全文获取类型
收费全文 | 41455篇 |
免费 | 1094篇 |
国内免费 | 1086篇 |
专业分类
测绘学 | 1080篇 |
大气科学 | 3294篇 |
地球物理 | 8804篇 |
地质学 | 15598篇 |
海洋学 | 3476篇 |
天文学 | 8632篇 |
综合类 | 257篇 |
自然地理 | 2494篇 |
出版年
2022年 | 310篇 |
2021年 | 525篇 |
2020年 | 510篇 |
2019年 | 527篇 |
2018年 | 1048篇 |
2017年 | 999篇 |
2016年 | 1240篇 |
2015年 | 851篇 |
2014年 | 1242篇 |
2013年 | 2116篇 |
2012年 | 1561篇 |
2011年 | 1927篇 |
2010年 | 1750篇 |
2009年 | 2210篇 |
2008年 | 1850篇 |
2007年 | 1872篇 |
2006年 | 1793篇 |
2005年 | 1300篇 |
2004年 | 1211篇 |
2003年 | 1090篇 |
2002年 | 1077篇 |
2001年 | 921篇 |
2000年 | 888篇 |
1999年 | 738篇 |
1998年 | 790篇 |
1997年 | 739篇 |
1996年 | 639篇 |
1995年 | 613篇 |
1994年 | 536篇 |
1993年 | 464篇 |
1992年 | 452篇 |
1991年 | 416篇 |
1990年 | 493篇 |
1989年 | 409篇 |
1988年 | 380篇 |
1987年 | 475篇 |
1986年 | 373篇 |
1985年 | 465篇 |
1984年 | 572篇 |
1983年 | 504篇 |
1982年 | 490篇 |
1981年 | 437篇 |
1980年 | 453篇 |
1979年 | 401篇 |
1978年 | 383篇 |
1977年 | 374篇 |
1976年 | 335篇 |
1975年 | 337篇 |
1974年 | 347篇 |
1973年 | 374篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
A high-velocity ionized outflow and XUV photosphere in the narrow emission line quasar PG1211+143 总被引:1,自引:0,他引:1
K. A. Pounds J. N. Reeves A. R. King K. L. Page P. T. O'Brien M. J. L. Turner 《Monthly notices of the Royal Astronomical Society》2003,345(3):705-713
We report on the analysis of a ∼60-ks XMM–Newton observation of the bright, narrow emission line quasar PG1211+143. Absorption lines are seen in both European Photon Imaging Camera and Reflection Grating Spectrometer spectra corresponding to H- and He-like ions of Fe, S, Mg, Ne, O, N and C. The observed line energies indicate an ionized outflow velocity of ∼24 000 km s−1 . The highest energy lines require a column density of N H ∼ 5 × 1023 cm−2 , at an ionization parameter of log ξ∼ 3.4 . If the origin of this high-velocity outflow lies in matter being driven from the inner disc, then the flow is likely to be optically thick within a radius of ∼130 Schwarzschild radii, providing a natural explanation for the big blue bump (and strong soft X-ray) emission in PG1211+143. 相似文献
82.
M.L. Litvak I.G. Mitrofanov A.B. Sanin W.V. Boynton D. Hamara R.S. Saunders 《Icarus》2006,180(1):23-37
In this paper, we have analyzed neutron spectroscopy data gathered by the High Energy Neutron Detector (HEND) instrument onboard Mars Odyssey for comparison of polar regions. It is known that observation of the neutron albedo of Mars provides important information about the distribution of water-ice in subsurface layers and about peculiarities of the CO2 seasonal cycle. It was found that there are large water-rich permafrost areas with contents of up to ∼50% water by mass fraction at both the north and south Mars polar regions. The water-ice layers at high northern latitudes are placed close to the surface, but in the south they are covered by a dry and relatively thick (10-20 cm) layer of soil. Analysis of temporal variations of neutron flux between summer and winter seasons allowed the estimation of the masses of the CO2 deposits which seasonally condense at the polar regions. The total mass of the southern seasonal deposition was estimated as 6.3×1015 kg, which is larger than the total mass of the seasonal deposition at the north by 40-50%. These results are in good agreement with predictions from the NASA Ames Research Center General Circulation Model (GCM). But, the dynamics of the condensation and sublimation processes are not quite as consistent with these models: the peak accumulation of the condensed mass of CO2 occurred 10-15 degrees of Ls later than is predicted by the GCM. 相似文献
83.
Ulrich Hopp Regina E. Schulte-Ladbeck Jürgen Kerp 《Astrophysics and Space Science》2003,284(2):647-650
We are investigating the hypothesis that Compact High-Velocity Clouds (CHVCs) are the left-over building blocks of Local Group
galaxies. To this end, we are searching for their embedded stellar populations using FORS at the VLT. The search is done with
single-star photometry in V and I bands, which is sensitive to both, young and old, stellar populations. Five CHVCs of our
sample have been observed so far down to I=24. We pointed the VLT towards the highest HI column density regions, as determined
in Effelsberg radio data. In an alternate approach, we searched 2MASS public data towards those 5 CHVCs down to K=16. While
the VLT data probe the central regions out to distance moduli of about 27, the 2MASS data are sensitive to a population of
red giant stars to distance moduli of about 20. The 2MASS data, on the other hand, cover a much wider field of view than the
VLT data (radius of 1 degree versus FORS field of 6.8 arcmin). We did not find a stellar population intrinsic to the CHVCs
in either data. In this paper, we illustrate our search methods.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
84.
85.
The evolution of a homogeneous, isotropic cosmological model driven by a nonminimally coupled scalar field is studied. The
potential for the quintessential inflation model proposed by Peebles and Vilenkin is selected as a scalar potential. Possible
scenarios for the cosmological dynamics are described in the conformal Einstein and Jordan representations. It is shown that,
unlike in models with a minimal scalar field, here a class of solutions exists for which the scalar field is fixed at finite
values during cosmological expansion.
__________
Translated from Astrofizika, Vol. 49, No. 3, pp. 487–498 (August 2006). 相似文献
86.
87.
E. Tóth F. Deak C. S. Gyurkócza Z. S. Kasztovszky R. Kuczi G. Marx B. Nagy S. Oberstedt L. Sajó-Bohus C. S. Sükösd G. Toth N. Vajda 《Environmental Geology》1997,31(1-2):123-127
A steady radon exhalation is assumed in most publications. In a village of North-East Hungary, however, high radon concentrations
have been measured, differing strongly in neighbouring houses and varying in time, due to the interplay of geochemical phenomena.
Received: 20 November 1995 · Accepted: 18 June 1996 相似文献
88.
89.
90.
Observations of the thermospheric wind at a mid-latitude station have been made using a Fabry-Perot interferometer to measure the Doppler shift of the nighttime OI emission at 630 nm. The results from 12 summer nights show that the zonal wind has a distinct feature associated with magnetic activity. The zonal wind first reverses and becomes westward. The maximum strength of the westward wind, its duration, and the maximum strength of the subsequent eastward wind all increase with increasing magnetic activity. The meridional wind is less consistent in its behaviour. It is normally equatorward but during magnetic activity it can increase, decrease, or even reverse, although it is consistently equatorward and of increased strength after 02.00 L.T. The initial reversal of the zonal wind is consistent with changes in the wind expected as a result of convective electric fields penetrating to mid-latitudes indicating that these electric fields modify the mid-latitude wind pattern before effects due to auroral heating reach mid-latitudes. The reversal of the zonal wind back to eastward may also be the result of electric field effects. The large variability of the meridional wind, to the extent that it becomes poleward at times, indicates the importance of wind sources equatorward of the observatory. 相似文献