全文获取类型
收费全文 | 9786篇 |
免费 | 851篇 |
国内免费 | 1639篇 |
专业分类
测绘学 | 341篇 |
大气科学 | 506篇 |
地球物理 | 778篇 |
地质学 | 3739篇 |
海洋学 | 920篇 |
天文学 | 5079篇 |
综合类 | 370篇 |
自然地理 | 543篇 |
出版年
2024年 | 25篇 |
2023年 | 69篇 |
2022年 | 203篇 |
2021年 | 219篇 |
2020年 | 252篇 |
2019年 | 310篇 |
2018年 | 239篇 |
2017年 | 214篇 |
2016年 | 247篇 |
2015年 | 275篇 |
2014年 | 493篇 |
2013年 | 555篇 |
2012年 | 566篇 |
2011年 | 690篇 |
2010年 | 742篇 |
2009年 | 992篇 |
2008年 | 934篇 |
2007年 | 824篇 |
2006年 | 773篇 |
2005年 | 675篇 |
2004年 | 553篇 |
2003年 | 463篇 |
2002年 | 366篇 |
2001年 | 331篇 |
2000年 | 282篇 |
1999年 | 261篇 |
1998年 | 177篇 |
1997年 | 67篇 |
1996年 | 58篇 |
1995年 | 47篇 |
1994年 | 50篇 |
1993年 | 65篇 |
1992年 | 19篇 |
1991年 | 30篇 |
1990年 | 28篇 |
1989年 | 20篇 |
1988年 | 24篇 |
1987年 | 6篇 |
1986年 | 17篇 |
1985年 | 21篇 |
1984年 | 19篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 6篇 |
1980年 | 11篇 |
1979年 | 2篇 |
1978年 | 6篇 |
1977年 | 15篇 |
1877年 | 1篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
123.
124.
125.
We present a 2D general circulation model of Titan's atmosphere, coupling axisymmetric dynamics with haze microphysics, a simplified photochemistry and eddy mixing. We develop a parameterization of latitudinal eddy mixing by barotropic waves based on a shallow-water, longitude-latitude model. The parameterization acts locally and in real time both on passive tracers and momentum. The mixing coefficient varies exponentially with a measure of the barotropic instability of the mean zonal flow. The coupled GCM approximately reproduces the Voyager temperature measurements and the latitudinal contrasts in the distributions of HCN and C2H2, as well as the main features of the zonal wind retrieved from the 1989 stellar occultation. Wind velocities are consistent with the observed reversal time of the North-South albedo asymmetry of 5 terrestrial years. Model results support the hypothesis of a non-uniform distribution of infrared opacity as the cause of the Voyager temperature asymmetry. Transport by the mean meridional circulation, combined with polar vortex isolation may be at the origin of the latitudinal contrasts of trace species, with eddy mixing remaining restricted to low latitudes most of the Titan year. We interpret the contrasts as a signature of non-axisymmetric motions. 相似文献
126.
J. P. Bernstein † Suketu P. Bhavsar ‡ 《Monthly notices of the Royal Astronomical Society》2001,322(3):625-630
The brightest, or first-ranked, galaxies (BCGs) in rich clusters show a very small dispersion in luminosity, making them excellent standard candles. This small dispersion raises questions about the nature of BCGs. Are they simply the extremes of normal galaxies formed via a stochastic process, or do they belong to a special class of atypical objects? If they do, are all BCGs special, or do normal galaxies compete for the first rank? To answer these questions, we undertake a statistical study of BCG magnitudes using results from extreme value theory. Two-population models do better than one-population models. A simple scenario where a random boost in the magnitude of a fraction of bright normal galaxies forms a class of atypical galaxies best describes the observed distribution of BCG magnitudes. 相似文献
127.
128.
N. Kameswara Rao V. R. Venugopal A. Patnaik 《Journal of Astrophysics and Astronomy》1985,6(3):153-157
VLA observations at 2 and 6 cm have been obtained for six hydrogen-deficient stars R CrB, HD 160641, BD — 9°4395, V348 Sgr,
MV Sgr and Sgrv Upper limits to the massloss rates have been estimated for some of these using the upper limits to the radio flux density.
National Radio Astronomy Observatory’s Very Large Array is operated by Associated Universities Inc. under contract with National
Science Foundation, USA 相似文献
129.
The gray crystalline hematite at Meridiani Planum first discovered by the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) instrument occurs as spherules that have been interpreted as concretions. Analysis of the TES and mini-TES spectra shows that no 390 cm−1 feature is present in the characteristic martian hematite spectrum. Here, we incorporate the mid-IR optical constants of hematite into a simple Fresnel reflectance model to understand the effect of emission angle and crystal morphology on the presence or absence of the 390 cm−1 feature in an IR hematite spectrum. Based on the results we offer two models for the internal structure of the martian hematite spherules. 相似文献
130.
Three dating techniques for metamorphic minerals using the Sm–Nd, Lu–Hf and Pb isotope systems are combined and interpreted in context with detailed petrologic data from crustal segments in NW Namibia. The combination of isochron ages using these different approaches is a valuable tool to testify for the validity of metamorphic mineral dating. Here, PbSL, Lu–Hf and Sm–Nd garnet ages obtained on low- to medium-grade metasedimentary rocks from the Central Kaoko Zone of the Neoproterozoic Kaoko belt (NW Namibia) indicate that these samples were metamorphosed at around 550–560 Ma. On the other hand, granulite facies metasedimentary rocks from the Western Kaoko Zone underwent two phases of high-grade metamorphism, one at ca. 660–625 Ma and another at ca. 550 Ma providing substantial evidence that the 660–625 Ma-event was indeed a major tectonothermal episode in the Kaoko belt. Our age data suggest that interpreting metamorphic ages by applying a single dating method only is not reliable enough when studying complex metamorphic systems. However, a combination of all three dating techniques used here provides a reliable basis for geochronological age interpretation. 相似文献