全文获取类型
收费全文 | 1322篇 |
免费 | 118篇 |
国内免费 | 243篇 |
专业分类
测绘学 | 18篇 |
大气科学 | 338篇 |
地球物理 | 107篇 |
地质学 | 257篇 |
海洋学 | 36篇 |
天文学 | 625篇 |
综合类 | 14篇 |
自然地理 | 288篇 |
出版年
2024年 | 3篇 |
2023年 | 7篇 |
2022年 | 18篇 |
2021年 | 21篇 |
2020年 | 26篇 |
2019年 | 31篇 |
2018年 | 24篇 |
2017年 | 26篇 |
2016年 | 27篇 |
2015年 | 22篇 |
2014年 | 52篇 |
2013年 | 48篇 |
2012年 | 47篇 |
2011年 | 70篇 |
2010年 | 54篇 |
2009年 | 93篇 |
2008年 | 117篇 |
2007年 | 124篇 |
2006年 | 125篇 |
2005年 | 103篇 |
2004年 | 112篇 |
2003年 | 88篇 |
2002年 | 87篇 |
2001年 | 62篇 |
2000年 | 48篇 |
1999年 | 40篇 |
1998年 | 50篇 |
1997年 | 47篇 |
1996年 | 11篇 |
1995年 | 30篇 |
1994年 | 13篇 |
1993年 | 9篇 |
1992年 | 11篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 11篇 |
1988年 | 7篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1982年 | 1篇 |
排序方式: 共有1683条查询结果,搜索用时 406 毫秒
81.
82.
83.
Variations in dust event frequency over the past century reflected by ice-core and lacustrine records in north China 总被引:1,自引:0,他引:1
WANG NingLian YAO TanDong YANG XiangDong SHEN Ji & WANG Yong Key Laboratory of Cryosphere Environment Cold Arid Regions Environmental Engineering Research Institute Chi- nese Academy of Sciences Lanzhou China Institute of Tibetan Plateau Research Chinese Academy of Sciences Beijing China Nanjing Institute of Geography Limnology Chinese Academy of Sciences Nanjing China 《中国科学D辑(英文版)》2007,50(5):736-744
In this paper, we analyzed the variations of dust proxies in the Dunde, Malan and Chongce ice cores from the northern Tibetan Plateau and the Hongjiannao lacustrine sediment core from north Shaanxi Province, and found that they all showed a general decrease trend over the past century. Owing to the fact that all these ice cores and lacustrine core were retrieved from the margins and/or the leeward sides of the major areas of dust events in north China, their records could suggest that the dust event frequency in north China declined over the study period. This decrease trend might be attributed to increasing precipitation and weakening westerly. However, human activities have made the areal extent of desertification expand acceleratingly in north China. This status could make it possible that dust events would occur on a large spatial scale under the future climate change, which would be a big environmental issue we shall face. 相似文献
84.
J. I. Davies P. Alton S. Bianchi & M. Trewhella 《Monthly notices of the Royal Astronomical Society》1998,300(4):1006-1014
We have constructed a numerical model of a galaxy that consists of a stellar, gas and dust disc imbedded within a dark halo. We have used this model to assess the radiation, gravitational and viscous forces on dust grains and to trace their motion through the interstellar medium over a period of 109 yr. We conclude that the disc opacity is a crucial factor in understanding the motion of the grains. Large grains (≈0.1 μm) with low disc opacity will lead to dust expulsion from the stellar disc, while high opacity leads to dust retention. Reasonable disc opacities lead to the recycling of the larger grains from the outer to the inner regions of the galaxy. The larger grains travel at higher velocities than small grains (0.01−0.001 μm), and so the smaller grains remain relatively close to their formation sites. Dust can 'leak' out over the entire surface of the disc because of the imbalance of radiation and gravitational forces. The dust is dynamically coupled to the gas and so although the gas lags behind the dust it is carried along with it. This explains the close correlation between the far-infrared emission from dust and the gas column density. We use a simple analytical model to show how the dust mass of a galaxy may evolve with time and how a significant fraction (90 per cent) of the total dust mass produced may have been expelled into the intergalactic medium. 相似文献
85.
太阳系尘埃等离子体研究 总被引:1,自引:0,他引:1
综述了太阳系尘埃等离子体中的充电机制和波动过程以及与之有关的若干空间物理现象,内容包括太阳系等离子体中尘埃表面的平衡电势,带电尘埃在空间环境中的受力与运动,行星环内沿的弥漫扩散,轮辐结构的成困,彗星环境中尘埃的静电爆裂,尘埃彗尾的形态演化,天王星窄环的稳定性等问题. 相似文献
86.
甘肃河西走廊沙尘暴与赤道中,东太平洋海温之间的遥相关分析 总被引:34,自引:14,他引:20
春、夏季甘肃河西走廊沙尘暴发生次数与赤道中、东太平洋海温之间遥相关分析结果表明:春、夏季河西走廊沙尘暴发生次数与前二年赤道中、东太平洋海温的负相关最好,时间过远或过近,相关性反而较差。利用前期的赤道中、东太平洋海温可以较好地预报出河西走廊春季沙尘暴发生的多寡。 相似文献
87.
