全文获取类型
收费全文 | 56560篇 |
免费 | 2220篇 |
国内免费 | 3250篇 |
专业分类
测绘学 | 1953篇 |
大气科学 | 5015篇 |
地球物理 | 10221篇 |
地质学 | 25738篇 |
海洋学 | 5343篇 |
天文学 | 9648篇 |
综合类 | 1180篇 |
自然地理 | 2932篇 |
出版年
2023年 | 350篇 |
2022年 | 959篇 |
2021年 | 1299篇 |
2020年 | 1170篇 |
2019年 | 1258篇 |
2018年 | 4425篇 |
2017年 | 4030篇 |
2016年 | 3139篇 |
2015年 | 1206篇 |
2014年 | 1678篇 |
2013年 | 2187篇 |
2012年 | 2668篇 |
2011年 | 4400篇 |
2010年 | 3950篇 |
2009年 | 4171篇 |
2008年 | 3472篇 |
2007年 | 4128篇 |
2006年 | 1803篇 |
2005年 | 1313篇 |
2004年 | 1205篇 |
2003年 | 1338篇 |
2002年 | 1273篇 |
2001年 | 964篇 |
2000年 | 768篇 |
1999年 | 692篇 |
1998年 | 597篇 |
1997年 | 623篇 |
1996年 | 505篇 |
1995年 | 509篇 |
1994年 | 462篇 |
1993年 | 360篇 |
1992年 | 364篇 |
1991年 | 286篇 |
1990年 | 281篇 |
1989年 | 288篇 |
1988年 | 231篇 |
1987年 | 240篇 |
1986年 | 224篇 |
1985年 | 244篇 |
1984年 | 235篇 |
1983年 | 229篇 |
1982年 | 225篇 |
1981年 | 206篇 |
1980年 | 202篇 |
1979年 | 204篇 |
1978年 | 171篇 |
1977年 | 155篇 |
1976年 | 150篇 |
1975年 | 157篇 |
1973年 | 179篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
We present the results of our analysis of the RXTE observations for two transient sources, IGR J17091-3624 and IGR J18539+0727, in April 2003. The derived energy spectra of the sources and the power-density spectra of their light curves make it possible to classify them as low/hard-state X-ray binaries. The parameters of the power spectrum for IGR J18539+0727 lead us to tentatively conclude that the compact object in this binary is a black hole. 相似文献
122.
In this paper we show that a change in the signs of some of the metric components of the solution of the field equations for
the classical cosmic string results in a solution which we interpret as a time-dependent wall composed of tachyons. We show
that the walls have the property of focusing the paths of particles which pass through them. As an illustration of this focusing,
we demonstrate the results of a simple simulation of the interaction between one such tachyon wall and a rotating disk of
point masses. This interaction leads to the temporary formation of spiral structures. These spiral structures exist for a
time on the order of one galactic rotation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
123.
Flux pile-up magnetic reconnection is traditionally considered only for incompressible plasmas. The question addressed in this paper is whether the pile-up scalings with resistivity are robust when plasma compressibility is taken into account. A simple analytical argument makes it possible to understand why the transition from a highly compressible limit to the incompressible one is difficult to discern in typical simulations spanning a few decades in resistivity. From a practical standpoint, however, flux pile-up reconnection in a compressible plasma can lead to anomalous electric resistivity in the current sheet and flare-like energy release of magnetic energy in the solar corona. 相似文献
124.
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
125.
V. K. Rosenbush 《Solar System Research》2005,39(4):312-321
Systematic and uniform sets of photometric and polarimetric observations of comet 1P/Halley have been analyzed. The phase dependence of brightness for comet Halley was obtained at phase angles α ranging from 1.4° ≤ α ≤ 65°. The following parameters were determined: the amplitude of the opposition effect Δm = 0.75m ± 0.06m; the half-width at a half-maximum of intensity HWHM = 6.4° ± 1.6°; the linear phase coefficient β = 0.0045 ± 0.0001 mag/deg for α from 30° ≤ α ≤ 65°; and the phase angle at which a nonlinear increase in brightness starts, α opp ≈ 31°. For the first time, the phase-angle dependence was obtained for the color of the dust of comet Halley: the color index BC-RC systematically decreases with increasing phase angle. Such a phase behavior of the dust color can be caused by the decrease in the mean size of dust particles that occurs when the comet approaches the Sun. For comet Halley, the negative polarization branch is almost symmetric; the minimal value of polarization is P
min = −1.54% at a phase angle αmin = 10.5°, and the inversion angle is αinv = 21.7°. A comparative analysis of the phase functions of brightness and polarization has been performed for the cometary dust and atmosphereless bodies. Among the latter are low-albedo asteroids of the P and C types (102 Miriam and 47 Aglaja, respectively), as well as Deimos; high-albedo objects, such as the E-type asteroid 64 Angelina and the icy satellite of Jupiter Europa; and the Moon with its intermediate albedo. The possibility of a weak depression in the negative polarization branch of comets Halley and 47P/Ashbrook-Jackson at phase angles smaller than 2° is discussed.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 4, 2005, pp. 353–363.Original Russian Text Copyright © 2005 by Rosenbush. 相似文献
126.
I.A. Crawford 《Astrophysics and Space Science》2003,285(3-4):661-675
I review the recent discoveries of variable interstellar absorption lines, widely taken to imply the existence of very small
scale(10–100 AU) structure in the interstellar medium. Possible origins of this structure are discussed, and attention is
drawn to the fact that most known examples seem to be associated with interstellar shells and bubbles of various kinds. On
the other hand, in at least one case (κ Velorum),the variation appears to occur in the supposedly quiescent cold neutral medium,
consistent with earlier studies of small scale structure in atomic hydrogen probed at radio wavelengths. Further work is urgently
required to determine the prevalence, mode of formation, and the physical and chemical state of these enigmatic structures.
Only then will their implications for our wider understanding of the interstellar medium become apparent.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
127.
We found the equilibrium conditions for a self-gravitating toroidal vortex by taking thermal pressure into account. These conditions are shown to significantly differ from those for a disk or a sphere. The evolution of a thin vortex turns it into a compact vortex that loses mechanical stability for low masses at a polytropic index γ<4/3 but retains stability for sufficiently high masses and densities determined by the velocity circulation in the vortex. 相似文献
128.
Subramanian Prasad Ananthakrishnan S. Janardhan P. Kundu M.R. White S.M. Garaimov V.I. 《Solar physics》2003,218(1-2):247-259
We present the first observations of a solar flare with the GMRT. An M2.8 flare observed at 1060 MHz with the GMRT on 17 November 2001 was associated with a prominence eruption observed at 17 GHz by the Nobeyama radioheliograph and the initiation of a fast partial halo CME observed with the LASCO C2 coronagraph. Towards the start of the eruption, we find evidence for reconnection above the prominence. Subsequently, we find evidence for rapid growth of a vertical current sheet below the erupting arcade, which is accompanied by the flare and prominence eruption. 相似文献
129.
M. G. Revnivtsev 《Astronomy Letters》2003,29(6):383-386
Analysis of the RXTE slew data in October 1996 revealed a weak X-ray burst from the millisecond pulsar SAX J 1808.4-3658. The 3–20-keV energy spectrum of the source can be described by a power law with an index of 2.0 and a(3-to 20 keV) luminosity of ~1.4×1035 erg s?1 (the distance to the source was taken to be 2.5 kpc). Because of the short exposure time, we failed to detect weak pulsations at a frequency of 401 Hz in the source. The (2σ) upper limit of the pulse fraction is ~13%. 相似文献
130.