全文获取类型
收费全文 | 7884篇 |
免费 | 1024篇 |
国内免费 | 2044篇 |
专业分类
测绘学 | 308篇 |
大气科学 | 3365篇 |
地球物理 | 630篇 |
地质学 | 924篇 |
海洋学 | 836篇 |
天文学 | 4084篇 |
综合类 | 343篇 |
自然地理 | 462篇 |
出版年
2024年 | 49篇 |
2023年 | 70篇 |
2022年 | 127篇 |
2021年 | 188篇 |
2020年 | 197篇 |
2019年 | 227篇 |
2018年 | 170篇 |
2017年 | 209篇 |
2016年 | 194篇 |
2015年 | 281篇 |
2014年 | 306篇 |
2013年 | 346篇 |
2012年 | 354篇 |
2011年 | 330篇 |
2010年 | 293篇 |
2009年 | 660篇 |
2008年 | 607篇 |
2007年 | 674篇 |
2006年 | 662篇 |
2005年 | 674篇 |
2004年 | 652篇 |
2003年 | 583篇 |
2002年 | 449篇 |
2001年 | 463篇 |
2000年 | 446篇 |
1999年 | 388篇 |
1998年 | 484篇 |
1997年 | 145篇 |
1996年 | 123篇 |
1995年 | 118篇 |
1994年 | 117篇 |
1993年 | 68篇 |
1992年 | 65篇 |
1991年 | 62篇 |
1990年 | 27篇 |
1989年 | 21篇 |
1988年 | 18篇 |
1987年 | 13篇 |
1986年 | 11篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 13篇 |
1980年 | 5篇 |
1977年 | 6篇 |
1973年 | 5篇 |
1972年 | 3篇 |
1971年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 859 毫秒
841.
We briefly review recent developments in black hole accretion disk theory, emphasizing the vital role played by magnetohydrodynamic
(MHD) stresses in transporting angular momentum. The apparent universality of accretion-related outflow phenomena is a strong
indicator that large-scale MHD torques facilitate vertical transport of angular momentum. This leads to an enhanced overall
rate of angular momentum transport and allows accretion of matter to proceed at an interesting rate. Furthermore, we argue
that when vertical transport is important, the radial structure of the accretion disk is modified at small radii and this
affects the disk emission spectrum. We present a simple model demonstrating how energetic, magnetically-driven outflows modify
the emergent disk emission spectrum with respect to that predicted by standard accretion disk theory. A comparison of the
predicted spectra against observations of quasar spectral energy distributions suggests that mass accretion rates inferred
using the standard disk model may be severely underestimated. 相似文献
842.
In this article, an asymmetric counterstreaming distribution function is investigated on the basis of three-dimensional relativistic
particle-in-cell simulations for wave propagation at an oblique angle with respect to the axis of the counterstream. For such
asymmetric distribution functions, any linear Weibel modes must be isolated and therefore restricted to discrete wavenumber
values. Using analytical linear Vlasov theory, this result has recently been proven generally, and has been illustrated by
the example of an electron beam counterstreaming with a positron beam. By the means of self-consistent particle-in-cell simulations,
in this paper a realistic distribution function is investigated that consists of neutral asymmetric Maxwellian counterstreams.
For this scenario, the existence of isolated modes can be confirmed, especially when compared to the case of symmetric counterstreams.
相似文献
843.
Mari Ito Myron J. Mitchell Charles T. Driscoll Robert M. Newton Chris E. Johnson Karen M. Roy 《水文研究》2007,21(10):1249-1264
The southwestern Adirondack region of New York receives among the highest rates of atmospheric nitrogen (N) deposition in the USA. Atmospheric N deposition to sensitive ecosystems, like the Adirondacks, may increase the acidification of soils through losses of exchangeable nutrient cations, and the acidification of surface waters associated with enhanced mobility of nitrate (NO3?). However, watershed attributes, including surficial terrestrial characteristics, in‐lake processing, and geological settings, have been found to complicate the relationships between atmospheric N deposition and N drainage losses. We studied two lake‐watersheds in the southwestern Adirondacks, Grass Pond and Constable Pond, which are located in close proximity (~26 km) and receive similarly high N deposition, but have contrasting watershed attributes (e.g. wetland area, geological settings). Since the difference in the influence of N deposition was minimal, we were able to examine both within‐ and between‐watershed influences of land cover, the contribution of glacial till groundwater inputs, and in‐lake processes on surface water chemistry with particular emphasis on N solutes and dissolved organic carbon (DOC). Monthly samples at seven inlets and one outlet of each lake were collected from May to October in 1999 and 2000. The concentrations of NO3? were high at the Grass Pond inlets, especially at two inlets, and NO3? was the major N solute at the Grass Pond inlets. The concentrations of likely weathering products (i.e. dissolved Si, Ca2+, Mg2+, Na+) as well as acid neutralizing capacity and pH values, were also particularly high at those two Grass Pond inlets, suggesting a large contribution of groundwater inputs. Dissolved organic N (DON) was the major N solute at the Constable Pond inlets. The higher concentrations of DON and DOC at the Constable Pond inlets were attributed to a large wetland area in the watershed. The DOC/DON ratios were also higher at the Constable Pond inlets, possibly due to a larger proportion of coniferous forest area. Although DON and DOC were strongly related, the stronger relationship of the proportion of wetland area with DOC suggests that additional factors regulate DON. The aggregated representation of watershed physical features (i.e. elevation, watershed area, mean topographic index, hypsometric‐analysis index) was not clearly related to the lake N and DOC chemistry. Despite distinctive differences in inlet N chemistry, NO3? and DON concentrations at the outlets of the two lakes were similar. The lower DOC/DON ratios at the lake outlets and at the inlets having upstream ponds suggest the importance of N processing and organic N sources within the lakes. Although an inverse relationship between NO3? and DOC/DON has been suggested to be indicative of a N deposition gradient, the existence of this relationship for sites that receive similar atmospheric N deposition suggest that the relationship between NO3? and the DOC/DON ratio is derived from environmental and physical factors. Our results suggest that, despite similar wet N deposition at the two watershed sites, N solutes entering lakes were strongly affected by hydrology associated with groundwater contribution and the presence of wetlands, whereas N solutes leaving lakes were strongly influenced by in‐lake processing. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
844.
