全文获取类型
收费全文 | 3531篇 |
免费 | 429篇 |
国内免费 | 509篇 |
专业分类
测绘学 | 147篇 |
大气科学 | 962篇 |
地球物理 | 570篇 |
地质学 | 414篇 |
海洋学 | 278篇 |
天文学 | 1669篇 |
综合类 | 116篇 |
自然地理 | 313篇 |
出版年
2024年 | 16篇 |
2023年 | 48篇 |
2022年 | 59篇 |
2021年 | 85篇 |
2020年 | 73篇 |
2019年 | 80篇 |
2018年 | 69篇 |
2017年 | 65篇 |
2016年 | 66篇 |
2015年 | 110篇 |
2014年 | 133篇 |
2013年 | 143篇 |
2012年 | 133篇 |
2011年 | 147篇 |
2010年 | 128篇 |
2009年 | 265篇 |
2008年 | 239篇 |
2007年 | 269篇 |
2006年 | 235篇 |
2005年 | 241篇 |
2004年 | 192篇 |
2003年 | 213篇 |
2002年 | 195篇 |
2001年 | 168篇 |
2000年 | 176篇 |
1999年 | 159篇 |
1998年 | 151篇 |
1997年 | 90篇 |
1996年 | 87篇 |
1995年 | 89篇 |
1994年 | 78篇 |
1993年 | 53篇 |
1992年 | 33篇 |
1991年 | 38篇 |
1990年 | 28篇 |
1989年 | 28篇 |
1988年 | 27篇 |
1987年 | 18篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1984年 | 7篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有4469条查询结果,搜索用时 15 毫秒
991.
充分认识外太阳系撞击体的来源类型和分布特征,对认识外太阳系固态天体上撞击过程,明确外太阳系天体上的撞击坑生成率和撞击坑定年等诸多方面具有重要的意义.得益于海量高质量探测数据的获取,如今我们对内太阳系主要天体表面的撞击分布和来源已经有了较为深入的了解,但对外太阳系天体的撞击分布和来源还知之甚少.不同大小频率的撞击体会在外... 相似文献
992.
We carry out a study of the interaction between electromagnetic fields produced innature and cells with no structure but with different shapes and sizes, considered asapproximations of those observed in the human body. Magnetic fields produced byvarious atmospheric and geomagnetic phenomena are compared with the fieldsgenerated by the cells themselves. Working with frequencies up to 105 Hz, we found that in most cases lightning is the natural phenomena capable of producingfield intensities comparable to those generated by the cells. As a first approximationwe assume that a cell might be affected only if a field of intensity comparable to thecell's field is present. Oscillating fields of less intensity and their harmonics can involveresonant interactions with the cells, however this situation is not considered here. Under the approximation of our study we found that the size and the shape are parameters of paramount importance: lightning represents a natural hazard only for the largest cells, while the quadrangular cells are not affected by any of the natural phenomena considered here. 相似文献
993.
与地震预报有关的电磁现象国际讨论会概况 总被引:3,自引:0,他引:3
与地震预报有关的电磁现象国际学术讨论会已于1993年9月6-8日在东京召开。这次学术讨论会的宗旨是,为世界各国学者提供一个公开讲坛以讨论与地震有关的电磁现象,并通过交叉学科与多科学相关领域相结合的方式探讨地震预报等问题。本文较详细地介绍了各国学者在这一领域中的最新成果。 相似文献
994.
初步探索了CE318型太阳光度计的室内和野外标定方法, 并对两种方法的标定结果进行对比。基于积分球辐射源的辐亮度标定方法, 对CE318型太阳光度计不同波段天空散射辐射通道进行标定实验; 分别采用Langley标定法和标准仪器相对标定方法, 对CE318型太阳光度计的不同波段太阳直接辐射通道进行了标定实验。实验结果显示:天空散射辐射通道的标定结果在670 nm, 870 nm和1020 nm波段与仪器出厂时给定的标定结果偏差不超过6%, 而440 nm波段处得到的标定结果要高于出厂时给定的结果, 偏离幅度约18.9%。太阳直接辐射各通道的标定结果普遍大于出厂时给定的结果, 说明滤光片老化较为严重, 需要进行更换, 以保证观测精度。由于标准仪器相对标定方法对太阳直接辐射通道的标定结果明显优于Langley法, 因此利用标准仪器相对标定方法对中国气象局太阳光度计站网仪器进行定期标定更能够保证仪器观测数据的准确性。 相似文献
995.
