全文获取类型
收费全文 | 97847篇 |
免费 | 1752篇 |
国内免费 | 1371篇 |
专业分类
测绘学 | 2651篇 |
大气科学 | 7435篇 |
地球物理 | 18612篇 |
地质学 | 36889篇 |
海洋学 | 8190篇 |
天文学 | 21027篇 |
综合类 | 460篇 |
自然地理 | 5706篇 |
出版年
2021年 | 698篇 |
2020年 | 743篇 |
2019年 | 777篇 |
2018年 | 4088篇 |
2017年 | 3892篇 |
2016年 | 3258篇 |
2015年 | 1384篇 |
2014年 | 2052篇 |
2013年 | 3962篇 |
2012年 | 3156篇 |
2011年 | 5322篇 |
2010年 | 4879篇 |
2009年 | 5686篇 |
2008年 | 4796篇 |
2007年 | 5380篇 |
2006年 | 3109篇 |
2005年 | 2888篇 |
2004年 | 2683篇 |
2003年 | 2610篇 |
2002年 | 2314篇 |
2001年 | 1886篇 |
2000年 | 1814篇 |
1999年 | 1610篇 |
1998年 | 1603篇 |
1997年 | 1567篇 |
1996年 | 1271篇 |
1995年 | 1254篇 |
1994年 | 1168篇 |
1993年 | 1068篇 |
1992年 | 1036篇 |
1991年 | 990篇 |
1990年 | 1128篇 |
1989年 | 966篇 |
1988年 | 922篇 |
1987年 | 1075篇 |
1986年 | 906篇 |
1985年 | 1163篇 |
1984年 | 1364篇 |
1983年 | 1261篇 |
1982年 | 1210篇 |
1981年 | 1162篇 |
1980年 | 1059篇 |
1979年 | 986篇 |
1978年 | 961篇 |
1977年 | 915篇 |
1976年 | 840篇 |
1975年 | 759篇 |
1974年 | 827篇 |
1973年 | 845篇 |
1972年 | 514篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
41.
42.
In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques. 相似文献
43.
44.
The 362.156 GHz absorption spectrum of H2O2 in the Mars atmosphere was observed on September 4 of 2003, employing the James Clerk Maxwell Telescope (JCMT) sub-millimeter facility on Mauna Kea, Hawaii. Radiative transfer analysis of this line absorption yields an average volume mixing ratio of 18±0.4 ppbv within the lower (0-30 km) Mars atmosphere, in general accordance with standard photochemical models (e.g., Nair et al., 1994, Icarus 111, 124-150). Our derived H2O2 abundance is roughly three times greater than the upper limit retrieved by Encrenaz et al. (2002, Astron. Astrophys. 396, 1037-1044) from infrared spectroscopy, although part of this discrepancy may result from the different solar longitudes (Ls) of observation. Aphelion-to-perihelion thermal forcing of the global Mars hygropause generates substantial (>200%) increases in HOx abundances above ∼10 km altitudes between the Ls=112° period of the Encrenaz et al. upper limit measurement and the current Ls=250° period of detection (Clancy and Nair, 1996, J. Geophys. Res. 101, 12785-12590). The observed H2O2 line absorption weakens arguments for non-standard homogeneous (Encrenaz et al., 2002, Astron. Astrophys. 396, 1037-1044) or heterogeneous (Krasnopolsky, 2003a, J. Geophys. Res. 108; 2003b, Icarus 165, 315-325) chemistry, which have been advocated partly on the basis of infrared (8 μm) non-detections for Mars H2O2. Observation of Mars H2O2 also represents the first measurement of a key catalytic specie in a planetary atmosphere other than our own. 相似文献
45.
Cygnus A was the first hyper-active galaxy discovered, and it
remains by far the closest of the ultra-luminous radio galaxies. As
such, Cygnus A has played a fundamental role in the study of virtually
all aspects of extreme activity in galaxies. We present a review of jet
theory for powering the double-lobed radio emitting structures in
powerful radio galaxies, followed by a review of observations of Cygnus
A in the radio, optical, and X-ray relevant to testing various aspects
of jet theory. Issues addressed include: jet structure from pc- to
kpc-scales, jet stability, confinement, composition, and velocity, the
double shock structure for the jet terminus and the origin of multiple
radio hotspots, the nature of the filamentary structure in the radio
lobes, and the hydrodynamic evolution of the radio lobes within a dense
cluster atmosphere, including an analysis of pressure balance between
the various gaseous components. Also discussed are relativistic
particle acceleration and loss mechanisms in Cygnus A, as well as
magnetic field strengths and geometries both within the radio source,
and in the intracluster medium. We subsequently review the
classification, cluster membership, and the emission components of the
Cygnus A galaxy. The origin of the activity is discussed.
Concentrating on the nuclear regions of the galaxy, we review the
evidence for an obscured QSO, also given the constraints on the
orientation of the radio source axis with respect to the sky plane. We
present an overview of models of central engines in AGN and observations
of Cygnus A which may be relevant to testing such models. We conclude
with a brief section concerning the question of whether Cygnus A is
representative of powerful high redshift radio galaxies.
Received October 10, 1995 相似文献
46.
47.
Long-term changes in growth of Norwegian spring-spawning herring 总被引:3,自引:1,他引:2
48.
We consider the particular solutions of the evolutionary system of equations in elements that correspond to planar and spatial circular orbits of the singly averaged Hill problem. We analyze the stability of planar and spatial circular orbits to inclination and eccentricity, respectively. We construct the instability regions of both particular solutions in the plane of parameters of the problem. 相似文献
49.
W. H. Jefferys G. F. Benedict P. D. Hemenway P. J. Shelus R. L. Duncombe 《Celestial Mechanics and Dynamical Astronomy》1985,37(3):299-305
The Hubble Space Telescope (HST), a large optical telescope having an aperture of 2.4 meters and a length of 8.8 meters, is being developed by the National Aeronautics and Space Administration. This telescope will be placed into earth orbit by the space shuttle. Astrometric observations with the HST are made using a Fine Guidance Sensor which is capable of measuring the position of one object relative to another with an accuracy of ±0.002 arcseconds. The astrometric user of HST will be provided with an Astrometric Data Reduction Software package (ADRS). The variety of astrometric problems to be investigated with HST is discussed. 相似文献
50.