全文获取类型
收费全文 | 45342篇 |
免费 | 241篇 |
国内免费 | 985篇 |
专业分类
测绘学 | 1820篇 |
大气科学 | 3366篇 |
地球物理 | 8087篇 |
地质学 | 20993篇 |
海洋学 | 2637篇 |
天文学 | 5752篇 |
综合类 | 2181篇 |
自然地理 | 1732篇 |
出版年
2022年 | 30篇 |
2021年 | 37篇 |
2020年 | 33篇 |
2019年 | 44篇 |
2018年 | 6886篇 |
2017年 | 6186篇 |
2016年 | 3834篇 |
2015年 | 373篇 |
2014年 | 272篇 |
2013年 | 267篇 |
2012年 | 1946篇 |
2011年 | 4808篇 |
2010年 | 4218篇 |
2009年 | 4436篇 |
2008年 | 3557篇 |
2007年 | 4566篇 |
2006年 | 303篇 |
2005年 | 814篇 |
2004年 | 838篇 |
2003年 | 895篇 |
2002年 | 537篇 |
2001年 | 147篇 |
2000年 | 159篇 |
1999年 | 94篇 |
1998年 | 101篇 |
1997年 | 91篇 |
1996年 | 73篇 |
1995年 | 47篇 |
1994年 | 32篇 |
1993年 | 60篇 |
1992年 | 36篇 |
1991年 | 46篇 |
1990年 | 30篇 |
1989年 | 35篇 |
1988年 | 37篇 |
1987年 | 46篇 |
1986年 | 50篇 |
1985年 | 45篇 |
1984年 | 48篇 |
1983年 | 49篇 |
1982年 | 49篇 |
1981年 | 57篇 |
1980年 | 62篇 |
1979年 | 36篇 |
1978年 | 18篇 |
1977年 | 23篇 |
1976年 | 28篇 |
1975年 | 25篇 |
1974年 | 20篇 |
1973年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
971.
Contemporary piece of writing devotes to the investigation of plane symmetric cosmological model with quark and strange quark matter in the deformations of the Einstein’s theory of General Relativity (GR). At small or large scales (ultraviolet or infrared gravity), deformations of the Einstein’s theory could provide a better handling of cosmic acceleration without magnetism (along with singularities). In particular, a proper deformation of GR in the ultraviolet regime could play the role of describing the transition between GR and quantum gravity. As a matter of fact, although with a different purpose in mind, it was Einstein himself who proposed in the 30’s the reformulation of GR by taking the field of orthonormal frames or tetrads as the dynamical variable instead of the metric tensor (Einstein, Phys. Math. Kl 217, 401, 1928). As per the observation, pressure and energy density of the model approaches the bag constant in negative and positive ways at \(t\rightarrow \infty \), i.e. \(p\rightarrow -B_c \) and \(\rho \rightarrow B_c \), the negative pressure due to the Dark Energy (DE) in the context of accelerated expansion of the universe. So the strange quark matter gives an idea of existence of dark energy in the universe and supports the observations of the SNe-I (Riess et al., Astron. J. 116,1009, 1998; Perlmutter et al., Astrophys. J. 517, 565, 1999). Also these results agree with the study of Aktas and Aygun (Chinese J. Phys. 55, 71, 2017) and Sahoo et al. (New. Astron. 60, 80, 2018). 相似文献
972.
