全文获取类型
收费全文 | 7835篇 |
免费 | 208篇 |
国内免费 | 311篇 |
专业分类
测绘学 | 213篇 |
大气科学 | 608篇 |
地球物理 | 1652篇 |
地质学 | 2932篇 |
海洋学 | 651篇 |
天文学 | 1851篇 |
综合类 | 30篇 |
自然地理 | 417篇 |
出版年
2023年 | 42篇 |
2022年 | 54篇 |
2021年 | 58篇 |
2020年 | 67篇 |
2019年 | 83篇 |
2018年 | 239篇 |
2017年 | 192篇 |
2016年 | 284篇 |
2015年 | 168篇 |
2014年 | 263篇 |
2013年 | 419篇 |
2012年 | 247篇 |
2011年 | 417篇 |
2010年 | 293篇 |
2009年 | 465篇 |
2008年 | 370篇 |
2007年 | 335篇 |
2006年 | 344篇 |
2005年 | 310篇 |
2004年 | 289篇 |
2003年 | 275篇 |
2002年 | 252篇 |
2001年 | 219篇 |
2000年 | 206篇 |
1999年 | 179篇 |
1998年 | 170篇 |
1997年 | 163篇 |
1996年 | 154篇 |
1995年 | 136篇 |
1994年 | 114篇 |
1993年 | 94篇 |
1992年 | 78篇 |
1991年 | 87篇 |
1990年 | 77篇 |
1989年 | 84篇 |
1988年 | 59篇 |
1987年 | 101篇 |
1986年 | 65篇 |
1985年 | 64篇 |
1984年 | 69篇 |
1983年 | 71篇 |
1982年 | 72篇 |
1981年 | 68篇 |
1980年 | 53篇 |
1979年 | 51篇 |
1977年 | 48篇 |
1976年 | 47篇 |
1975年 | 35篇 |
1974年 | 30篇 |
1973年 | 41篇 |
排序方式: 共有8354条查询结果,搜索用时 15 毫秒
151.
A great sunspot group of October 1991 (NOAA 6891 atL = 189,B = –10) was observed, in which an eruptive flare of importance (X2.5/31B) has been reported. A Caii-line 3933 filter was used, and during a time interval of 3 hours more than 90 frames were taken, using a CCD camera. The progress and the distribution in size of the two main flare points have been obtained; they covered an area of about 1800 millionths of the solar disk within a time interval of about 24 min. The average apparent horizontal velocity was 70 km s–1. 相似文献
152.
We derive a time series of two-dimensional velocity fields for a flare region on 1992 December 16, based on the asymmetries of the H line. The H spectra were obtained by an imaging spectrograph at the Solar Tower Telescope of Nanjing University. Four sites with evident chromospheric downflows are found to appear and decay consecutively in the studied region. The value of maximum velocities is 30–40 km s–1 and the lifetime of downflows is 2–3 min at these sites. It is also shown that the asymmetries only exist at the line wing, while the line center has nearly no shifts for this flare. Finally, we make a discussion on the characteristics of the velocity distribution and its correlations with the intensity distribution, as well as with the hard X-ray emission. 相似文献
153.
By use of the dispersion equation given by Song, Wu, and Dryer (1987) for a cylinder plasma with mass motion and gravity included, we investigate the linear current instabilities developed in loop prominences. The results indicate that the mode of linear instability depends mainly on whetherv
s
2
> or not, wherev
s
is the sonic velocity at heightz, =GM/(R +z) is the gravity potential,G the gravitational constant,M andR the mass and the radius of the Sun respectively. Ifv
s
2
> , then the sausage instability will be dominant. Otherwise, the kink instability will be more important. A possible explanation of knot structure, which appears sometimes in solar loop prominences has been given. 相似文献
154.
A. Brahic J. Breton J. Caubel A. Cazenave P. Cruvellier Y. Dupuis B. Lago J.F. Minster A. Perret A. Scribot 《Icarus》1979,40(3):423-433
Specific information on the surface morphology, composition, mean density, and internal structure of asteroids, which is necessary to advance our understanding of asteroids, can be obtained only by a detailed investigation of individual bodies: this will require space missions to individual targets. Since an essential characteristic of the asteroids is their variety, several objects must be visited. The Ariane launcher developed presently in Europe makes a multiple flyby mission possible. The first results of our feasibility study are particularly encouraging: during one revolution, five to six preselected main belt asteroids may be approached to within 1000 km with relative velocities which lie between 3 and 14 km/sec using a total impulse correction on the order of 1 to 2 km/sec. The weight of the spacecraft, excluding the engine and the propellant, would be at least 250 kg. This allows a scientific payload of 50 to 60 kg, in which priority will be given to an imaging system and radar altimeter. 相似文献
155.
This paper reports the study concerning the latitudinal dispalacement of the auroral oval as a function of the northward orientation of the Bz-component IMF and the relation between southward Bz and the auroral dynamics in the night sector. 相似文献
156.
157.
158.
A one-dimensional model for thinning of the plasma sheet is developed on the basis of launching a fast mode MHD rarefaction wave propagating in the tailward direction along the plasma sheet. Behind the rarefaction wave the pressure is reduced, leading to thinning of the plasma sheet and also to an Earthward plasma flow with a speed on the order of the sound speed a0. The plasma sheet thickness is reduced by a factor of 2 if an Earthward plasma flow speed of 0.8a0 is induced. The predictions of the model are in reasonable agreement with observations. 相似文献
159.
The propagation and interference of Alfvén waves in magnetic regions is studied. A multilayer approximation of the standard models of the solar atmosphere is used. In each layer, there is a linear law of temperature variation and a power law of Alfvén velocity variation. The analytical solutions of a wave equation are stitched at the layer boundaries. The low-frequency Alfvén waves (P > 1 s) are able to transfer the energy from sunspots into the corona by tunneling only. The chromosphere is not a resonance filter for the Alfvén waves. The interference and resonance of Alfvén waves are found to be important to wave propagation through the magnetic coronal arches. The transmission coefficient of Alfvén waves into the corona increases sharply on the resonance frequences. To take into account the wave absorption in the corona, a method of equivalent schemes is developed. The heating of a coronal arch by Alfvén waves is discussed. 相似文献
160.
Sawant SB Khan YS 《Population geography : a journal of the Association of Population Geographers of India》1982,4(1-2):67-75
This paper analyzes the impact of various factors on variations in the rate of natural increase of population in Western Maharashtra, India. By using district level data, coefficients of correlation have been calculated between the rate of natural increase, the birth rate and the death rate on the 1 hand and different factors affecting them on the other. The analysis shows that in Western Maharashtra the birth rate is high in response to the high death rate and the high infant mortality rate. It is also revealed that the rate of natural increase of the population declines with an increase in the proportion of working females as well as the proportion of educated males. All this signifies that the region under study is in the 2nd stage of the "demographic transition." 相似文献