全文获取类型
收费全文 | 45351篇 |
免费 | 2670篇 |
国内免费 | 3269篇 |
专业分类
测绘学 | 1913篇 |
大气科学 | 5072篇 |
地球物理 | 9484篇 |
地质学 | 18368篇 |
海洋学 | 4484篇 |
天文学 | 7597篇 |
综合类 | 1010篇 |
自然地理 | 3362篇 |
出版年
2023年 | 301篇 |
2022年 | 731篇 |
2021年 | 1031篇 |
2020年 | 955篇 |
2019年 | 989篇 |
2018年 | 1581篇 |
2017年 | 1557篇 |
2016年 | 1837篇 |
2015年 | 1315篇 |
2014年 | 1831篇 |
2013年 | 2772篇 |
2012年 | 2051篇 |
2011年 | 2323篇 |
2010年 | 2117篇 |
2009年 | 2379篇 |
2008年 | 2101篇 |
2007年 | 1989篇 |
2006年 | 1867篇 |
2005年 | 1452篇 |
2004年 | 1353篇 |
2003年 | 1212篇 |
2002年 | 1218篇 |
2001年 | 1124篇 |
2000年 | 1023篇 |
1999年 | 890篇 |
1998年 | 789篇 |
1997年 | 874篇 |
1996年 | 687篇 |
1995年 | 688篇 |
1994年 | 643篇 |
1993年 | 528篇 |
1992年 | 495篇 |
1991年 | 440篇 |
1990年 | 466篇 |
1989年 | 413篇 |
1988年 | 387篇 |
1987年 | 422篇 |
1986年 | 367篇 |
1985年 | 462篇 |
1984年 | 453篇 |
1983年 | 481篇 |
1982年 | 448篇 |
1981年 | 409篇 |
1980年 | 424篇 |
1979年 | 357篇 |
1978年 | 325篇 |
1977年 | 318篇 |
1976年 | 279篇 |
1975年 | 280篇 |
1974年 | 284篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
641.
642.
Leroy, Bommier, and Sahal-Bréchot (1984) determined the vector magnetic field in a large sample of quiescent prominences. The direction of the axial component is in general subject to a 180 deg uncertainty. We have selected those prominences in the sample whose field direction is unambiguous. For 95 such prominences, only 3 do not obey the hemispheric preferences of sinistral or dextral filaments, discovered by Martin, Tracadas, and Billamoria (1994). No explanation for the exceptional cases was found.A search of the Ottawa River Solar Observatory archives was made to check on the structural signatures of sinistral and dextral filaments. Of 32 filaments in common with the Leroy data set, 12 were classifiable as sinistral or dextral from their H fine structure and of these, 3 were exceptions to the hemispheric rule.Thus only a small percentage of quiescent filaments disobeys the hemispheric rule. 相似文献
643.
644.
The problem of designing low-energy transfers between the Earth and the Moon has attracted recently a major interest from the scientific community. In this paper, an indirect optimal control approach is used to determine minimum-fuel low-thrust transfers between a low Earth orbit and a Lunar orbit in the Sun–Earth–Moon Bicircular Restricted Four-Body Problem. First, the optimal control problem is formulated and its necessary optimality conditions are derived from Pontryagin’s Maximum Principle. Then, two different solution methods are proposed to overcome the numerical difficulties arising from the huge sensitivity of the problem’s state and costate equations. The first one consists in the use of continuation techniques. The second one is based on a massive exploration of the set of unknown variables appearing in the optimality conditions. The dimension of the search space is reduced by considering adapted variables leading to a reduction of the computational time. The trajectories found are classified in several families according to their shape, transfer duration and fuel expenditure. Finally, an analysis based on the dynamical structure provided by the invariant manifolds of the two underlying Circular Restricted Three-Body Problems, Earth–Moon and Sun–Earth is presented leading to a physical interpretation of the different families of trajectories. 相似文献
645.
