全文获取类型
收费全文 | 72058篇 |
免费 | 19736篇 |
国内免费 | 43752篇 |
专业分类
测绘学 | 3816篇 |
大气科学 | 24424篇 |
地球物理 | 20436篇 |
地质学 | 48614篇 |
海洋学 | 19430篇 |
天文学 | 9071篇 |
综合类 | 4852篇 |
自然地理 | 4903篇 |
出版年
2023年 | 385篇 |
2022年 | 851篇 |
2021年 | 1312篇 |
2020年 | 2432篇 |
2019年 | 5955篇 |
2018年 | 6588篇 |
2017年 | 6265篇 |
2016年 | 6522篇 |
2015年 | 5548篇 |
2014年 | 5365篇 |
2013年 | 6748篇 |
2012年 | 5947篇 |
2011年 | 6083篇 |
2010年 | 5863篇 |
2009年 | 5720篇 |
2008年 | 4686篇 |
2007年 | 4681篇 |
2006年 | 4236篇 |
2005年 | 3739篇 |
2004年 | 3958篇 |
2003年 | 3624篇 |
2002年 | 3440篇 |
2001年 | 3056篇 |
2000年 | 2544篇 |
1999年 | 2458篇 |
1998年 | 2526篇 |
1997年 | 2550篇 |
1996年 | 2039篇 |
1995年 | 1926篇 |
1994年 | 1744篇 |
1993年 | 1633篇 |
1992年 | 1435篇 |
1991年 | 1145篇 |
1990年 | 1163篇 |
1989年 | 972篇 |
1988年 | 862篇 |
1987年 | 820篇 |
1986年 | 666篇 |
1985年 | 674篇 |
1984年 | 805篇 |
1983年 | 616篇 |
1982年 | 651篇 |
1981年 | 541篇 |
1980年 | 524篇 |
1979年 | 463篇 |
1978年 | 388篇 |
1977年 | 378篇 |
1976年 | 337篇 |
1974年 | 327篇 |
1973年 | 356篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
Mallory S. E. Roberts Crystal L. Brogan Bryan M. Gaensler Jason W. T. Hessels C.-Y. Ng Roger W. Romani 《Astrophysics and Space Science》2005,297(1-4):93-100
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be
associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the
pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies
seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM. 相似文献
22.
A wide-field time-series CCD photometric survey of variable stars in the field of the open cluster NGC 2168 was carried out using the BATC Schmidt telescope. In total 13 new variable stars are discovered with three W UMa systems, one EA type and two EB type eclipsing binaries (one of them could be a W UMa system), and seven pulsating stars including three candidates of δScuti stars. 相似文献
23.
Zhi Xu Cheng Fang Wei-Qun Can Astronomy Department of Nanjing University Nanjing Purple Mountain Observatory Chinese Academy of Sciences Nanjing 《中国天文和天体物理学报》2005,5(5):519-530
Chromospheric lines, including Ha, Lyα, Lyβand CaⅡK, CaⅡ8542, are systemically and quantitatively investigated with respect to the non-thermal excitation and ionization due to particle beam bombardment for a series of solar semi-empirical atmospheric models. As a result we propose to use the contrast in the integrated intensity of hydrogen lines to estimate the total energy flux of the bombarding beam during the solar flare impulsive phase. Partial frequency redistribution is considered in the Lyαline calculation and a smaller intensity enhancement in the Ha line-centers is found than in the previous results of Fang et al. 相似文献
24.
Xue-Fei Chen Zhan-Wen HanNational Astronomical Observatories Yunnan Observatory Chinese Academy of Sciences. Kunming Graduate School of the Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2005,5(1):65-76
We have studied the influence of different choices of core-envelope transition point on the final merger of contact binaries with two main-sequence components. A binary of 1.00 + 0.90M⊙ with an initial orbital period of 0.35d is examined. The mass fraction of the primary mixed with the matter of the secondary, qmix, determined by the chosen core-envelope transition point, ranges from 0.04 to 1.00 in our analysis. If as qmix< 0.8, none of the 相似文献
25.
1 IntroductionBacteriaandtheiractivitiesplayanimportantroleintheelementalbiogeochemicalcyclesandenergytransformingintheocean (Zhenetal.1 997) .DortchandPackard(1 989) proposedthatfoodwebsintheeutrophicwatersaredominatedbythebiomassofprimaryproducerswhilefoodwebsintheoligotrophicwatersaredominatedbythebiomassofmicrobes.Heterotrophicbacteriahadbeenshowntoplayanimportantroleinthedecompositionoflarge ,rapidlysinkingorganicparticleswithinandbelowtheeuphot iczone ,andfurthertoaffecttheelementaldyn… 相似文献
26.
V. Delouille J. De Patoul J. F. Hochedez L. Jacques J. P. Antoine 《Solar physics》2005,228(1-2):301-321
The extreme ultraviolet imaging telescope (EIT) of SOHO offers a unique record of the solar atmosphere for its sampling in
temperature, field of view, resolution, duration, and cadence. To investigate globally and locally its topology and evolution
during the solar cycle, we consider a multi-scale approach, and more precisely we use the wavelet spectrum.
We present three results among the applications of such a procedure. First, we estimate the typical dimension of the supergranules
as seen in the 30.4 nm passband, and we show that the evolution of the characteristic network scale is almost in phase with
the solar cycle. Second, we build pertinent time series that give the evolution of the signal energy present in the corona
at different scales. We propose a method that detects eruptions and post-flaring activity in EUV image sequences. Third, we
introduce a new way to extract active regions in EIT images, with perspectives in, e.g., long-term irradiance analysis. 相似文献
27.
28.
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
29.
Histological study on the ontogeny of the lymphoid organs, kidney, thymus and spleen of Japanese flounder, Paralichthys olivaceus, from hatching to 40 d was carried out. The pronephric kidney duct appeared early in hatching although the primordial haemopoietic
stem cells were observed within a week after hatching. The spleen was first seen after 8d of hatching. The thymus appeared
after 15d, situated near the pronephric kidney. Small lymphoid cells appeared during the later phase of the post-larval stage
in the sequence of thymus, kidney and spleen. During the 40d of observations, there were no distinct inner or outer zones
in thymus and no red or white pulp in spleen. These results suggest that the nonspecific defense immune system plays a very
important role in the early larval stage of Japanese flounder. 相似文献