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   67272篇
  免费   1072篇
  国内免费   485篇
测绘学   1678篇
大气科学   4689篇
地球物理   13451篇
地质学   23885篇
海洋学   5794篇
天文学   15471篇
综合类   158篇
自然地理   3703篇
  2021年   614篇
  2020年   690篇
  2019年   681篇
  2018年   1627篇
  2017年   1515篇
  2016年   1835篇
  2015年   1031篇
  2014年   1779篇
  2013年   3450篇
  2012年   2032篇
  2011年   2683篇
  2010年   2378篇
  2009年   3222篇
  2008年   2709篇
  2007年   2704篇
  2006年   2596篇
  2005年   1924篇
  2004年   1916篇
  2003年   1800篇
  2002年   1744篇
  2001年   1579篇
  2000年   1518篇
  1999年   1254篇
  1998年   1262篇
  1997年   1293篇
  1996年   1074篇
  1995年   1089篇
  1994年   1010篇
  1993年   847篇
  1992年   829篇
  1991年   800篇
  1990年   891篇
  1989年   787篇
  1988年   720篇
  1987年   857篇
  1986年   742篇
  1985年   957篇
  1984年   1016篇
  1983年   997篇
  1982年   908篇
  1981年   890篇
  1980年   829篇
  1979年   750篇
  1978年   712篇
  1977年   671篇
  1976年   626篇
  1975年   622篇
  1974年   620篇
  1973年   638篇
  1971年   402篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
11.
12.
13.
The Cassini spacecraft, en route to Saturn, passed close to Jupiter while the Galileo spacecraft was completing its 28th and 29th orbits of Jupiter, thus offering a unique opportunity for direct study of the solar wind-Jovian interaction. Here evidence is given of response of the Jovian magnetopause and bow shock positions to changes of the north-south component of the solar wind magnetic field, a phenomenon long known to occur in equivalent circumstances at Earth. The period analyzed starts with the passage over Cassini of an interplanetary shock far upstream of Jupiter. The shock's arrival at Galileo on the dusk-flank of the magnetosphere caused Galileo to exit into the solar wind. Using inter-spacecraft timing based on the time delay established from the shock arrival at each spacecraft, we point out that Galileo's position with respect to the Jovian bow shock appears to correlate with changes in the disturbed north-south reversing field seen behind the shock. We specifically rule out the alternative of changes in the shape of the bow shock with rotations of the interplanetary magnetic field as the cause.  相似文献   
14.
15.
16.
17.
The dynamics of co-orbital motion in the restricted three-body problem are investigated by symplectic mappings. Analytical and semi-numerical mappings have been developed and studied in detail. The mappings have been tested by numerical integration of the equations of motion. These mappings have been proved to be useful for a quick determination of the phase space structure reflecting the main characteristics of the dynamics of the co-orbital problem.  相似文献   
18.
It has been shown that at certain frequencies the acoustic backscatter from elastic targets exhibits certain resonance behavior which closely relates to the physical properties of the target such as dimension, thickness, and composition. The purpose of this paper is to develop an automated approach for identifying the presence of resonance in the acoustic backscatter from an unknown underwater target by isolating the resonance part from the specular contribution. An adaptive transversal filter structure is used to estimate the specular part of the backscatter and consequently the error signal would provide an estimate of the resonance part. An important aspect of this scheme lies in the fact that it does not require an underlying model for the elastic return. The adaptation rule is based upon fast recursive least squares (RLS) learning. The approach taken in this paper is general in the sense that it can be applied to targets of unknown geometry and thickness and, further, does not require any a priori information about the target and/or the environment. Test results on acoustic data are presented which indicate the effectiveness of the proposed approach  相似文献   
19.
20.
Summary. Four box cores collected from the Ontong—Java plateau during the Eurydice expedition have been used to make relative geomagnetic palaeo-intensity measurements. Rock magnetic measurements on the sediments show that they are characterized by a uniform magnetic mineralogy, and that they are suitable for relative intensity estimates. These are obtained by normalizing the NRM by an ARM imparted in a low DC bias field. the palaeoceanographic event known as the preservation spike is used to establish a crude time-scale for the record so that it may be compared with other data from the same region, and also with global palaeointensity estimates. the marine sediment data are quite similar to Australian intensity data from lake sediments and archaeomagnetic sources, but as might be expected exhibit some obvious differences from the global record.  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