全文获取类型
收费全文 | 41395篇 |
免费 | 717篇 |
国内免费 | 317篇 |
专业分类
测绘学 | 984篇 |
大气科学 | 2948篇 |
地球物理 | 8015篇 |
地质学 | 14253篇 |
海洋学 | 3850篇 |
天文学 | 9751篇 |
综合类 | 75篇 |
自然地理 | 2553篇 |
出版年
2021年 | 404篇 |
2020年 | 401篇 |
2019年 | 453篇 |
2018年 | 947篇 |
2017年 | 847篇 |
2016年 | 1092篇 |
2015年 | 608篇 |
2014年 | 1007篇 |
2013年 | 2077篇 |
2012年 | 1135篇 |
2011年 | 1620篇 |
2010年 | 1474篇 |
2009年 | 1984篇 |
2008年 | 1646篇 |
2007年 | 1733篇 |
2006年 | 1589篇 |
2005年 | 1315篇 |
2004年 | 1273篇 |
2003年 | 1243篇 |
2002年 | 1193篇 |
2001年 | 1056篇 |
2000年 | 982篇 |
1999年 | 815篇 |
1998年 | 817篇 |
1997年 | 830篇 |
1996年 | 671篇 |
1995年 | 656篇 |
1994年 | 616篇 |
1993年 | 570篇 |
1992年 | 526篇 |
1991年 | 496篇 |
1990年 | 507篇 |
1989年 | 497篇 |
1988年 | 475篇 |
1987年 | 554篇 |
1986年 | 490篇 |
1985年 | 605篇 |
1984年 | 655篇 |
1983年 | 598篇 |
1982年 | 530篇 |
1981年 | 581篇 |
1980年 | 470篇 |
1979年 | 459篇 |
1978年 | 436篇 |
1977年 | 434篇 |
1976年 | 385篇 |
1975年 | 379篇 |
1974年 | 377篇 |
1973年 | 385篇 |
1971年 | 224篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
K.A. MAASCH P.A. MAYEWSKI E.J. ROHLING J.C. STAGER W. KARLEN L.D. MEEKER E.A. MEYERSON 《Geografiska Annaler: Series A, Physical Geography》2005,87(1):7-15
ABSTRACT. Although considerable attention has been paid to the record of temperature change over the last few centuries, the range and rate of change of atmospheric circulation and hydrology remain elusive. Here, eight latitudinally well-distributed (pole-equator-pole), highly resolved (annual to decadal) climate proxy records are presented that demonstrate major changes in these variables over the last 2000 years. A comparison between atmospheric 14 C and these changes in climate demonstrates a first-order relationship between a variable Sun and climate. The relationship is seen on a global scale. 相似文献
132.
133.
Nucleosynthetic yields and production rates of helium and heavy elements are derived using new initial mass functions which
take into account the recent revisions in O star counts and the stellar models of Maeder (1981a, b) which incorporate the
effects of massloss on evolution. The current production rates are significantly higher than the earlier results due to Chiosi
& Caimmi (1979) and Chiosi (1979), and a near-uniform birthrate operating over the history of the galactic disc explains the
currently observed abundances. However, the yields are incompatibly high, and to obtain agreement it is necessary to assume
that stars above a certain mass do not explode but proceed to total collapse. Further confirmation of this idea comes from
the consideration of the specific yields and production rates of oxygen, carbon and iron and the constraints imposed by the
observational enrichment history in the disc as discussed by Twarog & Wheeler (1982). Substantial amounts of4He and14C, amongst the primary synthesis species, are contributed by the intermediate mass stars in their wind phases. If substantial
numbers of them exploded as Type I SN, their contribution to the yields of12C and56Fe would be far in excess of the requirements of galactic nucleosynthesis. Either efficient massloss precludes such catastrophic
ends for these stars, or the current stellar models are sufficiently in error to leave room for substantial revisions in the
specific yields. The proposed upward revision of the12C (α,γ)16O rate may produce the necessary changes in stellar yields to provide a solution to this problem. Stars that produce most
of the metals in the Galaxy are the same ones that contribute most to the observed supernova rate. 相似文献
134.
Titan has been observed with UVES, the UV-Visual Echelle Spectrograph at the Very Large Telescope, with the aim of characterizing the zonal wind flow. We use a retrieval scheme originally developed for absolute stellar accelerometry [Connes, P., 1985. Astrophys. Space Sci., 110, 211-255] to extract the velocity signal by simultaneously taking into account all the lines present in the spectrum. The method allows to measure the Doppler shift induced at a given point by the zonal wind flow, with high precision. The short-wavelength channel (4200-5200 Å) probes one scale height higher than the long-wavelength one (5200-6200 Å), and we observe statistically significant evidence for stronger winds at higher altitudes. The results show a high dispersion. Globally, we detect prograde zonal winds, with lower limits of 62 and 50 m s−1 at the regions centered at 200 and 170 km altitude, but approximately a quarter of the measurements indicates null or retrograde winds. 相似文献
135.
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
136.
We present results of investigations into chromospheric velocity oscillations in sunspots, carried out at the Sayan Solar
Observatory. It is shown that the “chevron” structures in the space-time diagrams demonstrate wavetrain properties. Such structures
are indicators of a propagating wave process and they are typical of many sunspots. In the authors’ opinion, three-minute
umbral oscillations are not the source of running penumbral waves (RPW). It is very likely that umbral oscillations and RPW
initially propagate along different magnetic field lines. We explain the decrease in RPW propagation velocity and frequency
in the outer penumbra, as compared with the inner, by the combined action of different frequency modes. To better reveal the
properties of these modes, frequency filtering was used. Our measurements of the RPW (five-minute mode) wavelength and RPW
propagation velocity in different sunspots vary from 12″ to 30″ and from 28 to 60 – 70 km s−1 correspondingly. 相似文献
137.
Farmland on the Downs between Lewes and Brighton suffered severe erosion during the autumn of 1982. The erosion was widespread and affected a variety of topographic situations, but it was confined to areas of arable land and recently-sown grass leys. Erosion on the scale recorded during the autumn of 1982 has never previously been recorded from this area. Three major sites of erosion are described and explanations for the erosion are sought through an analysis of rainfall conditions experienced during autumn 1982 and in recent changes in agricultural land use on the Downs. It is concluded that, whilst total rainfall and the intensity of rainstorms were both unusually high, similar events are likely to recur several times a century. Evidence is presented which suggests that the ploughing up of permanent pasture, the removal of field boundaries and the increased adoption of autumn-sown cereals have all contributed to the onset of severe erosion. It appears that a major re-activation of erosion on the Downs may be commencing which threatens the long-term viability of farming in the area. 相似文献
138.
139.
We show how the Yarkovsky effect can be understood as a heat engine. The output of the engine, manifested in the rate of change in semimajor axis of the body, has a maximum at an intermediate heat capacity, depending on the rotation rate of the body. This maximum arises because the work output depends on the product of the solar heat absorbed by the body and transported from its morning to evening side (this am-pm heat flux increases with heat capacity) and the Carnot efficiency (which declines with heat capacity). 相似文献
140.
Recently, exact spatially-homogeneous solutions have been found for a conformally invariant massless scalar fieldS(t) coupled to a Robertson-Walker geometry. We investigate extending these solutions to inhomogeneous scalar fieldsS(t, x
l
). For an isotropic scalar fieldS(t, r) we find a solution satisfying three of the five field equations. If we use REDUCE, we show that the remaining equations forceS=S(t). 相似文献