全文获取类型
收费全文 | 92286篇 |
免费 | 1287篇 |
国内免费 | 1547篇 |
专业分类
测绘学 | 3073篇 |
大气科学 | 7052篇 |
地球物理 | 18427篇 |
地质学 | 35036篇 |
海洋学 | 6730篇 |
天文学 | 17577篇 |
综合类 | 2295篇 |
自然地理 | 4930篇 |
出版年
2021年 | 522篇 |
2020年 | 598篇 |
2019年 | 625篇 |
2018年 | 5937篇 |
2017年 | 5168篇 |
2016年 | 4166篇 |
2015年 | 1231篇 |
2014年 | 1760篇 |
2013年 | 3361篇 |
2012年 | 2780篇 |
2011年 | 5069篇 |
2010年 | 4128篇 |
2009年 | 5264篇 |
2008年 | 4398篇 |
2007年 | 4759篇 |
2006年 | 2549篇 |
2005年 | 2153篇 |
2004年 | 2341篇 |
2003年 | 2275篇 |
2002年 | 2056篇 |
2001年 | 1683篇 |
2000年 | 1661篇 |
1999年 | 1383篇 |
1998年 | 1377篇 |
1997年 | 1340篇 |
1996年 | 1179篇 |
1995年 | 1152篇 |
1994年 | 1022篇 |
1993年 | 896篇 |
1992年 | 859篇 |
1991年 | 864篇 |
1990年 | 943篇 |
1989年 | 842篇 |
1988年 | 785篇 |
1987年 | 983篇 |
1986年 | 857篇 |
1985年 | 1004篇 |
1984年 | 1180篇 |
1983年 | 1145篇 |
1982年 | 1005篇 |
1981年 | 1057篇 |
1980年 | 908篇 |
1979年 | 847篇 |
1978年 | 882篇 |
1977年 | 791篇 |
1976年 | 735篇 |
1975年 | 721篇 |
1974年 | 743篇 |
1973年 | 761篇 |
1972年 | 481篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
An object that is partially insonified by a collimated sound beam may have a scattering cross section sometimes much larger than when the object is totally covered by the incident beam. We quantitatively study this partial insonification problem here, under the classical method of physical optics. The importance of this study stems from the fact that partial coverage of the target by the beam is the situation most likely to occur in many cases of practical importance. We consider several basic target shapes partially insonified by finite beams. These shapes include the spherical, the infinite and finite cylinder, the flat plate, and the capped sphere. High-frequency approximations of the resulting integrals, obtained by means of the saddle-point method, show the relative importance of the scattering centers located at the beam's specular reflection points, or at the edges of the spots that the beam shines on the scatterers. The physical-optics method is extended to obtain formulas for the bistatic cross sections of partially insonified objects. The results are numerically evaluated and graphically displayed in many pertinent instances and compared to the predictions of approaches, such as the Fresnel-zones method and the Geometrical Theory of Diffraction (GTD). The predictions of the physical-optics method have all the advantages and deficiencies of this method and, with very minor modifications, hold equally well for the partial illumination of objects by beams of electromagnetic radiation. 相似文献
72.
CHUDongsheng LIANGMeng SHIXin ZHANGLing 《中国海洋大学学报(英文版)》2004,3(1):106-110
1 Introduction Inrecentyearstherehasbeengrowingresearchin terestinthetwo dimensional (2 D )systemtheory .The 2 Dsystemsmayfindapplicationsinareassuchasmarineseismicdataprocessingandimageprocessing .Althoughmoreandmorevaluableresultshavebeengained ,mos… 相似文献
73.
74.
