全文获取类型
收费全文 | 118083篇 |
免费 | 7025篇 |
国内免费 | 8422篇 |
专业分类
测绘学 | 4792篇 |
大气科学 | 12589篇 |
地球物理 | 24587篇 |
地质学 | 45727篇 |
海洋学 | 11409篇 |
天文学 | 22035篇 |
综合类 | 3861篇 |
自然地理 | 8530篇 |
出版年
2022年 | 1902篇 |
2021年 | 2480篇 |
2020年 | 2231篇 |
2019年 | 2556篇 |
2018年 | 3986篇 |
2017年 | 3704篇 |
2016年 | 4484篇 |
2015年 | 3052篇 |
2014年 | 4459篇 |
2013年 | 6360篇 |
2012年 | 4570篇 |
2011年 | 5478篇 |
2010年 | 5162篇 |
2009年 | 6057篇 |
2008年 | 5294篇 |
2007年 | 5253篇 |
2006年 | 4664篇 |
2005年 | 3764篇 |
2004年 | 3495篇 |
2003年 | 3326篇 |
2002年 | 3201篇 |
2001年 | 2977篇 |
2000年 | 3000篇 |
1999年 | 3093篇 |
1998年 | 2863篇 |
1997年 | 2683篇 |
1996年 | 2388篇 |
1995年 | 2228篇 |
1994年 | 1949篇 |
1993年 | 1742篇 |
1992年 | 1551篇 |
1991年 | 1457篇 |
1990年 | 1424篇 |
1989年 | 1210篇 |
1988年 | 1151篇 |
1987年 | 1164篇 |
1986年 | 1031篇 |
1985年 | 1216篇 |
1984年 | 1347篇 |
1983年 | 1215篇 |
1982年 | 1127篇 |
1981年 | 1080篇 |
1980年 | 945篇 |
1979年 | 909篇 |
1978年 | 871篇 |
1977年 | 748篇 |
1976年 | 708篇 |
1975年 | 711篇 |
1974年 | 665篇 |
1973年 | 716篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
731.
Fedorov GM 《Soviet geography》1983,24(4):289-297
"Oblast-level regions of the USSR are grouped and mapped in nine types and a number of subtypes to reflect a combination of demographic factors: level of urbanization, urban and rural population growth, and population density. The typology can be used to devise sets of spatially differentiated measures to carry out a demographic policy and to regulate the sociodemographic evolution of regions." 相似文献
732.
Recent Soviet research on commuting in the USSR is reviewed in the light of the General Strategy for a System of Settlement in the USSR, adopted in 1976. It is noted that rural-urban commuting to work has grown rapidly since 1975, particularly in the European parts of the USSR, and that most commuters prefer to live in rural or suburban areas within easy access of an urban center rather than in the central city itself. 相似文献
733.
Determinants and characteristics of female migration: a case study of Salem City in Tamil Nadu,India
Nangia S Samuel MJ 《Population geography : a journal of the Association of Population Geographers of India》1983,5(1-2):34-43
A random sample of 521 male and 481 female migrants surveyed in Salem city in Tamil Nadu, India, during 1980-1981 was analyzed on the basis of a classification related to reasons (association, marriage, distress, and voluntary) for migration. Women were found to outnumber males in associational migration, both in rural to urban and urban to urban streams. They also formed almost all the marriage migrants. They were more numerous in distress migration as well. There were, however, comparatively few women among voluntary migrants. The incidence of illiteracy among migrant women was generally high, except for voluntary migrants who had completed high school. The share of working women was not negligible, but in most cases present work status was achieved after migrating. 相似文献
734.
G. A. Pellegatti Franco 《Astrophysics and Space Science》1983,96(1):195-203
Analysis of observational data of OB stars show an, excellent agreement of the density distributions in space ?(x, y, z) as well as in velocity space \(\rho (\dot x,\dot y,\dot z)\) with the predictions of the density wave theory, the values for the density and velocity fluctuations are explained only by the non-linear theory. These theoretical calculations predict perturbations greater than ±10 km s?1, consistent with the observations for the velocity field. Thus one should disregard analytical treatments of the linearized equations since they predict maximum perturbations of ±5km s?1. Another consequence of this is the fact that the Gould's Belt is not a local anomaly, but a local feature of the density waves. The analysis of observational data show that the wave pattern is similar to that of the gas and dust. 相似文献
735.
