全文获取类型
收费全文 | 257603篇 |
免费 | 5191篇 |
国内免费 | 3340篇 |
专业分类
测绘学 | 6873篇 |
大气科学 | 19062篇 |
地球物理 | 54101篇 |
地质学 | 89522篇 |
海洋学 | 21434篇 |
天文学 | 56317篇 |
综合类 | 1003篇 |
自然地理 | 17822篇 |
出版年
2021年 | 2230篇 |
2020年 | 2583篇 |
2019年 | 2843篇 |
2018年 | 3492篇 |
2017年 | 3171篇 |
2016年 | 5707篇 |
2015年 | 4192篇 |
2014年 | 6930篇 |
2013年 | 14230篇 |
2012年 | 6476篇 |
2011年 | 7860篇 |
2010年 | 6984篇 |
2009年 | 9628篇 |
2008年 | 8345篇 |
2007年 | 7807篇 |
2006年 | 9603篇 |
2005年 | 7698篇 |
2004年 | 7594篇 |
2003年 | 7090篇 |
2002年 | 6684篇 |
2001年 | 5970篇 |
2000年 | 5925篇 |
1999年 | 5200篇 |
1998年 | 5226篇 |
1997年 | 5012篇 |
1996年 | 4674篇 |
1995年 | 4419篇 |
1994年 | 4105篇 |
1993年 | 3836篇 |
1992年 | 3625篇 |
1991年 | 3591篇 |
1990年 | 3759篇 |
1989年 | 3518篇 |
1988年 | 3300篇 |
1987年 | 3846篇 |
1986年 | 3407篇 |
1985年 | 4227篇 |
1984年 | 4741篇 |
1983年 | 4406篇 |
1982年 | 4310篇 |
1981年 | 3926篇 |
1980年 | 3643篇 |
1979年 | 3508篇 |
1978年 | 3494篇 |
1977年 | 3278篇 |
1976年 | 3039篇 |
1975年 | 2962篇 |
1974年 | 2913篇 |
1973年 | 3079篇 |
1972年 | 2027篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
891.
Electrons accelerated during solar flares are revealed by their electromagnetic radiation in different spectral ranges, emitted at different heights in the solar atmosphere. The observational analysis points to a common and continuous injection of particles. Based on this result, a quantitative investigation of the hard X-ray and microwave emissions observed during the 29 June, 1980 flare at 11: 40 UT has been performed. This is the first modelisation that takes into account both the inhomogeneity of the microwave source region and the dynamical evolution of the electron population. First results of our model computations demonstrate that during the most energetic phase of the event both hard X-rays and microwaves are described by electron populations resulting from the same injection function, and that the total numbers of electrons required for both emissions are compatible. Account for the inhomogeneity of the microwave source is shown to be a necessary condition for the interpretation of observed spectra.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985. 相似文献
892.
893.
Experimental results from an array of moored current meters and an HF ocean surface radar support the idea that line broadening on the radar spectra is caused by the velocity distribution within the radar target cell. The experiment was done in the wake of a small island where the velocity variations were severe. An estimate is made of the line broadening which can be expected. In a turbulent flow with dissipation rate of the orderepsilon sim 10^{-10}m^{2}s^{-3} and target cell size 1 3000 m, the line broadening isDeltaf sim 10^{-3} Hz. This would be resolved with a radar time series ofsim 20 min and indicates that the HF ocean surface radar technique has potential in the observation of surface velocity distributions. 相似文献
894.
During major geomagnetic storms, the interplanetary magnetic field angle φ (phi) changes often abruptly, either from 135° to 315° or 315° to 135°, suggesting that the heliospheric current sheet is pushed upward or downward by disturbed solar wind. The distortion of the heliospheric current sheet by three successive solar flares is simulated to show that such a flapping motion can occur. 相似文献
895.
Christopher J. Duffin 《Geology Today》2005,21(2):58-63
Nowadays, when someone refers to lapidary they usually mean the process of polishing, engraving or working stones. The term is much broader than that, however, encompassing an entire genre of literature which formed the basis of much geological observation even before the word 'geology' was ever coined. 相似文献
896.
The inception and growth of the active Carpino-Le Piane Basin Fault System (CLPBFS; central-southern Apennines, Italy) was analysed with respect to the neighbouring Isernia and Surrounding (ISFS) and Boiano Basin (BBFS) extensional Fault Systems. 39Ar–40Ar dating showed that the BBFS was already active 649 ± 21 ka bp and that the ISFS was active at least 476 ±10 ka bp , whereas the activity of the CLPBFS started certainly later than 253 ± 22 ka bp , and very probably as recently as <28 ka bp . These ages, combined with structural data (geometry and kinematics of the fault systems), indicate that the inception and development of the CLPBFS could be strictly related to the stress changes caused by earthquakes occurring on the BBFS. 相似文献
897.
Most nonlinear high-resolution bearing estimators are unstable in the presence of correlated noise, system phase errors, and multipath arrivals because they inadvertently place too much emphasis on unstable eigenvectors of the cross-sensor correlation matrix. For moderately correlated noise there will be sufficiently many stable eigenvectors to resolve and localize discrete sources. A method is given (the "stable nonlinear method" or SNLM) whereby reweighting of the eigenvectors is achieved implicitly, without actual calculation of the eigenvectors. This SNLM is compared with Capon's maximum likelihood method (MLM) in simulations of correlated noise, partially correlated signals, and phase errors, and is shown to provide good stability in the cases considered. 相似文献
898.
Summary Two co-existing plutonic rocks (diorite and granodiorite) were studied from an intrusion of Variscan age in the Raztocna Valley
– Nízke Tatry Mountains, Western Carpathians. Geochemical analyses of major and trace elements constrain a volcanic arc as
emplacement environment and give the first hints of a mixture of two magmatic end-members: the so-called Prasivá granodiorite
and the Raztocna diorite. The 87Sr/86Sr(0) ratios vary between 0.7075 and 0.7118, the ε Nd(0) values range from −1.4 to −5.0. Common Pb isotopes reveal a dominant crustal source with minor influences from a mantle and
a lower crustal source.
Modelling based on Sr and Nd isotope data and using three component mixing calculations indicates that mixing of 2/3 of upper
mantle material with 1/3 upper crustal material can produce the isotopic composition of the Raztocna diorite. Very minor amounts
of lower crust were incorporated in the diorite. For the Prasivá granodiorite, the mixing ratio of upper mantle and upper
crust is similar, but a lower crustal reservoir contributed about 5–10% of the source material. 相似文献
899.
900.
Coincident seismic reflection/refraction studies of the continental lithosphere: a global review 总被引:5,自引:0,他引:5
Summary. Vertical-incidence reflection profiling has identified several characteristic features of the continental Iithosphere including a generally transparent upper crust, a reflective lower crust, reflections from the crust-mantle boundary, and a commonly transparent upper mantle. The underlying physical causes of these characteristic features remain poorly understood. This review summarizes additional information brought to bear on the physical properties of these characteristic crustal structures through the use of coincident wide-angle refraction profiling. 相似文献