全文获取类型
收费全文 | 18197篇 |
免费 | 342篇 |
国内免费 | 151篇 |
专业分类
测绘学 | 473篇 |
大气科学 | 1247篇 |
地球物理 | 3712篇 |
地质学 | 6704篇 |
海洋学 | 1701篇 |
天文学 | 3671篇 |
综合类 | 77篇 |
自然地理 | 1105篇 |
出版年
2022年 | 174篇 |
2021年 | 282篇 |
2020年 | 276篇 |
2019年 | 347篇 |
2018年 | 702篇 |
2017年 | 588篇 |
2016年 | 703篇 |
2015年 | 355篇 |
2014年 | 607篇 |
2013年 | 1009篇 |
2012年 | 714篇 |
2011年 | 898篇 |
2010年 | 837篇 |
2009年 | 1019篇 |
2008年 | 907篇 |
2007年 | 864篇 |
2006年 | 758篇 |
2005年 | 585篇 |
2004年 | 572篇 |
2003年 | 482篇 |
2002年 | 493篇 |
2001年 | 361篇 |
2000年 | 394篇 |
1999年 | 269篇 |
1998年 | 295篇 |
1997年 | 252篇 |
1996年 | 200篇 |
1995年 | 211篇 |
1994年 | 204篇 |
1993年 | 170篇 |
1992年 | 159篇 |
1991年 | 166篇 |
1990年 | 150篇 |
1989年 | 118篇 |
1988年 | 134篇 |
1987年 | 147篇 |
1986年 | 133篇 |
1985年 | 175篇 |
1984年 | 158篇 |
1983年 | 178篇 |
1982年 | 143篇 |
1981年 | 147篇 |
1980年 | 151篇 |
1979年 | 151篇 |
1978年 | 138篇 |
1977年 | 112篇 |
1976年 | 87篇 |
1975年 | 89篇 |
1974年 | 97篇 |
1973年 | 95篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
841.
Abstract— The regolith evolution of the lunar meteorites Dhofar (Dho) 081, Northwest Africa (NWA) 032, NWA 482, NWA 773, Sayh al Uhaymir (SaU) 169, and Yamato (Y‐) 981031 was investigated by measuring the light noble gases He, Ne, and Ar. The presence of trapped solar neon in Dho 081, NWA 773, and Y‐981031 indicates an exposure at the lunar surface. A neon three‐isotope diagram for lunar meteorites yields an average solar 20Ne/22Ne ratio of 12.48 ± 0.07 representing a mixture of solar energetic particles neon at a ratio of 11.2 and solar wind neon at a ratio of 13.8. Based on the production rate ratio of 21Ne and 38Ar, the shielding depth in the lunar regolith of NWA 032, NWA 482, SaU 169, and Y‐981031 was obtained. The shielding depth of these samples was between 10.5 g/cm2 and >500 g/cm2. Based on spallogenic Kr and Xe, the shielding depth of Dho 081 was estimated to be most likely between 120 and 180 g/cm2. Assuming a mean density of the lunar regolith of 1.8 g/cm3, 10.5 g/cm2 corresponds to a depth of 5.8 cm and 500 g/cm2 to 280 cm below the lunar surface. The range of regolith residence time observed in this study is 100 Ma up to 2070 Ma. 相似文献
842.
We develop a radio astronomical approach to 3D‐reconstruction in few projections tomography. It is based on the 2‐CLEAN DSA method which consists of two clean algorithms by using a synthesized beam. In complex cases two extreme solutions are used for the analysis of the image structure. These solutions determine the limits of permissible energy redistribution on the image among the components of small and large scales. Two variants of 3D‐reconstruction proceeding from a set of two‐dimensional projections (3D2D) and from a set of one‐dimensional ones (3D1D) are considered. It is shown that the quality of 3D2D‐reconstruction should be similar to the quality of 2D1D‐reconstruction if the same number of equally spaced scans is used. But a doubled number of projections is required for 3D1D‐reconstruction. We have simulated 3D‐reconstruction of an optically thin emitting object. The present technique is a component of astrotomography and it has good prospects for a wide range of remote sensing. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
843.
