全文获取类型
收费全文 | 205篇 |
免费 | 8篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 2篇 |
大气科学 | 11篇 |
地球物理 | 77篇 |
地质学 | 87篇 |
海洋学 | 3篇 |
天文学 | 24篇 |
自然地理 | 14篇 |
出版年
2023年 | 1篇 |
2022年 | 3篇 |
2021年 | 4篇 |
2020年 | 6篇 |
2019年 | 7篇 |
2018年 | 12篇 |
2017年 | 11篇 |
2016年 | 8篇 |
2015年 | 9篇 |
2014年 | 6篇 |
2013年 | 13篇 |
2012年 | 8篇 |
2011年 | 16篇 |
2010年 | 16篇 |
2009年 | 8篇 |
2008年 | 9篇 |
2007年 | 14篇 |
2006年 | 8篇 |
2005年 | 1篇 |
2004年 | 9篇 |
2003年 | 3篇 |
2002年 | 7篇 |
2001年 | 7篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1998年 | 5篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1993年 | 2篇 |
1990年 | 1篇 |
1987年 | 1篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1940年 | 1篇 |
1925年 | 1篇 |
排序方式: 共有218条查询结果,搜索用时 15 毫秒
1.
Matteo Correnti Francesco Paresce Rossella Aversa Giacomo Beccari Guido De Marchi Marcella Di Criscienzo Xiaoying Pang Loredana Spezzi Elena Valenti Paolo Ventura 《Astrophysics and Space Science》2012,340(2):263-279
We have used new, deep, visible and near infrared observations of the compact starburst cluster in the giant HII region NGC 3603 and its surroundings with the WFC3 on HST and HAWK-I on the VLT to study in detail the physical properties of its intermediate mass (∼1–3 M⊙) stellar population. We show that after correction for differential extinction and actively accreting stars, and the study of field star contamination, strong evidence remains for a continuous spread in the ages of pre-main sequence stars in the range ∼2 to ∼30 Myr within the temporal resolution available. Existing differences among presently available theoretical models account for the largest possible variation in shape of the measured age histograms within these limits. We also find that this isochronal age spread in the near infrared and visible Colour-Magnitude Diagrams cannot be reproduced by any other presently known source of astrophysical or instrumental scatter that could mimic the luminosity spread seen in our observations except, possibly, episodic accretion. The measured age spread and the stellar spatial distribution in the cluster are consistent with the hypothesis that star formation started at least 20–30 Myrs ago progressing slowly but continuously up to at least a few million years ago. All the stars in the considered mass range are distributed in a flattened oblate spheroidal pattern with the major axis oriented in an approximate South-East–North-West direction, and with the length of the equatorial axis decreasing with increasing age. This asymmetry is most likely due to the fact that star formation occurred along a filament of gas and dust in the natal molecular cloud oriented locally in this direction. 相似文献
2.
Guido Travaglini 《Solar physics》2017,292(1):23
The Maunder Minimum (MM) was an extended period of reduced solar activity in terms of yearly sunspot numbers (SSN) during 1610?–?1715. The reality of this “grand minimum” is generally accepted in the scientific community, but the statistics of the SSN record suggest a need for data reconstruction. The MM data show a nonstandard distribution compared with the entire SSN signal (1610?–?2014). The pattern does not satisfy the weakly stationary solar dynamo approximation, which characterizes many natural events spanning centuries or even millennia, including the Sun and the stars. Over the entire observation period (1610?–?2014), the reported SSN exhibits statistically significant regime switches, departures from autoregressive stationarity, and growing trends. Reconstruction of the SSN during the pre-MM and MM periods is performed using five novel statistical procedures in support of signal analysis. A Bayesian–Monte Carlo backcast technique is found to be most reliable and produces an SSN signal that meets the weak-stationarity requirement. The computed MM signal for this reconstruction does not show a “grand” minimum or even a “semi-grand” minimum. 相似文献
3.
4.
Photoionization by stars cannot be the only cause for the diffuseionized gas component in the distant halos of galaxies.Quasistationary localized magnetic reconnection can efficientlycontribute to the ionization of the extraplanar halo gas. The gas isheated and re-ionized in numerous current filaments fast enough tocompensate recombination. The necessary magnetic energy is provided bythe disk activity. 相似文献
5.
6.
Although the logistics industry provides critical services to all sectors of the economy, few studies exist in economic geography that examine and explain the organizational dynamics of this industry. This article highlights the significance of the logistics industry in contemporary industrialization and argues that an enriched understanding of the interaction between technology and space can be achieved by examining the evolution of an industry that plays a central role in the contemporary economy. We focus on dimensions that are identified as particularly important: organizational, geographic, and risk and security. 相似文献
7.
