全文获取类型
收费全文 | 264714篇 |
免费 | 5193篇 |
国内免费 | 3362篇 |
专业分类
测绘学 | 6975篇 |
大气科学 | 19521篇 |
地球物理 | 55314篇 |
地质学 | 92783篇 |
海洋学 | 22176篇 |
天文学 | 57365篇 |
综合类 | 1040篇 |
自然地理 | 18095篇 |
出版年
2021年 | 2244篇 |
2020年 | 2593篇 |
2019年 | 2857篇 |
2018年 | 4412篇 |
2017年 | 4146篇 |
2016年 | 6180篇 |
2015年 | 4219篇 |
2014年 | 6906篇 |
2013年 | 14183篇 |
2012年 | 6841篇 |
2011年 | 8622篇 |
2010年 | 7727篇 |
2009年 | 10240篇 |
2008年 | 8929篇 |
2007年 | 8618篇 |
2006年 | 9701篇 |
2005年 | 7906篇 |
2004年 | 7769篇 |
2003年 | 7257篇 |
2002年 | 6801篇 |
2001年 | 6016篇 |
2000年 | 5961篇 |
1999年 | 5219篇 |
1998年 | 5234篇 |
1997年 | 5030篇 |
1996年 | 4679篇 |
1995年 | 4431篇 |
1994年 | 4104篇 |
1993年 | 3854篇 |
1992年 | 3636篇 |
1991年 | 3591篇 |
1990年 | 3765篇 |
1989年 | 3521篇 |
1988年 | 3305篇 |
1987年 | 3848篇 |
1986年 | 3407篇 |
1985年 | 4224篇 |
1984年 | 4737篇 |
1983年 | 4406篇 |
1982年 | 4312篇 |
1981年 | 3928篇 |
1980年 | 3637篇 |
1979年 | 3508篇 |
1978年 | 3488篇 |
1977年 | 3280篇 |
1976年 | 3044篇 |
1975年 | 2960篇 |
1974年 | 2914篇 |
1973年 | 3077篇 |
1972年 | 2025篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
351.
352.
353.
An investigation of 531 active regions was made to determine the correlation between energy released by flares and the available energy in magnetic fields of the regions. Regions with magnetic flux greater than 1021 maxwell during the years 1967–1969, which included sunspot maximum, were selected for the investigation. A linear regression analysis of flare production on magnetic flux showed that the flare energy is correlated with magnetic energy with a coeificient of correlation of 0.78. Magnetic classification and field configuration also significantly affect the production of flares.This work was supported by the Aerospace Sponsored Research Program. 相似文献
354.
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
355.
M.D. Morrison 《Planetary and Space Science》1985,33(1):135-139
Laboratory measurements of the Å (3s' 3D° → 2p41D, 3s' 3D° → 2p43P) branching ratio have been made with a value of 1.5 × 10t-4 indicated. This value makes the branching transition at 1173 Å an order of magnitude stronger than the branch at 7990 Å (3s' 3D° → 3p 3P). The 1173 Å branching loss is still too weak a loss process for multiply scattered 989 Å photons to resolve the 989 Å intensity problem in the dayglow. 相似文献
356.
357.
In previous papers, the present authors have shown that the galactic anisotropy is modulated due to cosmic ray orbital deflection in the heliomagnetosphere, and that the sidereal time daily variations of galactic origin can be expressed using the basic vectors, which have been obtained by calculating trajectories of cosmic rays in a model magnetosphere having Parker's Archimedian spiral structure with a flat or a wavy neutral sheet. In the present paper, the magnetic irregularities superposed on the Parker's spiral field have been taken into account, which cause the scattering of cosmic rays and disturb their orbits. We examined the fluctuations of asymptotic directions calculating their orbits by the Monte-Carlo simulation, based on the theory of the multiple scattering process. It is shown that the dispersion of the projected deviation angle is determined mainly by the scattering mean free path and by the structure of the order magnetic field, e.g. the polarity state of the heliomagnetosphere and the extent of the neutral sheet. We investigated also the influence of the fluctuations of asymptotic directions on the sidereal daily variation. It is found that, under some conditions, the scattering causes only the attenuation of the amplitude of the basic vector, and does not change its phase. The attenuation is negligibly small at high rigidities larger than ~ 1000 GV, but becomes more serious with decreasing rigidity. The rigidity dependence curve of the attenuation rate was calculated for various cases. A simple and approximate method is also presented for the derivation of those curves for any value of the magnitude of the mean free path and for various model magnetospheres. It is noted, however, that the lower limiting rigidity below which the present method is not applicable is relatively high in the Positive polarity state. 相似文献
358.
359.
Classical R-M and synthetic W-D analysis of V758 Centauri are presented. Two solutions (semi-detached and contact) were found from differential corrections approach. The semi-detached model is physically acceptable since the system is thermally decoupled. The solution for this case and the photometric data are consistent with a B9 primary and A9 secondary components having parameters close to Main-Sequence values. It is suggested that V758 Centauri is a B-type W UMa system at the brokencontact phase predicted by the Thermal Relaxation Oscillations theory. 相似文献
360.
Richard P. Binzel 《Icarus》1985,63(1):99-108
Photoelectric observations of asteroid 1220 Crocus (an Eos family member) show evidence for two distinct periods in its light variation: 30.7 days and 7.90 hr. The lightcurve amplitudes are 0.87 and 0.15 mag, respectively. The shorter period variation appears to be modulated over the longer period. Two periods are possible for a rigid body only if it is in a state of precession. The observations are shown to be compatible with a body in a forced precession state. This result leads us to hypothesize the existence of a satellite of Crocus as the source of the external torque. Calculations are presented which show that there are in fact dynamically possible “binary asteroid” solutions consistent with the observations. More photoelectric and perhaps direct (space telescope) observations are needed to resolve the true nature of this asteroid. 相似文献