全文获取类型
收费全文 | 20187篇 |
免费 | 457篇 |
国内免费 | 199篇 |
专业分类
测绘学 | 404篇 |
大气科学 | 1440篇 |
地球物理 | 4190篇 |
地质学 | 7566篇 |
海洋学 | 1968篇 |
天文学 | 3713篇 |
综合类 | 85篇 |
自然地理 | 1477篇 |
出版年
2022年 | 177篇 |
2021年 | 320篇 |
2020年 | 343篇 |
2019年 | 381篇 |
2018年 | 724篇 |
2017年 | 622篇 |
2016年 | 714篇 |
2015年 | 381篇 |
2014年 | 668篇 |
2013年 | 1070篇 |
2012年 | 741篇 |
2011年 | 962篇 |
2010年 | 926篇 |
2009年 | 1109篇 |
2008年 | 969篇 |
2007年 | 977篇 |
2006年 | 832篇 |
2005年 | 616篇 |
2004年 | 592篇 |
2003年 | 578篇 |
2002年 | 509篇 |
2001年 | 441篇 |
2000年 | 471篇 |
1999年 | 328篇 |
1998年 | 331篇 |
1997年 | 303篇 |
1996年 | 257篇 |
1995年 | 261篇 |
1994年 | 236篇 |
1993年 | 198篇 |
1992年 | 171篇 |
1991年 | 166篇 |
1990年 | 193篇 |
1989年 | 146篇 |
1988年 | 162篇 |
1987年 | 191篇 |
1986年 | 161篇 |
1985年 | 212篇 |
1984年 | 225篇 |
1983年 | 225篇 |
1982年 | 190篇 |
1981年 | 157篇 |
1980年 | 152篇 |
1979年 | 157篇 |
1978年 | 152篇 |
1977年 | 138篇 |
1976年 | 119篇 |
1975年 | 131篇 |
1974年 | 126篇 |
1973年 | 132篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
61.
Titan has been observed with UVES, the UV-Visual Echelle Spectrograph at the Very Large Telescope, with the aim of characterizing the zonal wind flow. We use a retrieval scheme originally developed for absolute stellar accelerometry [Connes, P., 1985. Astrophys. Space Sci., 110, 211-255] to extract the velocity signal by simultaneously taking into account all the lines present in the spectrum. The method allows to measure the Doppler shift induced at a given point by the zonal wind flow, with high precision. The short-wavelength channel (4200-5200 Å) probes one scale height higher than the long-wavelength one (5200-6200 Å), and we observe statistically significant evidence for stronger winds at higher altitudes. The results show a high dispersion. Globally, we detect prograde zonal winds, with lower limits of 62 and 50 m s−1 at the regions centered at 200 and 170 km altitude, but approximately a quarter of the measurements indicates null or retrograde winds. 相似文献
62.
Michael E. ZOLENSKY Andrei V. IVANOV S. Vincent YANG David W. MITTLEFEHLDT Kazumasa OHSUMI 《Meteoritics & planetary science》1996,31(4):484-493
Abstract Kaidun is a breccia of disparate enstatite and carbonaceous chondrite clasts that continues to provide real surprises. Many Kaidun clasts have been intensely altered by aqueous fluids, as evidenced by the widespread occurrence of ferromagnesian phyllosilicates and by the presence of carbonate- and phyllo-silicate-filled veins. In this report, we describe an unusual CM lithology containing many mineralogical features not previously reported from any meteorite, including pyrrhotite, with exclusive needlelike morphologies and thick mantles of phyllosilicate, and complex aggregates of phyllosilicate, melanite garnet, crosscut by pentlandite veins. The latter features appear to be due in large part to extensive hydrothermal alteration at temperatures on the order of 450 °C, which is significantly higher than that attained during secondary processing from other known CM material. 相似文献
63.
