首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1613篇
  免费   55篇
  国内免费   26篇
测绘学   32篇
大气科学   142篇
地球物理   335篇
地质学   586篇
海洋学   146篇
天文学   247篇
综合类   18篇
自然地理   188篇
  2023年   7篇
  2021年   21篇
  2020年   25篇
  2019年   29篇
  2018年   44篇
  2017年   32篇
  2016年   29篇
  2015年   51篇
  2014年   48篇
  2013年   91篇
  2012年   60篇
  2011年   94篇
  2010年   62篇
  2009年   96篇
  2008年   75篇
  2007年   89篇
  2006年   71篇
  2005年   57篇
  2004年   50篇
  2003年   44篇
  2002年   48篇
  2001年   24篇
  2000年   36篇
  1999年   26篇
  1998年   23篇
  1997年   23篇
  1996年   26篇
  1995年   16篇
  1994年   23篇
  1993年   19篇
  1992年   14篇
  1991年   18篇
  1990年   11篇
  1989年   25篇
  1988年   16篇
  1987年   22篇
  1986年   17篇
  1985年   18篇
  1984年   27篇
  1983年   18篇
  1982年   17篇
  1981年   16篇
  1980年   23篇
  1979年   15篇
  1978年   13篇
  1977年   13篇
  1976年   14篇
  1975年   15篇
  1974年   8篇
  1973年   12篇
排序方式: 共有1694条查询结果,搜索用时 4 毫秒
51.
The mean solar magnetic field as measured in integrated light has been observed since 1968. Since 1970 it has been observed both at Hale Observatories and at the Crimean Astrophysical Observatory. The observing procedures at both observatories and their implications for mean field measurements are discussed. A comparison of the two sets of daily observations shows that similar results are obtained at both observatories. A comparison of the mean field with the interplanetary magnetic polarity shows that the IMF sector structure has the same pattern as the mean field polarity.  相似文献   
52.
A recent estimate of tidal dissipation by turbulent viscosity in Jupiter's convective interior predicts that the current value of the planet's tidal Q ~ 5 × 106. We point out a fundamental error in this calculation, and show that turbulent dissipation alone implies that at present Q ~ 5 × 1013. Our reduced estimat for the rate of tidal dissipation shows conclusively that tidal torques have produced only negligible modifications of the orbits of the Galilean satellites over the age of the solar system.  相似文献   
53.
Photons in the energy region above 10 MeV can provide unique information about the highest energy particles produced in solar flares. Measurements of the energy spectra, temporal behavior and anisotropies of the incident photons, can give valuable insights into the particle acceleration process. In this paper we review the detection techniques used at these energies, typical instruments which use these techniques, and future solar missions both actual and potential.  相似文献   
54.
55.
Results of a survey of instrumentation and detector systems, either currently deployed or planned for use at telescopes larger than 3.5 m, in ground based observatories world-wide, are presented. This survey revealed a number of instrumentation design trends at optical, near, and mid-infrared wavelengths. Some of the most prominent trends include the development of vastly larger optical detector systems (> 109 pixels) than anything built to date, and the frequent use of mosaics of near-infrared detectors – something that was quite rare only a decade ago in astronomy. Some future science applications for detectors are then explored, in an attempt to build a bridge between current detectors and what will be needed to support the research ambitions of astronomers in the future.  相似文献   
56.
Philip B. James 《Icarus》1982,52(3):565-569
The 1979–1980 regression curve for the north polar cap of Mars, determined from Viking orbiter images, is compared to Viking observations of the same season 1 (Martin) year earlier and to telescopic observations. Differences between the two years cannot unambigously be attributed to dynamical effects because of uncertainties introduced by limited longitudinal coverage.  相似文献   
57.
