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   255905篇
  免费   5159篇
  国内免费   3333篇
测绘学   7022篇
大气科学   18893篇
地球物理   53480篇
地质学   89058篇
海洋学   21137篇
天文学   56015篇
综合类   1009篇
自然地理   17783篇
  2021年   2228篇
  2020年   2573篇
  2019年   2839篇
  2018年   3362篇
  2017年   3092篇
  2016年   5632篇
  2015年   4188篇
  2014年   6908篇
  2013年   14221篇
  2012年   6420篇
  2011年   7715篇
  2010年   6747篇
  2009年   9374篇
  2008年   8178篇
  2007年   7559篇
  2006年   9595篇
  2005年   7632篇
  2004年   7546篇
  2003年   7044篇
  2002年   6661篇
  2001年   5949篇
  2000年   5918篇
  1999年   5196篇
  1998年   5229篇
  1997年   5028篇
  1996年   4671篇
  1995年   4406篇
  1994年   4098篇
  1993年   3845篇
  1992年   3631篇
  1991年   3589篇
  1990年   3753篇
  1989年   3509篇
  1988年   3299篇
  1987年   3837篇
  1986年   3406篇
  1985年   4222篇
  1984年   4731篇
  1983年   4403篇
  1982年   4309篇
  1981年   3927篇
  1980年   3643篇
  1979年   3508篇
  1978年   3490篇
  1977年   3273篇
  1976年   3040篇
  1975年   2955篇
  1974年   2914篇
  1973年   3073篇
  1972年   2023篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
171.
172.
173.
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   
174.
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   
175.
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   
176.
177.
178.
Abstract Zircons have been studied from different layers of migmatites (from Arvika, western Sweden and Nelaug, southern Norway) and from a paragneiss (from Arvika) associated with one of the migmatites. The main purpose of the investigation is to establish whether or not information about zircons can help in the elucidation of the parentage and rock-forming processes of migmatites.
The elongation ratio of zircons from all layers is small and characteristic of sedimentary zircons. Further, the absence of characteristic colours and the growth trends of the zircons (indicated by the reduced major axes) observed in the various samples both support a sedimentary parentage for these rocks. The zircons of all layers exhibit secondary growth (overgrowth, outgrowth and multiple growth) due to metamorphism. Compared with the zircons from the paragneiss, those of the migmatite layers are more clouded and less rounded, some of them becoming opaque or even skeletal; this is especially true of the zircons from the leucosomes. These observations indicate an alteration of the original sedimentary zircons in the migmatite, especially in the leucosomes, in response to the migmatization process, previously interpreted as partial melting.  相似文献   
179.
We present new 1.45-1.75 μm spectra of Europa's dark non-ice material with a spectral resolution (λ/δλ) of 1200, obtained by using adaptive optics on the Keck telescope to spatially separate the spectrum of the non-ice material from that of the surrounding ice-rich regions. Despite the great increase in spectral resolution over the previous best spectra of the non-ice material, taken with Galileo's near-infrared mapping spectrometer (NIMS) with λ/δλ=66, no new fine-scale spectral structure is revealed. The smoothness of the spectra is inconsistent with available laboratory spectra of crystalline hydrated salts at Europa temperatures, but is more consistent with various combinations of non-crystalline hydrated salts and/or hydrated sulfuric acid, as have been matched to the lower-resolution NIMS spectra.  相似文献   
180.
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