844.
Michael De Becker Gregor Rauw Jean-Pierre Swings 《Astrophysics and Space Science》2005,297(1-4):291-298
We present the results of an intensive spectroscopic campaign in the optical waveband revealing that Cyg OB2 #8A is an O6+O5.5
binary system with a period of about 21.9 days. Cyg OB2 #8A is a bright X-ray source, as well as a non-thermal radio emitter.
We discuss the binarity of this star in the framework of a campaign devoted to the study of non-thermal emitters, from the
radio waveband to γ-rays. In this context, we attribute the non-thermal radio emission from this star to a population of relativistic
electrons, accelerated by the shock of the wind-wind collision. These relativistic electrons could also be responsible for
a putative γ-ray emission through inverse Compton scattering of photospheric UV photons, thus contributing to the yet unidentified
EGRET source 3EG J2033+4118.
Based partly on data Obtained at the Observatoire de Haute-Provence, France. 相似文献
845.
N. A. Lotova K. V. Vladimirskii V. N. Obridko B. P. Filippov O. A. Korelov 《Astronomy Letters》2005,31(8):546-556
We study the formation of solar-wind streams in the years of maximum solar activity 2000–2002. We use observations of the scattering of radio emission by solar-wind streams at distances of ~4–60RS from the Sun, data on the magnetic field structure and strength in the source region (R ~ 2.5RS), and observations with the LASCO coronagraph onboard the SOHO spacecraft. Analysis of these data allowed us to investigate the changes in the structure of circumsolar plasma streams during the solar maximum. We constructed radio maps of the solar-wind transition, transonic region in which the heliolatitudinal stream structure is compared with the structure of the white-light corona. We show that the heliolatitudinal structure of the white-light corona largely determines the structure of the solar-wind transition region. We analyze the correlation between the location of the inner boundary of the transition region Rin and the magnetic field strength on the source surface |BR|. We discuss the peculiarities of the Rin = F(|BR|) correlation diagrams that distinguish them from similar diagrams at previous phases of the solar cycle. 相似文献
846.
Roberto F. Viotti Rosario González-Riestra Takashi Iijima Stefano Bernabei Riccardo Claudi Jochen Greiner Michael Friedjung Vito Francesco Polcaro Corinne Rossi 《Astrophysics and Space Science》2005,296(1-4):435-439
We review the main characteristics of the symbiotic system AG Draconis, with special emphasis on its optical and X-ray variations.
We also discuss the X-ray to visual energy distribution during quiescence and outburst and describe our spectroscopic and
X-ray observations during the 2003 outburst.
Based on X-ray observations collected with the XMM–Newton Observatory, on INES data from the IUE satellite, and on optical
spectra collected with the Asiago–Cima Ekar, Bologna–Loiano and La Palma–Galileo Italian telescopes. 相似文献
847.
Marc W. Pound Jave O. Kane Bruce A. Remington Dmitri D. Ryutov Akira Mizuta Hideaki Takabe 《Astrophysics and Space Science》2005,298(1-2):177-181
Over the past few years, our group has been developing hydrodynamic models to simulate formation of the Eagle Nebula pillars.
The true test of any model is, of course, how well it can reproduce the observations. Here, we discuss how we go about testing
our models against observations. We describe the process by which we “observe” the model data to create synthetic maps. We
show an example of this technique using one of our model runs and compare the resultant synthetic map to the real one. 相似文献
848.