我国极端干旱区边缘阿羌砂尘堆积剖面的地质时代 总被引:11,自引:0,他引:11
根据对昆仑山北麓阿羌地区第四纪风成砂尘沉积物的调查、地层对比和107组古地磁标本(321块样品)的测定结果认为,阿羌地层剖面记录了Brunhes时以来我国极端干旱区最完整的风成砂尘堆积序列;剖面中间冰期形成的生草层1PE和细粒风尘堆积29FGL、31FGL、33FGL、35FGL、37FGL、39FGL、42FGL在形成的地质时代上可依次与黄土高原古土壤S_0、S_1、S_2、S_3、S_4、S_5、S_6、S_7进行对比,而与1PE和细粒风尘堆积29FGL、……、42FGL互为叠覆的冰期气候形成的风成极细砂和粗粒风尘堆积则可与黄土高原黄土L_1、……、L_8进行对比。此外,在阿羌剖面砂尘堆积底部42FGL层中发现有0.7 Ma B P前的宇宙尘埃物质,这对于该地风成砂尘下限年代的确定、研究极端干旱区气候的发展历史和晚新生代地外灾变事件的对比具有重要意义。 相似文献
88.
C. R.Benn M.Vigotti R.Carballo J. I.Gonzalez-Serrano S. F.Sánchez 《Monthly notices of the Royal Astronomical Society》1998,295(2):451-456
Webster et al. claimed that up to 80 per cent of QSOs may be obscured by dust. They inferred the presence of this dust from the remarkably broad range of B − K optical–infrared colours of a sample of flat-spectrum PKS radio QSOs. If such dust is typical of QSOs, it will have rendered invisible most of those which would otherwise have been detected by optical surveys. We used the William Herschel Telescope on La Palma to obtain K infrared images of 54 B3 radio QSOs selected at low frequency (mainly steep-spectrum), and we find that although several have very red optical–infrared colours, most of these can be attributed to an excess of light in K rather than a dust-induced deficit in B. We present evidence that some of the infrared excess comes from the light of stars in the host galaxy (some, as previously suggested, comes from synchrotron radiation associated with flat-spectrum radio sources). The B − K colours of the B3 QSOs provide no evidence for a large reddened population. Either the Webster et al. QSOs are atypical in having such large extinctions, or their reddening is not due to dust; either way, the broad range of their B − K colours does not provide evidence that a large fraction of QSOs has been missed from optical surveys. 相似文献
89.
G.A. Galazutdinov J. Krelowski C. Moutou F.A. Musaev 《Monthly notices of the Royal Astronomical Society》1998,295(2):437-442
We analyse spectra of slightly reddened, early-type stars in which the major diffuse interstellar bands at 5870 and 5797 Å are either strong or weak in relation to E(B – V) . It is demonstrated that among the low E(B – V) objects one can find stars obscured by 'sigma' as well as 'zeta' type clouds. The profiles of the diffuse bands seem not to be dependent on the total opacities of the interstellar clouds. We also discuss the physical conditions leading to the formation of the diffuse band carriers in low-opacity interstellar clouds; the carriers are apparently formed in media in which one can observe complex velocity fields. 相似文献
90.
M.Unavane GerardGilmore N.Epchtein G.Simon D.Tiphène B.de Batz 《Monthly notices of the Royal Astronomical Society》1998,295(1):119-144
We present the analysis of three-colour optical/near-infrared images, in IJK , taken for the DEep Near Infrared Southern Sky Survey (DENIS) project. The region considered covers 17.4 deg2 and lies within <5°, b <1.°5. The adopted methods for deriving photometry and astrometry in these crowded images, together with an analysis of the deficiencies nevertheless remaining, are presented. The numbers of objects extracted in I , J and K are 748 000, 851 000 and 659 000 respectively, to magnitude limits of 17, 15 and 13. Eighty per cent completeness levels typically fall at magnitudes 16, 13 and 10 respectively, fainter by about 2 mag than the usual DENIS limits as a result of the crowded nature of these fields. A simple model to describe the disc contribution to the number counts is constructed, and parameters for the dust layer are derived. We find that a formal fit of parameters for the dust plane, from these data in limited directions, gives a scalelength and scaleheight of 3.4±1.0 kpc and 40±5 pc respectively, and a solar position 14.0±2.5 pc below the plane. This latter value is likely to be affected by localized dust asymmetries. We convolve a detailed model of the systematic and random errors in the photometry with a simple model of the Galactic disc and dust distribution to simulate expected colourmagnitude diagrams. These are in good agreement with the observed diagrams, allowing us to isolate those stars from the inner disc and bulge. After correcting for local dust-induced asymmetries, we find evidence for longitude-dependent asymmetries in the distant J and K sources, consistent with the general predictions of some Galactic bar models. We consider complementary L -band observations in the companion paper. 相似文献