全文获取类型
收费全文 | 70959篇 |
免费 | 1398篇 |
国内免费 | 658篇 |
专业分类
测绘学 | 1714篇 |
大气科学 | 5029篇 |
地球物理 | 14214篇 |
地质学 | 25227篇 |
海洋学 | 6164篇 |
天文学 | 16332篇 |
综合类 | 208篇 |
自然地理 | 4127篇 |
出版年
2022年 | 396篇 |
2021年 | 704篇 |
2020年 | 777篇 |
2019年 | 825篇 |
2018年 | 1893篇 |
2017年 | 1759篇 |
2016年 | 2235篇 |
2015年 | 1318篇 |
2014年 | 2176篇 |
2013年 | 3766篇 |
2012年 | 2276篇 |
2011年 | 3118篇 |
2010年 | 2605篇 |
2009年 | 3504篇 |
2008年 | 3254篇 |
2007年 | 3045篇 |
2006年 | 2876篇 |
2005年 | 2353篇 |
2004年 | 2253篇 |
2003年 | 2110篇 |
2002年 | 1927篇 |
2001年 | 1800篇 |
2000年 | 1716篇 |
1999年 | 1377篇 |
1998年 | 1470篇 |
1997年 | 1387篇 |
1996年 | 1100篇 |
1995年 | 1142篇 |
1994年 | 962篇 |
1993年 | 872篇 |
1992年 | 855篇 |
1991年 | 759篇 |
1990年 | 856篇 |
1989年 | 717篇 |
1988年 | 650篇 |
1987年 | 814篇 |
1986年 | 663篇 |
1985年 | 855篇 |
1984年 | 922篇 |
1983年 | 862篇 |
1982年 | 831篇 |
1981年 | 707篇 |
1980年 | 663篇 |
1979年 | 606篇 |
1978年 | 603篇 |
1977年 | 552篇 |
1976年 | 539篇 |
1975年 | 499篇 |
1974年 | 506篇 |
1973年 | 468篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
331.
In this paper the unsteady laminar free-convection flow of a viscous incompressible fluid, past an accelerated infinite vertical porous plate subjected to a constant suction (or injection) in considered. Numerical results for the skin-friction on the plate are obtained for the class of accelerated motions whose velocity is of the formU
0
t
n
wheret is time,U
0 a constant, andn is a positive integer. The skin friction tends to zero with increasingt when the Grashof number Gr=2, the Prandtl number =1,n=0, and >0 which corresponds to suction.On leave of absence from the Department of Mathematics, University of Dhaka, Bangladesh.On leave from absence from the Department of Mathematics, University of Dar-es-Salaam, Tanzania. 相似文献
332.
M. Golay N. Cramer D. Huguenin B. Nicolet A. Blecha 《Astrophysics and Space Science》1985,109(1):191-212
The present paper is the introduction to a systematic analysis of 123 6-degree fields near the galactic plane, recorded in the medium ultraviolet (2000 Å) by the ballon-borne experiment SCAP 2000 of the Laboratoire d'Astronomie Spatiale, Marseille, and Geneva Observatory. The available data are presented and the general properties of the images are briefly discussed.It is shown that the high selectivity of the UV passband regarding spectral type, together with the strong interstellar extinction at that wavelength, provide the necessary conditions for an efficient application of Wolf's method to study the distribution of interstellar matter in the solar neighbourhood. The results of a fast analysis of the available data are presented here. 相似文献
333.
The continuum emission of an X1 flare on 26 March, 1970 observed close to the solar limb (N 05 E 64) was analyzed by a photometric determination of the contrast I()/I
0() in the wavelength range 3558–5920 Å. Two possible mechanisms for the emission were investigated, namely hydrogen Paschen and H– continua. We show the unlikeness of the Paschen possibility and derive strong constraints on the temperature structure and energy deposition mechanism imposed by the H– continuum process.We conclude that the emission must have originated in deep atmospheric layers. The relevance of this result with respect to flare energy release and transport processes is also discussed. 相似文献
334.
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
335.
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
336.
The solar causes of geomagnetic disturbances 总被引:1,自引:0,他引:1
Geomagnetic disturbances have been identified with respect to their sources for 1977–1983. The disturbance level was found using the daily planetary index A
p. High-amplitude ( 50), mean-amplitude (24) and low-amplitude ( 12) disturbances are caused by solar flares of importance 1, coronal holes, and filament cavities, respectively. The ranges of probable amplitudes of disturbances of different nature and their relative number are found from Poisson random distributions of amplitudes. 相似文献
337.
Using observations obtained with the Clark Lake radioheliograph we determined the diameter of the Sun in the decameter wavelength range. Both equatorial and polar diameters increase with decreasing frequency, as D=Af
. The eccentricity of the brightness distribution appears to remain constant in the frequency range (30–74 MHz) in good agreement with the optical results in a corresponding height range. The smaller size of the polar diameter is attributed to coronal holes covering the poles during the period of our observations, while streamers were observed at the equator most of the time. 相似文献
338.
The insulating effect of an evolving dust mantle is examined. The role of this mantle in determining the surface temperature of the ice core is studied as a function of the mass fraction of the dust in the ice-dust mixture and the thermal conductivity of the nucleus. Using the so-called “looselattice” model of D.A. Mendis and G.D. Brin (1977, Moon17, 359–372) (which was also extended to include cracks and pores in the mantle), it was found that both high dust to ice ratios and high core conductivities inhibit mantle blowoff. Indeed, it is often possible to build an essentially permanent dust mantle around an ice nucleus, so that the nucleus will take on an asteroidal appearance. 相似文献
339.
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
340.
Foggin PM 《Singapore journal of tropical geography》1983,4(1):11-24
This paper focuses on whether there is an organized structural pattern in the spatial distribution of oral contraceptive usage in rural Thailand and, if so, how this spatial pattern is linked to other related socioeconomic variables. The data concern the year 1975. The results suggest that the location of the health services infrastructure seems to have the most significant effect on the differences in level of oral contraceptive use observed. 相似文献