全文获取类型
收费全文 | 2714篇 |
免费 | 262篇 |
国内免费 | 433篇 |
专业分类
测绘学 | 79篇 |
大气科学 | 333篇 |
地球物理 | 276篇 |
地质学 | 582篇 |
海洋学 | 51篇 |
天文学 | 1806篇 |
综合类 | 58篇 |
自然地理 | 224篇 |
出版年
2024年 | 11篇 |
2023年 | 31篇 |
2022年 | 67篇 |
2021年 | 55篇 |
2020年 | 63篇 |
2019年 | 81篇 |
2018年 | 51篇 |
2017年 | 41篇 |
2016年 | 69篇 |
2015年 | 61篇 |
2014年 | 89篇 |
2013年 | 80篇 |
2012年 | 79篇 |
2011年 | 110篇 |
2010年 | 87篇 |
2009年 | 203篇 |
2008年 | 166篇 |
2007年 | 205篇 |
2006年 | 190篇 |
2005年 | 185篇 |
2004年 | 230篇 |
2003年 | 154篇 |
2002年 | 146篇 |
2001年 | 137篇 |
2000年 | 139篇 |
1999年 | 132篇 |
1998年 | 151篇 |
1997年 | 63篇 |
1996年 | 45篇 |
1995年 | 64篇 |
1994年 | 47篇 |
1993年 | 26篇 |
1992年 | 32篇 |
1991年 | 21篇 |
1990年 | 17篇 |
1989年 | 17篇 |
1988年 | 16篇 |
1987年 | 6篇 |
1986年 | 11篇 |
1985年 | 3篇 |
1984年 | 10篇 |
1983年 | 4篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有3409条查询结果,搜索用时 265 毫秒
971.
972.
973.
We present the results of a search for carbon recombination lines in the Galaxy at 34.5 MHz (C575α) made using the dipole
array at Gauribidanur near Bangalore. Observations made towards 32 directions resulted in detections of lines, in absorption
at nine positions. Followup observations at 328 MHz (C272α) using the Ooty Radio Telescope detected these lines in emission.
A VLA D-array observation of one of the positions at 330 MHz yielded no detection implying a lower limit of 10′ for the angular
size of the line forming region.
The longitude-velocity distribution of the observed carbon lines indicate that the line forming regions are located mainly
between 4 kpc and 7 kpc from the Galactic centre. Combining our results with published carbon recombination line data near
76 MHz (Erickson, McConnell & Anantharamaiah 1995), we obtain constraintson the physical parameters of the line forming regions.
We find thatif the angular size of the line forming regions is ≥ 4°, then the range of parameters that fit the data are:T
e
=20–40 K,n
e
∼ 0.1–0.3 cm−3 and pathlengths ∼ 0.07–0.9 pc which may correspond to thin photodissociated regions around molecular clouds. On the other
hand, if the line forming regions are ∼ 2° in extent, then warmer gas (T
e
∼ 60–300 K) with lower electron densities (n
e
∼ 0.03–0.05 cm−3) extending over several tens of parsecs along the line of sight and possibly associated with atomic HI gas can fit the data.
Based on the range of derived parameters, we suggest that the carbon line regions are most likely associated with photo-dissociation
regions. 相似文献
974.
975.
Elke Roediger Marcus Brüggen Matthias Hoeft 《Monthly notices of the Royal Astronomical Society》2006,371(2):609-618
We present a sample of 30 wide-angle tailed radio galaxies (WATs) that we use to constrain the jet speeds in these sources. We measure the distribution of the jet-sidedness ratios for the sample, and assuming that the jets are beamed, the jet speeds in the range (0.3–0.7) c are obtained. Whilst the core prominence of the sample, which ought to be a reliable indicator of beaming, shows little correlation with the jet sidedness, we argue that due to the peculiar nature of the WATs, core prominence is unlikely to be a good indicator of beaming in these sources. We further show that if the jets are fast and light, then the galaxy speeds required to bend the jets into C-shapes such as those seen in 0647+693 are reasonable for a galaxy in a merging or recently merged cluster. 相似文献
976.
L.L. Kedziora-Chudczer † D.L. Jauncey M.H. Wieringa A.K. Tzioumis J.E. Reynolds 《Monthly notices of the Royal Astronomical Society》2001,325(4):1411-1430
We present the results of an Australia Telescope Compact Array (ATCA) survey for intraday variability (IDV) of the total and polarized flux densities of 118 compact, flat-spectrum, extragalactic radio sources from the Parkes 2.7-GHz Survey. A total of 22 total flux density IDV sources were discovered and 15 sources were found to show IDV of their polarized flux density. We discuss the statistical properties of the IDV sources, including the distribution of source modulation indices, and the dependence of the variability amplitude on source spectral index and on Galactic position. We suggest interstellar scintillation (ISS) in the Galactic interstellar medium as the most likely mechanism for IDV. Even so, the inferred high brightness temperatures cannot be easily explained. 相似文献
977.
Vijay K. Kapahi 《Journal of Astrophysics and Astronomy》1981,2(1):43-58
A list has been compiled of 49 extragalactic sources, most of them identified with quasars, that appear to have a one-sided
(D2 type) radio structure characterized by a single outer component displaced from a compact central (nuclear) component coincident
with the optical object. The observed properties of a subsample of 28 D2 quasars that have an overall angular size larger than 5 arcsec are briefly discussed and compared with those of normal (D1
type) double quasars. It is found that the central components in most D2 sources account for more than half the total flux density at high frequencies in contrast to the D1 quasars which generally have less than 20 per cent of their total flux density in a central component. This makes it very
unlikely that D2 sources are just those D1s in which there is a large intrinsic difference in the flux densities or separations of the two
outer components. The observed properties of D2 sources are easier to understand in the relativistic beaming interpretation in which their axes are inclined at smaller angles
with the line of sight compared to D1 sources. 相似文献
978.
若干恒星形成区的^12CO(J=1—0)与^13CO(J=1—0)观测 总被引:1,自引:0,他引:1
首次利用紫金山天文台青海观测站13.7m毫米波射电望远镜对若干分子云与恒星形成区的~(12)CO(J=1—0)和~(13)CO(J=1—0)分子辐射进行了观测,得到了各自中心位置的谱线轮廓。作为一个实例本文将介绍如何通过对分子云~(12)CO(J=1—0)和~(13)CO(J=1—0)谱线的综合分析与计算得到云中的物理参数。 相似文献
979.
We calculated the ionization fraction for hydrogen and helium (He I) as a function of the redshift z by including the two-photon decays of high hydrogen and parahelium levels and the radiative transfer in the helium 23P1 ? 11S0 intercombination line. We show that this yields corrections of no more than a few percent to the ionization fraction for hydrogen and speeds up significantly the recombination for helium compared to the recent works by Seager et al. (1999, 2000), in which these effects were disregarded. 相似文献
980.
Kim Jik Su Yuan-Yong Deng Kim Kum Sok Kim Jin Song National Astronomical Observatories Chinese Academy of Sciences Beijing Pyongyang Astronomical Observatory Academy of Sciences DPR of Korea 《中国天文和天体物理学报》2001,(6)
1 INTRoDUCTIONIt is well known that solar proton flares are maiuly correlated with strong, bright H. flares,in particular, with two ribbon flares. Ellison et al. (l961) were the first to draw attelltion tothe faCt that all cosndc-ray flares had the typical two-ribbon shape. Then, Svestka and Simon(1976) drew uP the "Catalog of Solar Particle Evellts, 195ty1969", and using this Catalog,Dodson and Hedeman could safely identify the flare sources for 50 proton events. Out of the 50idellti… 相似文献