全文获取类型
收费全文 | 53832篇 |
免费 | 856篇 |
国内免费 | 566篇 |
专业分类
测绘学 | 1461篇 |
大气科学 | 3826篇 |
地球物理 | 9972篇 |
地质学 | 19330篇 |
海洋学 | 4931篇 |
天文学 | 12959篇 |
综合类 | 188篇 |
自然地理 | 2587篇 |
出版年
2022年 | 367篇 |
2021年 | 627篇 |
2020年 | 661篇 |
2019年 | 710篇 |
2018年 | 1587篇 |
2017年 | 1512篇 |
2016年 | 1880篇 |
2015年 | 995篇 |
2014年 | 1756篇 |
2013年 | 2881篇 |
2012年 | 1866篇 |
2011年 | 2384篇 |
2010年 | 2085篇 |
2009年 | 2695篇 |
2008年 | 2310篇 |
2007年 | 2358篇 |
2006年 | 2208篇 |
2005年 | 1630篇 |
2004年 | 1652篇 |
2003年 | 1557篇 |
2002年 | 1480篇 |
2001年 | 1303篇 |
2000年 | 1229篇 |
1999年 | 1003篇 |
1998年 | 1049篇 |
1997年 | 952篇 |
1996年 | 821篇 |
1995年 | 789篇 |
1994年 | 691篇 |
1993年 | 610篇 |
1992年 | 595篇 |
1991年 | 604篇 |
1990年 | 630篇 |
1989年 | 504篇 |
1988年 | 511篇 |
1987年 | 531篇 |
1986年 | 493篇 |
1985年 | 619篇 |
1984年 | 679篇 |
1983年 | 597篇 |
1982年 | 567篇 |
1981年 | 500篇 |
1980年 | 477篇 |
1979年 | 482篇 |
1978年 | 460篇 |
1977年 | 373篇 |
1976年 | 352篇 |
1975年 | 362篇 |
1974年 | 314篇 |
1973年 | 346篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
371.
A self-similar solution to the problem of the implosion of a cylindrical shock wave in the presence of a magnetic field has been investigated. A strong shock wave in a cylindrically-symmetric flow travels to the axis of symmetry through a gas of uniform initial density 0 and zero-pressure. A comparative study has been made between the results obtained in ordinary gasdynamics and magnetogasdynamics with transverse and axial components of the magnetic field. The value of similarity exponent has been assigned from that found in the paper of Whitham (1958). 相似文献
372.
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
373.
Recent observational and theoretical studies of the structure of umbral chromosphres have led us to propose new models of the region. We combine space and ground based determinations of the densitytemperature structure of the transition region with existing Caii H, K, and IR triplet spectral data to establish relatively high density, gradient models of the sunspot chromosphere.Avrett in 1981 presented theSunspot Sunspot Model which can be characterized as a physically extended, low density, plateau model. However, in a similar study, the authors (Beebeet al., 1982) pointed out that the relatively high transition region pressures derived from space observations indeed are required to reproduce high resolution Caii spectral features. Studies of the umbral chromosphere as a resonant cavity for slow-mode magneto-acoustic waves producing umbral oscillations (Zhugzhdaet al., 1984) also lead to atmospheres of relatively small physical thickness, thus higher densities.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984. 相似文献
374.
Observations on a grid of 900 radio sources have been used to map and to track large-scale structures in the solar wind at distances of 0.6–1.5 AU from the Sun. Most of the disturbances were shells of enhanced density followed by high-speed streams lasting for several days, although more stable corotating interaction regions were also observed. Ninety-six disturbances were mapped during August 1978–September 1979 and those of the erupting stream-type were usually accompanied by shocks and geomagnetic activity if they encountered the Earth. Back-projection to the Sun indicated sources that were always associated with coronal holes. Possible associations with solar flares and disappearing filaments occurred but on many occasions no flare or filament activity was evident anywhere on the disc within a suitable time interval. It is concluded that erupting streams are transients generated by coronal hole activity. Evidence is presented which further suggests that coronal mass ejections of the curved-front variety may be identified with these erupting streams. 相似文献
375.