The concentrations of major and trace elements have been determined in igneous rocks from Apollo 15. All materials analyzed have typical depletions of Eu except for minerals separated from sample 15085. Four samples have concentrations of trace elements that are similar to those of KREEP. The samples of mare basalt from Apollo 15 have higher concentrations of FeO, MgO, Mn, and Cr and lower concentrations of CaO, Na2O, K2O, and rare-earth elements (REE) as compared to the samples of mare basalt from Apollos 11, 12, and 14. The samples can be divided into two groups on the basis of their normative compositions. One group is quartz normative and has low concentrations of FeO while the other is olivine normative and has high concentrations of FeO. The trace element data indicate that the samples of olivine normative basalt could be from different portions of a single lava flow. At least two and possible three parent magmas can be identified from the samples of the quartz normative group on the basis of their concentration ratios of Sm to Eu. Within each group, the compositions of the samples appear to be related by crystallization of olivine or pyroxene. Significant variations of the ratio of concentration of Sm to Eu cannot be produced without plagioclase-liquid equilibrium. The source material of mare basalt may be depleted in Eu. Alternatively, the magmas may have assimilated a small volume of material similar to KREEP.Paper dedicated to Prof. Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.  相似文献   
58.
Measurements of sap flow, meteorological parameters, soil water content and tension were made for 4 months in a young cashew (Anacardium occidentale L.) plantation during the 2002 rainy season in Ejura, Ghana. This experiment was part of a sustainable water management project in West Africa. The Granier system was used to measure half‐hourly whole‐tree sap flow. Weather variables were observed with an automatic weather station, whereas soil moisture and tension were measured with a Delta‐T profile probe and tensiometers respectively. Clearness index (CI), a measure of the sky condition, was significantly correlated with tree transpiration (r2 = 0·73) and potential evaporation (r2 = 0·86). Both diurnal and daily stomata conductance were poorly correlated with the climatic variables. Estimated daily canopy conductance gc ranged from 4·0 to 21·2 mm s−1, with a mean value of 8·0 ± 3·3 mm s−1. Water flux variation was related to a range of environmental variables: soil water content, air temperature, solar radiation, relative humidity and vapour pressure deficit. Linear and non‐linear regression models, as well as a modified Priestley–Taylor formula, were fitted with transpiration, and the well‐correlated variables, using half‐hourly measurements. Measured and predicted transpiration using these regression models were in good agreement, with r2 ranging from 0·71 to 0·84. The computed measure of accuracy δ indicated that a non‐linear model is better than its corresponding linear one. Furthermore, solar radiation, CI, clouds and rain were found to influence tree water flux. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   
59.
Cores and surface samples of peats from the Everglades—Mangrove region of Southern Florida were analyzed for total sulfur and pyritic sulfur. These values were compared with the petrographic-botanical components of the peats as determined from point-counts of oriented microtome sections. Pyrite occurs as individual euhedral crystals, loosely packed framboids, and lenses or crusts of minute crystals. Framboids and minute crystals are often associated with organic matter and sometimes with bacteria and fungi. Pyrite tends to selectively occur in void spaces in or between peat tissues. In samples containing very small amounts of pyrite, framboidal pyrite is the prevalent form.Marine to brackish peats contain the highest pyrite and total sulfur contents, with brackish peats generally containing more pyrite than marine peats. Pyrite tends to be lower in all peats within 30 cm of the surface, whether marine or brackish.Burial of freshwater peats beneath marine or brackish peats tends to increase the total sulfur and pyritic sulfur in the underlying peats. Burial beneath brackish-water, clay-rich deposits (such as splays) tends to increase total sulfur and pyritic sulfur in the underlying deposits more than burial beneath less clay-rich deposits.  相似文献   
60.
The distribution of ammonium citrate-leachable lead, zinc and cadmium among size fractions in stream sediments is strongly influenced by the presence of hydrous Mn-Fe oxides in the form of coatings on sediment grains. Distribution curves showing leachable metals as a function of particle size are given for eight samples from streams in New York State. These show certain features in common; in particular two concentrations of metals, one in the finest fractions, and a second peak in the coarse sand and gravel fraction. The latter can be explained as a result of the increasing prevalence and thickness of oxide coatings with increasing particle size, with the oxides serving as collectors for the heavy metals. The distribution of Zn and Cd in most of the samples closely parallels that of Mn; the distribution of Pb is less regular and appears to be related to Fe in some samples and Mn in others. The concentration of metals in the coarse fractions due to oxide coatings, combined with the common occurrence of oxide deposition in streams of glaciated regions, raises the possibility of using coarse materials for geochemical surveys and environmental heavy-metal studies.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号