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   42337篇
  免费   757篇
  国内免费   357篇
测绘学   1300篇
大气科学   3130篇
地球物理   8637篇
地质学   13995篇
海洋学   3649篇
天文学   10372篇
综合类   115篇
自然地理   2253篇
  2021年   335篇
  2020年   368篇
  2019年   455篇
  2018年   979篇
  2017年   904篇
  2016年   1186篇
  2015年   676篇
  2014年   1122篇
  2013年   2045篇
  2012年   1237篇
  2011年   1644篇
  2010年   1492篇
  2009年   2060篇
  2008年   1801篇
  2007年   1822篇
  2006年   1714篇
  2005年   1295篇
  2004年   1317篇
  2003年   1211篇
  2002年   1246篇
  2001年   1102篇
  2000年   1037篇
  1999年   907篇
  1998年   899篇
  1997年   906篇
  1996年   723篇
  1995年   679篇
  1994年   647篇
  1993年   559篇
  1992年   487篇
  1991年   491篇
  1990年   472篇
  1989年   495篇
  1988年   440篇
  1987年   509篇
  1986年   463篇
  1985年   548篇
  1984年   670篇
  1983年   579篇
  1982年   571篇
  1981年   513篇
  1980年   447篇
  1979年   445篇
  1978年   453篇
  1977年   393篇
  1976年   352篇
  1975年   356篇
  1974年   335篇
  1973年   367篇
  1972年   269篇
排序方式: 共有10000条查询结果,搜索用时 468 毫秒
281.
A dominant source of errors in swath bathymetry is acoustic interference. In 1989 the author published an analysis of these errors and predicted depth accuracies for a system which reduced their effect by averaging. This present paper shows how a considerable improvement in performance may be obtained by a variety of signal processing strategies that include the use of several widely spaced receivers and the elimination of the most unsatisfactory measurements before averaging. Simulations show how impressive sea bed profiles can be produced with a single ping, even at low signal-to-interference ratios  相似文献   
282.
283.
284.
High Possil and Strathmore are the first (1804) and last (1917) meteorite falls, respectively, of the three recorded in Scotland. Olivine compositions and total Fe contents in High Possil (Fa25.2; 21.35 wt %) and Strathmore (Fa25.3; 20.6 wt %) confirm their classification as L-group chondrites (Mason, 1963), and the presence of abundant plagioclase feldspar shows that both chondrites belong to petrologic type 6. Both chondrites display thermal and mechanical alteration attributable to moderate shock-loading appropriate to facies c (High Possil) and c-d (Strathmore) (Dodd and Jarosewich, 1979). Incipient shock-melting of metal and troilite in both chondrites is the first described from Lc chondrites, and differences in the responses of metallic and silicate minerals to shock-loading are discussed.  相似文献   
285.
Observations of the thermospheric wind at a mid-latitude station have been made using a Fabry-Perot interferometer to measure the Doppler shift of the nighttime OI emission at 630 nm. The results from 12 summer nights show that the zonal wind has a distinct feature associated with magnetic activity. The zonal wind first reverses and becomes westward. The maximum strength of the westward wind, its duration, and the maximum strength of the subsequent eastward wind all increase with increasing magnetic activity. The meridional wind is less consistent in its behaviour. It is normally equatorward but during magnetic activity it can increase, decrease, or even reverse, although it is consistently equatorward and of increased strength after 02.00 L.T. The initial reversal of the zonal wind is consistent with changes in the wind expected as a result of convective electric fields penetrating to mid-latitudes indicating that these electric fields modify the mid-latitude wind pattern before effects due to auroral heating reach mid-latitudes. The reversal of the zonal wind back to eastward may also be the result of electric field effects. The large variability of the meridional wind, to the extent that it becomes poleward at times, indicates the importance of wind sources equatorward of the observatory.  相似文献   
286.
287.
The present fundamental reference system is defined by the FK5 Fundamental Reference Catalog and the associated IAU Resolutions of 1976 through 1982. The past is defined to be what preceded the present fundamental reference system and future is defined to be what ever will come after it. The resulting characteristics of the different fundamental reference systems are considered from the aspects of the observational data, the computational methods, and the underlying theories. Having developed an understanding of these operational fundamental reference systems, the desired properties of the ideal fundamental reference system are presented. Finally, some questions concerning the future fundamental reference system are presented.  相似文献   
288.
289.
A high resolution numerical sonagram of Pc-1 magnetic record has shown that single narrow lines of Pc-1 spectra produce lower frequency (“red-shifted”) satellite lines more than twice as often as their higher frequency (“violet”) counterparts. The violet satellites usually emerge only when at least two ancestor lines are present simultaneously.  相似文献   
290.
Accurate chronologies are essential for linking palaeoclimate archives. Carbon‐14 wiggle‐match dating was used to produce an accurate chronology for part of an early Holocene peat sequence from the Borchert (The Netherlands). Following the Younger Dryas–Preboreal transition, two climatic shifts could be inferred. Around 11 400 cal. yr BP the expansion of birch (Betula) forest was interrupted by a dry continental phase with dominantly open grassland vegetation, coeval with the PBO (Preboreal Oscillation), as observed in the GRIP ice core. At 11 250 cal. yr BP a sudden shift to a humid climate occurred. This second change appears to be contemporaneous with: (i) a sharp increase of atmospheric 14C; (ii) a temporary decline of atmospheric CO2; and (iii) an increase in the GRIP 10Be flux. The close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity, which may have forced the changes in climate and vegetation at around 11 250 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd.  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