全文获取类型
收费全文 | 149361篇 |
免费 | 2361篇 |
国内免费 | 1230篇 |
专业分类
测绘学 | 3860篇 |
大气科学 | 10236篇 |
地球物理 | 29063篇 |
地质学 | 54437篇 |
海洋学 | 12944篇 |
天文学 | 33694篇 |
综合类 | 506篇 |
自然地理 | 8212篇 |
出版年
2022年 | 847篇 |
2021年 | 1436篇 |
2020年 | 1567篇 |
2019年 | 1720篇 |
2018年 | 4854篇 |
2017年 | 4442篇 |
2016年 | 4808篇 |
2015年 | 2341篇 |
2014年 | 4154篇 |
2013年 | 7367篇 |
2012年 | 4822篇 |
2011年 | 6916篇 |
2010年 | 6106篇 |
2009年 | 7693篇 |
2008年 | 6659篇 |
2007年 | 6956篇 |
2006年 | 5507篇 |
2005年 | 4246篇 |
2004年 | 4247篇 |
2003年 | 4047篇 |
2002年 | 3918篇 |
2001年 | 3290篇 |
2000年 | 3161篇 |
1999年 | 2597篇 |
1998年 | 2703篇 |
1997年 | 2484篇 |
1996年 | 2213篇 |
1995年 | 2182篇 |
1994年 | 1879篇 |
1993年 | 1739篇 |
1992年 | 1659篇 |
1991年 | 1712篇 |
1990年 | 1728篇 |
1989年 | 1504篇 |
1988年 | 1412篇 |
1987年 | 1637篇 |
1986年 | 1446篇 |
1985年 | 1801篇 |
1984年 | 2031篇 |
1983年 | 1889篇 |
1982年 | 1759篇 |
1981年 | 1688篇 |
1980年 | 1522篇 |
1979年 | 1436篇 |
1978年 | 1387篇 |
1977年 | 1201篇 |
1976年 | 1182篇 |
1975年 | 1155篇 |
1974年 | 1120篇 |
1973年 | 1210篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
971.
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
972.
In this paper, the problem of the motion of n gyrostats in a central field, with relative momenta
k
r
(k=1, 2, ..., n) functions of the time, is considered.In these conditions, the existence of the linear and angular momentum integrals is established.Moreover, a sufficient condition for the existence of the Jacobi's Integral is given. This study generalizes the results given by Duboshin for n rigid bodies. 相似文献
973.
Bond AR 《Soviet geography》1985,26(1):26-47
This paper reviews labor planning and population policies in Noril'sk, a mining and metallurgical settlement in northern Siberia. When the settlement was established in 1935, planners were lacking in ideas about how to recruit workers to staff the mines and smelters and how to retain the labor force once it was in place. From 1935-79, planners followed a forced labor policy dependent upon the labor of prisoners. However, this solution was not adequate for meeting the manpower needs of an economy undergoing rapid technological sophistication. Northern wage increments were introduced after 1945 to recruit skilled workers from other regions. These increments built up over a 4-year period to a maximum of 80% of base pay. A special cost of living bonus was also awarded. Although these inducements facilitated labor recruitment, labor retention remained a major problem. Surveys indicated that workers would prefer improvements in housing and social services to further wage increases. Thus, policy was directed at the housing shortages, poor medical care, and inadequate child care facilities in Noril'sk. Such improvements facilitated labor retention but also contributed to overpopulation. The population doubled between 1956-80, exceeding 180,000 in the latter year. In the early 1980s, selective measures toward population control were implemented to ensure maintenance of living standards (e.g., encouragement of older workers to leave the area upon retirement, more careful screening of recruits). The goal is to stabilize city size at around 250,000. The Noril'sk case illustrates that quality of life investments can alleviate labor retention problems even in the harshest physical environments. Recent policies have advocated productivity-enriching technologies that do not require increments in the labor force. 相似文献
974.
"The paper reports the use of Australian Resources Information System to calculate two proximate populations (people living within (i) 150 and (ii) 500 km) for the centroids of each of 3,000 half by half degree geodetic grid cells covering Australia. The use of proximate population data is exemplified by computing a map of the eighteen regions collectively containing a maximum fraction of the Australian population." The data used are from the 1981 census. 相似文献
975.
"An attempt is made in the light of an analysis of population changes occurring in several sub-districts of Greater Santiago in the period between the last population censuses of 1970 and 1982, based on small-area census units (distritos and communas) to draw attention to the effects on the spatial growth of the city resulting from changed conditions in the political and juridical sphere since the change of power in 1973." The effect of changes in government policy concerning the liberalization of the property market and the illegal occupation of land are noted. (summary in ENG) 相似文献
976.
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
977.
M. E. Gedalin J. G. Lominadze L. Stenflo V. N. Tsytovich 《Astrophysics and Space Science》1985,108(2):393-400
Wave conversion mechanisms causing large-frequency shifts are considered for an electron-positron plasma in a strong magnetic field. In particular, we discuss the effects of the nonlinear erenkov as well as the cyclotron resonances in order to associate pulsar radio-emissions with our present model for nonlinear conversion of high-frequency radiation into the low-frequency region. 相似文献
978.
A. Čadež 《Astrophysics and Space Science》1985,109(2):399-402
We respond to comments made in a recent letter by Karimet al. (1984), and show that the examples of interstellar absorption at2800 Å that they have presented so far can all be attributed to overexposure of the IUE detectors. 相似文献
979.
This paper summarizes and analyzes the results of radio optical, infrared, and X-ray observations of a large sector of the sky in the constellation Cygnus (19h20m-22h, =30–50°;l
II=65–90°, |b
II|10°). This region is associated with an extended X-ray source referred to as the Cygnus superbubble. About a quarter of the superbubble region is occupied by the extensively investigated multicomponent thermal radio source Cyg X. The region contains eight OB-associations which, when projected on the sky, duplicate the outline of the X-ray superbubble. These associations contains 110 stars of high luminosity (about 40 Wolf-Rayet and Of stars). The observations suggest that the X-ray superbubble is not a single object. Between 50 and 75% of its X-ray emission can be ascribed to discrete sources, the rest being probably due to regions of coronal gas about 100 pc in diameter, created by stellar winds and, possibly, supernova explosions in individual associations. The objects that produce the X-ray and optical radiation of the presumed superbubble are located at distances from 0.5 to 2.5 kpc from the Sun in the Carina-Cygnus spiral arm. The eastern portion of the region presumed superbubble contains the associations Cyg OB7 and Cyg OB4 and is generally less than 1 kpc distant, while the western portion contains the associations Cyg OB1, 2, 3, 8, and 9 and is 1 to 2 kpc distant. 相似文献
980.
I. A. Issa 《Astrophysics and Space Science》1985,113(2):317-323
The size distribution of dark clouds and of H II regions were used to derive a distance of 440±22 kpc for M31. Some galactic H II regions and dark clouds were combined with the same ingredients in M31 to study the mass spectrum of both phenomena. A gas to dust ratio of 10001 has been derived for M31. Radii of the classical H II regions as derived in this work using distance of 440 kpc agrees with the galactic classical H II region radii. 相似文献