全文获取类型
收费全文 | 19608篇 |
免费 | 524篇 |
国内免费 | 180篇 |
专业分类
测绘学 | 429篇 |
大气科学 | 201篇 |
地球物理 | 525篇 |
地质学 | 1830篇 |
海洋学 | 227篇 |
天文学 | 16539篇 |
综合类 | 78篇 |
自然地理 | 483篇 |
出版年
2024年 | 41篇 |
2023年 | 74篇 |
2022年 | 143篇 |
2021年 | 88篇 |
2020年 | 90篇 |
2019年 | 153篇 |
2018年 | 55篇 |
2017年 | 44篇 |
2016年 | 69篇 |
2015年 | 166篇 |
2014年 | 140篇 |
2013年 | 127篇 |
2012年 | 202篇 |
2011年 | 214篇 |
2010年 | 267篇 |
2009年 | 1571篇 |
2008年 | 1506篇 |
2007年 | 1773篇 |
2006年 | 1780篇 |
2005年 | 1620篇 |
2004年 | 1730篇 |
2003年 | 1475篇 |
2002年 | 1297篇 |
2001年 | 1138篇 |
2000年 | 919篇 |
1999年 | 880篇 |
1998年 | 1056篇 |
1997年 | 213篇 |
1996年 | 96篇 |
1995年 | 258篇 |
1994年 | 283篇 |
1993年 | 120篇 |
1992年 | 68篇 |
1991年 | 84篇 |
1990年 | 75篇 |
1989年 | 130篇 |
1988年 | 81篇 |
1987年 | 81篇 |
1986年 | 74篇 |
1985年 | 37篇 |
1984年 | 26篇 |
1983年 | 22篇 |
1982年 | 3篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1977年 | 6篇 |
1905年 | 3篇 |
1900年 | 3篇 |
1897年 | 7篇 |
1877年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 7 毫秒
31.
On September 14–18, 2000, a medium-small solar active region was observed at Ganyu Station of Purple Mountain Observatory. Its spots were not large, but it had a peculiar active filament. On Sep.16, a flare of importance IIIb with rather intense geophysical effects was produced. Our computation of the magnetic structure of the active region reveals that the rope-shaped filament was concerned with a low magnetic arc close to magnetic neutral line. An intense shear of magnetic field occurred near magnetic rope. The QSL analysis shows that a 3-D magnetic reconnection might appear in the vicinity of filament, and this can be used to interpret the formation of a large flare. 相似文献
32.
33.
34.
35.
The integrated optical spectrum of the Crab nebula and evidence for its fading synchrotron continuum
Nathan Smith † 《Monthly notices of the Royal Astronomical Society》2003,346(3):885-889
A flux-calibrated optical spectrum integrated over the entire Crab nebula was obtained by making drift scans with a long-slit spectrograph. Compared to observations obtained over the past 40 years, these new data confirm an earlier controversial result that the [O iii ] λλ4959, 5007 equivalent width is increasing with time, although the rate of ∼0.9 per cent yr−1 is somewhat slower than that measured previously. Additionally, the Hβ equivalent width is increasing at a comparable rate, but the measured fluxes of both Hβ and [O iii ] have changed less than their respective equivalent widths. The different rates of change in the measured fluxes and equivalent widths of these lines suggest that the optical synchrotron continuum from the Crab nebula is indeed fading rapidly. The apparent decline is consistent with a rate around −0.5 (±0.2) per cent yr−1 at wavelengths near 5000 Å inferred independently from measurements of the optical continuum flux during the same time period. 相似文献
36.
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
37.
38.
39.
40.
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