全文获取类型
收费全文 | 2139篇 |
免费 | 73篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 36篇 |
大气科学 | 173篇 |
地球物理 | 514篇 |
地质学 | 740篇 |
海洋学 | 175篇 |
天文学 | 352篇 |
综合类 | 6篇 |
自然地理 | 229篇 |
出版年
2021年 | 16篇 |
2020年 | 19篇 |
2019年 | 22篇 |
2018年 | 51篇 |
2017年 | 42篇 |
2016年 | 51篇 |
2015年 | 44篇 |
2014年 | 67篇 |
2013年 | 102篇 |
2012年 | 81篇 |
2011年 | 92篇 |
2010年 | 81篇 |
2009年 | 77篇 |
2008年 | 88篇 |
2007年 | 82篇 |
2006年 | 83篇 |
2005年 | 75篇 |
2004年 | 67篇 |
2003年 | 60篇 |
2002年 | 74篇 |
2001年 | 51篇 |
2000年 | 49篇 |
1999年 | 40篇 |
1998年 | 51篇 |
1997年 | 32篇 |
1996年 | 33篇 |
1995年 | 26篇 |
1994年 | 28篇 |
1993年 | 33篇 |
1992年 | 26篇 |
1991年 | 18篇 |
1990年 | 28篇 |
1989年 | 18篇 |
1988年 | 18篇 |
1987年 | 28篇 |
1986年 | 33篇 |
1985年 | 34篇 |
1984年 | 44篇 |
1983年 | 30篇 |
1982年 | 37篇 |
1981年 | 22篇 |
1980年 | 30篇 |
1979年 | 26篇 |
1978年 | 36篇 |
1977年 | 22篇 |
1976年 | 30篇 |
1975年 | 21篇 |
1974年 | 23篇 |
1973年 | 20篇 |
1971年 | 12篇 |
排序方式: 共有2225条查询结果,搜索用时 31 毫秒
1.
2.
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
3.
Lawrence M. Mayer Peter T. Rahaim William Guerin Stephen A. Macko Les Watling Franz E. Anderson 《Estuarine, Coastal and Shelf Science》1985,20(4):491-503
A variety of measures of organic matter concentration and quality were made on samples collected from the top few mm of intertidal mudflat sediment over the course of a year, in order to assess the relative importance of biological and sedimentological influences on sedimentary organic matter. Winter and summer were times of relatively fine-grained sediment accumulation, caused by biological deposition or stabilization processes and resulting in higher organic matter concentrations. Stable carbon isotope and Br:C ratios indicated a planktonic source of bulk organic matter. Ratios of organic carbon to specific surface area of the sediments were consistent with an organic monolayer coverage of sediment grains. Correction for changing grain size during the year showed no change in the organic concentration per unit surface area, in spite of organic matter inputs by in situ primary production, buildup of heterotroph biomass and mucus coatings, and biodeposition of organic-rich seston. There were also no indications of changes in bulk organic quality, measured as hydrolyzable carbohydrates and amino acids, in response to these biological processes. It is concluded that biological processes on a seasonal time scale affect the bulk organic matter of these sediments via a modulation of grain size rather than creation or decay of organic matter. 相似文献
4.
5.
The occultation of Io by Ganymede as observed on 10 June 1985 is reported. The middle of the occultation minimum was found to occur at 14h 14m 5.7s UT. In the plane perpendicular to the line of sight the centres of the two satellite disks passed to within ~ 2530 km of each other, at a relative velocity, in this plane, of about 11 kms–1. The values of these last two quantities, however, depend on what assumptions are made about the light distribution over io's disk. 相似文献
6.
K. Iwasawa A.C. Fabian S.W. Allen S. Ettori 《Monthly notices of the Royal Astronomical Society》2001,328(1):L5-L10
We report on the metal distribution in the intracluster medium around the radio galaxy 4C+55.16 observed with the Chandra X-ray Observatory . The radial metallicity profile shows a dramatic change at 10 arcsec (∼50 kpc) in radius from half solar to twice solar at inner radii. Also found was a plume-like feature located at ∼3 arcsec to the south-west of the centre of the galaxy, which is mostly accounted for by a strong enhancement of iron L emission. The X-ray spectrum of the plume is characterized by the metal abundance pattern of Type Ia supernovae (SNeIa), i.e. large ratios of Fe to α elements, with the iron metallicity being unusually high at solar (90 per cent error). How the plume has been formed is not entirely clear. The inhomogeneous iron distribution suggested in this cluster provides important clues to understanding the metal enrichment process of the cluster medium. 相似文献
7.
8.
R. T. Schilizzi W. Aldrich B. Anderson A. Bos R. M. Campbell J. Canaris R. Cappallo J. L. Casse A. Cattani J. Goodman H. J. van Langevelde A. Maccafferri R. Millenaar R. G. Noble F. Olnon S. M. Parsley C. Phillips S. V. Pogrebenko D. Smythe A. Szomoru H. Verkouter A. R. Whitney 《Experimental Astronomy》2001,12(1):49-67
9.
S. W. Allen R. W. Schmidt A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2002,334(2):L11-L15
We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parametrizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White, we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r 2500 , where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe, , and measure a positive cosmological constant, . Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies. 相似文献
10.