M. J. Freyberg H. Bräuninger W. Burkert G. D. Hartner O. Citterio F. Mazzoleni G. Pareschi D. Spiga S. Romaine P. Gorenstein B. D. Ramsey 《Experimental Astronomy》2005,20(1-3):405-412
The Max-Planck-Institut für extraterrestrische Physik (MPE) in Garching, Germany, uses its large X-ray beam line facility PANTER for testing X-ray astronomical instrumentation. A number of telescopes, gratings, filters, and detectors, e.g. for astronomical satellite missions like Exosat, ROSAT, Chandra (LETG), BeppoSAX, SOHO (CDS), XMM-Newton, ABRIXAS, Swift (XRT), have been successfully calibrated in the soft X-ray energy range (< 15keV). Moreover, measurements with mirror test samples for new missions like ROSITA and XEUS have been carried out at PANTER. Here we report on an extension of the energy range, enabling calibrations of hard X-ray optics over the energy range 15–50 keV. Several future X-ray astronomy missions (e.g., Simbol-X, Constellation-X, XEUS) have been proposed, which make use of hard X-ray optics based on multilayer coatings. Such optics are currently being developed by the Osservatorio Astronomico di Brera (OAB), Milano, Italy, and the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, MA, USA. These optics have been tested at the PANTER facility with a broad energy band beam (up to 50 keV) using the XMM-Newton EPIC-pn flight spare CCD camera with its good intrinsic energy resolution, and also with monochromatic X-rays between C-K (0.277 keV) and Cu-Kα (8.04 keV).
PACS: 95.55.Ka, 95.55.Aq, 41 50.+h, 07.85.Fv 相似文献
849.
O. Sadot K. Levy A. Yosef-Hai D. Cartoon Y. Elbaz Y. Srebro G. Ben-Dor D. Shvarts 《Astrophysics and Space Science》2005,298(1-2):305-312
The hydrodynamic instability, which develops on the contact surface between two fluids, has great importance in astrophysical
phenomena such as the inhomogeneous density distribution following a supernova event. In this event acceleration waves pass
across a material interface and initiate and enhance unstable conditions in which small perturbations grow dramatically.
In the present study, an experimental technique aimed at investigating the above-mentioned hydrodynamic instability is presented.
The experimental investigation is based on a shock-tube apparatus by which a shock wave is generated and initiates the instability
that develops on the contact surface between two gases. The flexibility of the system enables one to vary the initial shape
of the contact surface, the shock-wave Mach number, and the density ratio across the contact surface.
Three selected sets of shock-tube experiments are presented in order to demonstrate the system capabilities: (1) large-initial
amplitudes with low-Mach-number incident shock waves; (2) small-initial amplitudes with moderate-Mach-number incident shock
waves; and (3) shock bubble interaction.
In the large-amplitude experiments a reduction of the initial velocity with respect to the linear growth prediction was measured.
The results were compared to those predicted by a vorticity-deposition model and to previous experiments with moderate- and
high-Mach number incident shock waves that were conducted by others. In this case, a reduction of the initial velocity was
noted. However, at late times the growth rate had a 1/t behavior as in the small-amplitude low-Mach number case. In the small-amplitude moderate-Mach number shock experiments a
reduction from the impulsive theory was noted at the late stages.
The passage of a shock wave through a spherical bubble results in the formation of a vortex ring. Simple dimensional analysis
shows that the circulation depends linearly on the speed of sound of the surrounding material and on the initial bubble radius. 相似文献
850.
O. Tamuz T. Mazeh S. Zucker † 《Monthly notices of the Royal Astronomical Society》2005,356(4):1466-1470
We suggest a new algorithm to remove systematic effects in a large set of light curves obtained by a photometric survey. The algorithm can remove systematic effects, such as those associated with atmospheric extinction, detector efficiency, or point spread function changes over the detector. The algorithm works without any prior knowledge of the effects, as long as they linearly appear in many stars of the sample. The approach, which was originally developed to remove atmospheric extinction effects, is based on a lower rank approximation of matrices, an approach which has already been suggested and used in chemometrics, for example. The proposed algorithm is especially useful in cases where the uncertainties of the measurements are unequal. For equal uncertainties, the algorithm reduces to the Principal Component Analysis (PCA) algorithm. We present a simulation to demonstrate the effectiveness of the proposed algorithm and we point out its potential, in the search for transit candidates in particular. 相似文献