全文获取类型
收费全文 | 22174篇 |
免费 | 3049篇 |
国内免费 | 4745篇 |
专业分类
测绘学 | 1980篇 |
大气科学 | 3142篇 |
地球物理 | 4324篇 |
地质学 | 11793篇 |
海洋学 | 3145篇 |
天文学 | 1901篇 |
综合类 | 1411篇 |
自然地理 | 2272篇 |
出版年
2024年 | 165篇 |
2023年 | 488篇 |
2022年 | 1081篇 |
2021年 | 1324篇 |
2020年 | 1170篇 |
2019年 | 1252篇 |
2018年 | 1396篇 |
2017年 | 1330篇 |
2016年 | 1329篇 |
2015年 | 1122篇 |
2014年 | 1303篇 |
2013年 | 1477篇 |
2012年 | 1445篇 |
2011年 | 1522篇 |
2010年 | 1409篇 |
2009年 | 1338篇 |
2008年 | 1301篇 |
2007年 | 1324篇 |
2006年 | 1225篇 |
2005年 | 713篇 |
2004年 | 628篇 |
2003年 | 592篇 |
2002年 | 723篇 |
2001年 | 645篇 |
2000年 | 478篇 |
1999年 | 416篇 |
1998年 | 302篇 |
1997年 | 321篇 |
1996年 | 271篇 |
1995年 | 261篇 |
1994年 | 240篇 |
1993年 | 194篇 |
1992年 | 169篇 |
1991年 | 117篇 |
1990年 | 104篇 |
1989年 | 112篇 |
1988年 | 93篇 |
1987年 | 66篇 |
1986年 | 58篇 |
1985年 | 50篇 |
1984年 | 42篇 |
1983年 | 45篇 |
1982年 | 47篇 |
1981年 | 29篇 |
1980年 | 24篇 |
1979年 | 30篇 |
1978年 | 19篇 |
1976年 | 18篇 |
1975年 | 26篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
33.
V. Yu. Terebizh 《Astrophysics and Space Science》1992,193(2):269-288
Image restoration, computerized tomography, and other similar problems are considered as a unified class of stochastic inverse problems. The conventional approach to these problems that proceeds from some integral or functional equations suffers from three main shortcomings: (i) subjectivity, (ii) inability to account for the inner (radiational) noise, and (iii) inability to include the fundamental concept of the natural limit of solution accuracy. A general approach is developed, the Statistical Parameterization of Inverse Problems (SPIPR), that takes into account both the inner and external random noise and gives an explicit form of the above-mentioned natural limit. Applications of the SPIPR to various problems show that the maximum likelihood method as the concrete way to obtain an object estimate has practically limiting efficiency.Two new fields of applications of the SPIPR are outlined along with the image restoration problem: the elimination of blurring due to atmosphere turbulence and reconstruction of an object structure in the computerized tomography. The expressions for the main distribution function in all these problems are found. The corresponding real examples and model cases are considered as well. 相似文献
34.
Natural and agricultural wetlands are considered to be the major sources of global atmospheric methane (CH4). A one‐dimensional model was developed to simulate methane emission and used to examine the influence of various physical processes on the rate of methane emission. Three processes involved in the methane emission are implemented in the model: production, reoxidation and transport. Three transport pathways were considered: diffusion across water–air or soil–air interfaces, ebullition and diffusion through plants. These pathways are influenced by soil properties, plant growth, water‐table conditions, temperature and external inputs (e.g. fertilizer). The model was used to examine the seasonal variation of the methane emission at a rice field in Hunan, China, which was observed during a field experiment for consecutive (early and late) rice seasons in 1992. The observed seasonal variations of methane emission, and role of plants in transporting methane to the atmosphere, are captured by the model simulation. Further model applications were conducted to simulate effects of fertilizer and water‐level condition on the methane emission. The results indicate that unfermented organic fertilizer produces a higher methane emission rate than mineral fertilizer. The simulations with treatments of a deep‐water covering and constant moisture reduced the methane emission. The rice field study provides a framework for further development of the model towards simulations based on spatially distributed variables (e.g. water table, soil temperature and vegetation) at a regional scale. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
35.
本文作者首次发现了EXO 2030+375的回旋吸收线迹象.这是迄今唯一一个同时有用回旋吸收线直接测量的磁场数据和用吸积演化方法间接测量的中子星磁矩数据的X射线脉冲星.两种测量方法的结果有很大差别,本文提出了可能导致这种差别的物理机制. 相似文献
36.
An analysis of our observations of the Geminga object with the GT-48 ground-based gamma-ray telescope has shown that its very-high-energy gamma-ray flux is modulated with a 59-s period. The 59-s period and its time derivative previously inferred from satellite data have been confirmed. According to our data, the period was 61.94 s in 1997 at MSD=50573. The statistical significance of this result is (1?4.5)×10?4. 相似文献
37.
介绍了近年来河外H2O超脉泽的主要观测结果。H2O超脉泽通常起源于活动星系核中央的拱核盘。它们主要寄生在 Serfert 2星系或低电离核区。至今为止,已有20个星系探测到H2O超脉泽。脉泽辐射的各向同性光度为10~6000L⊙。所有超脉泽星系显示出核的活动,显然,脉泽是由核活动所产生的射电和X射线光子或激波来抽运的。H2O超脉泽倾向存在于高倾斜度的星系,这使得沿视线上的分子柱密度增高,产生足够大的放大光深。最有可能产生H2O超脉泽辐射的星系应有一个包含着射电源的侧向的分子盘以及一个适当的抽运机制。 相似文献
38.
Yu. G. Shkuratov K. Muinonen E. Bowell K. Lumme J. I. Peltoniemi M. A. Kreslavsky D. G. Stankevich V. P. Tishkovetz N. V. Opanasenko L. Y. Melkumova 《Earth, Moon, and Planets》1994,65(3):201-246
About a dozen physical mechanisms and models aspire to explain the negative polarization of light scattered by atmosphereless celestial bodies. This is too large a number for the reliable interpretation of observational data. Through a comparative analysis of the models, our main goal is to answer the question: Does any one model have an advantage over the others? Our analysis is based on new laboratory polarimetric and photometric data as well as on theoretical results. We show that the widely used models due to Hopfield and Wolff cannot realistically explain the phase-angle dependence of the degree of polarization observed at small phase angles. The so-called interference or coherent backscattering mechanism is the most promising model. Models based on that mechanism use well-defined physical parameters to explain both negative polarization and the opposition effect. They are supported by laboratory experiments, particularly those showing enhancement of negative polarization with decreasing particle size down to the wavelength of light. According to the interference mechanism, pronounced negative branches of polarization, like those of C-class asteroids, may indicate a high degree of optical inhomogeneity of light-scattering surfaces at small scales. The mechanism also seems appropriate for treating the negative polarization and opposition effects of cometary dust comae, planetary rings, and the zodiacal light. 相似文献
39.
40.
We found the equilibrium conditions for a self-gravitating toroidal vortex by taking thermal pressure into account. These conditions are shown to significantly differ from those for a disk or a sphere. The evolution of a thin vortex turns it into a compact vortex that loses mechanical stability for low masses at a polytropic index γ<4/3 but retains stability for sufficiently high masses and densities determined by the velocity circulation in the vortex. 相似文献