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   49312篇
  免费   567篇
  国内免费   343篇
测绘学   1056篇
大气科学   3271篇
地球物理   9532篇
地质学   18515篇
海洋学   4489篇
天文学   10783篇
综合类   105篇
自然地理   2471篇
  2022年   377篇
  2021年   633篇
  2020年   628篇
  2019年   690篇
  2018年   1510篇
  2017年   1447篇
  2016年   1577篇
  2015年   776篇
  2014年   1453篇
  2013年   2662篇
  2012年   1674篇
  2011年   2138篇
  2010年   1938篇
  2009年   2403篇
  2008年   2068篇
  2007年   2189篇
  2006年   2062篇
  2005年   1330篇
  2004年   1290篇
  2003年   1240篇
  2002年   1257篇
  2001年   1156篇
  2000年   1056篇
  1999年   837篇
  1998年   795篇
  1997年   885篇
  1996年   683篇
  1995年   713篇
  1994年   672篇
  1993年   542篇
  1992年   563篇
  1991年   522篇
  1990年   554篇
  1989年   510篇
  1988年   486篇
  1987年   521篇
  1986年   477篇
  1985年   613篇
  1984年   588篇
  1983年   623篇
  1982年   581篇
  1981年   526篇
  1980年   545篇
  1979年   485篇
  1978年   442篇
  1977年   415篇
  1976年   391篇
  1975年   384篇
  1974年   378篇
  1973年   395篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
171.
The electron density observations made using ESRO-1 and ESRO-4 near solar maximum and solar minimum, respectively, show a strong longitudinal variation at middle latitudes in the southern hemisphere. The peak of this sinusoidal variation occurs at around 7 hr U.T. and decreases exponentially in size from about 300 km (depending on local time, season, solar flux) with increasing or decreasing altitude. During local summer conditions the amplitude is larger than during local winter conditions and particularly high values occur near the solar maximum. Selecting data from magnetically quiet periods, a quantitative model is constructed of the UT-eflect in the topside electron densities.  相似文献   
172.
173.
174.
河北平原水系密度与隐伏活动构造的关系   总被引:4,自引:1,他引:4  
以1926年出版的比例尺为1:5万的顺直地形图作为基础图件,用地理信息系统技术对河北平原作水系密度计量分析,揭示隐伏在第四纪巨厚的沉积物之下活动构造的信息,研究结果证实水系线密度统计图与使用其它地球科学方法获得的成果,例如基底活动断块图,地貌类型图和第四纪厚度图都有很好的对应,特别与布格重力图对应更好,研究结果发现水系密度图与布格重力异常图在总体一致性较好的前提下,多处反向异常区均为近代强震发生的区域,提出这种地表水系密度统计研究的成果不仅可以揭示深部隐伏的活动构造,而且具有较明确的时间含义。  相似文献   
175.
176.
Investigations on OH maser lines in comets have been performed with the RT-22 at CrAO. The results of observations of 9P/Temper1 and Lulin C/2007 N3 comets in the OH molecule line at a wavelength of 18 cm are presented. An original technique for observation data analysis has been developed. The gas production rate of OH molecules in these comets is estimated.  相似文献   
177.
For the first time, minor cyclicity is described from some limestones in the lower part of the Brigantian (D2) succession of the Mold district North Wales which can be traced throughout the area enabling a detailed correlation to be established. The minor cyclicity may have been caused by eustatic sea-level fluctuations. Periods of emergence associated with each regressive phase are demonstrated by the presence of subaerial features and terrestrial deposits. The lateral persistence of the cycles is confirmed by comparison with established faunal and lithological horizons. Correlation with other cyclic Brigantian strata in Yorkshire, Derbyshire and Bristol is briefly discussed. The Asbian/Brigantian (D1/D2) boundary in North Wales is described and distinctive faunal and lithological changes similar to those in the area of the stratotype in north England have been recorded.  相似文献   
178.
Izvestiya, Atmospheric and Oceanic Physics - The archaeal composition of permafrost samples taken during the drilling of frozen marine sediments in the area of the Barentsburg coal mine on the east...  相似文献   
179.
Comprehensive distribution patterns of physical and chemical characteristics have been obtained from a series of cruises in Liverpool Bay. The marked feature of these distributions was their temporal variability, suggesting that the surface residual circulation is also temporally variable. The influence of wind stress upon the circulation pattern and hence water quality of this sea area is illustrated.  相似文献   
180.
A thick (200 m) rock salt mass covering Late Archean granitoids was exposed for the first time in the Early Proterozoic volcanogenic-sedimentary formations in the Onega trough of the east Baltic Shield by the Onega parametric well. The mineral composition of salts, their geochemical features, and the isotopic composition of carbonate carbon and oxygen have been studied. After fluid inclusions present in salts, their metamorphism temperature and isotopic composition of helium and argon were determined. The obtained results give evidence of the fact that rock salts and magnesites associated with them were formed in an evaporate basin with participation of deep crustal processes. The age of the underlying granitoids (2.716 ± 9 Ma) is determined using the Pb—Pb method.  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