全文获取类型
收费全文 | 1050篇 |
免费 | 146篇 |
国内免费 | 139篇 |
专业分类
测绘学 | 53篇 |
大气科学 | 150篇 |
地球物理 | 269篇 |
地质学 | 288篇 |
海洋学 | 44篇 |
天文学 | 386篇 |
综合类 | 45篇 |
自然地理 | 100篇 |
出版年
2024年 | 7篇 |
2023年 | 20篇 |
2022年 | 24篇 |
2021年 | 42篇 |
2020年 | 27篇 |
2019年 | 32篇 |
2018年 | 26篇 |
2017年 | 22篇 |
2016年 | 31篇 |
2015年 | 28篇 |
2014年 | 49篇 |
2013年 | 33篇 |
2012年 | 53篇 |
2011年 | 41篇 |
2010年 | 38篇 |
2009年 | 55篇 |
2008年 | 67篇 |
2007年 | 63篇 |
2006年 | 74篇 |
2005年 | 62篇 |
2004年 | 68篇 |
2003年 | 49篇 |
2002年 | 35篇 |
2001年 | 37篇 |
2000年 | 37篇 |
1999年 | 64篇 |
1998年 | 43篇 |
1997年 | 27篇 |
1996年 | 30篇 |
1995年 | 31篇 |
1994年 | 15篇 |
1993年 | 22篇 |
1992年 | 17篇 |
1991年 | 16篇 |
1990年 | 14篇 |
1989年 | 10篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 5篇 |
1980年 | 3篇 |
1978年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有1335条查询结果,搜索用时 576 毫秒
141.
R. Ignace J. E. Bjorkman H. M. Bryce 《Monthly notices of the Royal Astronomical Society》2006,366(1):92-100
In recent years, it has been shown that microlensing is a powerful tool for examining the atmospheres of stars in the Galactic bulge and Magellanic Clouds. The high gradient of magnification across the source during both small impact parameter events and caustic crossings offers a unique opportunity for determining the surface brightness profile of the source. Furthermore, models indicate that these events can also provide an appreciable polarization signal: arising from differential magnification across the otherwise symmetric source. Earlier work has addressed the signal from a scattering photosphere for both point mass lenses and caustic crossings. In a previous paper, polarimetric variations from point lensing of a circumstellar envelope were considered, as would be suitable for an extended envelope around a red giant. In this work, we examine the polarization in the context of caustic crossing events, the scenario that represents the most easily accessible situation for actually observing a polarization signal in Galactic microlensing. Furthermore, we present an analysis of the effectiveness of using the polarimetric data to determine the envelope properties, illustrating the potential of employing polarimetry in addition to photometry and spectroscopy with microlensing follow-up campaigns. 相似文献
142.
143.
T. M. Gledhill 《Monthly notices of the Royal Astronomical Society》2005,356(3):883-898
In an imaging polarimetry survey of candidate post-AGB stars, scattering envelopes are detected around 20 objects. These envelopes represent the final mass-loss phases at the end of the AGB. In all cases, evidence for axisymmetry in the dust density is seen, suggesting that the presence of an axisymmetric outflow may be a ubiquitous phenomenon of the AGB to post-AGB transition. We use the polarized flux images to classify the objects into detached shell, bipolar and unresolved targets. Modelling based on a simple axisymmetric shell geometry supports the contention that post-AGB objects fall into one of two classes that differ in the amount of dust in the circumstellar environment: the detached shells correspond to stars with an optically thin expanding circumstellar envelope (CSE) whereas the bipolar and unresolved targets have optically thick dust structures, probably in the form of discs, which remain bound to the star, rather than partaking in the expansion of the AGB CSE. It is suggested that this bifurcation in morphology is rooted in the presence or absence of a binary companion, which determines whether or not a disc forms. Because the detached shell objects also appear axisymmetric, an additional mechanism for generating the axisymmetry, such as a magnetically shaped outflow, is needed if they do indeed have single star progenitors. 相似文献
144.
Jin-LinHan RichardWielebinski 《中国天文和天体物理学报》2002,2(4):293-294
Magnetic fields are observed everywhere in the universe. In this review, we concentrate on the observational aspects of the magnetic fields of Galactic and extragalactic objects. Readers can follow the milestones in the observations of cosmic magnetic fields obtained from the most important tracers of magnetic fields, namely, the star-light polarization, the Zeeman effect, the rotation measures (RMs, hereafter) of extragalactic radio sources, the pulsar RMs, radio polarization observations, as well as the newly implemented sub-mm and mm polarization capabilities. The magnetic field of the Galaxy was first discovered in 1949 by optical polarization observations. The local magnetic fields within one or two kpc have been well delineated by starlight polarization data. The polarization observations of diffuse Galactic radio background emission in 1962 confirmed unequivocally the existence of a Galactic magnetic field. The bulk of the present information about the magnetic fields in the Galaxy comes from anal 相似文献
145.
This review begins with a discussion of the techniques needed for observations of scattered light from cometary dust. After
an introduction into the basic concepts of the scattering process, observations of the phase curves of brightness, colour
and polarization are covered. Images of colour and polarization are presented and the observed relation of colour and polarization
in jets and shells is discussed. The interpretation of the measurements is based on the power law size distributions of dust
grains observed from space. The power index must lie between 2 and 4 to provide the mass budget and visibility of the dust
coma in accordance with the basic facts of cometary physics. Application of mechanical (radiation pressure) theory to cometary
images allows us to derive related power law distributions for comets not explored by spacecraft. Grain scattering models
are presented and compared with observations. A prediction is made of the spatial distribution of Stokes parameters U and
V in the presence of aligned particles. Up to now such patterns have not been observed. Future work should include the exploration
of comets at small and possibly very small phase angles and a detailed comparison of polarization and colour images of comets
with thermal images and with models based on mechanical theory.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
146.
Harris W. M. Nordsieck K. H. Scherb F. Mierkiewicz E. J. 《Earth, Moon, and Planets》1997,78(1-3):161-167
We report on the reduction and analysis of UVpolarimetric images of CI (λ1657 Å) and dust continuum (2696 Å emissions from C/1995 O1 (Hale-Bopp) taken using the Wide Field Imaging Survey Polarimeter (WISP) sounding rocket on 8 April, 1997. These observations represent the first imaging polarimetry of comets in the UV, and were performed in consort with ground based measurements of gas and dust polarization and distribution. The continuum results show 9% polarization across the image field with a polarization phase angle close to the 129° prediction. Comparison with ground based data implies minimal color dependence for Hale-Bopp in either the degree of polarization and in the position angle. The carbon polarimetry implies that most production occurs in the dense inner coma, and that it leaves that area in thermodynamic equilibrium. Its radial profile further constrains the carbon outflow speed to be sufficient to travel ≥5 × 106 km without photoionization. 相似文献
147.
M. Shimon Y. Rephaeli B. W. O'Shea M. L. Norman 《Monthly notices of the Royal Astronomical Society》2006,368(2):511-511
Scattering of the cosmic microwave background (CMB) in clusters of galaxies polarizes the radiation. We explore several polarization components which have their origin in the kinematic quadrupole moments induced by the motion of the scattering electrons, either directed or random. Polarization levels and patterns are determined in a cluster simulated by the hydrodynamical enzo code. We find that polarization signals can be as high as ∼1 μK , a level that may be detectable by upcoming CMB experiments. 相似文献
148.
149.
150.