Multi‐scale investigations aided by the discrete element method (DEM) play a vital role for current state‐of‐the‐art research on the elementary behaviour of granular materials. Similar to laboratory tests, there are three important aspects to be considered carefully, which are the proper stress/strain definition and measurement, the application of target loading paths and the designed experiment setup, to be addressed in the present paper. Considering the volume sensitive characteristics of granular materials, in the proposed technique, the deformation of the tested specimen is controlled and measured by deformation gradient tensor involving both the undeformed configuration and the current configuration. Definitions of Biot strain and Cauchy stress are adopted. The expressions of them in terms of contact forces and particle displacements, respectively, are derived. The boundary of the tested specimen consists of rigid massless planar units. It is suggested that the representative element uses a convex polyhedral (polygonal) shape to minimize possible boundary arching effects. General loading paths are described by directly specifying the changes in the stress/strain invariants or directions. Loading can be applied in the strain‐controlled mode by specifying the translations and rotations of the boundary units, or in the stress‐controlled mode by using a servo‐control mechanism, or in the combination of the two methods to realize mixed boundary conditions. Taking the simulation results as the natural consequences originated from a complex system, virtual experiments provide particle‐scale information database to conduct multi‐scale investigations for better understanding in granular material behaviours and possible development of the constitutive theories provided the qualitative similarity between the simulation results from virtual experiments and observations on real material behaviour. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
845.
The current study focuses on understanding key factors controlling geochemical export in eight diverse coastal watersheds at seasonal and annual time scales. Geochemical, atmospheric and hydrologic data across a range of hydro‐climatic regimes and varying land uses were investigated and relationships analysed. A hyperbolic dilution model was fitted for each watershed system to evaluate discharge–concentration relationships. Nitrate concentration effects were observed in watersheds exposed to high atmospheric deposition rates as well as agricultural watersheds, whereas urban watersheds showed nitrate dilution effects. Dilution patterns were observed for calcium, magnesium and sulfate for almost all watersheds. Seasonal loads for almost all constituents were noted to be mainly driven by hydrologic seasonality, but are also dependent on inputs (atmospheric deposition and land use sources). Understanding the primary controls on hydro‐chemical interactions is critical for developing and refining predictive water quality models, especially in coastal watersheds where sensitive downstream ecosystems act as receiving waters for upstream pollutant loads. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
846.
L. Michaille J.B. Clifford J.C. Dainty † T. Gregory J.C. Quartel F.C. Reavell R.W. Wilson N.J. Wooder 《Monthly notices of the Royal Astronomical Society》2001,328(4):993-1000
We present the results of a mesospheric sodium monitoring programme at La Palma carried out through five campaigns of one week each, from 1999 September to 2000 August. The yearly averaged parameters of the layer (the sodium column density and the width) are given. We show that the short time-scale dynamics of the layer are governed by the sporadic layers with an average frequency of one event per night. The influence of the short time-scale dynamics of the layer on an adaptive optics system working on the William Herschel Telescope is quantified. It appears that it is a small effect in terms of defocus error. Finally, we present data obtained during the Perseid meteor shower and show that the dynamics of the sodium layer undergoes a transition with the meteoric activity. 相似文献
847.
Robert J. Scherrer Enrique Gaztañaga 《Monthly notices of the Royal Astronomical Society》2001,328(1):257-265
We use the spherical collapse (SC) approximation to derive expressions for the smoothed redshift-space probability distribution function (PDF), as well as the p -order hierarchical amplitudes S p , in both real and redshift space. We compare our results with numerical simulations, focusing on the standard CDM model, where redshift distortions are strongest. We find good agreement between the SC predictions and the numerical PDF in real space even for , where σ L is the linearly evolved rms fluctuation on the smoothing scale. In redshift space, reasonable agreement is possible only for . Numerical simulations also yield a simple empirical relation between the real-space PDF and the redshift-space PDF: we find that for , the redshift-space PDF, [ P δ ( z ) ], is, to a good approximation, a simple rescaling of the real-space PDF, P [ δ ], i.e., where σ and σ ( z ) are the real-space and redshift-space rms fluctuations, respectively. This result applies well beyond the validity of linear perturbation theory, and it is a good fit for both the standard CDM model and the ΛCDM model. It breaks down for SCDM at , but provides a good fit to the ΛCDM models for σ L as large as 0.8. 相似文献
848.
849.
850.