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   1463篇
  免费   165篇
  国内免费   225篇
测绘学   178篇
大气科学   106篇
地球物理   218篇
地质学   689篇
海洋学   225篇
天文学   219篇
综合类   96篇
自然地理   122篇
  2024年   8篇
  2023年   12篇
  2022年   33篇
  2021年   37篇
  2020年   37篇
  2019年   53篇
  2018年   30篇
  2017年   54篇
  2016年   43篇
  2015年   65篇
  2014年   83篇
  2013年   72篇
  2012年   102篇
  2011年   79篇
  2010年   70篇
  2009年   87篇
  2008年   99篇
  2007年   117篇
  2006年   86篇
  2005年   96篇
  2004年   75篇
  2003年   80篇
  2002年   79篇
  2001年   46篇
  2000年   64篇
  1999年   52篇
  1998年   31篇
  1997年   57篇
  1996年   22篇
  1995年   12篇
  1994年   8篇
  1993年   12篇
  1992年   7篇
  1991年   8篇
  1990年   6篇
  1988年   5篇
  1987年   3篇
  1986年   3篇
  1985年   2篇
  1984年   2篇
  1979年   1篇
  1976年   1篇
  1971年   1篇
  1921年   1篇
  1919年   1篇
  1905年   1篇
  1897年   2篇
  1880年   2篇
  1877年   2篇
  1875年   1篇
排序方式: 共有1853条查询结果,搜索用时 750 毫秒
841.
We have developed a new technique to decouple the spectra of the host and the nucleus of type I AGNs using integral field spectroscopy data. The technique is a simple extension of methods widely tested in 2D imaging. We present here the results from applying the technique to data taken with INTEGRAL at the 4.2 m William Herschel Telescope on the Seyfert 1 radio-galaxy 3C 120. We obtained, for the first time, a clean spectrum of the host galaxy, without contamination from the nuclear source.  相似文献   
842.
We investigate sources of both dissolved and particulate organic carbon in the St Lawrence River from its source (the Great Lakes outlet) to its estuary, as well as in two of its tributaries. Special attention is given to seasonal interannual patterns by using data collected on a bi‐monthly basis from mid‐1998 to mid‐2003. δ13C measurements in dissolved inorganic carbon, dissolved organic carbon (DOC) and particulate organic carbon (POC), as well as molar C : N in particulate organic matter (POM), are used to bring insight into the dynamic between aquatic versus terrigenous sources. In addition, 14C activities of DOC were measured at the outlet of the St Lawrence River to its estuary to assess a mean age of the DOC exported to the estuary. In the St Lawrence River itself, aquatically produced POC dominates terrestrially derived POC and is depleted in 13C by approximately 12‰ versus dissolved CO2. In the Ottawa River, the St Lawrence River's most important tributary, the present dataset did not allow for convincing deciphering of POC sources. In a small tributary of the St Lawrence River, aquatically produced POC dominates in summer and terrestrially derived POC dominates in winter. DOC seems to be dominated by terrestrially derived organic matter at all sampling sites, with some influence of DOC derived from aquatically produced POC in summer in the St Lawrence River at the outlet of the Great Lakes and in one of its small tributaries. The overall bulk DOC is relatively recent (14C generally exceeding 100% modern carbon) in the St Lawrence River at its outlet to the estuary, suggesting that it derives mainly from recent organic matter from topsoils in the watershed. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   
843.
844.
LARDIÈRE  O.  GARRO  S.  MERLIN  J. -C. 《Earth, Moon, and Planets》1997,78(1-3):205-210
Comet Hale-Bopp was observed with the 80 cm reflector + CCD at the Haute-Provence observatory (OHP) and with the 62 cm reflector + CCD at the Saint-Véran observatory (Queyras, France). The morphology of the shells was followed from their first appearence on 1997 Jan. 30, until their disappearance on May 9. These shells spread from the nucleus region with a velocity in agreement with a nuclear rotation period of about 11.33 hours. We report also a short and bright dust ejection on May 8. CN images show a long spiral jet in the tailward side invisible on continuum images. The circumnuclear structures have been followed at Saint-Véran from Apr. 5 to Apr. 11, 1997 with a high spatial resolution (200 km/pixel). We have followed the emergence of a recurrent linear polar jet. Measurements of its expansion show a constant acceleration of material with typical expansion velocity of 1 km/s. The CCD frames show the interconnection between spiral jets and the successive shells. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
845.
