全文获取类型
| 收费全文 | 1202篇 |
| 免费 | 300篇 |
| 国内免费 | 121篇 |
专业分类
| 测绘学 | 54篇 |
| 大气科学 | 143篇 |
| 地球物理 | 566篇 |
| 地质学 | 271篇 |
| 海洋学 | 41篇 |
| 天文学 | 398篇 |
| 综合类 | 42篇 |
| 自然地理 | 108篇 |
出版年
| 2024年 | 6篇 |
| 2023年 | 20篇 |
| 2022年 | 23篇 |
| 2021年 | 44篇 |
| 2020年 | 31篇 |
| 2019年 | 40篇 |
| 2018年 | 35篇 |
| 2017年 | 29篇 |
| 2016年 | 35篇 |
| 2015年 | 38篇 |
| 2014年 | 64篇 |
| 2013年 | 51篇 |
| 2012年 | 56篇 |
| 2011年 | 51篇 |
| 2010年 | 49篇 |
| 2009年 | 76篇 |
| 2008年 | 77篇 |
| 2007年 | 75篇 |
| 2006年 | 86篇 |
| 2005年 | 72篇 |
| 2004年 | 80篇 |
| 2003年 | 66篇 |
| 2002年 | 49篇 |
| 2001年 | 48篇 |
| 2000年 | 48篇 |
| 1999年 | 79篇 |
| 1998年 | 51篇 |
| 1997年 | 33篇 |
| 1996年 | 33篇 |
| 1995年 | 41篇 |
| 1994年 | 19篇 |
| 1993年 | 20篇 |
| 1992年 | 26篇 |
| 1991年 | 18篇 |
| 1990年 | 10篇 |
| 1989年 | 15篇 |
| 1988年 | 9篇 |
| 1987年 | 5篇 |
| 1986年 | 3篇 |
| 1985年 | 2篇 |
| 1984年 | 4篇 |
| 1981年 | 1篇 |
| 1980年 | 3篇 |
| 1954年 | 2篇 |
排序方式： 共有1623条查询结果，搜索用时 15 毫秒
131.
SAR图像中散射目标的散射矩阵受极化方位角(POA)的影响会改变散射体的散射特性,散射矩阵是极化干涉SAR (PolInSAR)估计不同极化状态下复相干性的基础。本文根据极化方位角产生机制,建立了多视情况下基于极化方位角补偿的极化干涉相干性估计模型,分析了极化方位角补偿对相干性估计方法和不同散射机制下相干性估计的影响程度,研究了基于三阶段法与极化方位角补偿的植被参数反演。利用L波段SIRC全极化SAR图像为实验数据验证极化方位角补偿对极化干涉相干性估计和植被参数反演的可行性。实验结果表明,极化方位角补偿能够改变不同极化状态相干性分布规律,提高相干直线拟合精度,改善植被参数反演的可靠性和合理性。 相似文献
132.
Modern spectropolarimeters are capable of detecting subkilogauss field strengths using the Zeeman effect in line profiles from the static photosphere, but supersonic Doppler broadening makes it more difficult to detect the Zeeman effect in the wind lines of hot stars. Nevertheless, the recent advances in observational capability motivate an assessment of the potential for detecting the magnetic fields threading such winds. We incorporate the weak-field longitudinal Zeeman effect in the Sobolev approximation to yield integral expressions for the flux of circularly polarized emission. To illustrate the results, two specific wind flows are considered: (i) spherical constant expansion with v ( r ) = v ∞ and (ii) homologous expansion with v ( r ) ∝ r . Axial and split monopole magnetic fields are used to schematically illustrate the polarized profiles. For constant expansion, optically thin lines yield the well-known 'flat-topped' total intensity emission profiles and an antisymmetric circularly polarized profile. For homologous expansion, we include occultation and wind absorption to provide a more realistic observational comparison. Occultation severely reduces the circularly polarized flux in the redshifted component, and in the blueshifted component, the polarization is reduced by partially offsetting emission and absorption contributions. We find that for a surface field of approximately 100 G, the largest polarizations result for thin but strong recombination emission lines. Peak polarizations are approximately 0.05 per cent, which presents a substantial although not inconceivable sensitivity challenge for modern instrumentation. 相似文献
133.
134.
John R. Mattox 《Experimental Astronomy》1991,2(2):75-84
A Monte Carlo simulation shows that the EGRET gamma-ray telescope aboard the GRO satellite does not have sufficient sensitivity to detect linear polarization, even for 100% polarized gamma-ray sources. This is confirmed by analysis of calibration data. Several data selection techniques suggested to enhance polarization sensitivity have been evaluated and found to not significantly improve sensitivity.NRC/NAS RESEARCH ASSOCIATE 相似文献
135.
We present some interesting results from the mean pulse polarization observations of four southern pulsars made at the Australian National Radio Astronomy Observatory, Parkes, using the 64-m telescope in June and July, 1988. The 2 × 16 × 5 MHz filter system from Jodrell Bank has proved excellent in de-dispersing the pulse signals and measuring their polarization properties. We give the data for the four pulsars in some detail and discuss their spectral behaviour. 相似文献
136.
S. A. Petrova 《Monthly notices of the Royal Astronomical Society》2006,366(4):1539-1550
Propagation of radio waves in the ultrarelativistic magnetized electron–positron plasma of a pulsar magnetosphere is considered. The polarization state of the original natural waves is found to vary markedly on account of the wave mode coupling and cyclotron absorption. The change is most pronounced when the regions of mode coupling and cyclotron resonance approximately coincide. In cases when the wave mode coupling occurs above and below the resonance region, the resultant polarization appears essentially distinct. The main result of the paper is that in the former case the polarization modes become non-orthogonal. The analytical treatment of the equations of polarization transfer is accompanied by numerical calculations. The observational consequences of polarization evolution in pulsar plasma are discussed as well. 相似文献
137.
138.
139.
140.