Ram S 《Population geography : a journal of the Association of Population Geographers of India》1986,8(1-2):17-26
The author analyzes the spatial distribution of Indians living in Great Britain in 1981. The impact of factors such as demand for labor, location of manufacturing industries, and the attraction of urban areas is examined. 相似文献
376.
"A modification of the population potential model is used to analyze the structure of the potential of population for cities of the Moscow region, to develop a relationship between population potential and city size, and to use this procedure to forecast future population change in Moscow Oblast cities." Cities in the region with a population of 50,000 or more at the censuses of 1959, 1970, and 1979 are included. 相似文献
377.
A simple self-consistent model of a high-temperature turbulent current sheet (HTCS) is considered. The anomalous character of plasma conductivity in a sheet is assumed to be due to gradient instabilities. The possibility of a low threshold of their excitation is demonstrated by an example of temperature-drift instability.Application of the HTCS model to the hot or main phase of a solar flare is discussed. The model consistently explains many observed properties of this phase. 相似文献
378.
P. S. Goraya 《Astrophysics and Space Science》1985,112(2):325-336
A large sample of Be stars has been studied spectrophotometrically in the visible region. The continuum energy distribution data for 23 Be stars included in the list of Harmanecet al. (1983) are presented and discussed in the wavelength range 3200 Å–8000 Å. For 15 Be stars the observations reported in the present work are new. By comparing the observed continua with models, the effective temperatures of these stars have been estimated. It is found that, in general, Be stars have lower effective temperature than the corresponding normal B stars. The present study shows that the early-Be stars (B0–B5) possess near-ultraviolet and near-infrared excess emissions more frequently than the late-Be stars (B5–B9). The seven new Be stars are detected to show pole-on characteristics. 相似文献
379.
On the basis of a globular cluster study a crude estimate of the total mass of the galactic halo within 20 kpc from the centre is done. It gives a minimal halo mass of the order of
, yielding possibilities for a mass as large as
. The content of the interstellar matter in the halo is estimated too. It is found that the gas content is a few percents the minimal mass, the gas temperature is very high — about 1×106 K, the magnetic field weak — about 0.25 nT. A weak nonthermal radio emission might be expected from such a halo. 相似文献
380.
H. A. Taylor Jr. P. A. Cloutier M. Dryer S. T. Suess A. Barnes R. S. Wolff 《Earth, Moon, and Planets》1985,32(3):275-290
Corotating solar wind streams emanating from stable coronal structures provide an unique opportunity to compare the response of planetary ionospheres to the energy conveyed in the streams. For recurrent solar conditions the signal propagating outward along spiral paths in interplanetary space can at times exhibit rather similar content at quite different downstream locations in the ecliptic plane. Using solar wind measurements from plasma detectors on ISEE-3, Pioneer Venus Orbiter (PVO) and Helios-A, as well as in-situ ion composition measurements from Bennett Ion Mass Spectrometers on the Atmosphere Explorer-E and PVO spacecraft, corotating stream interactions are examined at Earth and Venus. During May–July 1979 a sequence of distinct, recurrent coronal regions developed at the Sun. Analysis of these regions and the associated solar wind characteristics indicates a corrresponding sequence of corotating streams, identifiable over wide distances. The time series of solar wind velocity variations observed at Earth, Venus, and the Helios-A positions during June–July attests to intervals of corotating stream propagation. The characteristics of the stream which passed Earth on July 3, are observed at Helios-A and at Venus (PVO) about 8 days later, consistent with the spiral path propagation delay times between the locations in the ecliptic plane. On July 3, Earth and Venus have a wide azimuthal separation of about 142 . Although the planetary environments are distinctly different, pronounced and somewhat analagous ionospheric responses to the stream passage are observed at both Earth and Venus. The response to the intercepted stream is consistent with independent investigations which have shown that the variability of the solar wind momentum flux is an important factor in the solar wind-ionosphere interaction at both planets. 相似文献