全文获取类型
收费全文 | 1198篇 |
免费 | 205篇 |
国内免费 | 158篇 |
专业分类
测绘学 | 103篇 |
大气科学 | 172篇 |
地球物理 | 434篇 |
地质学 | 200篇 |
海洋学 | 166篇 |
天文学 | 185篇 |
综合类 | 33篇 |
自然地理 | 268篇 |
出版年
2024年 | 3篇 |
2023年 | 8篇 |
2022年 | 20篇 |
2021年 | 20篇 |
2020年 | 27篇 |
2019年 | 42篇 |
2018年 | 23篇 |
2017年 | 45篇 |
2016年 | 41篇 |
2015年 | 44篇 |
2014年 | 41篇 |
2013年 | 71篇 |
2012年 | 39篇 |
2011年 | 52篇 |
2010年 | 59篇 |
2009年 | 70篇 |
2008年 | 83篇 |
2007年 | 69篇 |
2006年 | 113篇 |
2005年 | 76篇 |
2004年 | 66篇 |
2003年 | 44篇 |
2002年 | 53篇 |
2001年 | 59篇 |
2000年 | 49篇 |
1999年 | 48篇 |
1998年 | 36篇 |
1997年 | 20篇 |
1996年 | 42篇 |
1995年 | 21篇 |
1994年 | 24篇 |
1993年 | 33篇 |
1992年 | 21篇 |
1991年 | 15篇 |
1990年 | 20篇 |
1989年 | 14篇 |
1988年 | 22篇 |
1987年 | 5篇 |
1986年 | 7篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有1561条查询结果,搜索用时 15 毫秒
981.
982.
We have detected giant pulses from the millisecond pulsar PSR B1937+214 at the lowest frequency of 112 MHz. The observed flux density at the pulse peak is ~40 000 Jy, which exceeds the average level by a factor of 600. Pulses of such intensity occur about once per 300 000 periods. The brightness temperature of the observed giant pulses is T B≈1035 K. We estimated the pulse broadening by interstellar scattering to be τsc=3–10 ms. Based on this estimate and on published high-frequency measurements of this parameter, we determined the frequency dependence of the pulse broadening by scattering: τsc(f)=25 × (f/100)?4.0±02. 相似文献
983.
984.
985.
The Symmetrical One-dimensional Newtonian Four-body Problem: A Numerical Investigation 总被引:3,自引:1,他引:2
Winston L. Sweatman 《Celestial Mechanics and Dynamical Astronomy》2002,82(2):179-201
Numerical simulations of the one-dimensional Newtonian four-body problem have been conducted for the special case in which the bodies are distributed symmetrically about the centre of mass. Simulations show a great similarity between this problem and the one-dimensional Newtonian three-body problem. As in that problem the orbits can be divided into three different categories which form well-defined regions on a Poincaré section: there is a region of quasiperiodic orbits about a Schubart-like periodic orbit, there is a region of fast-scattering encounters and in between these two regions there is a chaotic scattering region. The Schubart-like periodic orbit's stability to perturbation is studied. It is apparently stable in one-dimension but is unstable in three-dimensions.This revised version was published online in October 2005 with corrections to the Cover Date. 相似文献
986.
Y. E. Lyubarsky 《Monthly notices of the Royal Astronomical Society》2002,332(1):199-204
Radiation transfer in a scattering medium in a superstrong magnetic field is considered. Because cross-sections depend on frequency, photons with different energies escape layers with different temperatures and therefore the spectrum of the outgoing radiation differs significantly from the equilibrium blackbody or Bose–Einstein spectrum. It is shown that the emergent spectrum (the photon flux per unit energy band) is flat at low energies. Applications of the result to soft gamma repeaters (SGRs) are discussed. Even though the spectrum is strongly distorted when the radiation propagates through the magnetosphere, a flat segment may be observed in the outgoing spectrum if the surface magnetic field of the neutron star is not too high, B <1015 G . 相似文献
987.
SELECTION OF BRILLOUIN SHIFT DISCRIMINATOR FOR BRILLOUIN LIDAR 总被引:1,自引:0,他引:1
For the measurement of vertical profiles of sound speed in the sea using laser excited Brillouin scattering, a high-resolution measurement of Brillouin frequency shift is required. In this work, a molecular absorption cell was selected as the frequency shift discriminator and several kinds of absorption gases were tried. It was found that the strong line ( # 1095) of ^127 I2 at 18783. 3297 cm^-1 and two absorption lines of ^129 I2 located at the two sides of the # 1095 line of ^127 I2 could be used as frequency shift discriminator to detect the changes of the Brillouin frequency shift. This selection is the best one within the range from 532.0131 run to 532.5154 nm. But it is not perfect and there is a lot of work to do before its practical application. 相似文献
988.
S. Yu. Sazonov E. M. Churazov R. A. Sunyaev 《Monthly notices of the Royal Astronomical Society》2002,333(1):191-201
It is known that resonant scattering can distort the surface brightness profiles of clusters of galaxies in X-ray lines. We demonstrate that the scattered line emission should be polarized and possibly detectable with future X-ray polarimeters. Spectrally resolved mapping of a galaxy cluster in polarized X-rays could provide valuable independent information on the physical conditions, in particular element abundances and the characteristic velocity of small-scale turbulent motions, in the intracluster gas. The expected degree of polarization is of the order of 10 per cent for the richest regular clusters (e.g. Coma) and clusters whose X-ray emission is dominated by a central cooling flow (such as Perseus and M87/Virgo). 相似文献
989.
990.
Don Groom 《Experimental Astronomy》2002,14(1):45-55
Cosmic-ray muons make recognizable straight tracks in the new-generation CCD's with thick sensitive regions. Wandering tracks
(‘worms’), which we identify with multiply-scattered low-energy electrons, are readily recognized as different from the muon
tracks. These appear to be mostly recoils from Compton-scattered gamma rays, although worms are also produced directly by
beta emitters in dewar windows and field lenses. The gamma rays are mostly byproducts of 40K decay and the U and Th decay chains. Trace amounts of these elements are nearly always present in concrete and other materials.
The direct betas can be eliminated and the Compton recoils can be reduced significantly by the judicious choice of materials
and shielding. The cosmic-ray muon rate is irreducible. Our conclusions are supported by tests at the Lawrence Berkeley National
Laboratory low-level counting facilities in Berkeley and 180 m underground at Oroville, California.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献