全文获取类型
收费全文 | 31979篇 |
免费 | 486篇 |
国内免费 | 373篇 |
专业分类
测绘学 | 1224篇 |
大气科学 | 2437篇 |
地球物理 | 6205篇 |
地质学 | 11296篇 |
海洋学 | 2485篇 |
天文学 | 7638篇 |
综合类 | 145篇 |
自然地理 | 1408篇 |
出版年
2021年 | 279篇 |
2020年 | 276篇 |
2019年 | 324篇 |
2018年 | 816篇 |
2017年 | 780篇 |
2016年 | 1019篇 |
2015年 | 591篇 |
2014年 | 957篇 |
2013年 | 1688篇 |
2012年 | 1040篇 |
2011年 | 1277篇 |
2010年 | 1074篇 |
2009年 | 1386篇 |
2008年 | 1203篇 |
2007年 | 1159篇 |
2006年 | 1177篇 |
2005年 | 978篇 |
2004年 | 871篇 |
2003年 | 867篇 |
2002年 | 856篇 |
2001年 | 770篇 |
2000年 | 743篇 |
1999年 | 672篇 |
1998年 | 617篇 |
1997年 | 629篇 |
1996年 | 568篇 |
1995年 | 534篇 |
1994年 | 506篇 |
1993年 | 428篇 |
1992年 | 384篇 |
1991年 | 411篇 |
1990年 | 411篇 |
1989年 | 387篇 |
1988年 | 359篇 |
1987年 | 432篇 |
1986年 | 361篇 |
1985年 | 444篇 |
1984年 | 478篇 |
1983年 | 463篇 |
1982年 | 445篇 |
1981年 | 356篇 |
1980年 | 360篇 |
1979年 | 311篇 |
1978年 | 306篇 |
1977年 | 295篇 |
1976年 | 258篇 |
1975年 | 254篇 |
1974年 | 281篇 |
1973年 | 305篇 |
1972年 | 194篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
G.C. Sethia G.J. Bailey R.J. Moffett J.K. Hargreaves 《Planetary and Space Science》1985,33(3):321-332
A modelling study of the electron content of the mid-latitude ionosphere and protonosphere has been carried out for solstice conditions using the mathematical model of Bailey (1983). In the model calculations coupled time-dependent O+, H+ continuity and momentum equations and O+, H+ and electron heat balance equations are solved for a magnetic shell extending over both hemispheres. The inclusion of interhemispheric flow of plasma and of heat balance has enabled us to investigate the role of interhemispheric coupling on the electron content and related shape parameters. The computed results are compared with results from slant path observations of the ATS-6 radio beacon made at Lancaster (U.K.) and Boulder, Colorado (U.S.A.).It has been found that the conjugate photoelectron heating has a major effect on the shape of the daily variation of slant slab thickness (τ) and also on the magnitude of the protonospheric content (Np). Some of the main features of τ are closely related to the sunrise and sunset times in the conjugate ionosphere. Also it is found that night-time increases in total electron content (NT) and F2 region peak electron density (Nmax) in winter are natural consequences of ionization loss at low altitudes causing an enhanced downward flow of plasma from the protonosphere which is coupled to the summer hemisphere. One other important consequence of the coupled protonosphere is that the effects on NT of the neutral air wind are not much different in winter from those in summer. 相似文献
102.
103.
An estimate for the anisotropy of the turbulent viscositys is given in a convective layer heated from below and rotating around a vertical axis. In the case of two-dimensional convection,
there is a stationary regime withs⊇2 regardless of the rotation. In the case of three-dimensional convection in a slowly rotating layer (with the Taylor number
equal to 1600), nonstationary turbulent regimes take place withs⊇1.6 forR=2.5×104 (R is the Rayleigh number) ands⊇1.2 forR=104.
The parameters plays an, important role in the theory of differential rotation of the convective solar or stellar envelopes. So far, it
has been evaluated empirically or semi-empirically. Some prospects in the development of the theory of differential rotation
are discussed here in terms of the moment theory of hydrodynamic fields. The relation between this strict approach and an
anisotropic viscosity approximation is considered. 相似文献
104.
105.
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
106.
107.
In this paper, the currently accepted correlation of the Early Pleistocene Ludhamian stage of England with the Tiglian‐A sub‐stage of the Netherlands is challenged. Recent investigations of Early Pleistocene marine North Sea deposits from a borehole near Noordwijk (the Netherlands) yielded evidence from molluscs, dinoflagellate cysts and sporomorphs for an alternation of warm‐temperate and arctic intervals within the Praetiglian and Tiglian stages. Marine equivalents of the terrestrial‐based pollen sub‐stages Tiglian A and B have been recognised in the upper part of the sequence. A Praetiglian age can be assigned to the lower part of the sequence on the basis of mollusc analysis. Within the Praetiglian, an alternation of warm and cold phases has been recognised from both the dinoflagellate cyst and molluscan records. Three cold phases within the Praetiglian are tentatively correlated with marine isotope stages (MIS) 96–100. The molluscan assemblages provide evidence for climate forcing of the sea level: highest sea levels are reached in the warm‐temperate intervals. Within the Praetiglian, an interval with an acme zone of the dinoflagellate cyst Impagidinium multiplexum, is correlated with the Ludhamian and tentatively linked to MIS 97 and/or MIS 96. The cold molluscan assemblages from the Noordwijk borehole include an acme zone of Megayoldia thraciaeformis, the first and only occurrence of this North Pacific bivalve in the North Sea Basin. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
108.
We found the equilibrium conditions for a self-gravitating toroidal vortex by taking thermal pressure into account. These conditions are shown to significantly differ from those for a disk or a sphere. The evolution of a thin vortex turns it into a compact vortex that loses mechanical stability for low masses at a polytropic index γ<4/3 but retains stability for sufficiently high masses and densities determined by the velocity circulation in the vortex. 相似文献
109.
110.
R. K. Manchanda 《Journal of Astrophysics and Astronomy》2001,22(2-3):145-154
Spectral measurement of Mkn 421 were made in the hard X-ray energy band of 20–200 keV using a high sensitivity, large area
scintillation counter telescope on November 21, 2000 and these coincided with the onset of an active X-ray phase as seen in
the ASM counting rates on board RXTE. The observed spectrum can not be fitted to a single power law similar to the PDS data
of BeppoSAX. The data can be fitted both by a two component power-law function or a combination of an exponential function
with a power law component at the high energies above 80 keV. We identify these components with those arising from the synchrotron
self compton and the high energy power-law tail arising from the upgrading of the thermal photons due to multiple Compton
scattering a la Cyg X-1. A comparison with the earlier data clearly suggests a spectral variability in the hard X-ray spectrum
of the source. We propose a continuously flaring geometry for the source as the underlying mechanism for energy release. 相似文献