全文获取类型
收费全文 | 6399篇 |
免费 | 171篇 |
国内免费 | 130篇 |
专业分类
测绘学 | 223篇 |
大气科学 | 599篇 |
地球物理 | 1354篇 |
地质学 | 2336篇 |
海洋学 | 526篇 |
天文学 | 1241篇 |
综合类 | 20篇 |
自然地理 | 401篇 |
出版年
2023年 | 32篇 |
2022年 | 59篇 |
2021年 | 62篇 |
2020年 | 75篇 |
2019年 | 68篇 |
2018年 | 197篇 |
2017年 | 187篇 |
2016年 | 255篇 |
2015年 | 162篇 |
2014年 | 210篇 |
2013年 | 326篇 |
2012年 | 266篇 |
2011年 | 361篇 |
2010年 | 287篇 |
2009年 | 417篇 |
2008年 | 326篇 |
2007年 | 282篇 |
2006年 | 299篇 |
2005年 | 287篇 |
2004年 | 375篇 |
2003年 | 312篇 |
2002年 | 205篇 |
2001年 | 186篇 |
2000年 | 166篇 |
1999年 | 125篇 |
1998年 | 147篇 |
1997年 | 104篇 |
1996年 | 83篇 |
1995年 | 67篇 |
1994年 | 80篇 |
1993年 | 59篇 |
1992年 | 67篇 |
1991年 | 35篇 |
1990年 | 45篇 |
1989年 | 20篇 |
1988年 | 28篇 |
1987年 | 27篇 |
1986年 | 25篇 |
1985年 | 19篇 |
1984年 | 29篇 |
1983年 | 25篇 |
1982年 | 20篇 |
1981年 | 22篇 |
1978年 | 24篇 |
1977年 | 21篇 |
1976年 | 17篇 |
1975年 | 19篇 |
1974年 | 17篇 |
1973年 | 17篇 |
1971年 | 13篇 |
排序方式: 共有6700条查询结果,搜索用时 15 毫秒
31.
We describe how a local plasma structure can be changed by a transverse shear flow using numerical simulation to investigate the disturbance process near the magnetopause. The results show that magnetic field lines are bent by transverse shear flow disturbance near the current sheet region. There are multiple bipolar structures of the normal magnetic field in the numerical simulation. We term this new feature as K-point magnetic reconnection, realistic for discussing space observations. 相似文献
32.
A. D. Biggs D. Rusin I. W. A. Browne A. G. de Bruyn N. J. Jackson L. V. E. Koopmans J. P. McKean S. T. Myers R. D. Blandford K.-H. Chae C. D. Fassnacht M. A. Norbury T. J. Pearson P. M. Phillips A. C. S. Readhead P. N. Wilkinson 《Monthly notices of the Royal Astronomical Society》2003,338(4):1084-1088
33.
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
34.
35.
36.
X. Bai T.K. Gaisser A. Karle K. Rawlins G.M. Spiczak Todor Stanev 《Astroparticle Physics》2006,25(6):361-367
The muon flux at the South Pole was measured for five zenith angles, 0°, 15°, 35°, 82.13° and 85.15° with a scintillator muon telescope incorporating ice Cherenkov tank detectors as the absorber. We compare the measurements with other data and with calculations. 相似文献
37.
The data such as the H-spectrum-spectroheliographic (SSHG) observations, the H-chromospheric observations, etc., of a flare loop prominence which occurred on the western solar limb on 1981 April 27 have been obtained at Yunnan Observatory. The distribution of the internal motions and the macroscopical motion of the flare loop prominence with time and space in the course of its eruption and ascension is derived from the comprehensive analysis of the data. The possible physical pictures and the instability of the motions of the loop are inferred and discussed. 相似文献
38.
Romain de Mesmay Pierre Metzger Vincent Grossi Sylvie Derenne 《Organic Geochemistry》2008,39(7):879-893
A mixture of C33–C37 botryococcenes and partially reduced derivatives was isolated from ca. 32,000 year old sediment from Lake Masoko, a freshwater crater lake in the Rungwe Range area (Tanzania). Botryococcenes and derivatives accounted for 246 μg/g dry sediment and for >92% of the hydrocarbon fraction; 1D and 2D nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry allowed the structure of the dominant botryococcene (43% of hydrocarbon fraction) to be established, after purification using high performance liquid chromatography (HPLC). The compound is a novel tetraunsaturated dicyclic C34 botryococcene and is named C34 masokocene. Overall, the structures of six other novel botryococcenes and four partially reduced derivatives were tentatively assigned. The structures of the new biomarkers, three dicyclic C34–C36 botryococcenes (or masokocenes) and seven monocyclic C34–C37 analogues are discussed along with their biosynthetic relationship. The high abundance of such polyunsaturated compounds preserved in 32,000 year old sediment from the lake indicates an aquatic ecosystem dominated at the time by the green alga Botryococcus braunii, as well very good preservation of the organic matter. 相似文献
39.
Manoj K. Pandit Helga de Wall Narendra K. Chauhan 《Journal of Earth System Science》2008,117(3):201-209
A number of fine-grained sericite bearing pelitic, schistose lithologies occur along the Archean (Banded Gneiss Complex)-Proterozoic
(Aravalli Supergroup) contact (APC) in the Udaipur valley in NW Indian craton. These Al-rich lithologies (subsequently metamorphosed)
have been described as ‘paleosols’, developed over a 3.3 Ga old Archean gneissic basement and are overlain by Paleoproterozoic
Aravalli quartzite. The paleosol was developed between 2.5 and 2.1, coincident with the globally recognized Great Oxidation
Event (GOE). In previous studies these paleosol sections were interpreted to have developed under reducing environment, however,
the finding of a ‘ferricrete’ zone in the upper part of Tulsi Namla section (east of Udaipur) during the present study (in
addition to earlier reported lithologies) has led to an alternative suggestion of oxygen-rich conditions during paleosol development.
The Tulsi Namla paleosol section shows all the features characteristic of a complete paleosol section described from other
Archean cratons. The paleosol includes sericite schist with kyanite as the prevalent Al-silicate in the lower part of profile
while chloritoid and Fe-oxides typify the Fe-rich upper part. Alumina has remained immobile during the weathering process
while Fe and Mn show a decrease in the lower part of the section and an abrupt rise in the upper part, in the ferricrete zone.
The field and geochemical data indicate that the Tulsi Namla section is an in situ weathering profile and at least the upper part shows evidence of oxidizing conditions. 相似文献
40.
Anne Marie de Grosbois 《Environmental Geology》2008,54(5):1121-1126