全文获取类型
收费全文 | 261377篇 |
免费 | 5202篇 |
国内免费 | 3413篇 |
专业分类
测绘学 | 6996篇 |
大气科学 | 19281篇 |
地球物理 | 54867篇 |
地质学 | 91336篇 |
海洋学 | 21741篇 |
天文学 | 56706篇 |
综合类 | 1026篇 |
自然地理 | 18039篇 |
出版年
2021年 | 2235篇 |
2020年 | 2619篇 |
2019年 | 2859篇 |
2018年 | 4017篇 |
2017年 | 3647篇 |
2016年 | 6000篇 |
2015年 | 4232篇 |
2014年 | 6947篇 |
2013年 | 14246篇 |
2012年 | 6661篇 |
2011年 | 8173篇 |
2010年 | 7184篇 |
2009年 | 9785篇 |
2008年 | 8658篇 |
2007年 | 8133篇 |
2006年 | 9697篇 |
2005年 | 7838篇 |
2004年 | 7739篇 |
2003年 | 7218篇 |
2002年 | 6791篇 |
2001年 | 6053篇 |
2000年 | 5975篇 |
1999年 | 5206篇 |
1998年 | 5245篇 |
1997年 | 5046篇 |
1996年 | 4684篇 |
1995年 | 4429篇 |
1994年 | 4118篇 |
1993年 | 3858篇 |
1992年 | 3644篇 |
1991年 | 3599篇 |
1990年 | 3763篇 |
1989年 | 3520篇 |
1988年 | 3303篇 |
1987年 | 3858篇 |
1986年 | 3422篇 |
1985年 | 4236篇 |
1984年 | 4742篇 |
1983年 | 4409篇 |
1982年 | 4319篇 |
1981年 | 3921篇 |
1980年 | 3646篇 |
1979年 | 3508篇 |
1978年 | 3487篇 |
1977年 | 3285篇 |
1976年 | 3055篇 |
1975年 | 2960篇 |
1974年 | 2917篇 |
1973年 | 3079篇 |
1972年 | 2024篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
871.
872.
We present a new Very Large Array (VLA) image of Saturn, made from data taken in October 1998 at a wavelength of λ3.6 cm. The moderate ring opening angle (B≈15°) allows us to explore direct transmission of microwave photons through the A and C rings. We find a strong asymmetry of photons transmitted through the A ring, but not in the C ring, a new diagnostic of wake structure in the ring particles. We also find a weak asymmetry between east and west for the far side of the ansae. To facilitate quantitative comparison between dynamic models of the A ring and radio observations, we extend our Monte Carlo radiative transfer code (described in Dunn et al., 2002, Icarus 160, 132-160) to include idealized wakes. We show the idealized model can reproduce the properties of dynamic simulations in directly transmitted light. We examine the model behavior in directly transmitted and scattered light over a range of physical and geometric wake parameters. Finally, we present a wake model with a plausible set of physical parameters that quantitatively reproduces the observed intensity and asymmetry of the A ring both across the planet and in the ansae. 相似文献
873.
Chaoborid and chironomid (Diptera) fossils were examined in sediment cores form nine Ontario Precambrian Shield lakes that were fishless in 1979. An abundance of Chaoborus americamus (intolerant of predation by fish) throughout cores from four lakes indicated that they were fishless historically. Occurrence of Chaoborus punctipennis and C. flavicans and absence of C. americanus in cores, except near the surface in two cases, indicated that four other lakes were inhabited by fish historically. One of these was probably periodically fishless as suggested by fluctuations among these Chaoborus species. The ninth lake was not suitable for Chaoborus; only one specimen of C. trivittatus was found in this core. Chironomid fossil associations were substantially different in historically fishless and inhabited lakes. Fishless lakes had chironomids typical of eutrophic conditions indicating that seasonal oxygen depletion likely was inimical to fish. Chironomid associations of oligotrophic waters occurred in lakes historically inhabited by fish. Both good water quality and accessibility were required for long-term habitation by fish. Partial recovery of alkalinity and rise in pH were accompanied by substantial re-invasion by fish in two lakes which had been inhabited historically by fish. Diptera associations provide valuable information on the historical suitability of lakes for fish and the presence/absence of fish in regions where acidification of surface waters has occurred. 相似文献
874.
