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   56137篇
  免费   2164篇
  国内免费   3096篇
测绘学   1918篇
大气科学   4964篇
地球物理   10143篇
地质学   25363篇
海洋学   5258篇
天文学   9643篇
综合类   1177篇
自然地理   2931篇
  2023年   249篇
  2022年   899篇
  2021年   1229篇
  2020年   1119篇
  2019年   1229篇
  2018年   4398篇
  2017年   3998篇
  2016年   3099篇
  2015年   1188篇
  2014年   1639篇
  2013年   2180篇
  2012年   2655篇
  2011年   4390篇
  2010年   3941篇
  2009年   4162篇
  2008年   3469篇
  2007年   4123篇
  2006年   1802篇
  2005年   1313篇
  2004年   1201篇
  2003年   1332篇
  2002年   1263篇
  2001年   963篇
  2000年   766篇
  1999年   691篇
  1998年   594篇
  1997年   622篇
  1996年   502篇
  1995年   508篇
  1994年   458篇
  1993年   358篇
  1992年   362篇
  1991年   283篇
  1990年   278篇
  1989年   286篇
  1988年   231篇
  1987年   240篇
  1986年   224篇
  1985年   241篇
  1984年   235篇
  1983年   229篇
  1982年   224篇
  1981年   204篇
  1980年   201篇
  1979年   203篇
  1978年   171篇
  1977年   155篇
  1976年   150篇
  1975年   157篇
  1973年   179篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
In connection with the celebration of the centennial of the Institute of Electrical and Electronics Engineers (UEEE) in 1984, the history of electrical engineering in relation to the world's oceans is traced, with emphasis on the 20-year period from the 1960's to the present. Note is taken of the breadth of the cross-disciplinary interests involved, as exemplified by the active participation of a score of IEEE Societies and Professional Groups, how they organized to cooperate, the periodic meetings and annual conferences they sponsored, the publications they undertook, the tokens of recognition they adopted, and how they adapted to changes in emphasis to reflect shifts in the areas of technology served. Finally, an assessment is made of the future of the recently organized IEEE Oceanic Engineering Society.  相似文献   
22.
Methyl-galactosides were oxidized at room temperature by galactose oxidase in a one-step reaction and afforded methyl-galactoaldehyde in excellent yield and high purity. The resulting galactoaldehyde as a useful intermediate can be directly used in glycopeptide synthesis.  相似文献   
23.
Using a differential method we have carried out observations of oscillations in six sunspots. Spectral lines Fe i 5434 Å and Fe i 5576 Å were used. Horizontal waves are not observed in the sunspot umbra photosphere. Results obtained indicate that, at least, the sunspot umbra oscillates as a single whole.  相似文献   
24.
本文通过对1980年以来长江上游10次洪涝过程的500hpa环流形势分析,并用车比雪夫多项式对500hpa高度场进行展开,研究了各类洪涝过程的中期演变特征,发现代表不同洪涝过程环流形势变化的不同的车氏系数值都有明显的特征变化,结合欧洲中心中期数值预告96小时500hpa高度场的车氏系数分析,可为洪涝暴雨的中期预报提供一种新的思路。  相似文献   
25.
Using one-minute cadence vector magnetograms from Big Bear Solar Observatory (BBSO), we analyze the temporal behavior of derived longitudinal electric currents associated with two flares on July 26, 2002. One of the events is an M1.0 flare which occurred in active region NOAA 10044, while the other is an M8.7 flare in the adjacent region 10039. Rapid changes of magnetic fields in the form of flux emergence are found to be associated with both of these events. However, the temporal behavior of electric currents are very different. For the M1.0 flare, the longitudinal electric current density drops rapidly near the flaring neutral line; while for the M8.7 flare, the current density rapidly increases, confirming the picture of the current-carrying flux emergence. We offer a possible explanation for such a difference: magnetic reconnection at different heights for the two events, near the photosphere for the M1.0 flare, and higher up for the M8.7 flare.  相似文献   
26.
27.
High resolution echelle spectroscopic observations taken with the FEROS spectrograph at the 2.2 m telescope ESO confirm the binary nature of the flare M3.5V star LU Vel (GJ 375, RE J0958-462) previously reported by Christian and Mathioudakis (2002). Emission of similar intensity from both components is detected in the Balmer, Na i D1&D2, He i D3, Ca ii H&K, and Ca ii IRT lines. We have determined precise radial velocities by cross correlation with radial velocity standard stars, which have allowed us to obtain for the first time the orbital solution of the system. The binary consists of two near-equal M3.5V components with an orbital period shorter than 2 days. We have analyzed the behaviour of the chromospheric activity indicators (variability and possible flares). In addition, we have determined its rotational velocity and kinematics.  相似文献   
28.
OSIRIS (OH-Suppressing Infra-Red Integral-field Spectrograph) is a new facility instrument for the Keck Observatory. After seeing first light in February 2005, OSIRIS is currently undergoing commissioning. OSIRIS provides the capability of performing three-dimensional spectroscopy in the near-infrared z, J, H, and K bands at the resolution limit of the Keck II telescope, which is equipped with adaptive optics and a laser guide star. The science case for OSIRIS is summarized, and the instrument and associated data reduction software are described.  相似文献   
29.
30.
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