全文获取类型
收费全文 | 21650篇 |
免费 | 845篇 |
国内免费 | 475篇 |
专业分类
测绘学 | 513篇 |
大气科学 | 260篇 |
地球物理 | 1375篇 |
地质学 | 2649篇 |
海洋学 | 335篇 |
天文学 | 17070篇 |
综合类 | 179篇 |
自然地理 | 589篇 |
出版年
2024年 | 52篇 |
2023年 | 81篇 |
2022年 | 188篇 |
2021年 | 142篇 |
2020年 | 151篇 |
2019年 | 224篇 |
2018年 | 112篇 |
2017年 | 110篇 |
2016年 | 143篇 |
2015年 | 232篇 |
2014年 | 231篇 |
2013年 | 221篇 |
2012年 | 300篇 |
2011年 | 296篇 |
2010年 | 335篇 |
2009年 | 1681篇 |
2008年 | 1640篇 |
2007年 | 1908篇 |
2006年 | 1908篇 |
2005年 | 1728篇 |
2004年 | 1851篇 |
2003年 | 1641篇 |
2002年 | 1409篇 |
2001年 | 1226篇 |
2000年 | 1012篇 |
1999年 | 954篇 |
1998年 | 1117篇 |
1997年 | 279篇 |
1996年 | 171篇 |
1995年 | 305篇 |
1994年 | 316篇 |
1993年 | 159篇 |
1992年 | 90篇 |
1991年 | 96篇 |
1990年 | 93篇 |
1989年 | 151篇 |
1988年 | 93篇 |
1987年 | 92篇 |
1986年 | 80篇 |
1985年 | 40篇 |
1984年 | 32篇 |
1983年 | 24篇 |
1982年 | 7篇 |
1980年 | 4篇 |
1979年 | 9篇 |
1977年 | 6篇 |
1905年 | 3篇 |
1900年 | 3篇 |
1897年 | 7篇 |
1877年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 4 毫秒
141.
H. Greiner-Mai 《Astronomische Nachrichten》1989,310(1):85-93
The influence of recently computed axial magnetic core-mantle coupling torques on the Earth's rotation was investigated. These torques derived from poloidal geomagnetic field within the mantle and at the core-mantle boundary are retarding torques. An accelerating torque due to the action of unknown parts of the core field was estimated by inverse solution of the equation of the mantle rotation for the periodic variations of the quantities of the magnetic field and the length of day. The variations of the drift rate of the Earth's core were compared with those of the mantle rotation velocity for a force-free Earth. The time constants of the coupling process were estimated and discussed in connection with the magnetic coupling of the mantle with an upper core layer. Der Einfluß kürzlich berechneter axialer Kern-Mantel-Kopplungsmomente auf die Erdrotation wurde untersucht. Diese Lorentz-Drehmomente, abgeleitet vom poloidalen geomagnetischen Feld im Mantel und an der Kern-Mantel-Grenze, sind retardierende Momente. Ein beschleunigendes Drehmoment, das der Wirkung unbekannter Feldanteile zugeordnet wird, wurde durch inverse Lösung der Mantelrotationsgleichung für die periodischen Variationen der Magnetfeldgrößen und der Tageslänge abgeschätzt. Die Variationen der Kerndriftgeschwindigkeit wurden mit denen der Mantelrotationsgeschwindigkeit für eine kräftefreie Erde verglichen. Die Zeitkonstanten des Kopplungsprozesses wurden ermittelt und im Zusammenhang mit der magnetischen Kopplung des Mantels mit einer oberen Kernschicht diskutiert. 相似文献
142.
张洪起 《中国天文和天体物理学报》1986,(4)
本文在用Unno-Beckers方程计算光球和黑子本影磁场内FeIλ5324.19谱线形成过程中,计算了该谱线Stokes参数随5000连续谱光学深度分布的贡献函数及形成深度随波长的变化。计算结果表明:磁光效应的存在给该线横向磁场定标参数Q、U的形成深度的确定带来一定的复杂性,对I和V的形成深度的确定没有明显的影响。结合北京天文台太阳磁场望远镜半宽0.15的双折射滤光器,确定所观测磁场信息的形成深度。当对日面中心观测,在滤光器调至线心时,I形成在光球层及黑子高度100公里左右,在偏离线心0.15时V分量形成高度亦如此,Q、U分量的情况较复杂。 相似文献
143.
朱杏芬 《中国天文和天体物理学报》1986,(1)
以Savage-Bolton的两个5°×5°天区的光学类星体巡天为样品,本文系统地分析了成线排列的类星体数目,并与由Monte Carlo方法产生的随机样品进行了比较。统计结果表明,类星体成线排列现象至少在这组样品中不明显。 相似文献
144.
