全文获取类型
收费全文 | 79426篇 |
免费 | 1481篇 |
国内免费 | 1217篇 |
专业分类
测绘学 | 2126篇 |
大气科学 | 5922篇 |
地球物理 | 15602篇 |
地质学 | 31351篇 |
海洋学 | 6410篇 |
天文学 | 15870篇 |
综合类 | 405篇 |
自然地理 | 4438篇 |
出版年
2021年 | 537篇 |
2020年 | 536篇 |
2019年 | 590篇 |
2018年 | 3907篇 |
2017年 | 3706篇 |
2016年 | 2851篇 |
2015年 | 1070篇 |
2014年 | 1404篇 |
2013年 | 2877篇 |
2012年 | 2733篇 |
2011年 | 4842篇 |
2010年 | 4397篇 |
2009年 | 5032篇 |
2008年 | 4180篇 |
2007年 | 4788篇 |
2006年 | 2411篇 |
2005年 | 2260篇 |
2004年 | 2090篇 |
2003年 | 2053篇 |
2002年 | 1816篇 |
2001年 | 1358篇 |
2000年 | 1386篇 |
1999年 | 1165篇 |
1998年 | 1172篇 |
1997年 | 1164篇 |
1996年 | 969篇 |
1995年 | 951篇 |
1994年 | 862篇 |
1993年 | 781篇 |
1992年 | 766篇 |
1991年 | 673篇 |
1990年 | 793篇 |
1989年 | 634篇 |
1988年 | 646篇 |
1987年 | 762篇 |
1986年 | 628篇 |
1985年 | 856篇 |
1984年 | 995篇 |
1983年 | 928篇 |
1982年 | 823篇 |
1981年 | 804篇 |
1980年 | 814篇 |
1979年 | 706篇 |
1978年 | 698篇 |
1977年 | 655篇 |
1976年 | 650篇 |
1975年 | 602篇 |
1974年 | 659篇 |
1973年 | 668篇 |
1972年 | 406篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
B. A. Ivanov 《Solar System Research》2005,39(5):381-409
Multi-ring impact basins have been found on the surfaces of almost all planetary bodies in the Solar system with solid crusts. The details of their formation mechanism are still unclear. We present results of our numerical modeling of the formation of the largest known terrestrial impact craters. The geological and geophysical data on these structures accumulated over many decades are used to place constraints on the parameters of available numerical models with a dual purpose: (i) to choose parameters in available mechanical models for the crustal response of planetary bodies to a large impact and (ii) to use numerical modeling to refine the possible range of original diameters and the morphology of partially eroded terrestrial craters. We present numerical modeling results for the Vredefort, Sudbury, Chicxulub, and Popigai impact craters and compare these results with available geological and geophysical information. 相似文献
33.
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
34.
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
35.
36.
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
37.
38.
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
39.
40.
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