全文获取类型
收费全文 | 51436篇 |
免费 | 6242篇 |
国内免费 | 7500篇 |
专业分类
测绘学 | 3821篇 |
大气科学 | 4573篇 |
地球物理 | 6396篇 |
地质学 | 20423篇 |
海洋学 | 4066篇 |
天文学 | 17602篇 |
综合类 | 2361篇 |
自然地理 | 5936篇 |
出版年
2024年 | 199篇 |
2023年 | 477篇 |
2022年 | 1286篇 |
2021年 | 1405篇 |
2020年 | 1315篇 |
2019年 | 1735篇 |
2018年 | 1164篇 |
2017年 | 1358篇 |
2016年 | 1401篇 |
2015年 | 1653篇 |
2014年 | 2057篇 |
2013年 | 2048篇 |
2012年 | 2254篇 |
2011年 | 2356篇 |
2010年 | 2128篇 |
2009年 | 3708篇 |
2008年 | 3578篇 |
2007年 | 3902篇 |
2006年 | 3863篇 |
2005年 | 3609篇 |
2004年 | 3436篇 |
2003年 | 3112篇 |
2002年 | 2732篇 |
2001年 | 2384篇 |
2000年 | 2192篇 |
1999年 | 1984篇 |
1998年 | 1997篇 |
1997年 | 970篇 |
1996年 | 732篇 |
1995年 | 788篇 |
1994年 | 779篇 |
1993年 | 547篇 |
1992年 | 378篇 |
1991年 | 351篇 |
1990年 | 263篇 |
1989年 | 276篇 |
1988年 | 204篇 |
1987年 | 147篇 |
1986年 | 119篇 |
1985年 | 77篇 |
1984年 | 46篇 |
1983年 | 43篇 |
1982年 | 18篇 |
1981年 | 15篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1978年 | 15篇 |
1977年 | 11篇 |
1954年 | 7篇 |
1897年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
521.
A flare of OH maser emission was discovered in W75N in 2000. Its location was determined with the Very Long Baseline Array (VLBA) to be within 110 au from one of the ultracompact H ii regions, Very Large Array 2 (VLA2). The flare consisted of several maser spots. Four of the spots were found to form Zeeman pairs, all of them with a magnetic field strength of about 40 mG. This is the highest ever magnetic field strength found in OH masers, an order of magnitude higher than in typical OH masers. Three possible sources for the enhanced magnetic field are discussed: (i) the magnetic field of the exciting star dragged out by the stellar wind; (ii) the general interstellar field in the gas compressed by the magnetohydrodynamic shock; and (iii) the magnetic field of planets which orbit the exciting star and produce maser emission in gaseous envelopes. 相似文献
522.
523.
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
524.
525.
M. T. Murphy S. J. Curran J. K. Webb 《Monthly notices of the Royal Astronomical Society》2003,342(3):830-836
We describe initial results of a search for redshifted molecular absorption towards four millimetre-loud, optically faint quasars. A wide frequency bandwidth of up to 23 GHz per quasar was scanned using the Swedish–ESO Submillimetre Telescope at La Silla. Using a search list of commonly detected molecules, we obtained nearly complete redshift coverage up to z abs = 5 . The sensitivity of our data is adequate to have revealed absorption systems with characteristics similar to those seen in the four known redshifted millimetre-band absorption systems, but none were found. Our frequency-scan technique nevertheless demonstrates the value of wide-band correlator instruments for searches such as these. We suggest that a somewhat larger sample of similar observations should lead to the discovery of new millimetre-band absorption systems. 相似文献
526.
527.
We present first observations of Mira stars obtained at the Very Large Telescope Interferometer (VLTI) at 2 microns, equipped
with the VINCI instrument, using different baselines with both the test siderostats and the 8.2 m unit telescopes. These observations,
collected in the course of the VLTI commissining program, have targeted so far about sixty cool giant stars. In this talk,
we list and describe the measurements obtained for 14 Mira stars, many of them representing first-time determinations of the
visibility. In particular, we devote special attention to a study of Mira itself, for which very accurate determinations of
the visibility at several baseline lenghts and orientations could be obtained. We find that a two-component model is more
consistent with the data than a single stellar disc. Further observations are needed for a better understanding of this source.
The VLTI will constitute an ideal instrument for such studies in the future. In particular it will permit detailed investigations
of southern AGB stars, such as accurate measurements of surface structureparameters (diameters, diameter variations, asymmetries,
center-to-limbvariations, special features like hot spots) and of circumstellar envelopes.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
528.
529.
We first present a self-consistent dynamical model in which ω Cen is formed from an ancient nucleated dwarf galaxy merging with the first generation of the Galactic thin disc in a retrograde manner with respect to the Galactic rotation. Our numerical simulations demonstrate that during merging between the Galaxy and the ω Cen host dwarf with M B ≃−14 mag and its nucleus mass of 107 M⊙ , the outer stellar envelope of the dwarf is nearly completely stripped, whereas the central nucleus can survive from the tidal stripping because of its compactness. The developed naked nucleus has a very bound retrograde orbit around the young Galactic disc, as observed for ω Cen, with apocentre and pericentre distances of ∼8 and ∼1 kpc, respectively. The Galactic tidal force can induce radial inflow of gas to the centre of the dwarf and consequently triggers moderately strong nuclear starbursts in a repetitive manner. This result implies that efficient nuclear chemical enrichment resulting from the later starbursts can be closely associated with the origin of the observed relatively young and metal-rich stars in ω Cen. Dynamical heating by the ω Cen host can transform the young thin disc into the thick disc during merging. 相似文献
530.