Joel Sanchez-Bermudez Florentin Millour Fabien Baron Roy van Boekel Laurent Bourgès Gilles Duvert Paulo J. V. Garcia Nuno Gomes Karl-Heinz Hofmann Thomas Henning Jacob W. Isbell Bruno Lopez Alexis Matter J-Uwe Pott Dieter Schertl Eric Thiébaut Gerd Weigelt John Young 《Experimental Astronomy》2018,46(3):457-473
During the last two decades, the first generation of beam combiners at the Very Large Telescope Interferometer has proved the importance of optical interferometry for high-angular resolution astrophysical studies in the near- and mid-infrared. With the advent of 4-beam combiners at the VLTI, the u ? v coverage per pointing increases significantly, providing an opportunity to use reconstructed images as powerful scientific tools. Therefore, interferometric imaging is already a key feature of the new generation of VLTI instruments, as well as for other interferometric facilities like CHARA and JWST. It is thus imperative to account for the current image reconstruction capabilities and their expected evolutions in the coming years. Here, we present a general overview of the current situation of optical interferometric image reconstruction with a focus on new wavelength-dependent information, highlighting its main advantages and limitations. As an Appendix we include several cookbooks describing the usage and installation of several state-of-the art image reconstruction packages. To illustrate the current capabilities of the software available to the community, we recovered chromatic images, from simulated MATISSE data, using the MCMC software SQUEEZE. With these images, we aim at showing the importance of selecting good regularization functions and their impact on the reconstruction. 相似文献
973.
A. V. Uryson 《Astronomy Letters》2018,44(8-9):541-545
The extragalactic sources of ultra-high-energy (E > 4 × 1019 eV) cosmic rays that make a small contribution to the flux of particles recorded by ground-based arrays are discussed. We show that cosmic rays from such sources can produce a noticeable diffuse gamma-ray flux in intergalactic space compared to the the data obtained with Fermi LAT (onboard the Fermi space observatory). A possible type of active galactic nuclei (AGNs) in which cosmi-ray protons can be accelerated to energies 1021 eV is considered as an illustration of such sources. We conclude that ultra-high-energy cosmic rays from the AGNs being discussed can contribute significantly to the extragalactic diffuse gamma-ray emission. In addition, a constraint on the fraction of the AGNs under consideration relative to the BL Lac objects and radio galaxies has been obtained from a comparison with the Fermi LAT data. 相似文献
974.
We studied the intermediate polar TVCol during and after its flare in November 1982 observed in the ultraviolet range with the International Ultraviolet Explorer. Two spectra revealing the variations of emission lines at different times are presented. We have estimated a new value of the reddening from the 2200 Å absorption feature, E (B ? V ) = 0.12 ± 0.02, and calculated the line fluxes of C IV and He II emission lines produced in the outer accretion disk. The average ultraviolet luminosity of emitting region during and after the flare is approximately 4 × 1032 erg s?1 and 9 × 1030 erg s?1, the corresponding average mass accretion rate is nearly 3 × 1015 erg s?1 (4.76 × 10?11M⊙ yr?1) and 5 × 1013 erg s?1 (7.93 × 10?13M⊙ yr?1), and the average temperature of the emitting region during and after flare is estimated to be of about 3.5 × 103 K and 2 × 103 K. We attribute this flare to a sudden increase in the mass accretion rate leading to the outburst activity. 相似文献
975.
On August 8, 2017, a Ms 7.0 earthquake occurred 5 km to the west of Jiuzhaigou National Park, causing 25 deaths and injuring 525. The objective of this study was to explore the seismogenic fault of the earthquake and tectonic dynamics of the source rupture. Field investigations, radon activity tests, remote sensing interpretations, and geophysical data analyses were carried out immediately after the earthquake. The Jiuzhaigou earthquake occurred at the intersection of the northern margin of the Minshan uplift belt and the south part of the Wenxian–Maqin fault in the south margin of the West Qinling geosyncline. There are two surface rupture zones trending northwest (NW), which are ground coseismic ruptures caused by concealed earthquake faults. The rupture on the southwest is the structure triggering the earthquake, along the Jiuzhaitiantang–Epicenter–Wuhuahai. The other one on the northeast (Shangsizhai–Zhongcha–Bimang) is a reactivation and extension of the secondary fault trending NW. The source rupture of this earthquake is a strike-slip shear fracture associated with the fault plane trending NW 331° and steeply dipping 75°, which is continuously expanding at both ends. The tectonic dynamics process of the source rupture is that the “Jiuzhaigou protrusion” is left-lateral sheared along the seismogenic fault in the NW direction. Finally, the Maqin fault and the arc fault system at the top of the “Wenxian protrusion” will be gradually broken through sometime in far future, as well as earthquaketriggered landslides will be further occurred along the narrow corridor between the seismogenic faults. The research results revealed the basic geological data and tectonic dynamic mechanism in this earthquake. 相似文献
976.
