全文获取类型
收费全文 | 707篇 |
免费 | 26篇 |
专业分类
测绘学 | 10篇 |
大气科学 | 28篇 |
地球物理 | 143篇 |
地质学 | 199篇 |
海洋学 | 67篇 |
天文学 | 202篇 |
综合类 | 4篇 |
自然地理 | 80篇 |
出版年
2021年 | 8篇 |
2020年 | 11篇 |
2019年 | 7篇 |
2018年 | 10篇 |
2017年 | 13篇 |
2016年 | 27篇 |
2015年 | 20篇 |
2014年 | 11篇 |
2013年 | 36篇 |
2012年 | 29篇 |
2011年 | 16篇 |
2010年 | 27篇 |
2009年 | 29篇 |
2008年 | 21篇 |
2007年 | 24篇 |
2006年 | 24篇 |
2005年 | 27篇 |
2004年 | 42篇 |
2003年 | 22篇 |
2002年 | 27篇 |
2001年 | 21篇 |
2000年 | 19篇 |
1999年 | 10篇 |
1998年 | 16篇 |
1997年 | 12篇 |
1996年 | 8篇 |
1995年 | 12篇 |
1994年 | 9篇 |
1993年 | 6篇 |
1992年 | 8篇 |
1991年 | 5篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1987年 | 5篇 |
1985年 | 8篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 11篇 |
1980年 | 11篇 |
1979年 | 13篇 |
1978年 | 13篇 |
1977年 | 7篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1973年 | 6篇 |
1972年 | 6篇 |
1971年 | 4篇 |
1968年 | 5篇 |
1966年 | 3篇 |
排序方式: 共有733条查询结果,搜索用时 0 毫秒
91.
Gravitational lensing allows us to probe the structure of matter on a broad range of astronomical scales, and as light from a distant source traverses an intervening galaxy, compact matter such as planets, stars, and black holes act as individual lenses. The magnification from such microlensing results in rapid brightness fluctuations which reveal not only the properties of the lensing masses, but also the surface brightness distribution in the source. However, while the combination of deflections due to individual stars is linear, the resulting magnifications are highly non-linear, leading to significant computational challenges which currently limit the range of problems which can be tackled. This paper presents a new and novel implementation of a numerical approach to gravitational microlensing, increasing the scale of the problems that can be tackled by more than two orders of magnitude, opening up a new regime of astrophysically interesting problems. 相似文献
92.
Stichtite, a rare (14 known localities worldwide) hydrated carbonate-hydroxide of Mg and Cr with ideal formula Mg6Cr2 (OH)16 CO3 · 4H2O, occurs exclusively in Cr-rich serpentinites of ophiolites or greenstone belts. Physical properties (hardness = 1.5–2, specific
gravity = 2.16–2.2, perfect basal [0001] cleavage, grain size commonly < 100 μm) resemble talc, but the mineral has an attractive
purple to lilac color; chemical analyses demonstrate it to be a non-silicate. Stichtite generally occurs as irregular to rounded
masses (< 1 cm – 30 cm across) and as veinlets (< 1 mm – > 2 cm wide) within serpentinite. Macroscopic and microscopic textures,
such as crosscutting veinlets and stringers, demonstrate that stichtite formation invariably post-dated serpentinization.
In some specimens stichtite surrounds relict grains of Cr-rich spinel; in others stichtite has completely replaced euhedral
or subhedral chromites. Chemical analyses of stichtites reveal substantial substitution of Al and Fe3+ for Cr in specimens from many localities, reflecting a possible compositional continuum between stichtite and rhombohedral
polymorphs hydrotalcite (Mg6Al2 (OH)16 CO3 · 4H2O) and pyroaurite (Mg6Fe2 (OH)16 CO3 · 4H2O). We report the first electron microprobe analyses of stichtites from seven localities, and summarize all available published
chemical data. Stichtites very likely inherited part of their trivalent cation chemistry from precursor Cr-rich spinels, but
stichtite growth apparently post-dated characteristic “ferritchromit” alteration, as demonstrated by the depletion of Al and
enrichment in Fe3+ in stichtite relative to primary chromite core compositions. Stichtite appears to form by reaction between serpentine and
altered chromite, during addition of substantial fluid, either as separate H2O and CO2 phases, or as a mixed volatile phase. Such reactions must involve removal of substantial SiO2, possibly by transport and remote deposition of silica by throughgoing aqueous and carbonic fluid.
Received: 4 April 1996 / Accepted: 16 September 1996 相似文献
93.