The IRAS and 2MASS associations for 193 T Tauri stars are identified in this paper. From the color–color diagrams and spectral index, it is found that the IR excesses for most samples are due to thermal emission from the circumstellar material, as suggested previously. It is also found that the IR excesses at IRAS region for few T Tauri stars and the near-IR excesses for some T Tauri stars are likely attributed to free-free emission or free-bound emission from the circumstellar ionized gas. Moreover, It is found in deredened J–H versus H–K color–color diagram that there is a slight separation in different spectral groups. The T Tauri stars locus equation in J–H versus H–K color–color diagram for our sample is also presented. 相似文献
145.
We present results from detailed analyses of new data combined with previously published light curves of ε Cra and ψ Ori. Based on the shape of the secondary minimum of ε CrA we found that the discrepancy between the photometric and spectroscopic mass ratio, although marginal, is statistically significant. We propose a third light as a possible solution and derive the absolute parameters of components. The physical parameters of components of the early-type binary system ψ Ori were also obtained from the light curve modelling. Our solution indicate that ψ Ori is a detached, grazing-eclipse system. 相似文献
146.
Valentin D. Ivanov G. Chauvin C. Foellmi M. Hartung N. Huélamo C. Melo D. Nürnberger M. Sterzik 《Astrophysics and Space Science》2006,304(1-4):247-249
The multiplicity of early-type stars is still not well established. The derived binary fraction is different for individual star forming regions, suggesting a connection with the age and the environment conditions. The few studies that have investigated this connection do not provide conclusive results. To fill in this gap, we started the first detailed adaptive-optic-assisted imaging survey of early-type field stars to derive their multiplicity in a homogeneous way. The sample has been extracted from the Hipparcos Catalog and consists of 341 BA-type stars within ∼300 pc from the Sun. We report the current status of the survey and describe a Monte-Carlo simulation that estimates the completeness of our companion detection. 相似文献
147.
148.
149.
M. Lyutikov 《Monthly notices of the Royal Astronomical Society》2006,373(1):73-78
Sharp fronts observed by the Chandra satellite between dense cool cluster cores moving with near-sonic velocity through the hotter intergalactic gas, require strong suppression of thermal conductivity across the boundary. This may be due to magnetic fields tangential to the contact surface separating the two plasma components. We point out that a super-Alfvenic motion of a plasma cloud (a core of a merging galaxy) through a weakly magnetized intercluster medium leads to 'magnetic draping', formation of a thin, strongly magnetized boundary layer with a tangential magnetic field. For supersonic cloud motion, M s ≥ 1 , magnetic field inside the layer reaches near-equipartition values with thermal pressure. Typical thickness of the layer is ∼ L / M 2 A ≪ L , where L is the size of the obstacle (plasma cloud) moving with Alfvén Mach number M A ≫ 1 . To a various degree, magnetic draping occurs for both subsonic and supersonic flows, random and ordered magnetic fields and it does not require plasma compressibility. The strongly magnetized layer will thermally isolate the two media and may contribute to the Kelvin–Helmholtz stability of the interface. Similar effects occur for radio bubbles, quasi-spherical expanding cavities blown up by active galactic nucleus jets; in this case, the thickness of the external magnetized layer is smaller, ∼ L / M 3 A ≪ L . 相似文献
150.
The accretion-induced neutron star (NS) magnetic field evolution is studied through considering the accretion flow to drag the field lines aside and dilute the polar-field strength, and as a result the equatorial field strength increases, which is buried inside the crust on account of the accretion-induced global compression of star crust. The main conclusions of model are as follows: (i) the polar field decays with increase in the accreted mass; (ii) the bottom magnetic field strength of about 108 G can occur when the NS magnetosphere radius approaches the star radius, and it depends on the accretion rate as ; and (iii) the NS magnetosphere radius decreases with accretion until it reaches the star radius, and its evolution is little influenced by the initial field and the accretion rate after accreting ∼0.01 M⊙ , which implies that the magnetosphere radii of NSs in low-mass X-ray binaries would be homogeneous if they accreted the comparable masses. As an extension, the physical effects of the possible strong magnetic zone in the X-ray NSs and recycled pulsars are discussed. Moreover, the strong magnetic fields in the binary pulsars PSR 1831−00 and PSR 1718−19 after accreting about 0.5 M⊙ in the binary-accretion phase, 8.7 × 1010 and 1.28 × 1012 G , respectively, can be explained through considering the incomplete frozen flow in the polar zone. As an expectation of the model, the existence of the low magnetic field (∼3 × 107 G) NSs or millisecond pulsars is suggested. 相似文献