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971.
Magnetic susceptibility (MS) data were obtained from 11 sections of the Doushantuo (Ediacaran) cap carbonate that directly overlies the Nantuo glacial diamictite in the southeastern margin of the Yangtze platform. The MS data revealed two regionally correlatable peaks at the bottom and top of the cap carbonate, separated by an interval of low values. The lower MS peak coincides with high percentage of insoluble siliciclastic residues that are compositionally identical to the matrix of the underlying diamictite, suggesting its origin controlled mainly by detrital components during the first phase of cap carbonate deposition at the end of the glaciation. The upper MS peak is associated with high clay content and iron sulfides, and can be interpreted as either derived from enhanced greenhouse weathering that could have brought more terrigenous components into the ocean, or the result of ocean anoxia at the late stage of cap carbonate deposition that could led to formation of abundant iron sulfides. The regionally consistent MS curves from the cap carbonates provided the first geophysical record for the rapid climate change from icehouse to greenhouse conditions in the aftermath of the Neoproterozoic “snowball Earth” event. 相似文献
972.
J. D. Landstreet 《Astronomy and Astrophysics Review》1992,4(1):35-77
Summary Magnetic fields have now been detected in stars in several parts of the Hertzsprung-Russell diagram. Roughly dipolar fields ranging in strength between 3× 102 and 3×104 G are found in many chemically peculiar A and B main sequence stars. Dipolar fields are also found in some 2–3% of white dwarfs, but with strengths between 1×106 and 5×108 G. In both these types of stars, the observed fields vary as the underlying star turns, but do not change in a secular manner. In solartype stars, structurally complex fields of a few kG are found with filling factors of the order of 0.1 to 0.8. Further indirect evidence of fields in cool main sequence stars is provided by detection of visible and ultraviolet line emission (chromospheric activity), x radiation (coronal matter), and giant starspots. In this review, we survey the observations of stellar magnetism in all these types of stars, as well as efforts to model the observed magnetic fields and associated photospheric peculiarities and activity.This article was processed by the author using the Springer-Verlag TEX AAR macro package 1991. 相似文献
973.
Masahiro N. Machida Tomoaki Matsumoto Kohji Tomisaka Tomoyuki Hanawa 《Monthly notices of the Royal Astronomical Society》2005,362(2):369-381
We discuss the evolution of the magnetic flux density and angular velocity in a molecular cloud core, on the basis of three-dimensional numerical simulations, in which a rotating magnetized cloud fragments and collapses to form a very dense optically thick core of >5 × 1010 cm−3 . As the density increases towards the formation of the optically thick core, the magnetic flux density and angular velocity converge towards a single relationship between the two quantities. If the core is magnetically dominated its magnetic flux density approaches 1.5( n /5 × 1010 cm−3 )1/2 mG , while if the core is rotationally dominated the angular velocity approaches 2.57 × 10−3 ( n /5 × 1010 cm−3 )1/2 yr−1 , where n is the density of the gas. We also find that the ratio of the angular velocity to the magnetic flux density remains nearly constant until the density exceeds 5 × 1010 cm−3 . Fragmentation of the very dense core and emergence of outflows from fragments will be shown in the subsequent paper. 相似文献
974.
J. E. Chambers 《Celestial Mechanics and Dynamical Astronomy》1993,57(1-2):131-136
The dynamical evolution of a low inclination, intermediate period comet in mean-motion resonance with a giant planet can be described by a simple mapping giving the comet's heliocentric energy and ecliptic longitude at successive aphelia. Provided that the energy oscillations produced by the resonance are not too large, this mapping is linear. When the amplitude of the oscillations is close to the maximum possible within the resonance, the mapping can be improved by adding non-linear terms. 相似文献
975.
