全文获取类型
收费全文 | 865篇 |
免费 | 27篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 14篇 |
大气科学 | 75篇 |
地球物理 | 128篇 |
地质学 | 276篇 |
海洋学 | 79篇 |
天文学 | 223篇 |
综合类 | 6篇 |
自然地理 | 98篇 |
出版年
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 16篇 |
2018年 | 14篇 |
2017年 | 19篇 |
2016年 | 18篇 |
2015年 | 28篇 |
2014年 | 16篇 |
2013年 | 30篇 |
2012年 | 18篇 |
2011年 | 26篇 |
2010年 | 27篇 |
2009年 | 47篇 |
2008年 | 37篇 |
2007年 | 28篇 |
2006年 | 31篇 |
2005年 | 39篇 |
2004年 | 25篇 |
2003年 | 24篇 |
2002年 | 23篇 |
2001年 | 17篇 |
2000年 | 23篇 |
1999年 | 12篇 |
1998年 | 18篇 |
1997年 | 9篇 |
1996年 | 8篇 |
1995年 | 11篇 |
1994年 | 11篇 |
1993年 | 8篇 |
1992年 | 13篇 |
1990年 | 15篇 |
1989年 | 15篇 |
1988年 | 10篇 |
1987年 | 13篇 |
1986年 | 6篇 |
1985年 | 18篇 |
1984年 | 17篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 11篇 |
1980年 | 15篇 |
1979年 | 21篇 |
1978年 | 16篇 |
1977年 | 12篇 |
1976年 | 9篇 |
1975年 | 10篇 |
1974年 | 7篇 |
1973年 | 14篇 |
1972年 | 6篇 |
1971年 | 6篇 |
排序方式: 共有899条查询结果,搜索用时 15 毫秒
811.
Asteroid 4 Vesta was detected on 1979 November 6 with the Arecibo Observatory's S-band (12.6-cm-wavelength) radar. The echo power spectrum, received in the circular polarization opposite to that transmmited,, yields a radar cross section of (0.2 ± 0.1)πa2, for a = 272 km. The data are too noisy to permit derivation of Vesta's rotation period. 相似文献
812.
Dr. Franz Baron Nopcsa 《International Journal of Earth Sciences》1921,12(1-2):1-19
Ohne Zusammenfassung 相似文献
813.
A model which possibly accounts for the unusual radar scattering behavior observed for Europa, Ganymede, and Callisto postulates a thick surface layer of ice saturated with nearly hemispherical craters. In the development of this model it is noted that a single reflection at normal incidence reverses the rotational sense of circularly polarized incident radiation, in conflict with the radar observations which show an echo predominantly not reversed. Furthermore, an ensemble of backscattering events, each the result of a large number of successive dielectric reflections, tends to produce a weak and unpolarized echo. However, two coupled reflections can produce the observed backscattering behavior, provided the angles of incidence lie between the Brewster angle and its complement. The effect is maximum when the angles equal 45°, and, for water ice, yields a ratio of 1.9 for components of the echo received in rotational senses the same as, and opposite to, the sense transmitted. Randomly oriented reflecting facets, either of ice on the surface or of rocks in the interior, cannot yield the observed behavior since too few of the total possible backscattering configurations meet the above requirement. Hemispherical surface craters, on the other hand, favor 45° dual reflection. A model consisting of such craters in ice is investigated and found capable of explaining the observed results, not only in respect to polarization, but in respect to albedo and angular scattering law as well. 相似文献
814.
We analyze Ca ii K-line profiles of one flare and EUV continuum observations of two other flares in order to infer values for the temperature enhancements (over active region values) produced in the upper photosphere around and above the temperature minimum region. The results, obtained through a partial redistribution calculation of the Ca ii K-line profiles and an LTE approach to the continuum observations, show that the flare temperature minimum is depressed some two scale heights below its preflare level, and that substantial temperature enhancements are produced even at this depth. Estimates for the energy release in these photospheric layers are given, and are found to be comparable with that released in chromospheric H and L emission.We then turn our attention to the investigation of possible heating mechanisms which might be responsible for the observed enhancements. Bombardment by both electrons and protons, and irradiation by soft X-rays, are each considered and found to be largely ineffective, due to the large attenuation of flux by photospheric depths, unless new ideas on the precise nature of these mechanisms are invoked, particularly if the same mechanism is also to explain the observed chromospheric emissions. We therefore conclude that it is most likely that some other mechanism must be advocated in order to explain the observed heating. Possibilities for this are (a) heating by EUV radiation, (b) proton beams with low dispersion energy spectra centered around 10–20 MeV, and (c) localized heating at temperature minimum levels.On leave from: Department of Astronomy, The University, Glasgow G12 8QQ, Scotland, United Kingdom. 相似文献
815.
