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   112941篇
  免费   1443篇
  国内免费   864篇
测绘学   2648篇
大气科学   7530篇
地球物理   21186篇
地质学   42307篇
海洋学   9846篇
天文学   25390篇
综合类   388篇
自然地理   5953篇
  2022年   723篇
  2021年   1222篇
  2020年   1340篇
  2019年   1459篇
  2018年   4512篇
  2017年   4081篇
  2016年   4045篇
  2015年   1741篇
  2014年   3150篇
  2013年   5402篇
  2012年   3928篇
  2011年   5826篇
  2010年   5165篇
  2009年   6340篇
  2008年   5432篇
  2007年   5835篇
  2006年   4203篇
  2005年   3170篇
  2004年   3057篇
  2003年   2936篇
  2002年   2842篇
  2001年   2349篇
  2000年   2304篇
  1999年   1820篇
  1998年   1893篇
  1997年   1750篇
  1996年   1500篇
  1995年   1487篇
  1994年   1314篇
  1993年   1202篇
  1992年   1144篇
  1991年   1158篇
  1990年   1163篇
  1989年   982篇
  1988年   923篇
  1987年   1095篇
  1986年   929篇
  1985年   1186篇
  1984年   1283篇
  1983年   1231篇
  1982年   1142篇
  1981年   1051篇
  1980年   991篇
  1979年   890篇
  1978年   886篇
  1977年   746篇
  1976年   757篇
  1975年   730篇
  1974年   709篇
  1973年   785篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
The causes of freshwater deficiency are considered, the main causes being poor environmental conditions and excessive water consumption. The problem of water deficiency is shown to be insoluble without the development of intense water use, conservation, and protection. Based on studying water as a commodity, it is asserted that the market mechanism is not sufficient to cope with water deficiency. The potential of the international water market is shown to be very limited in contrast to the market of water-efficient processes and water-retaining products.  相似文献   
962.
In this paper we report energetic ion behavior and its composition variations observed by the Cluster/RAPID instrument when the spacecraft was travelling in the high latitude magnetospheric boundary region on the day of the 31 March, 2001, strongest magnetic storm in the past 50 years. The Dst index reached −360 nT at about 09:00 UT. During its early recovery phase, large amounts of oxygen and helium ions were observed; the ratio of oxygen to hydrogen in the RAPID energy range reached as high as 250%, which suggests that the observed energetic particles might be of magnetospheric origin. The observations further show that enhanced energetic electron fluxes are confined in a very narrow region, while protons have occupied a larger region, and heavy ions have been observed in an even larger region. The flux of energetic electrons show a slight enhancement in a region where the magnetic field magnitude is around zero. These observed energetic ions could be quasi-trapped by the current sheet in the stagnation region of the cusp.  相似文献   
963.
964.
Seasonal evolution of the vertical thermal, halininc, and density structure of water in the phases of warming and heat loss is shown. The annual cycle of variability of seasonal and deep-water thermo-, halo-, and pycnocline is discussed. It is revealed that variations in the seasonal (subsurface) thermo-, halo-, and pycnocline depend on the surface environmental factors (air temperature, river runoff, and precipitation), while variations in the deep-water thermo-, halo-, and pycnocline depend on the dynamic factor impact (seasonal variations in the intensity of the general cyclonic circulation in the Black Sea).Translated from Vodnye Resursy, Vol. 32, No. 1, 2005, pp. 28–34.Original Russian Text Copyright © 2005 by Titov.  相似文献   
965.
The polar cusps of the magnetosphere are key regions for the transfer of mass, momentum, and energy from the solar wind into the magnetosphere. Understaning these key regions and the dynamical interactions that occur there are fundamentally important to determining the physical nature of the magnetosphere. In this paper we try to summarize many of the conclusions reached in the papers of this special issue emphasizing the present concepts and definition of the cusp, what variations could be temporal structures and what could be spatial structures. We address the need for further measurements and the role of present and planned projects to address these needs.  相似文献   
966.
We study the mineralogical changes suffered by specimens of natural miocene red and green continental sandstones (from Pozuelos Formation and Tiomayo Formation) cropping out in the Argentine Puna that increase their bulk magnetic susceptibility and change color when thermally treated. We hypothesize that on heating siderite, which is present in small quantities as cement in the studied sandstones, would oxidize and decompose into maghemite and/or magnetite. Subsequent heating to higher temperatures sometimes would bring about the conversion of maghemite and/or magnetite to hematite. Mössbauer spectroscopy proved to be a very valuable tool for the determination of the presence of siderite in small amounts in the studied samples. The present results show that further work is needed in order to fully understand the mineralogical changes suffered by continental sandstones during heating. The characterization of such changes occurred during laboratory routines is relevant, since they can help to better understand natural processes.  相似文献   
967.
We present results of an array study of seismic anisotropy beneath the Bohemian Massif (BM) showing distinct lateral and directional variations of the fast shear-wave polarization and split-delay time consistent with variations in the P-residual spheres, in which directional means of relative residuals are subtracted. Our analysis of the S- and P-wave anisotropy suggests that the mantle lithosphere of the BM consists of at least three large domains with different orientation of the large-scale fabric separated by sutures cutting most likely the whole lithosphere. Boundaries of the units are characterised by a null or small shear-wave splitting, as well as by smaller values in the P-residual spheres. We present self-consistent 3D anisotropic models of the lithosphere domains of the BM resulting from joint inversion of anisotropic parameters searching for a 3D orientation of mantle fabric. While in the Saxothuringian and Sudetes parts of the BM the (a, c) foliations dip prevailingly to the N-NW, they dip to the S and SW in the southern and eastern parts of the Moldanubian.  相似文献   
968.
Comparative characteristic of several microelements (Ni, Cu, Sr, Al, Zn, Co, Mn, Pb, Cd, Hg, As) found in fish (case study of breams in the Volga basin reservoirs) is presented. Regularities of accumulation of these elements in fish depending on their concentration in the water are analyzed. Main disturbances in fish caused by the accumulation of microelements in their organs and tissues are considered.  相似文献   
969.
970.
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