全文获取类型
收费全文 | 62811篇 |
免费 | 1149篇 |
国内免费 | 595篇 |
专业分类
测绘学 | 1831篇 |
大气科学 | 4645篇 |
地球物理 | 13049篇 |
地质学 | 21531篇 |
海洋学 | 5331篇 |
天文学 | 14382篇 |
综合类 | 146篇 |
自然地理 | 3640篇 |
出版年
2021年 | 433篇 |
2020年 | 477篇 |
2019年 | 560篇 |
2018年 | 1217篇 |
2017年 | 1119篇 |
2016年 | 1534篇 |
2015年 | 918篇 |
2014年 | 1465篇 |
2013年 | 3018篇 |
2012年 | 1639篇 |
2011年 | 2272篇 |
2010年 | 2011篇 |
2009年 | 2749篇 |
2008年 | 2449篇 |
2007年 | 2422篇 |
2006年 | 2329篇 |
2005年 | 1854篇 |
2004年 | 1890篇 |
2003年 | 1771篇 |
2002年 | 1802篇 |
2001年 | 1561篇 |
2000年 | 1495篇 |
1999年 | 1348篇 |
1998年 | 1323篇 |
1997年 | 1334篇 |
1996年 | 1092篇 |
1995年 | 1064篇 |
1994年 | 997篇 |
1993年 | 893篇 |
1992年 | 809篇 |
1991年 | 763篇 |
1990年 | 788篇 |
1989年 | 732篇 |
1988年 | 730篇 |
1987年 | 830篇 |
1986年 | 737篇 |
1985年 | 941篇 |
1984年 | 1092篇 |
1983年 | 1023篇 |
1982年 | 932篇 |
1981年 | 856篇 |
1980年 | 814篇 |
1979年 | 772篇 |
1978年 | 788篇 |
1977年 | 675篇 |
1976年 | 673篇 |
1975年 | 652篇 |
1974年 | 655篇 |
1973年 | 690篇 |
1972年 | 466篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
The structure, functioning and hydrodynamic properties of aquifers can be determined from an analysis of the spatial variability of baseflow in the streams with which they are associated. Such analyses are based on simple low‐cost measurements. Through interpreting the hydrological profiles (Q = f(A)) it is possible to locate the aquifer(s) linked to the stream network and to determine the type of interrelated flow, i.e. whether the stream drains or feeds the aquifer. Using an analytical solution developed for situations with a positive linear relationship, i.e. where the baseflow increases linearly with increasing catchment size, it is also possible to estimate the permeability of the aquifer(s) concerned at catchment scale. Applied to the hard‐rock aquifers of the Oman ophiolite, this method shows that the ‘gabbro’ aquifer is more permeable than the ‘peridotite’ aquifer. As a consequence the streams drain the peridotites and ‘leak’ into the gabbro. The hydrological profiles within the peridotite are linear and positive, and indicate homogeneity in the hydrodynamic properties of these formations at the kilometre scale. The permeability of the peridotite is estimated at 5 · 10?7 to 5 · 10?8 m/s. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
22.
The supply rates of Na and K to the atmosphere of Mercury by processes acting on the extreme surface—thermal vaporization, photon-stimulated desorption (PSD), and ion-sputtering—are limited by the rates at which atoms can be supplied to the extreme surface by diffusion from inside the regolith grains. Supply rates to the atmosphere are further regulated by ion retention and by gardening rates that supply new grains to the surface. We consider the limits on supply of sodium and potassium atoms to the atmosphere, and rates of photoion recycling to the surface. Thermal vaporization rates are severely limited by the ability of atoms to diffuse to the surface of the grain. Therefore, the diffusion-limited thermal vaporization rates on Mercury's surface are comparable to or less than the PSD rates. Ion sputtering is primarily due to highly ionized heavy ions, even though they represent a small fraction of the solar wind. We have shown that up to 60% of the Na photoions are deposited on the surface of Mercury. Ion recycling to the surface can have a long-term effect on the regolith abundance if an average recycling pattern persists such that more ions return to a particular area than are launched there. It is unknown whether the formation of latitude bands of >100% ion retention persist on average despite a rapidly changing magnetosphere. The total exospheric column of sodium observed at Mercury between 1997 to 2003 varied by a factor of 2-3 from perihelion to aphelion. 相似文献
23.
Parametric transduction offers valuable advantages for underwater acoustic communications. Perhaps the most significant benefit is the fact that high directivity is achieved by means of a physically small transmit transducer. This feature may, ultimately, be employed to permit long-range, low-frequency communication using a compact source. The high directivity is desirable to combat multipath propagation and to achieve data communications in water which is shallow by comparison with range. A real-time, high data-rate “model” differential phase shift keying (DPSK) communication system has been constructed and demonstrated. This system uses parametric transduction, with a 300-kHz primary frequency and a 50-kHz secondary frequency. Experimental results show that the system can be employed to combat multipath propagation in shallow water and can achieve high data-rate text and color image transmission at 10 and 20 kb s-1 for 2-DPSK and 4-DPSK, respectively, through a transmission bandwidth of 10 kHz. The “model” system was developed to confirm performance predictions for a future, operational long-range link employing a 50-kHz primary frequency and a 5-kHz secondary frequency 相似文献
24.
We consider the evolution of certain low-mass binaries, incorporating models of (a) internal evolution, (b) tidal friction,
(c) dynamo activity driven by an elementary α,Ω dynamo, (d) stellar wind driven by the activity, and (e) magnetic braking
as a consequence of wind and poloidal dynamo-generated magnetic field. In some circumstances the stellar wind is found to
remove mass on a nuclear timescale, as is necessary to explain some observed systems.
We can hope that various uncertainties in the model may be clarified by a careful comparison of the models with such observed
quantities as rotation periods. These are modified by processes (a), (b) and (e). Assuming that stellar evolution is slow,
rotation rate should in some circumstances represent a balance between magnetic braking trying to slow the star down and tidal
friction trying to spin it up. Preliminary attempts are promising, but indicate that some fine tuning is necessary.
When there is a third body present, in an orbit which is inclined but not necessarily of short period, the eccentricity of
a close binary can be strongly modified by ‘Kozai cycles’. We show that this may complicate attempts to account for spin rates
of stars in close binaries. 相似文献
25.
Daniela Lazzaro Marcos A. Florczak Alberto Betzler Othon C. Winter Silvia M. Giuliatti-Winter Claudia A. Angeli Dietmar W. Foryta 《Planetary and Space Science》1996,44(12):1547-1550
The results of photometric observations of comet/asteroid 2060 Chiron at the Observatório do Pico dos Dias (Brazil-OPD) and the Observatoire de Haute-Provence (France-OHP) during 1994 and 1995 are presented. The analysis of the data shows a decrease of 2060 Chiron brightness from its peak values of 1988–1991. The absolute magnitude, Hv, varies from a maximum of 6.6 in February 1994 up to a minimum of 6.8 in June 1995. Therefore 2060 Chiron is back to a minimum of activity close to that of 1983–1985. The slope parameter G is found to be G = 0.71 ± 0.15. It is suggested that the H-G magnitude system, generally adopted to present 2060 Chiron brightness, is not the most appropriate due to the cometary activity of this object. 相似文献
26.
27.
28.
29.
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