Speciation and colloid transport of arsenic from mine tailings 总被引:2,自引:0,他引:2
In addition to affecting biogeochemical transformations, the speciation of As also influences its transport from tailings at inoperative mines. The speciation of As in tailings from the Sulfur Bank Mercury Mine site in Clear Lake, California (USA) (a hot-spring Hg deposit) and particles mobilized from these tailings have been examined during laboratory-column experiments. Solutions containing two common, plant-derived organic acids (oxalic and citric acid) were pumped at 13 pore volumes d−1 through 25 by 500 mm columns of calcined Hg ore, analogous to the pedogenesis of tailings. Chemical analysis of column effluent indicated that all of the As mobilized was particulate (1.5 mg, or 6% of the total As in the column through 255 pore volumes of leaching). Arsenic speciation was evaluated using X-ray absorption spectroscopy (XAS), indicating the dominance of arsenate [As(V)] sorbed to poorly crystalline Fe(III)-(hydr)oxides and coprecipitated with jarosite [KFe3(SO4, AsO4)2(OH)6] with no detectable primary or secondary minerals in the tailings and mobilized particles. Sequential chemical extractions (SCE) of <45 μm mine tailings fractions also suggest that As occurs adsorbed to Fe (hydr)oxides (35%) and coprecipitated within poorly crystalline phases (45%). In addition, SCEs suggest that As is associated with 1 N acid-soluble phases such as carbonate minerals (20%) and within crystalline Fe-(hydr)oxides (10%). The finding that As is transported from these mine tailings dominantly as As(V) adsorbed to Fe (hydr)oxides or coprecipitated within hydroxysulfates such as jarosite suggests that As release from soils and sediments contaminated with tailings will be controlled by either organic acid-promoted dissolution or reductive dissolution of host phases. 相似文献
816.
We describe two small scoria cone volcanoes, Hidden Cone and Little Black Peak (ages between ~320–390 ka), in the Southwestern
Nevada Volcanic Field and discuss their eruption mechanisms and inferences about their plumbing systems. Cone-forming pyroclastic
deposits are consistent with eruptive styles ranging from Strombolian to violent Strombolian, and lavas emanated from near
the bases of the cones. The volcanoes are monogenetic (rather than polycyclic, as allowed by previous geomorphic interpretations).
Vents at each volcano appear to coincide with pre-existing normal faults, consistent with observations at older, deeply eroded
volcanoes in the region. The existence of these two volcanoes on a topographically high area (particularly Hidden Cone) provides
evidence for short feeder dike lengths (~500 m at the surface). We infer that this short length reflects the small length
scale of the mantle source region that was tapped to feed each volcano.
Editorial responsibility: J Stix 相似文献
817.
Benjamin J. Kadlec David A. Yuen Gordon Erlebacher 《Pure and Applied Geophysics》2006,163(11-12):2455-2465
Visualizing and analyzing datasets in the geosciences is becoming increasingly complicated as their volumes are growing explosively. This poses a challenging problem for researchers who must sift through terabytes of data to discover useful relationships inside the information. There is a great need for geophysicists to interactively explore their data sets. Conventional visualization systems lack adequate bandwidth and rendering capabilities necessary for the largest data sets. CAVE and Powerwall display devices are necessary for researchers to explore their data sets in an immersive setting. We describe a utilitarian system targeted specifically at the cost-effective interactive exploration of data sets tens of terabytes in size and harness this system for visualization and analysis of geophysical simulations. Webcams can be used as a steering device to track a local region of interest, which is useful for remote visualization of large data sets. This system will be employed as a web-service under the auspices of Narada-Brokering, while using webcam technologies to enable remote visualization for collaborating researchers. Webcams can be incorporated in a point-to-point network for rapid exchange of information and quickly announcing natural disasters, such as tsunamis, landslides and earthquakes. 相似文献
818.
Charles DeMets Richard G. Gordon Jean-Yves Royer 《Geophysical Journal International》2005,161(2):445-468
819.
820.
Stephen Zatman Richard G. Gordon Kartik Mutnuri 《Geophysical Journal International》2005,162(1):239-248