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   51101篇
  免费   499篇
  国内免费   312篇
测绘学   995篇
大气科学   2572篇
地球物理   9806篇
地质学   19545篇
海洋学   4855篇
天文学   12245篇
综合类   129篇
自然地理   1765篇
  2022年   576篇
  2021年   854篇
  2020年   896篇
  2019年   1006篇
  2018年   2103篇
  2017年   1914篇
  2016年   2043篇
  2015年   835篇
  2014年   1827篇
  2013年   2793篇
  2012年   2079篇
  2011年   2469篇
  2010年   2319篇
  2009年   2686篇
  2008年   2358篇
  2007年   2623篇
  2006年   2302篇
  2005年   1299篇
  2004年   1214篇
  2003年   1194篇
  2002年   1157篇
  2001年   1129篇
  2000年   936篇
  1999年   692篇
  1998年   704篇
  1997年   736篇
  1996年   548篇
  1995年   572篇
  1994年   535篇
  1993年   433篇
  1992年   448篇
  1991年   428篇
  1990年   478篇
  1989年   421篇
  1988年   389篇
  1987年   410篇
  1986年   336篇
  1985年   463篇
  1984年   468篇
  1983年   453篇
  1982年   432篇
  1981年   387篇
  1980年   395篇
  1979年   347篇
  1978年   378篇
  1977年   320篇
  1976年   287篇
  1975年   293篇
  1974年   274篇
  1973年   344篇
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
941.
942.
It is shown that, for the scalar-tensor cosmology (STC) by Jordan-Brans-Dicke (JBD), in general anisotropic solution the oscillatory mixmaster regime near the singularity will be destroyed by the scalar source-free field and replaced by monotonousV 3-collapse into the point or into the line and plane (only in caseG0) even in the presence of the primordial source-free electromagnetic (EM) field.  相似文献   
943.
The equation for the two-particles cosmic-ray distribution function is derived by means of the Boltzmann kinetic equation averaging. This equation is valid for arbitrary ratio of regular and random parts of the magnetic field. For small energy particles the guiding-center approximation is used. On the basis of the derived equation the dependence between power spectra of cosmic-ray intensity and random magnetic field is obtained. If power spectra are degree functions for high energy particles ( 10 GeV nucleon–1), then the spectral exponent of magnetic field lies between and –2, where is the spectral exponent of cosmic-ray power spectra. The experimental data concerning moderate energy particles are in accordance with =, which demonstrates that the magnetic fluctuations are isotropic or cosmic-ray space gradient is small near the Earth orbit.  相似文献   
944.
A study of the background corona near solar minimum   总被引:2,自引:0,他引:2  
The white light coronagraph data from Skylab is used to investigate the equatorial and polarK andF coronal components during the declining phase of the solar cycle near solar minimum. Measurements of coronal brightness and polarization brightness product between 2.5 and 5.5R during the period of observation (May 1973 to February 1974) lead to the conclusions that: (1) the equatorial corona is dominated by either streamers or coronal holes seen in projections on the limb approximately 50% and 30% of the time, respectively; (2) despite the domination by streamers and holes, two periods of time were found which were free from the influences of streamers or holes (neither streamers nor holes were within 30° in longitude of the limb); (3) the derived equatorial background density model is less than 15% below the minimum equatorial models of Newkirk (1967) and Saito (1970); (4) a spherically symmetric density model for equatorial coronal holes yields densities one half those of the background density model; and (5) the inferred brightness of theF-corona is constant to within ±10% and ±5% for the equatorial and polar values, respectively, over the observation period. While theF-corona is symmetric at 2R it begins to show increasing asymmetry beyond this radius such that at 5R the equatorialF-coronal brightness is 25% greater than the polar brightness.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   
945.
Solar wind and interplanetary magnetic field data were obtained by the PROGNOZ 1 and PROGNOZ 2 satellites during the period following the August 4, 1972 (0621 UT) solar flare. A thermalized plasma was recorded one hour after the shock followed two hours later by the plasma piston with a bulk velocity higher than 1700 km s-1. The comparison between the PROGNOZ and PIONEER 9 solar wind data shows an attenuation of the plasma properties with the deflection from the flare's meridian.  相似文献   
946.
Equivalent circuit method calculations using ‘2,2’ and ‘2,4’ mode semidiurnal winds yield sample variations of electric currents and fields at various latitudes. The positions of the current system foci are found to vary much more with longitude than with season. Observed seasonal changes in lunar current system foci must be due to changes in winds rather than in conductivities.  相似文献   
947.
948.
The resonant interaction between the whistler mode waves and the energetic electrons near the plasmapause boundary has been studied in the presence of field aligned currents which seem to exist during substorm activity. It is shown that the electrons which carry the current along the direction of the magnetic field enhance the whistler mode growth considerably if the streaming velocity is small compared to the phase velocity of the wave. It is likely that this is one of the mechanisms explaining the intense VLF emissions observed near the plasmapause during substorm activity.  相似文献   
949.
Theoretical results on the daily variation of O+ and H+ field-aligned velocities in the topside ionosphere are presented. The results are for an L = 3 magnetic field tube under sunspot minimum conditions at equinox. They come from calculations of time-dependent O+ and H+ continuity and momentum balance in a magnetic field tube which extends from the lower F2 region to the equatorial plane (Murphy et al., 1976).There are occasions when ion counterstreaming occurs, with the O+ velocity upward and H+ velocity downward. The conditions causing this counterstreaming are described: the H+ layer is descending whilst O+ is supplied from below either to increase the O+ concentration at fixed heights or to replace O+ ions lost by charge exchange with neutral H. It is suggested that the results of observations at Arecibo by Vickrey et al. (1976) of O+ and H+ concentrations and counterstreaming velocities are significantly affected by E×B drift.  相似文献   
950.
An upper limit has been calculated on the effective aperture separation or detector thickness of ion drift meters of two fundamental types. The limit applies to meters which compare currents collected by detectors with different view directions at the same retarding potential and to meters which measure the entire thermal ion distribution function. For both types, a single scanned detector may be employed instead of multiple detectors, in which case the limit applies to twice the radius of curvature of the path followed by the detector aperture during a scan, (i.e. the diameter of a spinning payload). The limit was found to be important in two cases. First, in the F region on spacecraft with stringent electrostatic cleanliness requirements, the 10% error limit was found to be 40 cm. Second, in the E region, the limit was found to be 1 cm.Originally submitted to the journalSpace Science Instrumentation.Deceased.  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