全文获取类型
收费全文 | 94042篇 |
免费 | 1149篇 |
国内免费 | 1432篇 |
专业分类
测绘学 | 3095篇 |
大气科学 | 6292篇 |
地球物理 | 18586篇 |
地质学 | 36049篇 |
海洋学 | 7403篇 |
天文学 | 18666篇 |
综合类 | 2321篇 |
自然地理 | 4211篇 |
出版年
2022年 | 574篇 |
2021年 | 888篇 |
2020年 | 976篇 |
2019年 | 1095篇 |
2018年 | 6801篇 |
2017年 | 5901篇 |
2016年 | 4883篇 |
2015年 | 1384篇 |
2014年 | 2314篇 |
2013年 | 3681篇 |
2012年 | 3409篇 |
2011年 | 5629篇 |
2010年 | 4753篇 |
2009年 | 5745篇 |
2008年 | 4942篇 |
2007年 | 5527篇 |
2006年 | 3073篇 |
2005年 | 2184篇 |
2004年 | 2324篇 |
2003年 | 2190篇 |
2002年 | 2047篇 |
2001年 | 1726篇 |
2000年 | 1565篇 |
1999年 | 1223篇 |
1998年 | 1260篇 |
1997年 | 1259篇 |
1996年 | 986篇 |
1995年 | 955篇 |
1994年 | 888篇 |
1993年 | 770篇 |
1992年 | 700篇 |
1991年 | 716篇 |
1990年 | 758篇 |
1989年 | 715篇 |
1988年 | 653篇 |
1987年 | 719篇 |
1986年 | 617篇 |
1985年 | 793篇 |
1984年 | 909篇 |
1983年 | 806篇 |
1982年 | 789篇 |
1981年 | 728篇 |
1980年 | 673篇 |
1979年 | 596篇 |
1978年 | 646篇 |
1977年 | 558篇 |
1976年 | 501篇 |
1975年 | 506篇 |
1974年 | 467篇 |
1973年 | 542篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
Christopher P. McKay 《Planetary and Space Science》1985,33(7):761-771
To investigate the occurrence of low temperatures and the formation of noctilucent clouds in the summer mésosphere a one-dimensional time-dependent photochemical-thermal numerical model of the atmosphere between 50 and 120 km has been constructed. The model includes the important chemistry of the hydrogen and oxygen species and transport by eddy and molecular processes. The thermal balance incorporates: heating by solar ultraviolet radiation; transport of chemical potential energy; eddy diffusion and dissipation; molecular conduction; airglow emissions; and infrared cooling by carbon dioxide. A non- LTE parameterization is used to calculate 15 μm band cooling by carbon dioxide. The model self-consistently solves the coupled photochemical and thermal equations as perturbation equations from a reference state assumed to be in equilibrium and is used to consider the effect of variability in water vapor in the lower mesosphere on the temperature in the region of noctilucent cloud formation. It is found that change in water vapor from an equilibrium value of 5 ppm at 50 km to a value of 10 ppm, a variation consistent with observations, can produce a ~ 15 K drop in temperature at 82 km. It is suggested that this process may produce long periods (weeks) of cold temperatures and influence noctilucent cloud formation. 相似文献
132.
133.
An HF radar called the Coastal Ocean Dynamics Applications Radar (CODAR) is presently being used in several forms to measure ocean surface parameters. The original version was developed by the National Oceanic and Atmospheric Administration (NOAA) and utilizes a four-element receive array. The array consists of four equally spaced elements arranged on a circle with a radius of 0.2151 wavelengths (at 25.4 MHz). It was designed to measure ocean currents using a direction-finding technique based on an extension to a simple two-element interferometer. The problem of determining the bearing of a radiating source can be readily shown to be equivalent to that incurred in spectral estimation. In an attempt to improve upon the processing of existing data, modern nonlinear spectral estimation techniques are applied in a beam-forming bearing estimation procedure and compared against several direction-finding algorithms. Enhancement of bearing estimators via analysis of the eigenstructure of a spatial correlation matrix is included. Antenna response patterns are calculated and used to investigate properties of direction-finding algorithms. Simulated data are used for a comparison of direction finding and beam forming. The asymmetrical bias of each method is investigated to determine its effect on the error in estimating the angle of arrival of a radar target. 相似文献
134.
The present work applies the method of characteristics to study the behaviour of planar and cylindrical wave-heads propagating through a perfectly electrically conducting and thermally radiating inviscid gas under the optically thin limit in the presence of a transverse magnetic field. The true nonlinear progress of the flow variable gradients at the wavefront is predicted and the critical distance at which the characteristics pile up at the wavefront to form a shock wave is obtained. It is investigated as to how the effects of radiative flux, the magnetic field strength and the specific heat ratio influence the process of steepening or flattening of the characteristic wavefront. 相似文献
135.
136.
We performed polarization observations of giant radio pulses from the millisecond pulsar B1937+21. The observations were carried out in July 2002 with the 64-m Kalyazin radio telescope at a frequency of 600 MHz in two polarization channels with left-and right-hand circular polarizations (RCP and LCP). We used the S2 data acquisition system with a time resolution of 125 ns. The duration of an observing session was 20 min. We detected twelve giant radio pulses with peak flux densities higher than 1000 Jy; five and seven of these pulses appeared in the RCP and LCP channels, respectively. We found no event that exceeded the established detection threshold simultaneously in the two polarization channels. Thus, we may conclude that the detected giant pulses have a high degree of circular polarization, with the frequency of occurrence of RCP and LCP pulses being the same. 相似文献
137.
G. Djurašević P. Rovithis H. Rovithis-Livaniou E. Fragoulopoulou 《Astrophysics and Space Science》2005,296(1-4):311-314
BV light curves of the eclipsing binary UV Leo obtained at the Kryonerion Astronomical Station of the National Observatory of
Athens, Greece, are analyzed. The analysis is based on a Roche configuration with two spots on the secondary surface. The
elements of the two components of the system are calculated and the spot characteristics are given. 相似文献
138.
139.
P. A. M. Van Hoof M. E. Foord R. F. Heeter J. E. Bailey H.-K. Chung M. E. Cuneo W. H. Goldstein V. Jonauskas F. P. Keenan R. Kisielius D. A. Liedahl C. Ramsbottom S. J. Rose P. T. Springer R. S. Thoe 《Astrophysics and Space Science》2005,298(1-2):147-153
In experiments at the high-power Z-facility at Sandia National Laboratory in Albuquerque, New Mexico, we have been able to
produce a low density photoionized laboratory plasma of Fe mixed with NaF. The conditions in the experiment allow a meaningful
comparison with X-ray emission from astrophysical sources. The charge state distributions of Fe, Na and F are determined in
this plasma using high resolution X-ray spectroscopy. Independent measurements of the density and radiation flux indicate
unprecedented values for the ionization parameter ξ = 20–25 erg cm s−1 under nearly steady-state conditions. First comparisons of the measured charge state distributions with X-ray photoionization
models show reasonable agreement, although many questions remain. 相似文献
140.