全文获取类型
收费全文 | 65922篇 |
免费 | 1418篇 |
国内免费 | 492篇 |
专业分类
测绘学 | 1634篇 |
大气科学 | 5281篇 |
地球物理 | 13741篇 |
地质学 | 21427篇 |
海洋学 | 5717篇 |
天文学 | 15216篇 |
综合类 | 134篇 |
自然地理 | 4682篇 |
出版年
2020年 | 473篇 |
2019年 | 495篇 |
2018年 | 933篇 |
2017年 | 918篇 |
2016年 | 1363篇 |
2015年 | 1011篇 |
2014年 | 1416篇 |
2013年 | 3237篇 |
2012年 | 1495篇 |
2011年 | 2276篇 |
2010年 | 1942篇 |
2009年 | 2921篇 |
2008年 | 2659篇 |
2007年 | 2392篇 |
2006年 | 2456篇 |
2005年 | 2133篇 |
2004年 | 2235篇 |
2003年 | 2059篇 |
2002年 | 1963篇 |
2001年 | 1774篇 |
2000年 | 1747篇 |
1999年 | 1505篇 |
1998年 | 1492篇 |
1997年 | 1480篇 |
1996年 | 1273篇 |
1995年 | 1208篇 |
1994年 | 1093篇 |
1993年 | 995篇 |
1992年 | 943篇 |
1991年 | 799篇 |
1990年 | 1009篇 |
1989年 | 848篇 |
1988年 | 753篇 |
1987年 | 926篇 |
1986年 | 817篇 |
1985年 | 1021篇 |
1984年 | 1182篇 |
1983年 | 1123篇 |
1982年 | 1017篇 |
1981年 | 976篇 |
1980年 | 834篇 |
1979年 | 815篇 |
1978年 | 868篇 |
1977年 | 788篇 |
1976年 | 749篇 |
1975年 | 695篇 |
1974年 | 703篇 |
1973年 | 709篇 |
1972年 | 440篇 |
1971年 | 384篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
101.
The dynamics of clumps observed in planetary nebulae are considered. The possibility that SiO maser spots in evolved stars and the planetary nebula clumps are formed by the Parker instability behind shocks in pulsating stars' atmospheres is raised. Molecular observations of the clumps are suggested. The effects of the ablation of clumps on the global flow structure of a more tenuous plasma in which they are embedded are reviewed. 相似文献
102.
J. I. Read M. I. Wilkinson N. W. Evans G. Gilmore Jan T. Kleyna 《Monthly notices of the Royal Astronomical Society》2006,366(2):429-437
We present an improved analytic calculation for the tidal radius of satellites and test our results against N -body simulations.
The tidal radius in general depends upon four factors: the potential of the host galaxy, the potential of the satellite, the orbit of the satellite and the orbit of the star within the satellite . We demonstrate that this last point is critical and suggest using three tidal radii to cover the range of orbits of stars within the satellite. In this way we show explicitly that prograde star orbits will be more easily stripped than radial orbits; while radial orbits are more easily stripped than retrograde ones. This result has previously been established by several authors numerically, but can now be understood analytically. For point mass, power-law (which includes the isothermal sphere), and a restricted class of split power-law potentials our solution is fully analytic. For more general potentials, we provide an equation which may be rapidly solved numerically.
Over short times (≲1–2 Gyr ∼1 satellite orbit), we find excellent agreement between our analytic and numerical models. Over longer times, star orbits within the satellite are transformed by the tidal field of the host galaxy. In a Hubble time, this causes a convergence of the three limiting tidal radii towards the prograde stripping radius. Beyond the prograde stripping radius, the velocity dispersion will be tangentially anisotropic. 相似文献
The tidal radius in general depends upon four factors: the potential of the host galaxy, the potential of the satellite, the orbit of the satellite and the orbit of the star within the satellite . We demonstrate that this last point is critical and suggest using three tidal radii to cover the range of orbits of stars within the satellite. In this way we show explicitly that prograde star orbits will be more easily stripped than radial orbits; while radial orbits are more easily stripped than retrograde ones. This result has previously been established by several authors numerically, but can now be understood analytically. For point mass, power-law (which includes the isothermal sphere), and a restricted class of split power-law potentials our solution is fully analytic. For more general potentials, we provide an equation which may be rapidly solved numerically.
Over short times (≲1–2 Gyr ∼1 satellite orbit), we find excellent agreement between our analytic and numerical models. Over longer times, star orbits within the satellite are transformed by the tidal field of the host galaxy. In a Hubble time, this causes a convergence of the three limiting tidal radii towards the prograde stripping radius. Beyond the prograde stripping radius, the velocity dispersion will be tangentially anisotropic. 相似文献
103.
104.
