全文获取类型
收费全文 | 413篇 |
免费 | 20篇 |
专业分类
测绘学 | 5篇 |
大气科学 | 26篇 |
地球物理 | 127篇 |
地质学 | 104篇 |
海洋学 | 16篇 |
天文学 | 102篇 |
自然地理 | 53篇 |
出版年
2021年 | 5篇 |
2020年 | 4篇 |
2019年 | 3篇 |
2018年 | 12篇 |
2017年 | 13篇 |
2016年 | 9篇 |
2015年 | 13篇 |
2014年 | 6篇 |
2013年 | 39篇 |
2012年 | 18篇 |
2011年 | 9篇 |
2010年 | 11篇 |
2009年 | 18篇 |
2008年 | 14篇 |
2007年 | 18篇 |
2006年 | 16篇 |
2005年 | 19篇 |
2004年 | 9篇 |
2003年 | 9篇 |
2002年 | 4篇 |
2001年 | 12篇 |
2000年 | 9篇 |
1999年 | 7篇 |
1998年 | 11篇 |
1997年 | 14篇 |
1996年 | 5篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1992年 | 3篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1988年 | 4篇 |
1987年 | 10篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1983年 | 3篇 |
1982年 | 13篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 2篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 9篇 |
1973年 | 8篇 |
1971年 | 2篇 |
1970年 | 2篇 |
1875年 | 2篇 |
排序方式: 共有433条查询结果,搜索用时 781 毫秒
371.
I. S. Bojii M. D. Filipovi Q. A. Parker J. L. Payne P. A. Jones W. Reid A. Kawamura Y. Fukui 《Monthly notices of the Royal Astronomical Society》2007,378(4):1237-1247
We present a new multiwavelength study of supernova remnant (SNR) B0513−692 in the Large Magellanic Cloud (LMC). The remnant also has a strong, superposed, essentially unresolved, but unrelated radio source at its north-western edge, J051324−691049. This is identified as a likely compact H ii region based on related optical imaging and spectroscopy. We use the Australia Telescope Compact Array (ATCA) at 4790 and 8640 MHz (λ≃ 6 cm and λ≃ 3.5 cm) to determine the large-scale morphology, spectral index and polarization characteristics of B0513−692 for the first time. We detect a strongly polarized region (49 per cent) in the remnant's southern edge (λ≃ 6 cm) . Interestingly, we also detect a small (∼40 arcsec) moderately bright, but distinct optical, circular shell in our Hα imagery which is adjacent to the compact H ii region and just within the borders of the north-eastern edge of B0513−692. We suggest that this is a separate new SNR candidate based on its apparently distinct character in terms of optical morphology in three imaged emission lines and indicative SNR optical spectroscopy (including enhanced optical [S ii ] emission relative to Hα). 相似文献
372.
Martin Cohen Quentin A. Parker Anne J. Green 《Monthly notices of the Royal Astronomical Society》2005,360(4):1439-1447
In a search for new Galactic planetary nebulae from our systematic scans of the Anglo-Australian Observatory/United Kingdom Schmidt Telescope (AAO/UKST) Hα Survey of the Southern Galactic Plane, we have identified a Population I Wolf–Rayet star of type WN7h associated with an unusual ring nebula that has a fractured rim. We present imagery in Hα, the 843-MHz continuum from the Molonglo Observatory Synthesis Telescope (MOST), the mid-infrared from the Midcourse Space Experiment ( MSX ), and confirmatory optical spectroscopy of the character of the nebula and of its central star. The inner edge of the Hα shell shows gravitational instabilities with a well-defined wavelength around its complete circumference. 相似文献
373.
374.
Janet E. Drew R. Greimel M. J. Irwin A. Aungwerojwit M. J. Barlow R. L. M. Corradi J. J. Drake B. T. Gänsicke P. Groot A. Hales E. C. Hopewell J. Irwin C. Knigge P. Leisy D. J. Lennon A. Mampaso M. R. W. Masheder M. Matsuura L. Morales-Rueda R. A. H. Morris Q. A. Parker S. Phillipps P. Rodriguez-Gil G. Roelofs I. Skillen J. L. Sokoloski D. Steeghs Y. C. Unruh K. Viironen J. S. Vink N. A. Walton A. Witham N. Wright A. A. Zijlstra A. Zurita 《Monthly notices of the Royal Astronomical Society》2005,362(3):753-776
375.
Kodak Technical Pan (Tech-Pan) film is a standard panchromatic emulsion that was used at the UK 1.2-m Schmidt Telescope (UKST) for a variety of broad-band exposures between 1992 and 2003. However, despite its versatility and frequent use, no formal presentation of its colour-term stability has been provided. A sample of 32 short-exposure Tech-Pan films was specifically acquired in five wavebands with a view to establishing the photometric relationships between Tech-Pan and the equivalent glass-based emulsions previously used. These films have been measured using the SuperCOSMOS measuring machine and the data reduced to provide the appropriate quantitative information. New colour terms relating these measurements to the Cousins photoelectric system are presented. They are shown to be stable, reproducible, generally small and similar to colour terms previously derived for the older emulsions on glass that Tech-Pan effectively replaced. These results give confidence in the use of Tech-Pan photometry from on-line UKST legacy survey archives. 相似文献
376.
377.