大地震前存在长波辐射异常已被许多震例所证实,为进一步分析强地震前的长波辐射异常变化特征,提取其异常的判定指标。以多年静止卫星长波辐射资料(OLR)为基础,应用小波变换和功率谱估计方法对中国大陆6级以上地震进行分析研究。结果表明,6次强地震前三个月内均出现不同程度的长波辐射异常现象,与以前震例分析结果较为一致。空间上在震中及其附近区域异常整体呈现开始-增强-极值-减弱-消失过程,异常最大值时大于多年均值的1倍标准偏差的范围面积达数万到十几万km2,异常最大值时相对功率谱幅值均在10倍以上;时间上震中附近小区域相对功率谱幅值持续偏离多年均值的1倍标准偏差,持续偏离时间介于40~75天。这些强地震的异常判定指标具有短期预测指示意义。 相似文献
996.
Kazuho Horiuchi Tomoko Uchida Yuko Sakamoto Aoi Ohta Hiroyuki Matsuzaki Yasuyuki Shibata Hideaki Motoyama 《Quaternary Geochronology》2008,3(3):253-261
The cosmogenic nuclide 10Be was analyzed by using accelerator mass spectrometry on an ice core drilled at the Dome Fuji station, inland Antarctica, for 700–1900 yr CE. The measured concentration of 10Be in the Dome Fuji ice core and the derived 10Be flux show similar fluctuations, with both increasing at known solar-activity minima over the last millennium in agreement with earlier observations of 10Be and 14C. Based on the similar nature of the 10Be flux to the reconstructed 14C production rate patterns, a 10Be–14C correlation age model for the Dome Fuji ice core was successfully constructed. This age model agrees well with the initial version of the tephrochronology of the core. The 10Be-flux record contains information on variability in the amount of cosmic radiation incident on the atmosphere, which is mainly attributable to high-frequency change in solar activity and low-frequency background intensity adjustment of the geomagnetic field. High-resolution 10Be analyses of the Dome Fuji ice cores promise to provide potentially important information on the history of cosmic radiation intensity over the past several hundred thousand years. 相似文献
997.
We would like to comment on a recent paper by Sturrock et al. (2014) in which the authors analyze decay data acquired by an ionization chamber in our institute. They interpret the variations in the data as solar-driven changes in the decay rates of the radionuclides under study. In brief we would like to discuss and elucidate the properties and the origin of the data used by the authors and explain why these data are not a sound basis for claiming evidence for new physics. 相似文献
998.
The theory of optimal control is applied to obtain minimum-time trajectories for solar sail spacecraft for interplanetary
missions. We consider the gravitational and solar radiation forces due to the Sun. The spacecraft is modelled as a flat sail
of mass m and surface area A and is treated dynamically as a point mass. Coplanar circular orbits are assumed for the planets. We obtain optimal trajectories
for several interrelated problem families and develop symmetry properties that can be used to simplify the solution-finding
process. For the minimum-time planet rendezvous problem we identify different solution branches resulting in multiple solutions
to the associated boundary value problem. We solve the optimal control problem via an indirect method using an efficient cascaded
computational scheme. The global optimizer uses a technique called Adaptive Simulated Annealing. Newton and Quasi-Newton Methods
perform the terminal fine tuning of the optimization parameters. 相似文献
999.
Saleem Zaroubi Rajat M. Thomas Naoshi Sugiyama Joseph Silk 《Monthly notices of the Royal Astronomical Society》2007,375(4):1269-1279
A simple analytical model is used to calculate the X-ray heating of the intergalactic medium (IGM) for a range of black hole masses. This process is efficient enough to decouple the spin temperature of the IGM from the cosmic microwave background (CMB) temperature and produce a differential brightness temperature of the order of ∼ 5–20 mK out to distances as large as a few comoving Mpc, depending on the redshift, black hole mass and lifetime. We explore the influence of two types of black holes, those with and without ionizing ultraviolet radiation. The results of the simple analytical model are compared to those of a full spherically symmetric radiative transfer code. Two simple scenarios are proposed for the formation and evolution of black hole mass density in the Universe. The first considers an intermediate mass black hole that form as an end-product of pop III stars, whereas the second considers supermassive black holes that form directly through the collapse of massive haloes with low spin parameter. These scenarios are shown not to violate any of the observational constraints, yet produce enough X-ray photons to decouple the spin temperature from that of the CMB. This is an important issue for future high-redshift 21-cm observations. 相似文献
1000.
I.observedResultsandtheEmpiricalEstimatedFormulasforDirect'UltraviolctRadiationl.observedresultsofdirectultravioletradiationDuetostrongabsorptionofo2,N2,O,N,andO3inthehighlayeroftheatmosphere,solarradiationatwavelengthlessthan29onmcannotarriveateartl1suthee.Betweenthewavelength29Onmand4Oonm,thedirectultravioletirradia11ceSuvcanbewrittenas:Where,S.xissolarsPectralirradiancearrivedattopofatmosphere,Tlxisoptlcalthicknessofatmosphericmolecularabovesea1evel,Whicl1isaknownfunctiollofwavelengt… 相似文献