全文获取类型
收费全文 | 140201篇 |
免费 | 2243篇 |
国内免费 | 1100篇 |
专业分类
测绘学 | 3452篇 |
大气科学 | 9553篇 |
地球物理 | 27318篇 |
地质学 | 51444篇 |
海洋学 | 12320篇 |
天文学 | 30994篇 |
综合类 | 475篇 |
自然地理 | 7988篇 |
出版年
2022年 | 789篇 |
2021年 | 1408篇 |
2020年 | 1540篇 |
2019年 | 1667篇 |
2018年 | 4760篇 |
2017年 | 4403篇 |
2016年 | 4666篇 |
2015年 | 2249篇 |
2014年 | 3906篇 |
2013年 | 6924篇 |
2012年 | 4638篇 |
2011年 | 6830篇 |
2010年 | 5958篇 |
2009年 | 7468篇 |
2008年 | 6559篇 |
2007年 | 6825篇 |
2006年 | 5214篇 |
2005年 | 4135篇 |
2004年 | 4015篇 |
2003年 | 3832篇 |
2002年 | 3587篇 |
2001年 | 3135篇 |
2000年 | 3044篇 |
1999年 | 2430篇 |
1998年 | 2550篇 |
1997年 | 2353篇 |
1996年 | 2026篇 |
1995年 | 2022篇 |
1994年 | 1725篇 |
1993年 | 1612篇 |
1992年 | 1537篇 |
1991年 | 1484篇 |
1990年 | 1563篇 |
1989年 | 1348篇 |
1988年 | 1227篇 |
1987年 | 1478篇 |
1986年 | 1274篇 |
1985年 | 1570篇 |
1984年 | 1757篇 |
1983年 | 1682篇 |
1982年 | 1576篇 |
1981年 | 1428篇 |
1980年 | 1317篇 |
1979年 | 1238篇 |
1978年 | 1201篇 |
1977年 | 1050篇 |
1976年 | 1037篇 |
1975年 | 985篇 |
1974年 | 989篇 |
1973年 | 1030篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
A. Greco G. Zimbardo P. Veltri A.L. Taktakishvili L.M. Zelenyi 《Astrophysics and Space Science》2001,277(1-2):35-38
We study the ion dynamics in a magnetic field reversal with a constant electric field and with a model of three dimensional
magnetic turbulence. By computing the mean square displacements in the plane of the current sheet we find superdiffusive and
superballistic transport regimes. Since velocity increases with the length of the free path, we have accelerated Lévyflights.
The possibility to generate power law velocity distribution functions is pointed out, as well as the long memory effects and
non local properties of ion transport.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
162.
M.M. Montgomery 《Monthly notices of the Royal Astronomical Society》2001,325(2):761-766
We compare analytical expressions of precession rates from apsidal (positive) superhumps in close binary systems with numerical disc simulation results and observed values. In the analytical expressions, we include both the dynamical effects on the precession of the disc and effects caused by pressure forces that have been theorized to provide a retrograde effect (i.e. slowing) on the prograde disc precession. We establish new limits on density wave pitch angle to a normalized disc sound speed 60≥Ωorb d tan i / c >2.214 . Using average values for the density wave pitch angle i and speed of sound c , we find good correlation between numerical simulations and the analytical expression for the apsidal superhump period excess, which includes both the prograde and retrograde effects, for mass ratios of 0.025≤ q ≤0.33 . We also show good correlations with the four known eclipsing systems, OY Car, Z Cha, HT Cas, and WZ Sge. Our analytical expression for apsidal superhump period excess as a function of orbital period is consistent with the trend found in observed systems. 相似文献
163.
Yulia A. Chernetenko 《Celestial Mechanics and Dynamical Astronomy》2001,80(3-4):185-194
International programs of observations of selected minor planets have lasted about 50 years and the last one comes to the end in 2000. The main aim of these observational programs consists in obtaining the orientation of the stellar reference frame with respect to the dynamical one using observations of the bright minor planets. The observations are also useful for the orbital improvements of the asteroids themselves. They are available from the author via e-mail at the address .During the above mentioned period more than 23 000 observations of minor planets, referred to different reference star catalogues, have been obtained. The reduction procedure of observations to the PPM star catalogue is described. The orientation parameters are given and discussed. 相似文献
164.