Eugene N.Parker 《天文和天体物理学研究(英文版)》2014,(1):1-14
Reminiscing begins with childhood and passes on to student days through graduate school and the first real contact with research.Then early academic positions and stumbling efforts to pursue my ideas.The first significant progress came as a research associate with Prof.W.M.Elsasser at the University of Utah,beginning with an introduction to magnetohydrodynamics and the generation of the geomagnetic field through induction in the liquid metal core of Earth.A move to the University of Chicago to work with Prof.J.A.Simpson,on the implications of cosmic ray variations and interplanetary magnetic fields,led to the theory of coronal expansion and the solar wind and then to exploring the dynamical effects of cosmic rays on the galactic magnetic field.Spontaneous current sheets and intrinsic rapid reconnection in interlaced magnetic field line topologies were the next big project,leading up to retirement.Finally,it is a pleasure to recall my many associates,whose fresh thinking helped stimulate the daily research activities. 相似文献
378.
Qina Yan Toshiki Iwasaki Andrew Stumpf Patrick Belmont Gary Parker Praveen Kumar 《地球表面变化过程与地形》2018,43(1):218-228
Floodplains and terraces in river valleys play important roles in the transport dynamics of water and sediment. While flat areas in river valleys can be identified from LiDAR data, directly characterizing them as either floodplain or terraces is not yet possible. To address this challenge, we hypothesize that, since geomorphic features are strongly coupled to hydrological and hydraulic dynamics and their associated variability, there exists a return frequency, or possibly a narrow band of return frequencies, of flow that is associated with floodplain formation; and this association can provide a distinctive signature for distinguishing them from terraces. Based on this hypothesis we develop a novel approach for distinguishing between floodplains and terraces that involves transforming the transverse cross‐sectional geometry of a river valley into a curve, named a river valley hypsometric (RVH) curve, and linking hydraulic inundation frequency with the features of this curve. Our approach establishes that the demarcation between floodplains and terraces can be established from the structure of steps and risers in the RVH curves which can be obtained from the DEM data. Further, it shows that these transitions may themselves be shaped by floods with 10‐ to 100‐year recurrence. We additionally show that, when floodplain width and height (above channel bottom) are normalized by bankfull width and depth, the ratio lies in a narrow range independent of the scale of the river valley. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
379.
When fine sediments are present in gravel streambeds (gravel‐framework beds), the gravel can be more easily removed from its original position, compared with gravel in a streambed without fine sediment but otherwise under the same hydraulic conditions. In this study, the effect of the presence of sand on the initiation of gravel motion in gravel riverbeds was investigated using flume experiments. The relationship between the critical Shields stress for gravel motion initiation and the fraction of sand in the bed was determined experimentally. The results can be summarized as follows. (1) When the fraction of sand in the bed is smaller than about 0.4, the critical Shields stress for the initiation of gravel motion decreases with increasing fraction of sand. The critical Shields stress increases, however, with increasing fraction of sand when it is larger than about 0.4. (2) The difference between the value of the critical Shields stress predicted by the Egiazaroff equation and the value obtained from the experimental data becomes maximum at about 0.4 of the fraction of sand. Here an empirical relation between the critical Shields stress and the fraction of sand is proposed so as to consider the effects of the ratio of the characteristic gravel size to the mean size of the bed material on the critical Shields stress. (3) Gravel in armored beds can be more easily mobilized by supplying sand as part of a sediment augmentation scheme. The sand fraction in the subsurface layer of the bed appears to reduce the friction angle of exposed particles. Sediment augmentation using sand has been recently demonstrated to be a viable alternative for mobilizing gravel for the restoration of gravel‐bed rivers downstream of dams. The quantitative evaluation obtained through the experiments reported here may be useful for the design of augmentation schemes. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
380.
The Laguna Madre of South Texas is a shallow coastal lagoon whose dominant primary producers shifted from seagrasses to phytoplankton with the onset of the Texas brown tide, which persisted from 1990 through 1997. Acartia tonsa is the dominant component of the mesozooplankton and forms an important link in both the phytoplankton and detritus-based pelagic food webs. Stable carbon isotope ratios of A. tonsa, as well as the two major primary producers: phytoplankton (as particulate organic carbon) and seagrasses, were measured from March 1989 to October 1991. Zooplankton samples were collected at four locations in the Laguna Madre: two in shallow water (c. 1 m) over seagrass beds and two in slightly deeper water (c. 2–3 m) over a muddy bottom in a secondary bay without seagrasses. We found seasonal trends in the isotopic composition of A. tonsa collected within both habitats as well as distinct differences between the average {ie995-1} values of individuals collected in the two regions. Isotopic ratios of animals collected during the summer months were generally 4–8‰ enriched in 13C compared with those collected in the winter, at all stations. A. tonsa collected over seagrass beds were 2–5‰ more enriched in 13C than those collected over muddy bottoms. These observations suggest carbon derived from seagrasses can be an important source of nutrition for these copepods in summer, especially for copepods living over seagrass beds. The effects of the persistent brown tide decreased the contribution of seagrasses as a carbon source for A. tonsa during the summer of 1991. The pathway by which seagrass carbon enters the diet of A. tonsa is unclear, but the two pathways considered most likely are through copepods feeding on microzooplankton that have fed on bacteria nourished on seagrass carbon, or by copepods feeding directly on particles of seagrass detritus. 相似文献