全文获取类型
收费全文 | 92866篇 |
免费 | 1237篇 |
国内免费 | 812篇 |
专业分类
测绘学 | 2434篇 |
大气科学 | 6836篇 |
地球物理 | 17474篇 |
地质学 | 32370篇 |
海洋学 | 8230篇 |
天文学 | 21998篇 |
综合类 | 271篇 |
自然地理 | 5302篇 |
出版年
2021年 | 831篇 |
2020年 | 923篇 |
2019年 | 969篇 |
2018年 | 2180篇 |
2017年 | 2088篇 |
2016年 | 2632篇 |
2015年 | 1503篇 |
2014年 | 2648篇 |
2013年 | 4855篇 |
2012年 | 2677篇 |
2011年 | 3516篇 |
2010年 | 3171篇 |
2009年 | 4114篇 |
2008年 | 3624篇 |
2007年 | 3660篇 |
2006年 | 3431篇 |
2005年 | 2784篇 |
2004年 | 2799篇 |
2003年 | 2613篇 |
2002年 | 2507篇 |
2001年 | 2208篇 |
2000年 | 2083篇 |
1999年 | 1839篇 |
1998年 | 1865篇 |
1997年 | 1777篇 |
1996年 | 1508篇 |
1995年 | 1464篇 |
1994年 | 1350篇 |
1993年 | 1211篇 |
1992年 | 1205篇 |
1991年 | 1149篇 |
1990年 | 1264篇 |
1989年 | 1059篇 |
1988年 | 1056篇 |
1987年 | 1160篇 |
1986年 | 1016篇 |
1985年 | 1332篇 |
1984年 | 1477篇 |
1983年 | 1372篇 |
1982年 | 1309篇 |
1981年 | 1213篇 |
1980年 | 1104篇 |
1979年 | 1085篇 |
1978年 | 1060篇 |
1977年 | 921篇 |
1976年 | 879篇 |
1975年 | 815篇 |
1974年 | 844篇 |
1973年 | 873篇 |
1971年 | 548篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
731.
732.
The possible avenues for photoelectron transport were determined during southern hemisphere winter at Mars by using a mapping analysis of the theoretical magnetic field. Magnetic field line tracing was performed by superposing two magnetic field models: (1) magnetic field derived from a three-dimensional (3D) self-consistent quasi-neutral hybrid model which does not contain the Martian crustal magnetic anomalies and (2) a 3D map of the magnetic field associated with the magnetic anomalies based on Mars Global Surveyor magnetic field measurements. It was found that magnetic field lines connected to the nightside of the planet are mainly channeled within the optical shadow of the magnetotail whereas magnetic field lines connected to the dayside of the planet are observed to form the remainder of the magnetosphere. The simulation suggests that the crustal anomalies create “a magnetic shield” by decreasing the region near Mars which is magnetically connected to the Martian magnetosphere. The rotation of Mars causes periodic changes in magnetic connectivity, but not to qualitative changes in the overall magnetic field draping around Mars. 相似文献
733.
R. E. Gershberg A. V. Terebizh A. A. Shlyapnikov 《Bulletin of the Crimean Astrophysical Observatory》2011,107(1):11-19
A catalogue of dwarf stars with different manifestations of Solar-type activity is compiled based on publications of the last
10–15 years. The catalogue includes objects with dark spots, hydrogen and calcium chromospheric emission, short-lived flares
in different wavelength ranges, and radio and X-ray emission of stellar corona. The resulting compiled list includes 5535
objects. 相似文献
734.
Sherry K. Fieber-Beyer Michael J. Gaffey Michael S. Kelley Vishnu Reddy Chalbeth M. Reynolds 《Icarus》2011,213(2):524-537
We present a mineralogical assessment of 12 Maria family asteroids, using near-infrared spectral data obtained over the years 2000-2009 combined with visible spectral data (when available) to cover the spectral interval of 0.4-2.5 μm. Our analysis indicates the Maria asteroid family, which is located adjacent to the chaotic region of the 3:1 Kirkwood Gap, appears to be a true genetic family composed of assemblages analogous to mesosiderite-type meteorites. Dynamical models by Farinella et al. (Farinella, P., Gunczi, R., Froeschlé, Ch., Froeschlé, C., [1993]. Icarus 101, 174-187) predict this region should supply meteoroids into Earth-crossing orbits. Thus, the Maria family is a plausible source of some or all of the mesosiderites in our meteorite collections. These individual asteroids were most likely once part of a larger parent object that was broken apart and dispersed. One of the Maria dynamical family members investigated, ((695) Bella), was found to be unrelated to the genetic Maria family members. The parameters of (695) Bella indicate an H-chondrite assemblage, and that Bella may be a sister or daughter of Asteroid (6) Hebe. 相似文献
735.
