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   33274篇
  免费   1031篇
  国内免费   487篇
测绘学   1020篇
大气科学   2612篇
地球物理   7521篇
地质学   11835篇
海洋学   2844篇
天文学   7037篇
综合类   243篇
自然地理   1680篇
  2021年   226篇
  2020年   265篇
  2019年   326篇
  2018年   1311篇
  2017年   1190篇
  2016年   1069篇
  2015年   623篇
  2014年   868篇
  2013年   1507篇
  2012年   1503篇
  2011年   1536篇
  2010年   1148篇
  2009年   1458篇
  2008年   1318篇
  2007年   1346篇
  2006年   1308篇
  2005年   1886篇
  2004年   2057篇
  2003年   1613篇
  2002年   934篇
  2001年   709篇
  2000年   710篇
  1999年   601篇
  1998年   582篇
  1997年   568篇
  1996年   429篇
  1995年   423篇
  1994年   437篇
  1993年   325篇
  1992年   335篇
  1991年   297篇
  1990年   348篇
  1989年   297篇
  1988年   276篇
  1987年   296篇
  1986年   243篇
  1985年   337篇
  1984年   369篇
  1983年   350篇
  1982年   329篇
  1981年   276篇
  1980年   292篇
  1979年   234篇
  1978年   227篇
  1977年   230篇
  1976年   191篇
  1975年   206篇
  1974年   189篇
  1973年   183篇
  1972年   124篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
The magnetic-field distribution outside a flat, infinitely conductive unbounded disk in the field of a point magnetic dipole is determined. A relationship is established between the problem of magnetic-field determination and the problem of the flow of an ideal incompressible fluid around an infinitely thin disk.  相似文献   
3.
4.
5.
This paper presents an introductory overview of recently developed stochastic theories for tackling spatial variability problems in predicting groundwater flow and solute transport. Advantages and limitations of the theories are discussed. Lastly, strategies based on the stochastic approaches to predict solute transport in aquifers are recommended.  相似文献   
6.
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM.  相似文献   
7.
8.
A spectacular change in the lower corona on the south-west limb has been found in solar images taken by the Yohkoh soft X-ray telescope. The event is characterized by a large topological change in magnetic field and a large intensity decrease observed after the X1. 1/1B flare on 9 November, 1991. A coronal mass ejection (CME) was observed by the Mark III K-coronameter (MK3) at the HAO/Mauna Loa Observatory. Both the MK3 (white-light) and soft X-ray observations showed that one leg of this CME was located above the flare site. An interplanetary shock associated with this event was observed by Pioneer Venus Orbiter, and, possibly, by IMP-8.Also Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, U.S.A.  相似文献   
9.
10.
We examine a siphon-like mechanism for moving mass from the chromosphere to a gravitational well at the top of a magnetic loop to form a prominence. The calculations assume no apriori flow velocity at the loop base. Instead heating in the loop legs drives the flow. The prominence formation process requires two steps. First, the background heating rate must be reduced to on the order of 1 % of the initial heating rate required to maintain the coronal loop. This forms an initial condensation at the top of the loop. Second, the heating must take place only in the loop legs in order to produce a pressure differential which drives mass up into the well at the top of the loop. The heating rate in the loop must be increased once the prominence has begun to form or full prominence densities can not be achieved in a reasonable time. We conclude that this heating driven siphon-like mechanism is feasible for producing and maintaining prominences.  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