首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   133493篇
  免费   1731篇
  国内免费   1700篇
测绘学   3790篇
大气科学   9297篇
地球物理   25760篇
地质学   49794篇
海洋学   11232篇
天文学   26987篇
综合类   2393篇
自然地理   7671篇
  2022年   680篇
  2021年   1228篇
  2020年   1306篇
  2019年   1475篇
  2018年   7297篇
  2017年   6399篇
  2016年   5650篇
  2015年   1941篇
  2014年   3234篇
  2013年   5802篇
  2012年   4344篇
  2011年   6979篇
  2010年   6008篇
  2009年   7471篇
  2008年   6381篇
  2007年   6888篇
  2006年   4535篇
  2005年   3526篇
  2004年   3640篇
  2003年   3437篇
  2002年   3260篇
  2001年   2705篇
  2000年   2610篇
  1999年   2085篇
  1998年   2153篇
  1997年   2011篇
  1996年   1700篇
  1995年   1691篇
  1994年   1467篇
  1993年   1429篇
  1992年   1317篇
  1991年   1314篇
  1990年   1295篇
  1989年   1194篇
  1988年   1108篇
  1987年   1290篇
  1986年   1135篇
  1985年   1386篇
  1984年   1563篇
  1983年   1478篇
  1982年   1350篇
  1981年   1333篇
  1980年   1143篇
  1979年   1096篇
  1978年   1052篇
  1977年   967篇
  1976年   926篇
  1975年   903篇
  1974年   887篇
  1973年   961篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
We use the recently presented group sunspot number series to show that a persistent 22-year cyclicity exists in sunspot activity throughout the entire period of about 400 years of direct sunspot observations. The amplitude of this cyclicity is about 10% of the present sunspot activity level. A 22-year cyclicity in sunspot activity is naturally produced by the 22-year magnetic polarity cycle in the presence of a relic dipole magnetic field. Accordingly, a persistent 22-year cyclicity in sunspot activity gives an evidence for the existence of such a relic magnetic field in the Sun. The stable phase and the roughly constant amplitude of this cyclicity during times of very different sunspot activity level strongly support this interpretation.  相似文献   
942.
The near and mid-IR properties of barred and unbarred spiral galaxies are discussed on the basis of complete samples that were compiled earlier. The two types of spirals are shown not to differ from one another in emission power in the near and mid-IR ranges. Multiple regression and principal component analysis have been applied to investigate near and mid-IR properties of SB and SA galaxies, particularly their relation to X-ray and radio continuum emissions. There are definite differences between SB and SA spirals in the near and mid-IR. In the case of SB galaxies, the compactness of 10 m emission is closely related to the J - H color index, and the redder J - H color corresponds to relatively more extended emission at 10 m. It is assumed that these are caused by the bar itself, which stimulates enhanced star formation in a barred galaxy with respect to unbarred spiral.  相似文献   
943.
944.
945.
Using a sample of 57 VLT FORS spectra in the redshift range 1.37< z < 3.40 and a comparison sample with 36 IUE spectra of local ( ) starburst galaxies we derive CIV equivalent width values and estimate metallicities of starburst galaxies as a function of redshift. Assuming that a calibration of the CIV equivalent widths in terms of the metallicity based on the local sample of starburst galaxies is applicable to high-z objects, we find a significant increase of the average metallicities from about 0.16 Z at the cosmic epoch corresponding to z ≈ 3.2 to about 0.42 Z at z ≈ 2.3. A significant further increase in metallicity during later epochs cannot be detected in our data. Compared to the local starburst galaxies our high-redshift objects tend to be overluminous for a given metallicity. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
946.
Hypothesis of possible superconductivity of the iced matter of the rings of Saturn (based on the data of Voyager and Pioneer space missions) allow us to explain many phenomena which have not been adequately understood earlier. Introducing into planetary physics force of magnetic levitation of the superconducting iced particle of the rings, which interact with magnetosphere of the planet, becomes to be possible to explain origin, evolution, and dynamics of the rings; to show how the consequent precipitation of the rings’ matter upon the planet was concluded; how the rings began their rotation; how they were compressed by the magnetic field into the thin disc, and how this disc was fractured into hundreds of thousands of separated rings; why in the ring B do exist “spokes”; why magnetic field lines have distortion near by ring F; why there is a variable azimuth brightness of the ring A; why the rings reflected radio waves so efficiently; why exists strong electromagnetic radiation of the rings in the 20.4 kHz–40.2 MHz range and Saturnian kilometric radiation; why there is anomalous reflection of circularly polarized microwaves; why there are spectral anomalies of the thermal radiation of the rings; why the matter of the various rings does not mix but preserves its small-scale color differences; why there is an atmosphere of unknown origin nearby the rings of Saturn; why there are waves of density and bending waves within Saturn’s rings; why planetary rings in the solar system appear only after the Belt of Asteroids (and may be the Belt of Asteroids itself is a ring for the Sun); why our planet Earth has no rings of its own.  相似文献   
947.
948.
949.
We have made near-infrared photometric observations of nine β-Cephei and eight δ-Scuti stars inJ, H, andK bands. The observed fluxes are in good agreement with those expected according to their spectral types. We conclude that these stars do not have any anomalous emission in these near-infrared bands.  相似文献   
950.
Excitation of electron cyclotron waves and whistlers by reflected auroral electrons which possess a loss-cone distribution is investigated. Based on a given magnetic field and density model, the instability problem is studied over a broad region along the auroral field lines. This region covers altitudes ranging from one quarter of an Earth radius to five Earth radii. It is found that the growth rate is significant only in the region of low altitude, say below the source region of the auroral kilometric radiation. In the high altitude region the instability is insignificant either because of low refractive indices or because of small loss cone angles.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号