全文获取类型
收费全文 | 33869篇 |
免费 | 645篇 |
国内免费 | 417篇 |
专业分类
测绘学 | 837篇 |
大气科学 | 3073篇 |
地球物理 | 7060篇 |
地质学 | 11855篇 |
海洋学 | 2674篇 |
天文学 | 7166篇 |
综合类 | 80篇 |
自然地理 | 2186篇 |
出版年
2020年 | 215篇 |
2019年 | 239篇 |
2018年 | 539篇 |
2017年 | 539篇 |
2016年 | 751篇 |
2015年 | 514篇 |
2014年 | 735篇 |
2013年 | 1561篇 |
2012年 | 824篇 |
2011年 | 1155篇 |
2010年 | 1006篇 |
2009年 | 1385篇 |
2008年 | 1170篇 |
2007年 | 1047篇 |
2006年 | 1142篇 |
2005年 | 969篇 |
2004年 | 924篇 |
2003年 | 940篇 |
2002年 | 936篇 |
2001年 | 808篇 |
2000年 | 820篇 |
1999年 | 705篇 |
1998年 | 676篇 |
1997年 | 708篇 |
1996年 | 634篇 |
1995年 | 576篇 |
1994年 | 528篇 |
1993年 | 467篇 |
1992年 | 467篇 |
1991年 | 466篇 |
1990年 | 468篇 |
1989年 | 437篇 |
1988年 | 425篇 |
1987年 | 511篇 |
1986年 | 473篇 |
1985年 | 520篇 |
1984年 | 633篇 |
1983年 | 639篇 |
1982年 | 564篇 |
1981年 | 533篇 |
1980年 | 492篇 |
1979年 | 479篇 |
1978年 | 492篇 |
1977年 | 433篇 |
1976年 | 391篇 |
1975年 | 384篇 |
1974年 | 430篇 |
1973年 | 415篇 |
1972年 | 265篇 |
1971年 | 240篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
761.
Eight rare-earth elements (REE) have been determined in Cambro-Silurian sediments and volcanics from nine stratigraphic units in the Trondheim Region and ten stratigraphic units in the Oslo Region by neutron-activation analyses. In the eugeosynclinal sediments from the Trondheim Region the REE variations are related to variations in source material, which mainly are volcanic debris of local origin. In the foreland sediments from the Oslo Region significant development of clays has occurred and therefore the REE variations reflect the deposition environment. 相似文献
762.
763.
R. Mewe J. Heise E. H. B. M. Gronenschild A. C. Brinkman J. Schrijver A. J. F. den Boggende 《Astrophysics and Space Science》1976,42(1):217-222
With the soft X-ray detector (0.2–0.284 keV) aboard the Astronomical Netherlands Satellite (ANS) we have searched for X-ray emission from hot star coronae and peculiar stars. On Sirius ( CMa) and Capella ( Aur) X-ray emission has been measured at 6 and 5 level, respectively, above background. In all other cases the search revealed no evidence for soft X-ray emission. Upper limits to the luminosities of about 25 star coronae (main-sequence stars, (sub)giants, and supergiants) and of 4 peculiar stars ( Sco, Lyr, P Cyg, and Car) have been obtained.Paper presented at the COSPAR/IAU Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975. 相似文献
764.
The expansion of the solar wind in divergent flux tubes is calculated by taking into account a magnetic acceleration of the particles, analogous to the magnetic mirror effect.The resulting force term included in the magnetohydrodynamical equations describes a conversion of thermal into kinetic energy. This causes an additional acceleration of the solar wind plasma which has never been taken into account before. The force is directed opposite to the magnetic field gradient. Consequently, in this case the solar wind velocity increases faster to its asymptotic value than it does for corresponding nonmagnetic solutions. Therefore inside and close to the solar corona markedly higher velocities are found. Compared to strictly hydrodynamical models, the critical point is shifted towards the Sun, and the radial decrease of the ratio of thermal to kinetic energy is faster.The necessary prerequisites for these calculations are (a) that the gyroperoid
g
of the plasma particles is much shorter than the Coulomb collision time
c
, and (b) that the collision time
c
is shorter than the characteristic time
d
in which an appreciable amount of thermal anisotropy is built up. Thus it is (a) insured that the particles have established magnetic moments and follow the guiding center approximation, and (b) an almost isotropic velocity distribution function is maintained which, in this first approximation of a purely radial expansion, justifies the use of isotropic pressures and temperatures.Both (a) and (b) are shown to be fulfilled in a region around the Sun out to about 20R
, and thermal anisotropies developing outside of this region could explain the observed magnetically aligned anisotropies at 1 AU. 相似文献
765.