This study explores the tradeoff relationship between the number of initial attack firefighting resources and the level of fire ignition prevention efforts mitigating the probability of human-made fires in the Republic of Korea, where most fires are caused by human activities. To examine this tradeoff relationship, we develop a hybrid model that combines a robust optimization model with a stochastic simulation model. The robust optimization minimizes the expected number of fires not receiving a pre-defined response, such as the number of firefighting resources that must arrive at the fire within half an hour, subject to budget constraints and uncertainty about the daily number and location of fires. The simulation model produces a set of fire scenarios in which a combination of number, location, ignition time, and intensity of fires occur. Results show that fire ignition prevention is as cost-effective as initial attack firefighting resources given the current budget in the Republic of Korea for reducing the expected number of fires not covered by the predefined response. The mixed policy of fire suppression and fire prevention may produce some gains in efficiency relative to the dominant policy of strong fire suppression strategies. 相似文献
977.
A landslide susceptibility mapping study was performed using dynamic hillslope hydrology. The modified infinite slope stability model that directly includes vadose zone soil moisture (SM) was applied at Cleveland Corral, California, US and Krishnabhir, Dhading, Nepal. The variable infiltration capacity (VIC-3L) model simulated vadose zone soil moisture and the wetness index hydrologic model simulated groundwater (GW). The GW model predictions had a 75% NASH-Sutcliffe efficiency when compared to California’s in-situ GW measurements. The model performed best during the wet season. Using predicted GW and VIC-3L vadose zone SM, the developed landslide susceptibility maps showed very good agreement with mapped landslides at each study region. Previous quasi-dynamic model predictions of Nepal’s hazardous areas during extreme rainfall events were enhanced to improve the spatial characterization and provide the timing of hazardous conditions. 相似文献
978.
We study the spreading of wastewaters from an underwater source in a stratified coastal environment using the results of satellite monitoring and mathematical modeling. The problem is considered as applied to deepwater discharge in the region of Golubaya Bay of the Black Sea near Sebastopol. The main factors preventing upwelling of pollution to the sea surface are analyzed on the basis of a numerical model. It is shown that peculiarities of wastewater spreading depend on the character of stratification and velocity of the background current. The main factor influencing the uplift of these waters to the surface is the existence of water layers with high vertical gradients of water density. We reveal the structure of the wastewater field consisting of a plume and a jet extended in the direction of the background current, which is located in the density interface. If stratification is weak, the plume may reach the sea surface and form a local region of water pollution, which is recorded in multispectral satellite images. It is found that the mass of polluted waters is characterized by negative anomalies of temperature and salinity. 相似文献
979.
Obtaining absolute seafloor backscatter measurements from hydrographic multibeam echosounders is yet to be achieved. We propose a low-cost experiment to calibrate the various acquisition modes of a 30-kHz Kongsberg EM 302 multibeam echosounder in a range of water depths. We use a 38-kHz Simrad EK60 calibrated fisheries split-beam echosounder mounted at 45° angle on the vessel’s hull as a reference for the calibration. The processing to extract seafloor backscatter from the EK60 requires bottom detection, ray tracing and motion compensation to obtain acceptable geo-referenced backscatter measurements from this non-hydrographic system. Our experiment was run in Cook Strait, New Zealand, on well-known seafloor patches in shallow, mid, and deep-water depths. Despite acquisition issues due to weather, our results demonstrate the strong potential of such an approach to obtain system’s absolute calibration which is required for quantitative use of backscatter strength data. 相似文献
980.