ANISOTROPY OF MAGNETIC SUSCEPTIBILITY AND IDENTIFICATION OF TILTED SEDIMENT CORES FROM THE BOHAI SEA
Anisotropy of magnetic susceptibility (AMS) is a well-developed technique increaseingly used in petro-fabric determination. ItS application to two Mackereth cores JX91-2A and JX91-2B, taken from thesame site in the central Bohai Sea, revealed 3D magnetic fabrics of the sediments as expressed with susce-ptibility ellipsoids. The flat seafloor and the relatively quiet depositional environment in the semi-isolatedBohai Sea should asult in a primary fabric with horisontally obate susocptibility el1ipsoids. Mormwr,the inaxing downcore totaI anisotropy comprising mainly of magntic foliation nsulting fIrom sedimentcompaction should also lead to flat ellipsoids. Therefore, the off-pole meam dindions of minimurn axes ofaseptibility ellipsoids on stermnets must indicate that the two coas are ti]ted re1ative to the flat anfloor.Use of Fisher statisties showh the tilt angles to be 14° for JX91-2A and 23°for JX91 -2B from whichthe tna depths of the coas were dedded accordingly. 相似文献
976.
我们从四个方面综述星系闭的射电研究进展。首先介绍了星系闭中的分立射电源,特别是cD星系研究的一些最新进展和结果。继而介绍了星系闭中射电晕的分类、目前的观测结果和理论解释。星闭中的磁场主要由射电研究得出,在本文中对此也作了适当的介绍和讨论。最后还简介了在星系闭射观测中发现的relic射电源。 相似文献
977.
湖泊(水库)沉积物分析在土壤侵蚀研究中的运用 总被引:1,自引:1,他引:0
土壤侵蚀研究对于防治自然灾害,实现土地的可持续利用具有重要的现实意义。由于一些地区长期以来水文观测资料较为匮乏,针对过去时间段内的土壤侵蚀研究通常需要寻找替代指标。湖泊(水库)作为流域内侵蚀产物的“汇”,其沉积物可不同程度地记录较长时期土壤侵蚀的具体特征,因此,基于湖泊(水库)沉积物分析的土壤侵蚀研究具有较强的应用价值和发展潜力。本文主要围绕确定土壤侵蚀年代,反演侵蚀过程,探寻沉积物来源的研究主线,详细论述了湖泊(水库)沉积物中放射性同位素、矿物磁性、粒度以及地球化学指标的应用原理和方法,分别总结了国内外学者利用湖泊(水库)沉积物在上述3个领域所取得的研究成果,在此基础上讨论了目前研究中存在的不确定性,并对未来的研究方向进行了展望。 相似文献
978.
构造叠加弱应变沉积岩地区的磁组构研究——以川西北盆地为例 总被引:3,自引:1,他引:2
磁组构是一种灵敏的应变指示计。单一方向应力作用下,在平行层缩短的初始阶段,磁线理与地层走向是一致的。然而,在构造叠加背景下弱变形的沉积岩地区,另一个(多个)不同方向的应力使得已经产生定向排列的磁性矿物发生旋转,表现为磁线理和与地层走向斜交。川西北盆地在新生代是一个典型的构造叠加区域,来自龙门山和米仓山的变形在此相互作用。本文在川西北盆地分3条剖面在18个采样点中采集了172个样品进行了磁组构研究。研究区内观察到3种弱变形的磁组构类型:沉积磁组构、初始变形磁组构和铅笔状磁组构。由于应变的叠加,由盆地内部向造山带前缘没有出现应变由弱到强的变化趋势,同时磁线理的方向也不一致。由盆地向造山带,来自米仓山的变形逐渐增强,磁线理从与地层走向一致转变成与地层走向斜交。 相似文献
979.
980.
基于1950~2011年的NCEP/NCAR再分析资料,对渤海10 m风场的风速与风向变化进行多尺度分析。利用小波分析、交叉谱分析等方法对渤海海域的海表风速、风向的变化趋势以及周期进行研究。分析发现:渤海地区海表风的风向与风速除了存在显著的季节性变化特征外,在年际、年代际的变化尺度上也有明显的周期性。风向存在1 a、8.7 a、15.8 a的显著周期,风速存在1 a、6.3 a、15 a的显著周期。风向与风速在时间尺度分别为20 a、5.71 a、2.67 a时存在显著共振周期;共振周期受东亚季风、西太平副热带高压的年际、年代际变化的影响呈现出多尺度变化周期。 相似文献