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   260398篇
  免费   4998篇
  国内免费   3349篇
测绘学   7046篇
大气科学   19196篇
地球物理   54339篇
地质学   90906篇
海洋学   21620篇
天文学   56677篇
综合类   1031篇
自然地理   17930篇
  2021年   2240篇
  2020年   2603篇
  2019年   2842篇
  2018年   3852篇
  2017年   3601篇
  2016年   5952篇
  2015年   4237篇
  2014年   6956篇
  2013年   14256篇
  2012年   6632篇
  2011年   8113篇
  2010年   7183篇
  2009年   9754篇
  2008年   8525篇
  2007年   7967篇
  2006年   9625篇
  2005年   7767篇
  2004年   7641篇
  2003年   7150篇
  2002年   6736篇
  2001年   6003篇
  2000年   5954篇
  1999年   5201篇
  1998年   5228篇
  1997年   5032篇
  1996年   4691篇
  1995年   4420篇
  1994年   4108篇
  1993年   3864篇
  1992年   3652篇
  1991年   3598篇
  1990年   3762篇
  1989年   3516篇
  1988年   3300篇
  1987年   3847篇
  1986年   3407篇
  1985年   4226篇
  1984年   4739篇
  1983年   4413篇
  1982年   4317篇
  1981年   3924篇
  1980年   3641篇
  1979年   3509篇
  1978年   3492篇
  1977年   3278篇
  1976年   3044篇
  1975年   2956篇
  1974年   2913篇
  1973年   3075篇
  1972年   2023篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
681.
Orbital parameters of the satellite NIMBUS 1 ROCKET (1964-52B) have been determined at 285 epochs using about 14,500 optical and radar observations during the last 2233 days of its life. 1964-52B was in an orbit inclined at about 98.7° to the Equator and it decayed on 13 August 1974. The orbital elements were found using the RAE orbit refinement program PROP 6 giving average orbital accuracies in perigee height and inclination at 110 m and 0.002° respectively.  相似文献   
682.
In many astrophysical problems, the study of the stability of an atmosphere in the presence of a magnetic field is of importance. In most cases the MHD instabilities of atmospheres are studied by energy principle of Bernsteinet al. (1958). In this paper, a general method for studying the stability of a system subject to MHD equations of conditions has been proposed. This is based on the local potential concept put forward by Glansdorff and Prigogine (1964). The scheme for securing stability criteria has been demonstrated in two particular cases.  相似文献   
683.
684.
The quadrupole mass spectrometer flown by the Air Force Geophysics Laboratory on STS-4 in 1982 detected large intensities of several ions, primarily O+, H2O+ and H3O+, with energies less than 1.5 e V with respect to the Shuttle Orbiter. Ion-molecule reactions and non-reactive scattering between the outgassing neutral flux from the Orbiter surfaces and the ambient ionic species are identified as the primary source of these low energy ions.  相似文献   
685.
The propagation of axisymmetric magnetohydrodynamic waves near the equatorial plane of the crust of a neutron star in a transverse magnetic field is considered. The magnetic field is perpendicular to the equatorial plane. The magnetic fields and electric currents excited by this wave beam at the stellar surface are determined.  相似文献   
686.
Thaw modification is the general process whereby frost-fissure wedges are modified during thaw, and by which frost-fissure pseudomorphs may develop. Specific processes of thaw modification are inferred from ice-wedge pseudomorphs, composite-wedge pseudomorphs and deformed sand wedges in the Pleistocene Mackenzie Delta: i.e. thermal erosion, collapse, subsidence, refreezing, loading, buoyancy, spreading, folding and shearing. Thaw modification is believed to result in selective preservation of pseudomorphs and wedges. Sand wedges are more likely to be preserved than are ice-wedge pseudomorphs or compositewedge pseudomorphs, because only those sand wedges that penetrate massive ice or icy sediments are prone to thaw modification. Furthermore, whereas ice wedges preferentially develop in ice-rich, fine-grained sediments (thaw-sensitive), their pseudomorphs appear to be selectively preserved in ice-poor, coarse-grained sediments (thaw-stable).  相似文献   
687.
688.
Miniature gravity corer for recovering short sediment cores   总被引:12,自引:0,他引:12  
  相似文献   
689.
    
  相似文献   
690.
The purpose of this work was to reinvestigate the existing hydrogeological conceptual model of the basin of Madrid, Spain. A cumulative chemical isotopic diagram which enabled the distinction between different groups of water as well as calculation of the mode of their blending was used for this investigation. It was found that the groups of discharge were lighter in their isotopic composition than that of recharge. The previous explanation of this fact, backed by carbon-14 dating, was the long residence time due to flow lines going down to depths of more than 1000 m. This flow model assumes homogenous conditions to these depths. This assumption can not be supported by evidence from deep wells. Thus a modified model is suggested which maintains homogenous conditions only to about 300 m and a deep confined aquifer below containing paleowater. The higher degree of depletion of this water has been explained by a colder climate on top of an altitude effect. Another interesting observation was the correlation between the isotopic composition of the rains, the month of the rain event and the composition of the recharge group groundwater. It could be seen that the winter rains resemble the groundwater composition, which shows that practically all the spring and summer rains were evapotranspirated.  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