全文获取类型
收费全文 | 91560篇 |
免费 | 1084篇 |
国内免费 | 765篇 |
专业分类
测绘学 | 2363篇 |
大气科学 | 5968篇 |
地球物理 | 17082篇 |
地质学 | 35075篇 |
海洋学 | 7876篇 |
天文学 | 20826篇 |
综合类 | 382篇 |
自然地理 | 3837篇 |
出版年
2022年 | 667篇 |
2021年 | 1041篇 |
2020年 | 1124篇 |
2019年 | 1219篇 |
2018年 | 4337篇 |
2017年 | 3914篇 |
2016年 | 3788篇 |
2015年 | 1489篇 |
2014年 | 2702篇 |
2013年 | 4184篇 |
2012年 | 3483篇 |
2011年 | 5225篇 |
2010年 | 4561篇 |
2009年 | 5452篇 |
2008年 | 4648篇 |
2007年 | 5192篇 |
2006年 | 3402篇 |
2005年 | 2453篇 |
2004年 | 2424篇 |
2003年 | 2361篇 |
2002年 | 2204篇 |
2001年 | 1876篇 |
2000年 | 1726篇 |
1999年 | 1299篇 |
1998年 | 1342篇 |
1997年 | 1288篇 |
1996年 | 1066篇 |
1995年 | 1068篇 |
1994年 | 937篇 |
1993年 | 813篇 |
1992年 | 808篇 |
1991年 | 815篇 |
1990年 | 877篇 |
1989年 | 689篇 |
1988年 | 713篇 |
1987年 | 733篇 |
1986年 | 640篇 |
1985年 | 844篇 |
1984年 | 910篇 |
1983年 | 807篇 |
1982年 | 773篇 |
1981年 | 696篇 |
1980年 | 700篇 |
1979年 | 633篇 |
1978年 | 649篇 |
1977年 | 531篇 |
1976年 | 499篇 |
1975年 | 504篇 |
1974年 | 450篇 |
1973年 | 507篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
371.
In this paper we seek the origin of the axial component of the magnetic field in filaments by adapting theory to observations.
A previous paper (Mackay, Gaizauskas, and van Ballegooijen, 2000) showed that surface flows acting on potential magnetic fields
for 27 days – the maximum time between the emergence of magnetic flux and the formation of large filaments between the resulting
activity complexes – cannot explain the chirality or inverse polarity nature of the observed filaments. We show that the inclusion
of initial helicity, for which there is observational evidence, in the flux transport model results in sufficiently strong
dextral fields of inverse polarity to account for the existence and length of an observed filament within the allotted time.
The simulations even produce a large length of dextral chirality when just small amounts of helicity are included in the initial
configuration. The modeling suggests that the axial field component in filaments can result from a combination of surface
(flux transport) and sub-surface (helicity) effects acting together. Here surface effects convert the large-scale helicity
emerging in active regions into a smaller-scale magnetic-field component parallel to the polarity inversion line so as to
form a magnetic configuration suitable for a filament. 相似文献
372.
L. A. Nolan J. S. Dunlop R. Jimenez A. F. Heavens 《Monthly notices of the Royal Astronomical Society》2003,341(2):464-476
We explore whether the rest-frame near-ultraviolet spectral region, observable in high-redshift galaxies via optical spectroscopy, contains sufficient information to allow the degeneracy between age and metallicity to be lifted. We do this by first testing the ability of evolutionary synthesis models to reclaim the correct metallicity when fitted to the near-ultraviolet spectra of F stars of known (subsolar and supersolar) metallicity. F stars are of particular interest because the rest-frame near-ultraviolet spectra of the oldest known elliptical galaxies at z > 1 appear to be dominated by F stars near to the main-sequence turn-off.
We find that, in the case of the F stars, where the Hubble Space Telescope ultraviolet spectra have a high signal-to-noise ratio, fitting models in which the metallicity is allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turn-off ages of these stars yielded by model-fitting are well constrained. Encouraged by this we have fitted these same variable-metallicity models to the deep, optical spectra of the z ≃ 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age and metallicity are not so easily constrained for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still ≥3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein–de Sitter universe, and favour a Λ-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3–5 . 相似文献
We find that, in the case of the F stars, where the Hubble Space Telescope ultraviolet spectra have a high signal-to-noise ratio, fitting models in which the metallicity is allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turn-off ages of these stars yielded by model-fitting are well constrained. Encouraged by this we have fitted these same variable-metallicity models to the deep, optical spectra of the z ≃ 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age and metallicity are not so easily constrained for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still ≥3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein–de Sitter universe, and favour a Λ-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3–5 . 相似文献
373.
374.
We have studied the variations of the height of polar crown prominences according to daily observations of the Sun at the Kodaikanal Observatory (India) during 1905–1975. Polar ring filaments at latitudes 60°–80° are related to the polar magnetic field reversal. A double decrease of the height of polar ring filaments was found in the course of their migration from 40°to the poles. We estimated the limiting height of the equilibrium of polar ring filaments from the stability condition of a strong electric current. We found that the transition from large-scale to small-scale ring filaments reduces the critical height of the stability for the prominences. A model of an inverse-polarity filament was used. 相似文献
375.
376.
377.
378.
379.
380.
The three-dimensional geometry of the heliospheric current sheet seen from fixed points in interplanetary space is constructed for idealized (sinusoidal) magnetic neutral lines (equators) and for an observed magnetic equator on the basis of the “kinematic method” developed by Hakamada and Akasofu (1982). The cross-sections of the wavy current sheet at distances 1, 2 and 5 a.u. are also constructed for the idealized magnetic neutral lines. 相似文献