J. CAMPSIE M. H. RASMUSSEN L. C. KOVACS F. DITTMER J. C. BAILEY N. O. HANSEN J. LAURSEN L. JOHNSON 《Polar research》1990,8(2):237-243
New aeromagnetic data, K-Ar age determinations of dredged marine igneous rocks, as well as other geophysical evidence have shed light on the chronology, nature and evolution of the northern Iceland Plateau. Correspondence between seismic refraction profiles taken on the Jan Mayen Ridge and westward through Jan Mayen Island, suppressed aeromagnetic anomalies, earthquake surface wave studies, and ages of dredged igneous rocks suggest these strata may form an extended region of thickened crust, possibly of Caledonian age, extending westward toward the Kolbeinsey Ridge and northwest to the south wall of the Jan Mayen Fracture Zone. 相似文献
64.
65.
We present a new Very Large Array (VLA) image of Saturn, made from data taken in October 1998 at a wavelength of λ3.6 cm. The moderate ring opening angle (B≈15°) allows us to explore direct transmission of microwave photons through the A and C rings. We find a strong asymmetry of photons transmitted through the A ring, but not in the C ring, a new diagnostic of wake structure in the ring particles. We also find a weak asymmetry between east and west for the far side of the ansae. To facilitate quantitative comparison between dynamic models of the A ring and radio observations, we extend our Monte Carlo radiative transfer code (described in Dunn et al., 2002, Icarus 160, 132-160) to include idealized wakes. We show the idealized model can reproduce the properties of dynamic simulations in directly transmitted light. We examine the model behavior in directly transmitted and scattered light over a range of physical and geometric wake parameters. Finally, we present a wake model with a plausible set of physical parameters that quantitatively reproduces the observed intensity and asymmetry of the A ring both across the planet and in the ansae. 相似文献
66.
Chaoborid and chironomid (Diptera) fossils were examined in sediment cores form nine Ontario Precambrian Shield lakes that were fishless in 1979. An abundance of Chaoborus americamus (intolerant of predation by fish) throughout cores from four lakes indicated that they were fishless historically. Occurrence of Chaoborus punctipennis and C. flavicans and absence of C. americanus in cores, except near the surface in two cases, indicated that four other lakes were inhabited by fish historically. One of these was probably periodically fishless as suggested by fluctuations among these Chaoborus species. The ninth lake was not suitable for Chaoborus; only one specimen of C. trivittatus was found in this core. Chironomid fossil associations were substantially different in historically fishless and inhabited lakes. Fishless lakes had chironomids typical of eutrophic conditions indicating that seasonal oxygen depletion likely was inimical to fish. Chironomid associations of oligotrophic waters occurred in lakes historically inhabited by fish. Both good water quality and accessibility were required for long-term habitation by fish. Partial recovery of alkalinity and rise in pH were accompanied by substantial re-invasion by fish in two lakes which had been inhabited historically by fish. Diptera associations provide valuable information on the historical suitability of lakes for fish and the presence/absence of fish in regions where acidification of surface waters has occurred. 相似文献
67.
68.
This paper considers the problem of estimatingm, the number of components in a finite mixture of distributions from a parametric family. A step-up procedure using the bootstrap method is proposed. Some properties of the procedure are illustrated with simulation studies. An example of the method, applied to orientation of beach clasts, is given. 相似文献
69.
70.
Andrew J. L. Harris Anna L. Butterworth Richard W. Carlton Ian Downey Peter Miller Pedro Navarro David A. Rothery 《Bulletin of Volcanology》1997,59(1):49-64
Satellite data offer a means of supplementing ground-based monitoring during volcanic eruptions, especially at times or locations
where ground-based monitoring is difficult. Being directly and freely available several times a day, data from the advanced
very high resolution radiometer (AVHRR) offers great potential for near real-time monitoring of all volcanoes across large
(3000×3000 km) areas. Herein we describe techniques to detect and locate activity; estimate lava area, thermal flux, effusion
rates and cumulative volume; and distinguish types of activity. Application is demonstrated using data for active lavas at
Krafla, Etna, Fogo, Cerro Negro and Erebus; a pyroclastic flow at Lascar; and open vent systems at Etna and Stromboli. Automated
near real-time analysis of AVHRR data could be achieved at existing, or cheap to install, receiving stations, offering a supplement
to conventional monitoring methods.
Received: 21 January 1997 / Accepted: 3 April 1997 相似文献