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   2964篇
  免费   74篇
  国内免费   52篇
测绘学   302篇
大气科学   282篇
地球物理   525篇
地质学   1309篇
海洋学   140篇
天文学   418篇
综合类   55篇
自然地理   59篇
  2023年   16篇
  2022年   54篇
  2021年   64篇
  2020年   64篇
  2019年   67篇
  2018年   261篇
  2017年   235篇
  2016年   227篇
  2015年   135篇
  2014年   200篇
  2013年   261篇
  2012年   164篇
  2011年   164篇
  2010年   147篇
  2009年   144篇
  2008年   124篇
  2007年   86篇
  2006年   74篇
  2005年   54篇
  2004年   43篇
  2003年   37篇
  2002年   28篇
  2001年   23篇
  2000年   31篇
  1999年   20篇
  1998年   17篇
  1997年   22篇
  1996年   11篇
  1995年   11篇
  1994年   21篇
  1993年   15篇
  1992年   9篇
  1991年   28篇
  1990年   19篇
  1989年   24篇
  1988年   14篇
  1987年   22篇
  1986年   17篇
  1985年   16篇
  1984年   13篇
  1982年   6篇
  1981年   7篇
  1979年   13篇
  1978年   7篇
  1977年   6篇
  1976年   7篇
  1975年   8篇
  1974年   11篇
  1972年   8篇
  1969年   5篇
排序方式: 共有3090条查询结果,搜索用时 421 毫秒
11.
The equivalent width calculations for the fundamental vibration rotation band lines of HeH+ have been carried out for a non-DA white dwarf model with an effective temperature of 12 000 K. BothP andR branch lines with rotational quantum numbersJ=3 to 18 were included in the calculations. A search for these lines in helium rich white dwarfs is suggested.  相似文献   
12.
Introduction The Himalaya is considered to be the youngest mountains on the earth, and is tectonically very active, and hence inherently (geologically) vulnerable to hazards. Extreme rainfall events, landslides, debris flows, torrents and flash floods due…  相似文献   
13.
A model of the predawn bulge ionosphere composition and structure is constructed and compared with the ion mass spectrometer measurements from the Pioneer Venus Orbiter during orbits 117 and 120. Particular emphasis is given to the identification of the mass-2 ion which we find unequivocally due to D+ (and not H2+). The atmospheric D/H ratio of 1.4% and 2.5% is obtained at the homopause (~ 130 km) for the two orbits. The H2+ contribution to the mass-2 ion density is less than 10%, and the H2 mixing ratio must be <0.1 ppm at 130 km altitude. The He+ data require a downward He+ flux of ~2 × 107 cm?2 sec?1 in the predawn region which suggest that the light ions also flow across the terminator from day to night along with the observed O+ ion flow.  相似文献   
14.
The Oppenheimer-Penney theory, as developed by Percival and Seaton (1958), is applied to calculate the polarization of resonance lines from Li-like ions. Two laws for the pitch-angle distribution of electrons around the magnetic field are accounted. The degrees of polarization are averaged over the energy of non-thermal electrons generated during the initial phase of solar flares. It is found that for the full space pitch-angle distribution, as adopted by Chandra and Joshi (1984), the degrees of polarization are nearly independent of the atomic number of ion. Whereas for the forward-come distribution used by Haug (1981), they depend on the choice of the free parameterE 0. The polarization of the resonance lines from Li-like ions is two times larger than that of the L radiations from H-like ions. Hence, under favourable conditions, it may be detected during solar flares.  相似文献   
15.
An analytical study is performed to examine the effects of temperature-dependent heat source on free-convective flow of non-Newtonian fluid (Walters's liquidB). The expression for the velocity field has been obtained by the Laplace transform technique. The influence of the various parameter entering into the problem is extensively discussed.  相似文献   
16.
Spectrophotometric observations of the eclipsing binary system Capricorni, covering the wavelength interval 3300–7300 Å, have been presented. Comparison of the standard spectral scans of Cap with the spectral scans of the stars of known spectral types and luminosity classes taken from the Breger (1976) catalogue shows that, near the phase of secondary eclipse, Cap shows the spectral-luminosity type as A8-9III. This is in agreement with the photometric findings of Srivastava (1987b). H emission is not visible convincingly, however, some irregularities are apparent, at least in two scans, around the H region.  相似文献   
17.
Four spectrograms of the cepheid Geminorum at different phases have been analysed for the determination of the abundances of various elements. The analysis shows that the atmosphere of Gem has an essentially solar composition.  相似文献   
18.
There are great spatial variations in the distribution of the fast growing populations of India. About 70% of the 1981 population is concentrated in only 40% of the geographical area. Such a pattern of population concentration, with its spatio-temporal dimension, can be understood better if examined at the macro-regional level considering the physico-climatic and socio-economic diversity of the country. This is done by way of analyzing district level data for India as a whole and also for its various macro-regions. The study reveals that the Great Plains and Coastal Plains and Islands have a homogenous distribution and a higher population increase. The Deccan Plateau shows the most homogenous dispersion of population. The population is declining in the Northern Mountains and the Great Plains but accentuating in the Coastal Plains and Islands and the Deccan Plateau. The north and south of India are showing opposite trends in population concentration.  相似文献   
19.
The continuum energy distribution of the emission line star X Per (O9.5III-V) has been obtained in the wavelength range 340–710 nm and has been compared with the energy distribution of Cam in the same wavelength range. The continuum of the star is found to be modified by the circumstellar envelope. A number density of the order of 1011 in the envelope has been obtained from the observations of H in emission.  相似文献   
20.
Modified similarity method has been used to study the propagation of spherical-variable energy blast waves through a self-gravitating gas. For an energy inputE =E 0t4/3, whereE is the energy released up to timet andE 0 is a functional constant, the similarity solutions correct up to third approximation have been obtained. It is found that the effects of self-gravitational forces are of third order. An increase in the parameterA 2 (characterising the gravitational field) increases the shock velocity.  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