M. G. Abrahamyan 《Astrophysics》2004,47(4):555-567
The equilibrium of elliptical Riemann disks with a polytropic equation of state and their evolution under the influence of viscosity and gravitational radiation inside spheroidal halos with a relative surface mass density k. The evolutionary trajectory of a disk inside a halo with k<0.5, which is analogous to the evolution of an isolated disk, differs from that of a disk inside a denser halo. 相似文献
875.
M. J. Carson J. C. Davies E. Daw R. J. Hollingworth V. A. Kudryavtsev T. B. Lawson P. K. Lightfoot J. E. McMillan B. Morgan S. M. Paling M. Robinson N. J. C. Spooner D. R. Tovey 《Astroparticle Physics》2004,21(6):667-687
Simulations of the neutron background for future large-scale particle dark matter detectors are presented. Neutrons were generated in rock and detector elements via spontaneous fission and (α,n) reactions, and by cosmic-ray muons. The simulation techniques and results are discussed in the context of the expected sensitivity of a generic liquid xenon dark matter detector. Methods of neutron background suppression are investigated. A sensitivity of 10−9–10−10 pb to WIMP-nucleon interactions can be achieved by a tonne-scale detector. 相似文献
876.
M. Sereno 《Monthly notices of the Royal Astronomical Society》2005,356(3):937-943
Constraints on an exact quintessence scalar-field model with an exponential potential are derived from gravitational lens statistics. An exponential potential can account for data from both optical quasar surveys and radio-selected sources. Based on the Cosmic Lens All-Sky Survey (CLASS) sample, lensing statistics provides, for the pressureless matter density parameter, an estimate of ΩM0 = 0.31+0.12 −0.14 . 相似文献
877.
878.
A tachometer for measuring the displacements of the center of gravity of spectral line profiles is developed. The implementation of the device, basically a Fabry-Pérot etalon, is given. The spectral shifts are determined, using the frequency of a laser as reference. 相似文献
879.
S. C. Trager Guy Worthey S. M. Faber Alan Dressler 《Monthly notices of the Royal Astronomical Society》2005,362(1):2-8
We present numerical investigations into the formation of massive stars from turbulent cores of density structure ρ∝ r −1.5 . The results of five hydrodynamical simulations are described, following the collapse of the core, fragmentation and the formation of small clusters of protostars. We generate two different initial turbulent velocity fields corresponding to power-law spectra P ∝ k −4 and P ∝ k −3.5 , and we apply two different initial core radii. Calculations are included for both completely isothermal collapse, and a non-isothermal equation of state above a critical density (10−14 g cm−3 ) . Our calculations reveal the preference of fragmentation over monolithic star formation in turbulent cores. Fragmentation was prevalent in all the isothermal cases. Although disc fragmentation was largely suppressed in the non-isothermal runs due to the small dynamic range between the initial density and the critical density, our results show that some fragmentation still persisted. This is inconsistent with previous suggestions that turbulent cores result in the formation of a single massive star. We conclude that turbulence cannot be measured as an isotropic pressure term. 相似文献
880.
L. A. L. Da Silva 《Astrophysics and Space Science》1990,165(2):255-260
A comparison is made of the light and colour curves of the SN 1987A in the first 280 days with the mean properties of type II supernovae. The conclusion is that even though it has H lines, and was not a typical type II supernova, but a peculiar object from a photometric point of view. The implications for inclusion of SN 1987A in the context of the usual Minkowski-Zwicky classification are explicitly discussed. 相似文献