving the planets in a final configuration, there is likely to be a low probability of seeing resonant planets at small radii as well as a sensitive dependence on past history.We have also considered a multiplanet system in secular apsidal resonance. We consider the system as being in just one secular normal mode and include the effects of a gaseous disc. It is suggested that a normal mode may be selected by adding in some weak dissipative process in the disc and that it may remain, involving only the planets, when the disc is slowly removed. 相似文献
148.
M. R. Bate S. H. Lubow G. I. Ogilvie K. A. Miller 《Monthly notices of the Royal Astronomical Society》2003,341(1):213-229
We analyse the non-linear, three-dimensional response of a gaseous, viscous protoplanetary disc to the presence of a planet of mass ranging from 1 Earth mass (1 M⊕ ) to 1 Jupiter mass (1 MJ ) by using the zeus hydrodynamics code. We determine the gas flow pattern, and the accretion and migration rates of the planet. The planet is assumed to be in a fixed circular orbit about the central star. It is also assumed to be able to accrete gas without expansion on the scale of its Roche radius. Only planets with masses M p ≳ 0.1 MJ produce significant perturbations in the surface density of the disc. The flow within the Roche lobe of the planet is fully three-dimensional. Gas streams generally enter the Roche lobe close to the disc mid-plane, but produce much weaker shocks than the streams in two-dimensional models. The streams supply material to a circumplanetary disc that rotates in the same sense as the orbit of the planet. Much of the mass supply to the circumplanetary disc comes from non-coplanar flow. The accretion rate peaks with a planet mass of approximately 0.1 MJ and is highly efficient, occurring at the local viscous rate. The migration time-scales for planets of mass less than 0.1 MJ , based on torques from disc material outside the Roche lobes of the planets, are in excellent agreement with the linear theory of type I (non-gap) migration for three-dimensional discs. The transition from type I to type II (gap) migration is smooth, with changes in migration times of about a factor of 2. Starting with a core which can undergo runaway growth, a planet can gain up to a few MJ with little migration. Planets with final masses of the order of 10 MJ would undergo large migration, which makes formation and survival difficult. 相似文献
149.
150.