G. A. Gurzadyan 《Astrophysics and Space Science》1992,193(1):105-107
The emission lines in the spectra of some planetary nebula experience variations with the time. These variations (due to evolution effects and by their nature) differ from the spectral changes which we usually observe in stars. It appears that the long-scale and systematic observations on the intensity behaviour of different emission lines of nebula may be used as an independent indicator to receive a principally new and quite unpredicted category of information related to the physics of nebulae and their nuclei. Particularly, the analysis of long standing observations carried out in relation to the forbidden lines 4363 [Oiii] andN
1+N
2 [Oiii] of double-ionized oxygen lead to a conception about the possible existence ofrelativistic electrons of moderate energy in planetary nebulae and the generation oftransition radiation as a result of electrodynamic interaction of these electrons with the dust particles in nebula (Gurzadyan, 1991). Just on the basis of the same observational material, the long-scale periodicity in the activity or variations of a star-like planetary nebula IC 4997 is discovered about which is the present note. 相似文献
75.
76.
77.
78.
J.H. Krolik 《Astrophysics and Space Science》1997,248(1-2):207-215
A short survey is presented of dynamical (and thermodynamical) issues relevant to outflows in Seyfert galaxies like NGC 1068.
Numerical simulations incorporating both realistic radiative heating and cooling, and angular momentum, show that the fraction
of injected gas which escapes in a wind is very sensitive to both the injection pressure and the gas's angular momentum. Radiation
pressure acting through a variety of atomic opacity processes may also be important.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
79.
Several physical and observational effects may contribute to the significant imbalances of magnetic flux that are often observed in active regions. We consider an effect not previously treated: the influence of electric currents in the photosphere. Electric currents can cause a line-of-sight flux imbalance because of the directionality of the magnetic field they produce. Currents associated with magnetic flux tubes produce larger imbalances than do smoothly-varying distributions of flux and current. We estimate the magnitude of this effect for current densities, total currents, and magnetic geometry consistent with observations. The expected imbalances lie approximately in the range 0–15%, depending on the character of the current-carrying fields and the angle from which they are viewed. Observationally, current-induced flux imbalances could be indicated by a statistical dependence of the imbalance on angular distance from disk center. A general study of magnetic flux balance in active regions is needed to determine the relative importance of other - probably larger -effects such as dilute flux (too weak to measure or rendered invisible by radiative transfer effects), merging with weak background fields, and long-range connections between active regions.Operated for the National Science Foundation by the Association of Universities for Research in Astronomy. 相似文献
80.
Donald D. BOGARD Daniel H. GARRISON Jozef MASARIK 《Meteoritics & planetary science》2001,36(1):107-122
Abstract— The Monahans H‐chondrite is a regolith breccia containing light and dark phases and the first reported presence of small grains of halite. We made detailed noble gas analyses of each of these phases. The 39Ar‐40Ar age of Monahans light is 4.533 ± 0.006 Ma. Monahans dark and halite samples show greater amounts of diffusive loss of 40Ar and the maximum ages are 4.50 and 4.33 Ga, respectively. Monahans dark phase contains significant concentrations of He, Ne and Ar implanted by the solar wind when this material was extant in a parent body regolith. Monahans light contains no solar gases. From the cosmogenic 3He, 21Ne, and 38Ar in Monahans light we calculate a probable cosmic‐ray, space exposure age of 6.0 ± 0.5 Ma. Monahans dark contains twice as much cosmogenic 21Ne and 38Ar as does the light and indicates early near‐surface exposure of 13–18 Ma in a H‐chondrite regolith. The existence of fragile halite grains in H‐chondrites suggests that this regolith irradiation occurred very early. Large concentrations of 36Ar in the halite were produced during regolith exposure by neutron capture on 35Cl, followed by decay to 36Ar. The thermal neutron fluence seen by the halite was (2–4) × 1014 n/cm2. The thermal neutron flux during regolith exposure was ~0.4‐0.7 n/cm2/s. The Monahans neutron fluence is more than an order of magnitude less than that acquired during space exposure of several large meteorites and of lunar soils, but the neutron flux is lower by a factor of ≤5. Comparison of the 36Arn/21Necos ratio in Monahans halite and silicate with the theoretically calculated ratio as a function of shielding depth in an H‐chondrite regolith suggests that irradiation of Monahans dark occurred under low shielding in a regolith that may have been relatively shallow. Late addition of halite to the regolith can be ruled out. However, irradiation of halite and silicate for different times at different depths in an extensive regolith cannot be excluded. 相似文献