全文获取类型
收费全文 | 64982篇 |
免费 | 2219篇 |
国内免费 | 3184篇 |
专业分类
测绘学 | 2218篇 |
大气科学 | 5261篇 |
地球物理 | 11966篇 |
地质学 | 28359篇 |
海洋学 | 6077篇 |
天文学 | 12148篇 |
综合类 | 1235篇 |
自然地理 | 3121篇 |
出版年
2023年 | 265篇 |
2022年 | 1093篇 |
2021年 | 1453篇 |
2020年 | 1413篇 |
2019年 | 1537篇 |
2018年 | 4652篇 |
2017年 | 4148篇 |
2016年 | 3522篇 |
2015年 | 1403篇 |
2014年 | 2197篇 |
2013年 | 2797篇 |
2012年 | 3075篇 |
2011年 | 4685篇 |
2010年 | 4106篇 |
2009年 | 4426篇 |
2008年 | 3778篇 |
2007年 | 4377篇 |
2006年 | 2355篇 |
2005年 | 1499篇 |
2004年 | 1403篇 |
2003年 | 1495篇 |
2002年 | 1396篇 |
2001年 | 1198篇 |
2000年 | 974篇 |
1999年 | 775篇 |
1998年 | 705篇 |
1997年 | 757篇 |
1996年 | 582篇 |
1995年 | 599篇 |
1994年 | 547篇 |
1993年 | 448篇 |
1992年 | 427篇 |
1991年 | 382篇 |
1990年 | 407篇 |
1989年 | 369篇 |
1988年 | 325篇 |
1987年 | 314篇 |
1986年 | 237篇 |
1985年 | 335篇 |
1984年 | 334篇 |
1983年 | 323篇 |
1982年 | 302篇 |
1981年 | 280篇 |
1980年 | 295篇 |
1979年 | 220篇 |
1978年 | 250篇 |
1977年 | 213篇 |
1976年 | 187篇 |
1975年 | 195篇 |
1973年 | 213篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
181.
It was found recently that fast electrons travel through the plasma of the solar corona in the form of beam-plasma structure
(BPS), which consists of electrons and Langmuir waves. In this paper the influence of scattering BPS Langmuir waves off plasma
ions (l+i=l+i) on BPS velocity is studied. We show that the maximum BPS velocity equals 0.35c, which is close to the velocity of Type III bursts sources. 相似文献
182.
Andrei V. Ivanov Nataliya N. Kononkova S. Vincent Yang Michael E. Zolensky 《Meteoritics & planetary science》2003,38(5):725-737
Abstract— Clasts of alkaline (the second find in meteorites) and subalkaline rocks were found in the Kaidun meteorite. One of them (#d4A) is a large crystal of albite with inclusions of fluorapatite, arfvedsonite, aenigmatite, and wilkinsonite. The two latter minerals were previously unknown in meteorites. Another clast (#d[3–5]D) has a melt crystallization texture of mainly feldspar (oligoclase) composition and contains relict grains of both high‐Ca and low‐Ca pyroxene and fluorapatite. The mineralogical characteristics of these clasts suggest a genetic relationship and an origin from the same parent body. The textural and mineralogical characteristics of the clasts indicate origin by extensive igneous differentiation. Such processes most likely took place in a rather large differentiated body. The material of clast #d(3–5)D is similar in some mineralogical respects to basaltic shergottites. 相似文献
183.
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
184.
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
185.
Andrés E. Piatti Juan J. Clariá Andrea V. Ahumada 《Monthly notices of the Royal Astronomical Society》2003,346(2):390-402
Charge-coupled device (CCD) photometry in the Johnson V , Kron–Cousins I and Washington CMT 1 systems is presented in the field of the poorly known open cluster NGC 2627. Four independent Washington abundance indices yield a mean cluster metallicity of [Fe/H]=−0.12 ± 0.08 , which is compatible with the existence of a radial gradient in the Galactic disc. The resultant colour–magnitude diagrams indicate that the cluster is an intermediate-age object of 1.4 Gyr. Based on the best fits of the Geneva group's isochrones to the ( V , V − I ) and ( T 1 , C − T 1 ) diagrams, we estimate E ( V − I ) = 0.25 ± 0.05 and V − MV = 11.80 ± 0.25 for log t = 9.15 , and E ( C − T 1 ) = 0.23 ± 0.07 and T 1 − M T 1 = 11.85 ± 0.25 for log t = 9.10 , respectively, assuming solar metal content. The derived reddening value E ( C − T 1 ) implies E ( B − V ) = 0.12 ± 0.07 and a distance from the Sun of 2.0 ± 0.4 kpc . Using the WEBDA data base and the available literature, we re-examined the overall properties of all the open clusters with ages between 0.6 and 2.5 Gyr. We identified peaks of cluster formation at 0.7–0.8, 1.0–1.1, 1.6–1.7 and 2.0–2.1 Gyr, separated by relative quiescent epochs of ∼0.2–0.3 Gyr. We also estimated a radial abundance gradient of −0.08 ± 0.02 , which is consistent with the most recent determinations for the Galactic disc, but no clear evidence for a gradient perpendicular to the Galactic plane is found. 相似文献
186.
187.
The dynamical evolution of meteoroid streams associated with cornets Encke, Halley, Machholz 1986 VIII and asteroid Phaethon is discussed. It is shown that the planetary perturbations can greatly increase the streams thickness and each stream may produce several couples of meteor showers active in different seasons of the year. The theoretical and observed data are in a satisfactory accordance. 相似文献
188.
Nonstationary hydrodynamic models of a viscous accretion disk around a central compact object were constructed. Two different numerical methods (TVD and SPH) are used to study the dynamics of dissipatively unstable acoustic perturbations at the nonlinear stage in terms of the standard α-disk model. The standard disk accretion in the Shakura-Sunyaev model is unstable against acoustic waves for various parameters of the system. If the α parameter, which specifies the level of turbulent viscosity, exceeds α?0.03, then a complex nonstationary system of small-scale weak shock waves is formed. The growth rate of the perturbations is higher in the central disk region. For α?0.2, the relative shock amplitude can exceed 50% of the equilibrium disk parameters. The reflection of waves from the disk boundaries and their nonlinear interaction are important factors that can produce unsteady accretion. The luminosity of such a disk undergoes quasi-periodic oscillations at a level of several percent (?5%) of the equilibrium level. 相似文献
189.
The effect of particle-particle interaction on the adiabatic index γ for an electron-positron plasma is considered. An improved method for numerically calculating the Hartree-Fock exchange integral is presented and its relativistic asymptotics is determined. An approximation formula is derived for the correlation part of the interaction in the low-density limit. This formula includes degeneracy and the positron component. 相似文献
190.