Experimental solidification of anhydrous latitic and trachytic melts at different cooling rates: The role of nucleation kinetics 总被引:1,自引:0,他引:1
Gianluca Iezzi Silvio Mollo Guido Ventura Andrea Cavallo Claudia Romano 《Chemical Geology》2008,253(3-4):91-101
Two sets of cooling experiments were run at atmospheric conditions for two anhydrous starting latitic and trachytic melts: 1) five cooling rates (25, 12.5, 3, 0.5, and 0.125 °C/min) between 1300° and 800 °C, and 2) a 0.5 °C/min cooling rate from 1300 °C with quench temperatures at 1200°, 1100°, 1000° and 900 °C. Trachytic run-products are invariably glassy. Nucleation is also suppressed in the latitic run-products at the three highest cooling rates. Conversely, in the 0.5 and 0.125 °C/min runs, latites have a crystal content of 90 vol.%. The phases are: plagioclase, clinopyroxene, glass and iron-bearing oxide (in order of abundance). The variable quench temperatures, investigated by coupling experiments with Pt wire and Pt capsule sample containers in set 2, again did not produce crystallization of trachyte, whereas latitic samples are characterized by 10 vol.% of oxides, pyroxenes and plagioclase (in order of appearance), at temperature < 1000 °C. Effects of (preferential) heterogeneous nucleation on sample holders, of superheating degree, and chemical species loss during cooling are absent for both melt compositions. The difference of solidification paths between these two silicate melts can be ascribed only to their small chemical differences. In comparison with calculated equilibrium conditions all the experimental latitic and trachytic run-products revealed strong kinetic effects, interpretable in the light of the nucleation theory. The glass-forming ability (GFA) of trachyte is higher, whereas their critical cooling rate (Rc) is lower (< 0.125 °C/min), in comparison to latitic melts (Rc > 0.5 °C/min). The experimental results carried out in this study can be applied to lava flows and domes; trachytic lavas are able to flow for longer period with respect to latitic ones in a metastable condition. Glass-rich terrestrial lavas, i.e. obsidians, can be the result of sluggish nucleation kinetics due to the relative high polymerisation of evolved silicate melts. 相似文献
8.
Luigi Bruno Michela Marchi Ilaria Bertolini Guido Gottardi Alessandro Amorosi 《第四纪科学杂志》2020,35(4):559-571
Paleosols are recurrent features in alluvial successions and provide information about past sedimentary dynamics and climate change. Through sedimentological analysis on six sediment cores, the mud-dominated succession beneath the medieval ‘Two Towers’ of Bologna was investigated down to 100 m depth. A succession of weakly developed paleosols (Inceptisols) was identified. Four paleosols (P1, P2, P3 and PH) were radiocarbon-dated to 40–10 cal ka bp . Organic matter and CaCO3 determinations indicate low groundwater levels during soil development, which spanned periods < 5 ka. The development and burial of soils, which occurred synchronously in the Bologna region and in other sectors of the Po Plain, are interpreted to reflect climatic and eustatic variations. Climatic oscillations, at the scale of the Bond cycles, controlled soil development and burial during Marine Isotope Stage (MIS) 3 (P1 and P2). Rapid sea-level oscillations probably induced soil development at the MIS 3/2 transition (P3) and favored burial of PH after 10 ka bp . Weakly developed paleosols in alluvial successions can provide clues to millennial-scale climatic and environmental variations. In particular, the paleosol-bearing succession of the Po Plain represents an unprecedent record of environmental changes across the Late Pleistocene (MIS 3 and 2) in the Mediterranean region. 相似文献
9.
Guido G Weigend 《Geoforum》1975,6(2):151-161
The Danube River, Europe's longest west of the Soviet Union, connects central and southeastern Europe. It flows through areas of great diversity of climates and land forms. Navigation has, in fact, been hindered by fluctuating water levels, ice, and defiles. Primarily for political reasons the river, prior to the middle of the 20th century, had never been a viable link of transportation and communication. Since the second world war, however, effective internationalization of the river, extensive navigation improvement projects, construction of dams and hydroelectric power plants, and greatly accelerated economic development in the Danube River basin in general, all point toward the river becoming one of Europe's principal axes and regional bonds. 相似文献
10.
Upper Pollara eruption products (13 ka, Salina Island, Italy) include both homogeneous and heterogeneous pumices resulting from mixing/mingling processes between an HK andesite and a high-SiO2 rhyolite. Representative samples of heterogeneous pumices are collected and analyzed in order to check the correspondence between glass composition and morphological features of the mingling/mixing structures. Image analysis techniques are applied and eight grey color ranges (classes) are extracted from high-resolution scans of pumice. Class 1 (lighter colors) and class 8 (darker colors) show end-member glass compositions, i.e. HK andesite and high-SiO2 rhyolite, respectively. These two classes show spot- to cluster-like morphological structures. Intermediate classes show an HK dacitic to rhyolitic composition and a banding- to fold-like morphology. Fractal analysis by box-counting of the boundary pattern of eight grey classified images is performed over a length scale of 0.028–1.8 cm. Fractal dimension D is between 1.01 and 1.84. Coupled fractal analysis and geochemical data reveal that D increases as the degree of magma interaction (homogenization) increases. This feature well fits the results from numerical models on the convective mixing of fluids driven by thermal convection. We conclude that the increase of D observed in the Upper Pollara samples reflects the transition from fractal mixing to homogenization. End-member magmas (HK andesite and high-SiO2 rhyolite) represent isolated mixing regions, while homogenized magmas represent active mixing regions. In the analyzed pumices, isolated and active mixing regions coexist at scales between 10−4 and 10−2 m. Morphological and compositional features of the Upper Pollara pumices result from turbulence. 相似文献