105.
F. J. Ormeling Sr. 《GeoJournal》1986,13(4):413-416
Conclusion The above account is just a selection of Perthes' multifarious activities. As such it is fragmentary and incomplete. Many other publications and collaborators could have been mentioned. Even in its incompleteness the report may justify the statement that the success of Justus Perthes Geographische Verlagsanstalt, was apart from the managing qualities of its governors, undoubtedly due to the long series of scholarly competent collaborators whom they succeeded in engaging. As illustrated above most of theme were among the avant garde of the profession. Apparently they were at ease with Justus Perthes where they were granted enough freedom of action to realise their ambitious projects under the vigilant eye of the management. In fact the history of the firm is a continuous story of their achievements. Their combined effort initiated modern scientific atlas-making, laid down the foundation of methodic school cartography and accelerated the growth of thematic cartography. The Justus Perthes maps and atlasses stand for intellectual honestly and have an authoritative appearance of truth and exactness. For a long time to come the name of the firm will be considered as a classical example of the promotion of geography and cartography by private enterprise. 相似文献
106.
N.J.P. Owens 《Estuarine, Coastal and Shelf Science》1985,20(4):505-510
In this study, the 14N:15N ratio of suspended particulate material collected from the Tamar river estuary, south-west England, is described. Three populations of particles, distinguishable by their 15N content, were observed. This investigation has shown that populations of estuarine particles are generated by biological transformations in situ and that the 15N content of estuarine particles does not merely reflect hydrodynamic mixing of the freshwater and seawater source particulate material. 相似文献
107.
108.
R.E. Hartle E.C. Sittler R.E. Johnson F. Crary D.T. Young D. Simpson D. Reisenfeld J.J. Berthelier J. Vilppola N. Andre 《Planetary and Space Science》2006,54(12):1211-1224
The Cassini plasma spectrometer (CAPS) instrument made measurements of Titan's plasma environment when the Cassini Orbiter flew through the moon's plasma wake October 26, 2004 (flyby TA). Initial CAPS ion and electron measurements from this encounter will be compared with measurements made by the Voyager 1 plasma science instrument (PLS). The comparisons will be used to evaluate previous interpretations and predictions of the Titan plasma environment that have been made using PLS measurements. The plasma wake trajectories of flyby TA and Voyager 1 are similar because they occurred when Titan was near Saturn's local noon. These similarities make possible direct, meaningful comparisons between the various plasma wake measurements. They lead to the following: (A) The light and heavy ions, H+and N+/O+, were observed by PLS in Saturn's magnetosphere in the vicinity of Titan while the higher mass resolution of CAPS yielded H+ and H2+as the light constituents and O+/CH4+ as the heavy ions. (B) Finite gyroradius effects were apparent in PLS and CAPS measurements of ambient O+ ions as a result of their absorption by Titan's extended atmosphere. (C) The principal pickup ions inferred from both PLS and CAPS measurements are H+, H2+, N+, CH4+ and N2+. (D) The inference that heavy pickup ions, observed by PLS, were in narrow beam distributions was empirically established by the CAPS measurements. (E) Slowing down of the ambient plasma due to pickup ion mass loading was observed by both instruments on the anti-Saturn side of Titan. (F) Strong mass loading just outside the ionotail by a heavy ion such as N2+ is apparent in PLS and CAPS measurements. (G) Except for the expected differences due to the differing trajectories, the magnitudes and structures of the electron densities and temperatures observed by both instruments are similar. The high-energy electron bite-out observed by PLS in the magnetotail is consistent with that observed by CAPS. 相似文献
109.
Ocean dynamical processes exist over a wide range of temporal and spatial scales. Remotely stationed acoustic devices are being used to "sense" the interior of the oceans at previously unattainable scales. The method is similar to computer-aided tomography (CAT) scans of the brain, but the measurement difficulties are far more severe. The results of a demonstration experiment are reviewed and some directions for acoustic oceanography are discussed. 相似文献
110.
Bennett R. Burns J. Nastav F. Lipkin J. Percival C. 《Oceanic Engineering, IEEE Journal of》1985,10(1):17-22
Two single-sensor piezometer probes, 8 mm in diameter, were developed for deep-ocean geotechnical investigations. These probes were tested in a hyperbaric chamber pressurized to 55 MPa (8000 psi). Testing was performed for a period of five weeks under high hydrostatic pressure with the probes inserted in reconstituted illitic marine sediment. Small differential pore-water pressures were generated in response to both mechanically and thermally generated forcing functions. During deep-ocean simulated pressure tests, the sensors exhibited excellent sensitivity and stability. These developments in piezometer-probe technology provide a quantitative means of assessing important geotechnical parameters of fine-grained seabed deposits. 相似文献