全文获取类型
收费全文 | 67807篇 |
免费 | 1069篇 |
国内免费 | 494篇 |
专业分类
测绘学 | 1714篇 |
大气科学 | 4709篇 |
地球物理 | 13484篇 |
地质学 | 24076篇 |
海洋学 | 5800篇 |
天文学 | 15712篇 |
综合类 | 170篇 |
自然地理 | 3705篇 |
出版年
2021年 | 625篇 |
2020年 | 692篇 |
2019年 | 684篇 |
2018年 | 1644篇 |
2017年 | 1524篇 |
2016年 | 1851篇 |
2015年 | 1044篇 |
2014年 | 1793篇 |
2013年 | 3466篇 |
2012年 | 2042篇 |
2011年 | 2698篇 |
2010年 | 2391篇 |
2009年 | 3234篇 |
2008年 | 2716篇 |
2007年 | 2714篇 |
2006年 | 2605篇 |
2005年 | 1931篇 |
2004年 | 1921篇 |
2003年 | 1804篇 |
2002年 | 1748篇 |
2001年 | 1583篇 |
2000年 | 1521篇 |
1999年 | 1256篇 |
1998年 | 1263篇 |
1997年 | 1300篇 |
1996年 | 1076篇 |
1995年 | 1091篇 |
1994年 | 1011篇 |
1993年 | 849篇 |
1992年 | 830篇 |
1991年 | 802篇 |
1990年 | 891篇 |
1989年 | 792篇 |
1988年 | 746篇 |
1987年 | 886篇 |
1986年 | 763篇 |
1985年 | 976篇 |
1984年 | 1036篇 |
1983年 | 1019篇 |
1982年 | 923篇 |
1981年 | 909篇 |
1980年 | 840篇 |
1979年 | 762篇 |
1978年 | 726篇 |
1977年 | 686篇 |
1976年 | 647篇 |
1975年 | 639篇 |
1974年 | 633篇 |
1973年 | 641篇 |
1971年 | 405篇 |
排序方式: 共有10000条查询结果,搜索用时 750 毫秒
361.
362.
Electrons accelerated during solar flares are revealed by their electromagnetic radiation in different spectral ranges, emitted at different heights in the solar atmosphere. The observational analysis points to a common and continuous injection of particles. Based on this result, a quantitative investigation of the hard X-ray and microwave emissions observed during the 29 June, 1980 flare at 11: 40 UT has been performed. This is the first modelisation that takes into account both the inhomogeneity of the microwave source region and the dynamical evolution of the electron population. First results of our model computations demonstrate that during the most energetic phase of the event both hard X-rays and microwaves are described by electron populations resulting from the same injection function, and that the total numbers of electrons required for both emissions are compatible. Account for the inhomogeneity of the microwave source is shown to be a necessary condition for the interpretation of observed spectra.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985. 相似文献
363.
G. Palladino A. Basili G. Di Cocco T. Franceschini G. Landini S. Silvestri A. Barbini M. Galimberti L. A. Gizzi 《Experimental Astronomy》2006,21(3):169-187
This paper describes the design of a star sensor based upon a high dynamic range CCD in order to reach an arcsec-level attitude
determination in balloon-borne missions. A custom star identification software was developed and laboratory-tested on a prototype
assembled using commercial components. A set of numerical simulations have been carried out to study the dependence on the
pointing precision of the centroid position accuracy, the number of detected stars and the effect of the image focusing. Moreover,
the role of the electronic noise and the discrete pixel structure on the light signals is identified by the analysis of numerical
simulations. Laboratory tests confirm that the arcsec pointing accuracy with a 1 Hz update rate can be achieved with our combination
of custom-developed software and selected hardware components. 相似文献
364.
365.
Anita Joshi 《Solar physics》2001,198(1):149-161
The correlation between the presence of coronal holes and flare indices has been investigated for the period from 1976 to 1995. The analysis shows that in the cases of 227 Carrington rotations (CRs) backward time lags yield the highest correlation between the coronal holes and flare indices. The maximum correlations were found at time lags of 222 and 142 CRs for polar and equatorial coronal holes, respectively. The period of study covers the past two solar cycles (21 and 22). Correlation analysis of both solar cycles has also been studied individually. The correlation analysis reveals that there is in general a forward shift in the maximum correlation for polar coronal holes, but it cannot be recommended to use polar coronal hole numbers for forecasting the next solar cycle. 相似文献
366.
M. Guainazzi F. Fiore G. Matt G.C. Perola 《Monthly notices of the Royal Astronomical Society》2001,327(1):323-328
We have studied the correlation among X-ray absorption, optical reddening and nuclear dust morphology in Seyfert 2 galaxies. Two main conclusions emerge: (i) the Balmer decrement and the amount of X-ray absorption are anticorrelated over a wide range of column density, – the correlation no longer applies to Compton-thick objects , although they span a comparable range in Balmer decrement; (ii) Compton-thin Seyfert 2s seem to prefer nuclear environments, which are rich in dust on scales of hundreds of parsecs. On the other hand, Compton-thick Seyferts indifferently exhibit 'dust-poor' and 'dust-rich' environments. These results support an extension of the Seyfert unification scenario (as recently proposed by Matt ), where Compton-thick Seyfert 2s are observed through compact 'torii', whereas Compton-thin ones are obscured by dust on much larger scales. 相似文献
367.
Based on our 1997 observations with a CCD camera and narrow-band filters attached to the 1-m Assy Obsev vatory telescope, we extensively study the latitudinal variations in limb darkening and normal albedo on Jupiter's disk at wavelengths of 387, 445, 502, 619, and 702 nm. In addition, we carried out observations in 1998 with broad-band red, green, and blue filters. Apart from a general regularity—the increase in darkening coefficient with normal albedo of Jupiter's cloud cover—there is an appreciable scatter of darkening coefficients for the same albedo, which is most pronounced in the ultraviolet. This scatter may result from differences in the optical depth of the aerosol haze above the clouds. The lack of any wavelength dependence of the darkening coefficients is confirmed for Jupiter's polar regions, while at other latitudes, they decrease with decreasing wavelength. 相似文献
368.
Bjerrum-Niese C. Bjorno L. Pinto M.A. Quellec B. 《Oceanic Engineering, IEEE Journal of》1996,21(2):143-149
The paper discusses the development of a simulation tool to model high data-rate acoustic communication in shallow water. The simulation tool is able to generate synthetic time series of signals received at a transducer array after transmission across a shallow-water communication channel. The simulation tool is suitable for testing advanced signal processing techniques for message recovery. A channel model has been developed based on the physical aspects of the acoustic channel. Special emphasis has been given to fluctuations of the signal transmission caused by time-varying multipath effects. At shorter ranges, the temporal variations are dominated by acoustic scattering from the moving sea surface. Therefore, the channel model produces a coherence function which may be interpreted as a time-varying reflection coefficient for the surface scattered acoustical path. A static, range-independent ray model identifies the significant multipaths, and the surface path is modulated with the time-varying reflection coefficient. The advantages and limitations of the channel model are discussed and assumptions necessary to overcome the limitations are emphasised. Based on the assumptions, an algorithm has been developed and implemented to model how a binary message will be modulated when transmitted by a transducer, is distorted in the channel and finally is received by a transducer array 相似文献
369.