全文获取类型
收费全文 | 85003篇 |
免费 | 1439篇 |
国内免费 | 817篇 |
专业分类
测绘学 | 2187篇 |
大气科学 | 6061篇 |
地球物理 | 16393篇 |
地质学 | 30025篇 |
海洋学 | 7783篇 |
天文学 | 19874篇 |
综合类 | 246篇 |
自然地理 | 4690篇 |
出版年
2022年 | 474篇 |
2021年 | 871篇 |
2020年 | 949篇 |
2019年 | 1024篇 |
2018年 | 2218篇 |
2017年 | 2066篇 |
2016年 | 2632篇 |
2015年 | 1442篇 |
2014年 | 2458篇 |
2013年 | 4437篇 |
2012年 | 2638篇 |
2011年 | 3524篇 |
2010年 | 3142篇 |
2009年 | 4085篇 |
2008年 | 3483篇 |
2007年 | 3578篇 |
2006年 | 3317篇 |
2005年 | 2589篇 |
2004年 | 2596篇 |
2003年 | 2451篇 |
2002年 | 2351篇 |
2001年 | 2093篇 |
2000年 | 1941篇 |
1999年 | 1634篇 |
1998年 | 1676篇 |
1997年 | 1590篇 |
1996年 | 1368篇 |
1995年 | 1309篇 |
1994年 | 1175篇 |
1993年 | 1080篇 |
1992年 | 1025篇 |
1991年 | 1000篇 |
1990年 | 1039篇 |
1989年 | 918篇 |
1988年 | 903篇 |
1987年 | 996篇 |
1986年 | 903篇 |
1985年 | 1125篇 |
1984年 | 1231篇 |
1983年 | 1102篇 |
1982年 | 1010篇 |
1981年 | 991篇 |
1980年 | 879篇 |
1979年 | 868篇 |
1978年 | 849篇 |
1977年 | 753篇 |
1976年 | 684篇 |
1975年 | 684篇 |
1974年 | 644篇 |
1973年 | 692篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
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
33.
34.
35.
We discuss long-time changes of polar activity of the Sun using the new observational data sets in the optical range during
1872–2001. A study of the secular and cycle variations of the magnetic activity at the high-latitude regions is the main goal
that includes polar magnetic field reversals during 1872–2001 and secular changes of the duration of polar activity cycles.
The secular increase of the area of polar zones during the minimum activity in the last 120 years and as consequence a decrease
of coronal temperature of the Sun in the high-latitude zones during the last 50 years. Correlation between the polar cycles
of Caii-K bright points with the Wolf sunspot numbers cycles, W(t), and the 22-year polar magnetic cycles of Caii-K bright points at the high latitudes during 1905–1995 is discussed. 相似文献
36.
Abstract The transition from impact to post‐impact rocks in the Yaxcopoil‐1 (Yax‐1) core is marked by a 2 cm‐thick clay layer characterized by dissolution features. The clay overlies a 9 cm‐thick hardground, overlying a 66 cm‐thick crossbedded unit, consisting of dolomite sandstone alternating with thin micro‐conglomerate layers with litho‐ and bioclasts and the altered remains of impact glass, now smectite. The micro‐conglomerates mark erosion surfaces. Microprobe and backscatter SEM analysis of the dolomite rhombs show an early diagenetic, complex‐zoned, idiomorphic overgrowth, with Mn‐rich zones, possibly formed by hot fluids related to cooling melt sheet in the crater. The pore spaces are filled with several generations of coelestite, barite, K‐feldpar, and sparry calcite. XRF core scanning analysis detected high Mn values in the crossbedded sediments but no anomalous enrichment of the siderophile elements Cr, Co, Fe, and Ni in the clay layer. Shocked quartz occurs in the crossbedded unit but is absent in the clay layer. The basal Paleocene marls are strongly dissolved and do not contain a basal Paleocene fauna. The presence of a hardground, the lack of siderophile elements, shocked quartz, or Ni‐rich spinels in the clay layer, and the absence of basal Paleocene biozones P0 and Pa all suggest that the top of the ejecta sequence and a significant part of the lower Paleocene is missing. Due to the high energy sedimentation infill, a hiatus at the top of the impactite is not unexpected, but there is nothing in the biostratigraphy, geochemistry, and petrology of the Yax‐1 core that can be used to argue against the synchroneity of the end‐Cretaceous mass‐extinctions and the Chicxulub crater. 相似文献
37.
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
38.
Similarity solutions, describing the flow of a perfect gas behind spherical shock waves, are investigated including the radiation heat flux. The shock is assumed to be propagating in a medium at rest. Shock radius varies exponentially with time and density is inversely proportional to fifth power of the shock radius immediately ahead of the shock front. 相似文献
39.
40.