介绍了利用IMB-PC 8088CPU的单板机控制检测多路直流电源过压、欠压。在出现此类故障时能发出报警声,并在显示器上显示故障电源编号,同时切断故障电源,启动同类备用电源,以便维修。  相似文献   
846.
    
The concept of simultaneous multifrequency continuum observations, successfully tested on Comet Hyakutake, was applied to Comet Hale-Bopp, using the Heinrich Hertz Submillimeter Telescope (HHT) with the four color bolometer between 250 and 870 GHz, the IRAM 30m telescope at 240 Ghz, the MPIfR 100-m telescope at 32 GHz, and the IRAM interferometer near 90 and 240 GHz. Near-simultaneous measurements were done between February 15 and April 26, 1997, mainly concentrated in mid March shortly before perigee of the comet. The measurements gave the following preliminary results: Interferometer detection of the nuclear thermal emission. If the signal at the longest interferometer spacing of 170 mis due to thermal emission from the nucleus only, its equivalent diameter is ∼49 km. If, however, this signal contains a contribution from a strongly centrally peaked halo distribution(e.g., r−2 density variation) the diameter may be as low as 35 km. The emission found interferometrically was always 5″ north and 0.1 sec east from the position predicted by Yeoman's solution 55.The comparison of the interferometric continuum emission with the simultanously obtained molecular line observations (reported on this conference) shows the origin of the strongest line emission concentrated on the nucleus. The 30-m observations show a radio halo with a gaussian FWHP of ∼11, corresponding to a diameter of 11000 km at geocentric distance of 1.2 a.u. A spectral index of ∼3.0 for the total signal, which may indicate a smaller mean particle size than for Hyakutake. Assuming an average cometary density of 0.5 gcm−3, the mass contained in the nucleus is ∼1$#x2013;3 1019 g and 1012 g in the particle halo. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
847.
We present results from a Keck optical and near IR spectroscopic study of the giant emission line halos of the z>3 High Redshift Radio Galaxies (HiZRGs) 4C 41.17, 4C 60.07 and B2 0902+34. The outer regions of these halos show quiet kinematics with typical velocity dispersions of a few hundred km s−1 and velocity shears consistent with rotation. The inner regions contain shocked, clumpy cocoons of gas closely associated with the radio lobes with disturbed kinematics and expansion velocities and/or velocity dispersions >1000 km s−1. We also find evidence for the ejection of chemically enriched material in 4C 41.17 up to a distance of ∼60 kpc along the radio-axis. We infer that these HiZRGs are undergoing a final jet-induced phase of star formation with the ejection of most of their interstellar medium before evolving to become “red and dead” Elliptical galaxies.  相似文献   
848.
借助L[a,b]空间和选择收敛好列的对角线法则,研究了一类显含一阶导数的二阶奇异边值问题非C^1正解的存在性和唯一性。  相似文献   
849.
Comet C/2001 A2 experienced several splitting events duringits 2001 perihelion passage. The first break-upevent was observed in March 2001 (IAUC 7616).In this paper we report the first results of ourextensive imaging and spectroscopic monitoring campaignwith ESO telescopes over several weeks before andafter the perihelion passage on May 25 2001.  相似文献   
850.
We present the results of polarimetric and photometricobservations of split Comet C/2001 A2 (LINEAR), which wereobtained at the 70-cm telescope of the Astronomical Observatoryof Kharkiv National University between 30 June and 31 July 2001.The brightest fragment of the comet, nucleus B, was observed.Eight narrowband cometary filters in the continuum and in emissionbands and a wideband red filter have been used. The comet wasobserved through apertures of 88, 33, and 19 arcsec. Polarizationphase curves were obtained for the continuum and for the firsttime, for NH2(0, 7, 0) emission. The degree of polarization ofthe light scattered by the dust decreases with the increase ofaperture size. An important temporal variation of the polarizationwith a rotation of the polarization plane was observed at twophase angles (26.5° and 36.2°). Molecular column densities and production rates of CN, C2,C3, and NH2 species are calculated in the framework of theHaser model. A comparative analysis of the temporal variations ofthe visual magnitudes, gas and dust production rates, dust colorand polarization are presented.  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