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   98727篇
  免费   1143篇
  国内免费   797篇
测绘学   2947篇
大气科学   7100篇
地球物理   19923篇
地质学   33775篇
海洋学   8384篇
天文学   22703篇
综合类   349篇
自然地理   5486篇
  2021年   901篇
  2020年   977篇
  2019年   1161篇
  2018年   2325篇
  2017年   2137篇
  2016年   2809篇
  2015年   1575篇
  2014年   2593篇
  2013年   4923篇
  2012年   2805篇
  2011年   3809篇
  2010年   3402篇
  2009年   4447篇
  2008年   3825篇
  2007年   3854篇
  2006年   3643篇
  2005年   2887篇
  2004年   2869篇
  2003年   2754篇
  2002年   2727篇
  2001年   2398篇
  2000年   2296篇
  1999年   1977篇
  1998年   1974篇
  1997年   1942篇
  1996年   1676篇
  1995年   1574篇
  1994年   1486篇
  1993年   1309篇
  1992年   1191篇
  1991年   1214篇
  1990年   1210篇
  1989年   1197篇
  1988年   1099篇
  1987年   1287篇
  1986年   1158篇
  1985年   1380篇
  1984年   1553篇
  1983年   1409篇
  1982年   1353篇
  1981年   1273篇
  1980年   1130篇
  1979年   1080篇
  1978年   1054篇
  1977年   988篇
  1976年   900篇
  1975年   873篇
  1974年   878篇
  1973年   959篇
  1972年   605篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
This paper presents error propagation equations for modeling of radiogenic isotopes during mixing of two components or end-members. These equations can be used to estimate errors on an isotopic ratio in the mixture of two components, as a function of the analytical errors or the total errors of geological field sampling and analytical errors. Two typical cases (“Small errors” and “Large errors”) are illustrated for mixing of Sr isotopes. Similar examples can be formulated for the other radiogenic isotopic ratios. Actual isotopic data for sediment and basalt samples from the Cocos plate are also included to further illustrate the use of these equations. The isotopic compositions of the predicted mixtures can be used to constrain the origin of magmas in the central part of the Mexican Volcanic Belt. These examples show the need of high quality experimental data for them to be useful in geochemical modeling of magmatic processes.  相似文献   
242.
243.
The synorogenic intrusive activity of the eastern part of the Seiland province evolved from tholeiitic basalt low in K and Ti, through high-K calc-alkaline magmas and possible transitional basalt, to alkaline olivine basalt and picrite; finally, highly differentiated alkaline magmas and carbonatites were emplaced. For intrusive rocks with equivalent SiO2 contents, K2O, K2O + Na2O and K2O/Na2O increased with time, and the degree of iron enrichment in basaltic suites dimished. The western part of the province shows no equivalent evolution, tholeiitic magmas being emplaced at the same time as calc-alkaline magmas to the east.The magmatism is believed to have stemmed from a diapiric complex established in the mantle above a Benioff zone dipping to the east beneath the deforming Andean-type margin of the Baltic plate. Tectonic shortening of the continental edge and rearward movement of the underthrust plate relative to the asthenosphere resulted in migration of the plate junction, steepening of the seismic zone, and increasing depth to the magmagenetic region.  相似文献   
244.
245.
We have applied the method of Wiener-Hopf technique to obtain the Green's functions of one-group transport equations (with anisotropic unit plane source and with isotroic unit plane source) for a finite slab of passive and multiplying media.  相似文献   
246.
247.
Results of the studies of the ionospheric D region during spring periods of 2004 and 2005 using partial reflections and radiowave scattering by artificial periodical irregularities are presented. It has been found that the vertical profile of the electron density during this period has a local minimum at a height of ~75 km, which is absent in summer. The causes of the minimum appearance are discussed.  相似文献   
248.
A method has been proposed for detecting hidden periodicities in nonstationary finite time series with strong noise. The successive application of the spectral singular expansion and wavelet transform makes it possible to calculate the energy of short signals with an arbitrary form against a suppressing noise back-ground, the amplitude of which is several times as large as that of a useful signal. This method has been used to identify variations in the galactic cosmic ray (GCR) flux related to IMF disturbances due to the earthward propagation of solar magnetic clouds in the heliosphere.  相似文献   
249.
We report multi-instrument observations during an isolated substorm on 17 October 1989. The EISCAT radar operated in the SP-UK-POLI mode measuring ionospheric convection at latitudes 71°-78°. SAMNET and the EISCAT Magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intensifications, as well as the location of the field aligned and ionospheric currents associated with the substorm current wedge. IMP-8 magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turning of the interplanetary magnetic field (IMF). We infer that the polar cap expanded as a result of the addition of open magnetic flux to the tail lobes during this interval. The flow reversal boundary, which is a lower limit to the polar cap boundary, reached an invariant latitude equatorward of 71° by the time of the expansion phase onset. A westward electrojet, centred at 65.4°, occurred at the onset of the expansion phase. This electrojet subsequently moved poleward to a maximum of 68.1° at 2000 UT and also widened. During the expansion phase, there is evidence of bursts of plasma flow which are spatially localised at longitudes within the substorm current wedge and which occurred well poleward of the westward electrojet. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radially earthward of the boundary between open and closed magnetic flux, the “distant” neutral line. Thus the substorm was not initiated at the distant neutral line, although there is evidence that it remained active during the expansion phase. It is not obvious whether the electrojet mapped to a near-Earth neutral line, but at its most poleward, the expanded electrojet does not reach the estimated latitude of the polar cap boundary.  相似文献   
250.
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