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   258292篇
  免费   5286篇
  国内免费   3466篇
测绘学   6880篇
大气科学   19196篇
地球物理   53948篇
地质学   90092篇
海洋学   21642篇
天文学   56336篇
综合类   1009篇
自然地理   17941篇
  2021年   2245篇
  2020年   2636篇
  2019年   2865篇
  2018年   3598篇
  2017年   3399篇
  2016年   5765篇
  2015年   4246篇
  2014年   6928篇
  2013年   14229篇
  2012年   6538篇
  2011年   7920篇
  2010年   7083篇
  2009年   9645篇
  2008年   8414篇
  2007年   7905篇
  2006年   9615篇
  2005年   7699篇
  2004年   7602篇
  2003年   7096篇
  2002年   6664篇
  2001年   5929篇
  2000年   5909篇
  1999年   5204篇
  1998年   5231篇
  1997年   5014篇
  1996年   4666篇
  1995年   4409篇
  1994年   4099篇
  1993年   3843篇
  1992年   3632篇
  1991年   3585篇
  1990年   3756篇
  1989年   3510篇
  1988年   3301篇
  1987年   3849篇
  1986年   3405篇
  1985年   4224篇
  1984年   4734篇
  1983年   4410篇
  1982年   4321篇
  1981年   3925篇
  1980年   3645篇
  1979年   3512篇
  1978年   3483篇
  1977年   3279篇
  1976年   3042篇
  1975年   2964篇
  1974年   2917篇
  1973年   3079篇
  1972年   2024篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
991.
992.
993.
994.
995.
The seasonal variation of the surface temperature is calculated for various idealized paleogeographic conditions with a 1.5-dimensional (1.5-D) coupled climate-sea ice model. The sensitivity of the annual and summer polar temperatures to the meridional oceanic heat transport and to the parameterizations adopted for the snow and sea ice albedos is examined in connection with the location and size of a polar global super-continent. It is shown that the high latitude summer temperatures remain below the freezing point in all numerical simulations with a polar super-continent, thus suggesting the potential role played by a large polar continental mass in the initiation of glaciations. These results are in agreement with a previous 1.5-D energy balance model (EBM) study but in conflict with two-dimensional (2-D) EBMs suggesting above-freezing high latitude summer temperatures in the case of a polar-centered super-continent. It is also found that the amount of seasonality is strongly dependent on the details of the surface albedo feedback parameterizations and could explain the various model diverging results.If a simplified temperature dependence of the silicate weathering rate controlling the long-term carbon cycle is included, the atmospheric CO2 level is significantly increased in the case of a polar-centered continent but summer temperatures still remain below freezing.  相似文献   
996.
997.
998.
Short-time variations of gravitational potential derivatives (otonic gravity-impulses) are described which are produced by fast-moving otons (objects of general relativity) in the Earth. Expressions for oton mass are obtained through measurable physical quantities. The question of otonic gravity-impulses registration is analysed.  相似文献   
999.
Mission and hardware constraints make the Cassini radar altimeter working in the beam limited or pulse limited mode dependent on the radar operative mode (Low and High Resolution, respectively), but never allows work in a condition such that the pulsewidth limited circle is much smaller than the beamwidth limited circle. Unfortunately this latter condition is vital for the application of the so-called Brown model widely and successfully used in Earth (ocean) observation missions where the quoted condition is really met. In the paper a new model is discussed which is based on the same general hypotheses of the Brown model but is worked out by means of a different approach which makes it more general and independent of the specific operative conditions. An extension of the new model to take into account large mispointing angles is considered as well based on a series expansion of the Bessel function and on the analysis of the truncation error. Finally a comparison with the classical Brown model is discussed too.  相似文献   
1000.
We study the dynamics of extended shells of relatively low-mass particles around and inside the orbit of two heavy centres of gravity (a binary) by computer simulations. The binary components are surrounded byN = 16 000 small mass particles in uniform random distribution on few spherical envelopes with different radii expanding with respective velocities. Some shells are inside the orbit of binary.We apply this model to binary galaxy systems with baryonic dark matter, e.g., massive black holes. In principle, we can apply this model to different kinds of objects (from binary star systems until superclusters of galaxies).It is shown that the shell expands homologously with a decreasing velocity and then, falls back into the binary system forming zones of compressed matter. At some moment of time there could be a collapse of these particles on to the heavier component of the binary. Further in time, some part of particles which were outside the binary orbit escape from the system. Other particles which were initially inside of the orbit are captured by binary components.We consider a number of different models with different initial parameters. For models with smaller radii of shells, about one-half of the particles escape from systems; whereas for larger values the shell disrupts as a whole. Escaping particles form collimated flows in planes of orbits of binaries. Positions of flows and directions of motion depend on positions of heavier components of binaries at the moment of a closest approach of particles and on ratios of masses of binary components.We show that during evolution of our models different kinds of structures of systems often are very similar to the observed structures of galaxies: spiral and elliptical galaxies, interacting galaxies, different kinds of flows and jets. Totally systems are expanding - after 40 periods of rotation of the binary the system expands by 300 times.  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