A.H. Delsemme 《Icarus》1975,24(1):95-110
In order to prepare a flyby mission to Comet Encke, six different sources of information on the possible chemical composition of the cometary nucleus are compared. These are: the neutral and charged radicals and molecules observed in cometary spectra; the chemical composition of type I carbonaceous chondrites; the meteor spectra; the metallic ions collected in the upper atmosphere and correlated with the meteor shower associated with Comet Encke; and finally the volatile molecules observed in a volatile-rich sample of lunar soil, that were interpreted as a possible cometary impact. Possible molecular abundances for the volatile fraction of Comet Encke are tentatively proposed. 相似文献
766.
We here investigate the motion of particles in a dipole magnetic field under the assumption of conservation of the first two adiabatic invariants. The results are then combined with Liouville's theorem to obtain the variation of the distribution function, and hence the plasma bulk parameters with L-shell. A comparison of the numerical results with recently published analytical approximations is made. Finally, the results are used to describe the structure of the ring current plasma in the outer radiatoin zone, the effects of the Alfvén layers being quantitatively evaluated for a simple electric field model. 相似文献
767.
H.E. Mitler 《Icarus》1975,24(2):256-268
It is shown how it is possible to explain the low abundance of iron and siderophiles in the Moon in a natural way. This is done by an extension of Öpik's mechanism, whereby one or more planetoids pass through Earth's Roche zone, are broken up, and have part of their material captured. Assuming the planetoids are differentiated, the iron core can easily escape capture. This process does not involve any dissipation mechanisms and goes a long way toward explaining the peculiar bulk composition of the Moon. The picture is consistent with that of Urey, in which the early solar system contained more (and smaller) planets than it does today. In the second part of the paper, dynamical considerations are applied for two models, and it is shown that capture from one (or a few) planetoids is quite feasible, whereas a large number of contributors is highly unlikely. 相似文献
768.
We present a preliminary analysis of CH4 absorptions near 6800 Å in new high resolution spectra of Uranus. A curve of growth analysis of the data yields a rotational temperature near 100 K and a CH4/H2 ratio that is 1 to 3 times that expected for a solar type composition. The long pathlengths of CH4, apparently demanded by absorptions near 4700 Å, are qualitatively shown to be the result of line formation in a deep, predominantly Rayleigh scattering atmosphere in which continuum absorption is a strong function of wavelength. The analysis of the CH4 also yields a minimum value for the effective pressure of line formation (~ 2 atm). This value is shown to be twice that expected on Uranus if the atmosphere were predominantly H2. It is speculated that large amounts of some otherwise optically inert gas is present in the Uranus atmosphere. N2 is suggested as a possible candidate since there are cosmogonic reasons why Uranus should contain large amounts of N relative to C, He, and H, and also because the pressure-induced pure rotation spectrum of N2 could possibly account for the low brightness temperatures that have recently been observed at 33 and 350 μm. If N2 is present the planet probably possesses a surface at the 10–100 atmosphere level. 相似文献
769.
From simultaneous spectroscopic and photometric observations of the T Tauri star RU Lup, which was followed for nine consecutive nights, it was found that most if not all of the light variations observed on this star were caused by variable circumstellar extinction. The character and the time-scale of the variations imply that the variations are due to dust concentrations of stellar dimension crossing the line of sight to the star. The implications of this interpretation and its possible bearing on problems of protoplanetary systems are discussed. 相似文献
770.
G.H. Rieke 《Icarus》1975,26(1):37-44
Infrared observations of Saturn from 5 to 40 μm are described. There is intense limb brightening at 12.35 μm over the southern polar cap. The C ring is anomalously bright at 10 and 20 μm and has bluer (hotter) colours than the A and B rings. The ring spectra have been extrapolated beyond 40 μm and subtracted from low-resolution far-infrared measurements to show that the far-infrared spectrum of the disk of Saturn is qualitatively similar to that of Jupiter and that Saturn radiates 2.5 ± 0.6 times the energy it absorbs from the Sun. 相似文献