全文获取类型
收费全文 | 28087篇 |
免费 | 4055篇 |
国内免费 | 5741篇 |
专业分类
测绘学 | 3292篇 |
大气科学 | 4670篇 |
地球物理 | 6886篇 |
地质学 | 11209篇 |
海洋学 | 3509篇 |
天文学 | 2301篇 |
综合类 | 1860篇 |
自然地理 | 4156篇 |
出版年
2024年 | 111篇 |
2023年 | 301篇 |
2022年 | 757篇 |
2021年 | 971篇 |
2020年 | 1113篇 |
2019年 | 1384篇 |
2018年 | 947篇 |
2017年 | 1248篇 |
2016年 | 1244篇 |
2015年 | 1351篇 |
2014年 | 1709篇 |
2013年 | 1979篇 |
2012年 | 1695篇 |
2011年 | 1867篇 |
2010年 | 1432篇 |
2009年 | 1958篇 |
2008年 | 1918篇 |
2007年 | 2034篇 |
2006年 | 1936篇 |
2005年 | 1586篇 |
2004年 | 1461篇 |
2003年 | 1221篇 |
2002年 | 980篇 |
2001年 | 862篇 |
2000年 | 762篇 |
1999年 | 713篇 |
1998年 | 703篇 |
1997年 | 596篇 |
1996年 | 502篇 |
1995年 | 459篇 |
1994年 | 381篇 |
1993年 | 366篇 |
1992年 | 262篇 |
1991年 | 211篇 |
1990年 | 160篇 |
1989年 | 152篇 |
1988年 | 119篇 |
1987年 | 66篇 |
1986年 | 63篇 |
1985年 | 54篇 |
1984年 | 32篇 |
1983年 | 15篇 |
1982年 | 26篇 |
1981年 | 21篇 |
1980年 | 25篇 |
1978年 | 15篇 |
1977年 | 20篇 |
1976年 | 25篇 |
1973年 | 17篇 |
1971年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
Z. Q. Qu C. L. Xu X. Y. Zhang X. L. Yan C. L. Jin 《Monthly notices of the Royal Astronomical Society》2006,370(4):1790-1796
The influence of magnetic fields on the energy level populations of atoms is firstly studied by analysing the Stokes profiles of Fe i 6302.5 forming in the solar magnetized atmosphere, with the aid of a departure factor defined to evaluate the deviation from the normal Boltzmann distribution without a magnetic field. This factor is directly related to the ratio of line-source function to the continuum one. The relationship between the departure and the magnetic field reveals an effect that the magnetic field induces an exponential increase in the level population of the lower level of Fe i 6302.5 (Landé factor g = 2.5 ) with the field strength. This indicates that the magnetic field can cause the redistribution of populations of those levels whose Landé factors are non-zero. Therefore, this effect should be included in the calculation of the statistical equilibrium. Secondly, an experiment utilizing the Hg5461 line in the laboratory on the Earth is carried out to reveal that the exponential relation is independent of variations in temperature, and the excitation is completely magneto-induced. Finally, the exponential relation is explained by taking account of the magnetic energy in the Boltzmann distribution. 相似文献
103.
104.
本文通过数值求解带电粒子与Alfven波湍动相互作用的动力学方程,得到了相对论电子在射电喷流中被加速随时间演化的解.高能电子可以加速到Lorentz因子γy~106,且形成稳态的幂律谱,尽管其谱指数S≈l比观测值小,但粒子加速时间约为1012-1014秒,小于射电斑的寿命107年.粒子能谱指数几乎与Alfven波谱指数和能量损失函数无关.能量损失对加速上限有较大影响. 相似文献
105.
山区短波反射辐射的计算模式 总被引:2,自引:0,他引:2
本文提出计算山区测点可接受到的来自周围山地的短波反射辐射日平均通量密度的通用计算机模式,并以大别山南段赵公岭山区为例进行了试验,结果表明,该式通用性强、精度较高。 相似文献
106.
107.
C. S. Crawford A. C. Fabian P. Gandhi R. J. Wilman R. M. Johnstone 《Monthly notices of the Royal Astronomical Society》2001,324(2):427-442
We present observations of a sample of optically faint, hard X-ray sources of the kind likely to be responsible for much of the hard X-ray background. We confirm that such sources are easily detected in the near-infrared, and find that they have a featureless continuum suggesting that the active nucleus is heavily obscured. The infrared colours of the majority of the targets observed are consistent with absorbed elliptical host galaxies at z =1–2. It is likely that we are observing some of the brighter members of the important new class of X-ray type II quasars. 相似文献
108.
Using a set of compilations of measurements for extragalactic radio sources, we construct all-sky maps of the Faraday rotation produced by the Galactic magnetic field. In order to generate the maps, we treat the radio source positions as a kind of 'mask' and construct combinations of spherical harmonic modes that are orthogonal on the masked sky. As long as relatively small multipoles are used, the resulting maps are quite stable to changes in the selection criteria for the sources, and show clearly the structure of the local Galactic magnetic field. We also suggest the use of these maps as templates for cosmic microwave background (CMB) foreground analysis, illustrating the idea with a cross-correlation analysis between the Wilkinson Microwave Anisotropy Probe ( WMAP ) data and our maps. We find a significant cross-correlation, indicating the presence of a significant residual contamination. 相似文献
109.
110.
《The Professional geographer》1987,39(1):85-94
Reviews of geographic software in this article: DEMO-GRAPHICS: WORLD POPULATIONS AND PROJECTIONS. ESP GAUSS. CEMODEL S. Damus LIMDEP. William H. Greene MICROSTAT 4.1 OTIS PCIPS. (Personal Computer Image Processing System) . H.J. Meyers and R. Bernstein. REGRESSION ANALYSIS OF TIME SERIES (RATS) SPSS/PC+ URBAN DATA MANAGEMENT SOFTWARE (UDMS) 相似文献