The roughness of a planetary surface offers clues to its past geologic history. We apply a surface roughness model developed by Buratti and Veverka (Buratti, B.J., Veverka, J. [1985]. Icarus 64, 320-328) to Cassini ISS data from the January 1st, 2005 flyby of Iapetus. This model uses the observed scattering behavior to provide a depth to radius factor q quantifying the size of idealized craters on the surface. Our findings indicate that the surface on the dark side is significantly smoother than the surfaces of other icy low-albedo saturnian satellites. We have found that the average depth to radius on the leading (dark) side is 0.084, corresponding to a Hapke mean slope angle of 6°. As compared to the 13-33° Hapke mean slope angle of other icy satellites (Buratti, B.J., and 10 colleagues [2008]. Icarus 193, 309-322), our results present a clearly different picture for the leading surface of Iapetus, suggesting that the dark deposit contributes to the decrease in macroscopic surface roughness of the leading side. Attempts were made to obtain an average depth to radius value for the trailing (bright) side; however the scans of the bright side from this flyby exhibited large variations in albedo, resulting in results that were physically unrealistic. 相似文献
736.
R.M. Kulsrud 《Astronomische Nachrichten》2010,331(1):22-26
A serious difficulty with the standard alpha‐omega theory of the origin of galactic magnetic fields involves the question of flux expulsion. This is intimately related to flux freezing. The alpha‐omega theory is shown in the context of the giant superbubble explosions that have a large impact on the physics of the interstellar medium. It is shown that superbubbles alone can duplicate the processes of the alpha‐omega dynamo and produce exponential growth of the galactic magnetic field. The possibility of the blow‐out of pieces of the magnetic field is discussed and it is shown that they have the potential to solve the flux‐expulsion problem. However, such an explanation must lead to apparent ‘gaps’ in the field in the galactic disc. These gaps are probably unavoidable in any dynamo theory and should have important observable consequences, one of which is an explanation for the escape of cosmic rays from the disc (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
737.
738.
Y. N. Gnedin S. D. Buliga N. A. Silant’ev T. M. Natsvlishvili M. Y. Piotrovich 《Astrophysics and Space Science》2012,342(1):137-145
In this paper we demonstrate that the wavelength dependence of polarization degree and position angle allows us to derive the distribution of magnetic field in accretion disc. The polarized radiation arises due to scattering of emission light by electrons in a magnetized optically thick accretion disc. Faraday rotation of polarization plane is taken into consideration. Through wavelength dependence of polarization it is possible to derive the value of the magnetic Prandtl number in the accretion disc plasma. The power law index of the polarization wavelength dependence is related with the radial distribution of magnetic field in an accretion disc. This allows us to test the various models of an accretion disc around the central black hole. 相似文献
739.
An intensive survey has been conducted of the distributions of some chemical properties (dissolved oxygen, nutrients and carbonate
properties) in the Kuroshio/Oyashio Interfrontal Zone. Many low-salinity water patches were found down to depths of 640 m.
Each chemical property also showed anomalies in these patches, but the degree of variation showed a low correlation with salinity.
This may be due to the high variability of biological processes in the surface waters where these patches are formed. Vertical
profiles of the chemical properties were also observed along the Kuroshio extension axis from 140.50°E to 146.75°E. The concentrations
of nutrients and total carbonate (TC) in the water having densities greater than σθ=26.60 can be regarded as being formed by the isopycnal mixing of the Kuroshio component water and Oyashio component water
and biological degradation within the density surfaces. This implies that the transport of chemical properties by the diapycnal
mixing is negligible in these density layers in the K/O zone. 相似文献
740.
The processes of space debris formation are described; the urgency of the problem of its removal from near-Earth space is noted. A method to prevent the clogging of space by equipping a spacecraft to be launched into orbit with additional devices, simple in design and mode of operation, is presented. The results of the estimation of aerothermodynamics, bulk–mass characteristics and trajectories of low-orbit spacecraft equipped with a space debris disposal system are presented. 相似文献