T. W. B. Muxlow P. N. Wilkinson A. M. S. Richards K. I. Kellermann E. A. Richards M. A. Garrett 《New Astronomy Reviews》1999,43(8-10)
42 hours of A-array VLA data and 18 days MERLIN data at 1.4 GHz have been combined to image a 10 arcminute field centred on the Hubble Deep Field (HDF). This area encloses both the Hubble Deep and Flanking Fields. A complete sample of 87 sources have been detected with flux densities above 40 μJy. All these have been imaged with the MERLIN+VLA combination to produce images with 0.2, 0.3 and 0.5 arcsecond resolution. These are the most sensitive 1.4 GHz images yet made with rms noise levels of 3.3 μJy/beam in the 0.2 arcsecond images. About 70% of the microJy sources are found to be starburst type systems associated with major disk galaxies in the redshift range 0.4–1. Some 20% are found to be low-luminosity AGN systems identified with field ellipticals at redshifts close to 1. The remaining 10% are associated with optically faint systems close to or beyond the HDF limit; many of these may be dust-shrouded starbursts at high redshift. We propose to extend this study to include VLBI data of comparable sensitivity to investigate the compact radio structures found in the microJy source population. 相似文献
646.
Li-Hong Geng Cheng-Ming Tan Yi-Hua Yan Bao-Lin Tan Dong-Hao Liu Jin-Ping Dun 《天文和天体物理学研究(英文版)》2021,(6):219-228
This work analyzes the annual fluctuation of the observation data of the Mingantu Solar radio Telescope(MST) in S, C and X bands. It is found that the data vary with local air temperature as the logarithmic attenuation of equipment increases with temperature and frequency. A simplified and effective calibration method is proposed, which is used to calibrate the MST data in 2018–2020, while the correction coefficients are calculated from data in 2018–2019. For S, C and X bands, the root mean square errors of one polarization are 2.7, 5.7 and 20 sfu, and the relative errors are 4%, 6% and 8% respectively. The calibration of MUSER and SBRS spectra is also performed. The relative errors of MUSER at 1700 MHz,SBRS at 2800 MHz, 3050 MHz and 3350 MHz are 8%, 8%, 11% and 10% respectively. We found that several factors may affect the calibration accuracy, especially at X-band. The method is expected to work for other radio telescopes with similar design. 相似文献
647.
Adiabatic indices for a non-dissipative anisotropic convecting plasma are analyzed, and general expressions for the effective adiabatic index and the partial adiabatic indices parallel (γ∥) and perpendicular (γ⟂) to the magnetic field are obtained. It is shown that, in the general case, the value of the effective adiabatic index is not an universal constant and depends on the plasma temperature anisotropy and on the properties of the plasma motion. The values of γ⟂ and γ∥ are shown to be independent of the plasma parameters being completely determined by the characteristics of the plasma flow. 相似文献
648.
There are three major issues in modeling solar evanescent oscillations: the variation of the intensity [I]–velocity [V] phase difference of p-modes close to the base of photosphere; the existence of a plateau of negative I–V phase differences below and between the ridges of the low-frequency p-modes; the explanation of the I–V cross-spectra of the evanescent oscillations. We present new interpretations for the first two issues, based on modeling intensity fluctuations taking steep temperature gradients, opacity, and non-adiabatic cooling into account. We also discuss consequences of our model for the explanation of power spectra and cross-power spectra of p-modes. In particular, we present evidence that the acoustic sources that generate evanescent waves produce a coherent background that explains the plateau–interridge regime of negative I–V phase difference. 相似文献
649.
The dynamical properties of electromagnetic (EM) waves in ultra-relativistic electron-positron (EP) plasmas are analytically investigated on the basis of the nonlinear governing equations obtained from a kinetic way. It is shown that the EM wave envelope will collapse and be trapped into a localized region for the modulation interaction with low frequency density variation induced by ponderomotive force. The correlation between the localized strong wave field and the pulsar radio emission is discussed. 相似文献
650.
Kopylov A. I. Kosolapov D. B. Zabotkina E. A. Romanenko A. V. Sazhin A. F. 《Oceanology》2021,61(2):220-232
Oceanology - The distribution of heterotrophic bacteria, viruses, and heterotrophic nanoflagellates was studied in the shelf waters of the East Siberian Sea along the meridional transect from the... 相似文献