The physical nature of the material responsible for the high-velocity broad absorption line (BAL) features seen in a small fraction of quasar spectra has been the subject of debate since their discovery. This has been especially compounded by the lack of observational probes of the absorbing region. In this paper we examine the role of 'microlenses' in external galaxies in the observed variability in the profiles of BALs in multiply imaged quasars. Utilizing realistic models for both the BAL region and the action of an ensemble of microlensing masses, we demonstrate that stars at cosmological distances can provide an important probe of the physical state and structure of material at the heart of these complex systems. Applying these results to the macrolensed BAL quasar system H1413+117, the observed spectral variations are readily reproduced, but without the fine-tuning requirements of earlier studies which employ more simplistic models. 相似文献
94.
Meandering river sinuosity increases until the channel erodes into itself (neck cutoff) or forms a new channel over the floodplain (chute cutoff) and sinuosity is reduced. Unlike neck cutoff, which can be measured or modelled without considering overbank processes, chute cutoff must be at least partially controlled by channel-forming processes on the floodplain. Even though chute cutoff controls meandering river form, the processes that cause chute cutoff are not well understood. This study analyses the morphology of two incipient chute cutoffs along the East Fork White River, Indiana, USA, using high temporal and spatial resolution UAS-based LiDAR and aerial photography. LiDAR and aerial imagery obtained between 1998 and 2019 reveals that large scour holes formed in the centre of both chutes sometime after chute channel initiation. A larger analysis within the study watershed reveals that scour holes within incipient chutes can be stable or unstable, and tend to stabilize when the chute is colonized by native vegetation and forest. When the scour holes form in farmed floodplain, they enlarge rapidly after initial formation and contribute to complete chute cutoff. In addition, this study shows that the formation of scour holes can occur in response to common, relatively low-magnitude floods and that the amount of incipient chute erosion does not depend on peak flood magnitude. The role of scour holes in enlarging chute channels could be an important mechanism for chute channel evolution in meandering rivers. This study also confirms that understanding the relationships among flow, land cover, and cutoff morphology is substantially improved with on-demand remote sensing techniques like integrated UAS and LiDAR. © 2020 John Wiley & Sons, Ltd. 相似文献
95.
96.
97.
98.
Peder Norberg Carlton M. Baugh Ed Hawkins Steve Maddox John A. Peacock Shaun Cole Carlos S. Frenk Joss Bland-Hawthorn Terry Bridges Russell Cannon Matthew Colless Chris Collins Warrick Couch Gavin Dalton Roberto De Propris Simon P. Driver George Efstathiou Richard S. Ellis Karl Glazebrook Carole Jackson Ofer Lahav Ian Lewis Stuart Lumsden Darren Madgwick Bruce A. Peterson Will Sutherland Keith Taylor 《Monthly notices of the Royal Astronomical Society》2001,328(1):64-70
We investigate the dependence of the strength of galaxy clustering on intrinsic luminosity using the Anglo-Australian two degree field galaxy redshift survey (2dFGRS). The 2dFGRS is over an order of magnitude larger than previous redshift surveys used to address this issue. We measure the projected two-point correlation function of galaxies in a series of volume-limited samples. The projected correlation function is free from any distortion of the clustering pattern induced by peculiar motions and is well described by a power law in pair separation over the range . The clustering of galaxies in real space is well-fitted by a correlation length and power-law slope . The clustering amplitude increases slowly with absolute magnitude for galaxies fainter than M *, but rises more strongly at higher luminosities. At low luminosities, our results agree with measurements from the Southern Sky Redshift Survey 2 by Benoist et al. However, we find a weaker dependence of clustering strength on luminosity at the highest luminosities. The correlation function amplitude increases by a factor of 4.0 between and −22.5, and the most luminous galaxies are 3.0 times more strongly clustered than L * galaxies. The power-law slope of the correlation function shows remarkably little variation for samples spanning a factor of 20 in luminosity. Our measurements are in very good agreement with the predictions of the hierarchical galaxy formation models of Benson et al. 相似文献
99.
In this introduction to the Special Issue ‘Practising a New Regional Geography in Northland’, we call for a new regional geography. We use the experience of a field course to reflect upon the opportunities and challenges associated with doing ‘regional geography’ in the regions, a subdiscipline that has in recent decades been pushed into the backwaters of the discipline. We reinterpret the maxim ‘geography is what geographers do’ in a new way that emphasises pedagogy and research practice. The case of Northland and helping our students to learn experientially about community and environment under the rubric of sustainability allows us to argue the case for a reinterpretation of regional geography in these terms. We position the contributions in this Special Issue in terms of our call. 相似文献
100.