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   257948篇
  免费   5180篇
  国内免费   3377篇
测绘学   6939篇
大气科学   19157篇
地球物理   54045篇
地质学   89849篇
海洋学   21383篇
天文学   56202篇
综合类   1008篇
自然地理   17922篇
  2021年   2203篇
  2020年   2600篇
  2019年   2857篇
  2018年   3554篇
  2017年   3257篇
  2016年   5760篇
  2015年   4217篇
  2014年   6922篇
  2013年   14247篇
  2012年   6525篇
  2011年   7860篇
  2010年   6887篇
  2009年   9573篇
  2008年   8364篇
  2007年   7786篇
  2006年   9635篇
  2005年   7717篇
  2004年   7613篇
  2003年   7102篇
  2002年   6742篇
  2001年   6019篇
  2000年   5955篇
  1999年   5189篇
  1998年   5222篇
  1997年   5026篇
  1996年   4677篇
  1995年   4427篇
  1994年   4112篇
  1993年   3844篇
  1992年   3608篇
  1991年   3592篇
  1990年   3761篇
  1989年   3507篇
  1988年   3297篇
  1987年   3844篇
  1986年   3414篇
  1985年   4220篇
  1984年   4731篇
  1983年   4419篇
  1982年   4325篇
  1981年   3929篇
  1980年   3658篇
  1979年   3514篇
  1978年   3489篇
  1977年   3282篇
  1976年   3047篇
  1975年   2957篇
  1974年   2933篇
  1973年   3080篇
  1972年   2026篇
排序方式: 共有10000条查询结果,搜索用时 609 毫秒
351.
352.
353.
354.
An investigation of 531 active regions was made to determine the correlation between energy released by flares and the available energy in magnetic fields of the regions. Regions with magnetic flux greater than 1021 maxwell during the years 1967–1969, which included sunspot maximum, were selected for the investigation. A linear regression analysis of flare production on magnetic flux showed that the flare energy is correlated with magnetic energy with a coeificient of correlation of 0.78. Magnetic classification and field configuration also significantly affect the production of flares.This work was supported by the Aerospace Sponsored Research Program.  相似文献   
355.
A variety of measures of organic matter concentration and quality were made on samples collected from the top few mm of intertidal mudflat sediment over the course of a year, in order to assess the relative importance of biological and sedimentological influences on sedimentary organic matter. Winter and summer were times of relatively fine-grained sediment accumulation, caused by biological deposition or stabilization processes and resulting in higher organic matter concentrations. Stable carbon isotope and Br:C ratios indicated a planktonic source of bulk organic matter. Ratios of organic carbon to specific surface area of the sediments were consistent with an organic monolayer coverage of sediment grains. Correction for changing grain size during the year showed no change in the organic concentration per unit surface area, in spite of organic matter inputs by in situ primary production, buildup of heterotroph biomass and mucus coatings, and biodeposition of organic-rich seston. There were also no indications of changes in bulk organic quality, measured as hydrolyzable carbohydrates and amino acids, in response to these biological processes. It is concluded that biological processes on a seasonal time scale affect the bulk organic matter of these sediments via a modulation of grain size rather than creation or decay of organic matter.  相似文献   
356.
Laboratory measurements of the OI1173989 Å (3s' 3D° → 2p41D, 3s' 3D° → 2p43P) branching ratio have been made with a value of 1.5 × 10t-4 indicated. This value makes the branching transition at 1173 Å an order of magnitude stronger than the branch at 7990 Å (3s' 3D° → 3p 3P). The 1173 Å branching loss is still too weak a loss process for multiply scattered 989 Å photons to resolve the 989 Å intensity problem in the dayglow.  相似文献   
357.
358.
In previous papers, the present authors have shown that the galactic anisotropy is modulated due to cosmic ray orbital deflection in the heliomagnetosphere, and that the sidereal time daily variations of galactic origin can be expressed using the basic vectors, which have been obtained by calculating trajectories of cosmic rays in a model magnetosphere having Parker's Archimedian spiral structure with a flat or a wavy neutral sheet. In the present paper, the magnetic irregularities superposed on the Parker's spiral field have been taken into account, which cause the scattering of cosmic rays and disturb their orbits. We examined the fluctuations of asymptotic directions calculating their orbits by the Monte-Carlo simulation, based on the theory of the multiple scattering process. It is shown that the dispersion of the projected deviation angle is determined mainly by the scattering mean free path and by the structure of the order magnetic field, e.g. the polarity state of the heliomagnetosphere and the extent of the neutral sheet. We investigated also the influence of the fluctuations of asymptotic directions on the sidereal daily variation. It is found that, under some conditions, the scattering causes only the attenuation of the amplitude of the basic vector, and does not change its phase. The attenuation is negligibly small at high rigidities larger than ~ 1000 GV, but becomes more serious with decreasing rigidity. The rigidity dependence curve of the attenuation rate was calculated for various cases. A simple and approximate method is also presented for the derivation of those curves for any value of the magnitude of the mean free path and for various model magnetospheres. It is noted, however, that the lower limiting rigidity below which the present method is not applicable is relatively high in the Positive polarity state.  相似文献   
359.
360.
Classical R-M and synthetic W-D analysis of V758 Centauri are presented. Two solutions (semi-detached and contact) were found from differential corrections approach. The semi-detached model is physically acceptable since the system is thermally decoupled. The solution for this case and the photometric data are consistent with a B9 primary and A9 secondary components having parameters close to Main-Sequence values. It is suggested that V758 Centauri is a B-type W UMa system at the brokencontact phase predicted by the Thermal Relaxation Oscillations theory.  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