全文获取类型
收费全文 | 140038篇 |
免费 | 2329篇 |
国内免费 | 1088篇 |
专业分类
测绘学 | 3448篇 |
大气科学 | 9553篇 |
地球物理 | 27283篇 |
地质学 | 51411篇 |
海洋学 | 12318篇 |
天文学 | 30982篇 |
综合类 | 473篇 |
自然地理 | 7987篇 |
出版年
2022年 | 789篇 |
2021年 | 1391篇 |
2020年 | 1538篇 |
2019年 | 1667篇 |
2018年 | 4760篇 |
2017年 | 4403篇 |
2016年 | 4663篇 |
2015年 | 2249篇 |
2014年 | 3905篇 |
2013年 | 6923篇 |
2012年 | 4638篇 |
2011年 | 6830篇 |
2010年 | 5957篇 |
2009年 | 7467篇 |
2008年 | 6559篇 |
2007年 | 6825篇 |
2006年 | 5214篇 |
2005年 | 4135篇 |
2004年 | 4015篇 |
2003年 | 3831篇 |
2002年 | 3585篇 |
2001年 | 3135篇 |
2000年 | 3042篇 |
1999年 | 2429篇 |
1998年 | 2547篇 |
1997年 | 2351篇 |
1996年 | 2021篇 |
1995年 | 2019篇 |
1994年 | 1723篇 |
1993年 | 1610篇 |
1992年 | 1528篇 |
1991年 | 1483篇 |
1990年 | 1558篇 |
1989年 | 1348篇 |
1988年 | 1227篇 |
1987年 | 1478篇 |
1986年 | 1273篇 |
1985年 | 1567篇 |
1984年 | 1755篇 |
1983年 | 1682篇 |
1982年 | 1576篇 |
1981年 | 1428篇 |
1980年 | 1317篇 |
1979年 | 1238篇 |
1978年 | 1201篇 |
1977年 | 1050篇 |
1976年 | 1037篇 |
1975年 | 985篇 |
1974年 | 989篇 |
1973年 | 1030篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
871.
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
872.
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
873.
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
874.
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
875.
R. A. Serafin 《Celestial Mechanics and Dynamical Astronomy》1985,36(3):273-280
We consider the collision probability for comets with the Sun under the suppositions of different velocity distributions and various initial conditions. We solve the problem applying Laplace's method and using Schiaparelli's hyperboloid of visibility. The probabilities obtained in this manner are given separately for elliptic and hyperbolic orbits. 相似文献
876.
C. Baixeras M. Casas F. Fernandez A. Vidal-Quadras M. Ortega 《Astrophysics and Space Science》1983,96(2):321-331
Elemental abundances of the VH group of cosmic radiation have been measured in the energy interval 250–550 MeV nucl–1 in a balloon exposure at Sioux Falls (South Dakota) of a plastic detector LeXAN stack. The so obtained abundances have been extrapolated to the sources in the frame of the homogeneous model correcting for energy loss. After taking into account solar modulation, the best fit to model values has led to a escape mean free path e = 5E
–0.4 g cm–2, whereE is the energy in GeV nucl–1, forE>1 GeV nucl–1, and a constant e = 5 g cm–2 forE1 GeV nucl–1. When turning to the diffusion model, also including an energy loss term, a diffusion coefficientD=3×1028 cm2 s–1 has been estimated. 相似文献
877.
Angle-averaged partial frequency redistributionR II has been employed in obtaining a simultaneous solution of radiative transfer equation in the comoving frame and the statistical equilibrium equation for a non-LTE two level atom. We have obtained the ratios of population densities of the upper and lower levels of the resonance line of PV by utilizing the data given in Bernacca and Bianchi (1979). Line source functions are also obtained for different types of variations of density and velocity of the expanding gases. We have considered the atmosphere to be 11 times as thick as the stellar radius. The first iteration was started by putting the density of the upper level (N 2) equal to zero. However, the convergent solution shows a substantial increase inN 2 although it is still much less than the equilibrium value. The line source function and the ratio of the densities of the particles in the upper and lower levels fall sharply from a maximum at τ=τmax to minimum at τ=0. We have studied the scattering integral \(\int {_{ - \infty }^{ + \infty } J_x \phi _x } dx\) and found that this quantity also varies quite similar to the ratioN 2/N 1 and the line source functionS L. 相似文献
878.
The effect of the Coriolis force on the Kelvin-Helmholtz instability has been investigated, in which basic velocities of fluid are varying slowly with lateral coordinatey. The problem is solved by J.W.K.B. approximation method. Parabolic and other types of profiles for wide long channel flow have also been studied in detail. 相似文献
879.
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
880.
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