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   79953篇
  免费   4478篇
  国内免费   5409篇
测绘学   2545篇
大气科学   8765篇
地球物理   16314篇
地质学   33563篇
海洋学   7218篇
天文学   13221篇
综合类   2664篇
自然地理   5550篇
  2023年   350篇
  2022年   958篇
  2021年   1198篇
  2020年   1096篇
  2019年   1211篇
  2018年   4748篇
  2017年   4483篇
  2016年   3666篇
  2015年   1656篇
  2014年   2069篇
  2013年   3071篇
  2012年   3243篇
  2011年   5334篇
  2010年   4903篇
  2009年   5484篇
  2008年   4573篇
  2007年   5132篇
  2006年   2193篇
  2005年   2603篇
  2004年   2129篇
  2003年   2201篇
  2002年   1921篇
  2001年   1563篇
  2000年   1671篇
  1999年   1915篇
  1998年   1631篇
  1997年   1708篇
  1996年   1524篇
  1995年   1320篇
  1994年   1117篇
  1993年   1016篇
  1992年   897篇
  1991年   761篇
  1990年   701篇
  1989年   591篇
  1988年   572篇
  1987年   570篇
  1986年   536篇
  1985年   588篇
  1984年   630篇
  1983年   597篇
  1982年   555篇
  1981年   501篇
  1980年   486篇
  1979年   423篇
  1978年   427篇
  1977年   368篇
  1975年   349篇
  1974年   340篇
  1973年   341篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Y.C. Minh  W.M. Irvine   《New Astronomy》2006,11(8):594-599
The large-scale structure associated with the 2′N HNCO peak in Sgr B2 [Minh, Y.C., Haikala, L., Hjalmarson, Å., Irvine, W.M., 1998. ApJ 498, 261 (Paper I)] has been investigated. A ring-like morphology of the HNCO emission has been found; this structure may be colliding with the Principal Cloud of Sgr B2. This “HNCO Ring” appears to be centered at (l,b) = (0.7°,−0.07°), with a radius of 5 pc and a total mass of 1.0 × 105 to 1.6 × 106 M. The expansion velocity of the Ring is estimated to be 30–40 km s−1, which gives an expansion time scale of 1.5 × 105 year. The morphology suggests that collision between the Ring and the Principal Cloud may be triggering the massive star formation in the Sgr B2 cloud sequentially, with the latest star formation taking place at the 2′N position. The chemistry related to HNCO is not certain yet, but if it forms mainly via reaction with the evaporated OCN from icy grain mantles, the observed enhancement of the HNCO abundance can be understood as resulting from shocks associated with the collision between the Principal Cloud and the expanding HNCO Ring.  相似文献   
82.
83.
Cosmic radiation is an important problem for human interplanetary missions. The “Radiation Exposure and Mission Strategies for Interplanetary Manned Missions–REMSIM” study is summarised here. They are related to current strategies and countermeasures to ensure the protection of astronauts from radiation during interplanetary missions, with specific reference to: radiation environment and its variability; radiation effects on the crew; transfer trajectories and associated fluences; vehicle and surface habitat concepts; passive and active shielding concepts; space weather monitoring and warning systems.  相似文献   
84.
85.
The D-CIXS Compact X-ray Spectrometer will provide high quality spectroscopic mapping of the Moon, the primary science target of the ESA SMART-1 mission. D-CIXS consists of a high throughput spectrometer, which will perform spatially localised X-ray fluorescence spectroscopy. It will also carry a solar monitor, to provide the direct calibration needed to produce a global map of absolute lunar elemental abundances, the first time this has been done. Thus it will achieve ground breaking science within a resource envelope far smaller than previously thought possible for this type of instrument, by exploiting two new technologies, swept charge devices and micro-structure collimators. The new technology does not require cold running, with its associated overheads to the spacecraft. At the same time it will demonstrate a radically novel approach to building a type of instrument essential for the BepiColombo mission and potential future planetary science targets.  相似文献   
86.
The dunes in estuary of Tumen River in China lie to the area between the Jiushaping and Fangchuan in the left bank of Tumen River (Fig.1). The dis-tance is about 15~20 km between the dunes and the coast. It ranges from China to D. P. R. Korea and Russia. The range of the height of dunes is about 15~20 m and the width is 100~200 m. By the re-connaissance, we protracted 9 section planes of the dunes and collected 40 sand samples of the dunes, and identified the sedimentary environment …  相似文献   
87.
Three Super Active Regions in the Descending Phase of Solar Cycle 23   总被引:2,自引:0,他引:2  
We analyze the magnetic configurations of three super active regions, NOAA 10484, 10486 and 10488, observed by the Huairou Multi-Channel Solar Telescope (MCST) from 2003 October 18 to November 4. Many energetic phenomena, such as flares (including a X-28 flare) and coronal mass ejections (CMEs), occurred during this period. We think that strong shear and fast emergence of magnetic flux are the main causes of these events. The question is also of great interest why these dramatic eruptions occurred so close together in the descending phase of the solar cycle.  相似文献   
88.
Mavromichalaki  H.  Plainaki  C.  Zouganelis  I.  Petropoulos  B. 《Solar physics》2003,218(1-2):63-78
Global changes of the solar activity can be expressed by the coronal index that is based upon the total irradiance of the coronal 530.3 nm green line from observations at five stations. Daily mean values of the coronal index of solar activity and other well-correlated solar indices are analyzed for the period 1966–1998 covering over three solar cycles. The significant correlation of this index with the sunspot number and the solar flare index have led to an analytical expression which can reproduce the coronal index of solar activity as a function of these parameters. This expression explains well the existence of the two maxima during the solar cycles taking into account the evolution of the magnetic field that can be expressed by some sinusoidal terms during solar maxima and minima. The accuracy between observed and calculated values of the coronal index on a daily basis reaches the value of 71%. It is concluded that the representative character of the coronal index is preserved even when using daily data and can therefore allow us to study long-term, intermediate and short-term variations for the Sun as a star, in association with different periodical solar–terrestrial phenomena useful for space weather studies.  相似文献   
89.
90.
卫星星座与编队飞行问题综述   总被引:3,自引:0,他引:3  
胡松杰  王歆  刘林 《天文学进展》2003,21(3):231-240
卫星应用的日益发展需要由多颗卫星组成的卫星系统(形成各种类型的星座)来替代单颗卫星,这就给卫星轨道力学的应用增加了一些新内容。重点阐述了卫星星座整体结构的演化、卫星星座编队飞行中的星-星相对几何构形及其保持问题,以及星座卫星定轨所涉及的测控方式和天地基网联合定轨问题。这些内容都是卫星星座轨道力学中的重要组成部分。  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