Absolute spectrophotometry of Neptune from 3390 to 7800 Å, with spectral resolution of 10 Å in the interval 3390–6055 and 20 Å in the interval 6055–7800 Å, is reported. The results are compared with filter photometry (Appleby, 1973; Wamsteker, 1973; Savage et al., 1980) and with synthetic spectra computed on the basis of a parameterization proposed by Podolak and Danielson (1977) for aerosol scattering and absorption. A CH4/H2 ratio of is derived for the convectively mixed part of Neptune's atmosphere, and constrains optical properties of hypothetical aerosol layers. 相似文献
736.
High-velocity impacts of interplanetary meteoroids on Saturn's rings are discussed. It is shown that the neutral gas emitted by impact vaporization may be responsible, to a large part, for the observed neutral ring atmosphere. Both the predicted neutral gas injection rate and the gas temperature (or kinetic energy) are compatible with the measurements (see Broadfoot, A. L., B. R. Sandel, D. E. Shemansky, J. B. Holberg, G. R. Smith, D. F. Strobel, J. C. McConnell, S. Kumar, D. M. Hunten, S. K. Atreya, T. M. Dohnahne, H. W. Moos, J. L. Bertaux, J. E. Blamont, R. B. Pomphrey, and S. Linik, Science212, 206–211, 1981). Heavy ejecta particles produce a particulate ring “halo”. The physical properties of this halo are calculated, and it appears to be identical with the tenous particle population discussed by Baum and Kreidl (1982). Erosion of Saturn's ring particles, the resulting mass balance, and regolith formation are estimated. This provides some constraints on surface properties and optical albedo. 相似文献
737.
Exact solutions have been obtained for a massive fluid sphere under the extreme causality condition (dP/dρ)=1. Radial pulsational stability of these structures has been discussed. It is found that for pulsationally stable configurations the surface to central density ratio is greater than 0.30, the maximum values for surface and central redshifts are 0.85 and 3.40 respectively in the extreme case, and the maximum mass and size are respectively 4.8M ⊙ and 20.1 km. It has also been shown that these structures are gravitationally bound, with a maximum binding energy per unit rest mass equal to 0.25 for a surface to central density ratio ?0.40. Slow rotation of these configurations has also been considered, and the relative drag and moment of inertia have been calculated. These results have been applied to the Crab pulsar and the mass of the pulsar has also been calculated based upon this model. 相似文献
738.
It is shown that emission lines, first of all the ultraviolet resonance doublet 2800 Mgii, can be used to decipher the internal dynamic structure of QSOs and active galactic nuclei. The electron temperatures of the emission regions of these objects are determined. A hypothesis is proposed explaining the observed broad profiles of the 2800 Mgii line as a summary effect of emission clouds with a Maxwellian velocity distribution. The main physical parameters of emission clouds in QSOs are obtained. A mechanism of formation of emission clouds is suggested: namely, tidal disruption of stars near to a massive compact object. 相似文献
739.
We establish limits on the total radiant energy of solar flares during the period 1980 February – November, using the solar-constant monitor (ACRIM) on board the Solar Maximum Mission. Typical limits amount to 6 × 1029 erg/s for a 32-second integration time, with 5σ statistical significance, for an impulsive emission; for a gradual component, about 4 × 1032 ergs total radiant energy. The limits lie about an order of magnitude higher than the total radiant energy estimated from the various known emission components, suggesting that no heretofore unknown dominant component of flare radiation exists. 相似文献
740.
It is proposed that the solar flare phenomenon can be understood as a manifestation of the electrodynamic coupling process of the photosphere-chromosphere-corona system as a whole. The system is coupled by electric currents, flowing along (both upward and downward) and across the magnetic field lines, powered by the dynamo process driven by the neutral wind in the photosphere and the lower chromosphere. A self-consistent formulation of the proposed coupling system is given. It is shown in particular that the coupling system can generate and dissipate the power of 1029 erg s#X2212;1 and the total energy of 1032 erg during a typical life time (103 s) of solar flares. The energy consumptions include Joule heat production, acceleration of current-carrying particles along field lines, magnetic energy storage and kinetic energy of plasma convection. The particle acceleration arises from the development of field-aligned potential drops of 10–150 kV due to the loss-cone constriction effect along the upward field-aligned currents, causing optical, X-ray and radio emissions. The total number of precipitating electrons during a flare is shown to be of order 1037–1038. 相似文献