M. J. Hardcastle M. Birkinshaw D. M. Worrall 《Monthly notices of the Royal Astronomical Society》2001,323(2):L17-L22
Chandra X-ray Observatory observations of the powerful, peculiar radio galaxy 3C 123 have resulted in an X-ray detection of the bright eastern hotspot, with a 1-keV flux density of ∼5 nJy. The X-ray flux and spectrum of the hotspot are consistent with the X-rays being inverse-Compton scattering of radio synchrotron photons by the population of electrons responsible for the radio emission ('synchrotron self-Compton emission') if the magnetic fields in the hotspot are close to their equipartition values. 3C 123 is thus the third radio galaxy to show X-ray emission from a hotspot which is consistent with being in equipartition. Chandra also detects emission from a moderately rich cluster surrounding 3C 123, with L X (2–10 keV)=2×1044 erg s−1 and kT ∼5 keV, and absorbed emission from the active nucleus, with an inferred intrinsic column density of 1.7×1022 cm−2 and an intrinsic 2–10 keV luminosity of 1044 erg s−1 . 相似文献
165.
K. Nagamine † M. Fukugita R. Cen J.P. Ostriker 《Monthly notices of the Royal Astronomical Society》2001,327(1):L10-L14
The luminosity function of galaxies is derived from a cosmological hydrodynamic simulation of a Λ cold dark matter universe with the aid of a stellar population synthesis model. At , the resulting B -band luminosity function has a flat faint-end slope of with the characteristic luminosity and the normalization in fair agreement with observations, while the dark matter halo mass function is steep with a slope of . The colour distribution of galaxies also agrees well with local observations. We also discuss the evolution of the luminosity function, and the colour distribution of galaxies from to 5. A large evolution of the characteristic mass in the stellar mass function as a result of number evolution is compensated by luminosity evolution; the characteristic luminosity increases only by 0.8 mag from to 2, and then declines towards higher redshift, while the B -band luminosity density continues to increase from to 5 (but only slowly at . 相似文献
166.
We report on SWS and LWS observations of the circumstellar disks of young stars of a few solar masses. The ISO spectra of
these objects present a diversity of emission features of carbon-rich and oxygen-rich grains. The similarity of the forsterite
spectra observed for Comet Hale-Bopp and the Haebe star HD100546 is particularly striking and provides a new argument that
huge comet swarms are formed in the disks surrounding young stars. While the data suggest that the formation of crystalline
silicates in the dust disks essentially occurs when a Haebe star has already reached the main sequence, no clear correlation
with stellar age only is apparent.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
167.
Generally speaking, variable stars are monitored through observing campaigns which coordinate multi-site telescopes at various
longitudes. A new practice is in progress: devoted networks involving robotic telescopes. We will review these two technologies
and will emphasize the NORT (Network of Oriental Robotic Telescopes) project which we are promoting in North Africa, Middle-Eastern
countries and Asia.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
168.
A. Kučinskas 《Astrophysics and Space Science》1997,251(1-2):231-234
We have calculated terminal gas outflow velocities from the circumstellar shells of the AGB stars. Our results confirm that,
in case of the stationary dust driven winds, the dynamics of the outflow is governed essentially by the dust properties and
stellar luminosities, while only marginally by other stellar parameters. Although being far from comprehensive, our calculations
also indicate that the dust driven winds may only occur in cool, luminous, low mass objects.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
169.
An optoelectronic instrument has been developed to analyze the scales and scale patterns of chum salmon. This device, with specially developed software, has been used to study populations of salmon. Fish from two different rivers approximately 200 km apart from each other were grouped with an accuracy of 82% or more. Machine-measured ages coincided with those derived from visual estimation in approximately 80% of the fish sampled. These results suggest the proposed system could be of practical use for measuring and analyzing the scale patterns that allow one to accurately measure both natural and farmed populations of salmon 相似文献
170.
Published in Astrofizika, Vol. 38, No. 4, pp. 677–680, October–December, 1995. 相似文献