全文获取类型
收费全文 | 94774篇 |
免费 | 1272篇 |
国内免费 | 650篇 |
专业分类
测绘学 | 2406篇 |
大气科学 | 6422篇 |
地球物理 | 18204篇 |
地质学 | 35622篇 |
海洋学 | 7887篇 |
天文学 | 20964篇 |
综合类 | 286篇 |
自然地理 | 4905篇 |
出版年
2021年 | 712篇 |
2020年 | 784篇 |
2019年 | 823篇 |
2018年 | 4128篇 |
2017年 | 3816篇 |
2016年 | 3292篇 |
2015年 | 1260篇 |
2014年 | 1973篇 |
2013年 | 3859篇 |
2012年 | 2997篇 |
2011年 | 5076篇 |
2010年 | 4649篇 |
2009年 | 5651篇 |
2008年 | 4674篇 |
2007年 | 5228篇 |
2006年 | 2973篇 |
2005年 | 2686篇 |
2004年 | 2559篇 |
2003年 | 2508篇 |
2002年 | 2317篇 |
2001年 | 1904篇 |
2000年 | 1760篇 |
1999年 | 1489篇 |
1998年 | 1496篇 |
1997年 | 1482篇 |
1996年 | 1250篇 |
1995年 | 1222篇 |
1994年 | 1110篇 |
1993年 | 997篇 |
1992年 | 930篇 |
1991年 | 915篇 |
1990年 | 996篇 |
1989年 | 923篇 |
1988年 | 856篇 |
1987年 | 1052篇 |
1986年 | 896篇 |
1985年 | 1115篇 |
1984年 | 1234篇 |
1983年 | 1147篇 |
1982年 | 1038篇 |
1981年 | 1093篇 |
1980年 | 909篇 |
1979年 | 864篇 |
1978年 | 836篇 |
1977年 | 809篇 |
1976年 | 744篇 |
1975年 | 719篇 |
1974年 | 723篇 |
1973年 | 752篇 |
1971年 | 453篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Iapetus (S8) is unique in our solar system in that the albedo of its leading hemisphere is only 0.05 while that of the trailing side is 0.5. Several existing hypotheses are examined and found inadequate. Photometric studies of the dark side are compared to comet nuclei and class D asteroids. It is hypothesized that in the last 106–108 yrs the leading side suffered a high-velocity collision with a cometary body of mass 1013–1015 kg and traveling at a speed of 20 km s–1. About 5–16% of the excavated material was ejected into space, where the vaporized ices dissipated while the dark carbonaceous/silicate material was reaccreted on the leading side. The collision, although not sufficient to break Iapetus' tidal lock, resulted in a period of oscillation of about 5 yr. Until tidal friction reasserted a lock, the oscillation gave rise to the longitude effect, viz., the observed fact that the dark material covers more than 220 of longitude but only 110 of latitude. 相似文献
952.
A special fine structure (slowly drifting chains of narrowband fiber bursts), firstly observed during the solar type-IV radio burst on April 24, 1985, is interpreted as the radio signature of whistler waves periodically excited by a switch-on/switch-off process of a loss-cone instability in a localized wave packet of the fast magnetoacoustic mode. 相似文献
953.
C. L. Goudas M. leftaki E. G. Petsagourakis 《Celestial Mechanics and Dynamical Astronomy》1989,47(1):1-14
The zero-velocity curves and surfaces created by two parallel or antiparallel magnetic dipoles moving on concentric circles are computed. It is found that for all combinations of values of the mass parameter µ and the ratio of the magnitudes of the magnetic moments of the two dipoles, there exist closed regions inside of which charged particles are permanently trapped. 相似文献
954.
K. T. Brazier A. Carramiñana P. M. Chadwick T. R. Currell N. A. Dipper E. W. Lincoln V. G. Mannings T. J. L. McComb K. J. Orford S. M. Rayner K. E. Turver 《Experimental Astronomy》1989,1(2):77-99
The design of very high energy (VHE) gamma ray telescopes is discussed and it is shown that a triple-mirror telescope operating in threefold fast coincidence provides an effective instrument. The design, construction and operation of a large VHE gamma ray telescope with an energy threshold of 300 GeV (at the zenith) and incorporating several novel features is described. The use of a coincidence guard ring technique is shown to result in the rejection of about 50% of the off-source (proton) events which form a high background and limit the sensitivity of such telescopes. 相似文献
955.
The radiative thermal instability is investigated taking into account finite-, or electromagnetic, effects. The two-fluid model for magnetized plasmas together with the Maxwell equations are used to derive a general dispersion relation valid for compressional perturbations with frequency below the electron-cyclotron frequency. The growth rates of the radiative thermal instabilities involving fast magnetosonic flute-like and low-frequency hydromagnetic perturbations are presented. 相似文献
956.
We analyze the emission component of galaxy nuclei at very low intensity levels (W(H)2Å). This emission level is considerably lower than that of classical LINERS like NGC 1052. We have access to weaker emission lines by averaging spectra with similar line ratios for H [NII], and [SII]. From the resulting spectrum for very low level emission nuclei, the [SII] 6717, 6731/[SIII] 9069, 9532 line ratio criterion (Diazet al., 1985a) unambiguously shows that shock-wave heating is the mechanism responsible for the ionization in such objects. 相似文献
957.
L. D. Barr 《Astrophysics and Space Science》1989,160(1-2):71-85
The National Optical Astronomy Observatories (NOAO) are planning to buld two 8 m telescopes, one for Mauna Kea, Hawaii, the other for a site in Chile. Optical configurations, primary mirror systems, and the telescope mounting are discussed. A new optical testing method is outlined. The system imaging goal is 0.25 FWHM. Construction could begin in the early 1990's.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.Operated by the Association of Universities for Research in Astronomy Inc., under contract with the National Science Foundation. 相似文献
958.
Veeder GJ Hanner MS Matson DL Tedesco EF Lebofsky LA Tokunaga AT 《The Astronomical journal》1989,97(4):1211-1219
We report 10 micrometers infrared photometry for 22 Aten, Apollo, and Amor asteroids. Thermal models are used to derive the corresponding radiometric albedos and diameters. Several of these asteroids appear to have surfaces of relatively high thermal inertia due to the exposure of bare rock or a coarse regolith. The Apollo asteroid 3103, 1982 BB, is recognized as class E. The Jupiter-crossing Amor asteroid 3552, 1983 SA, is confirmed as class D, but low albedos remain rare for near-Earth asteroids. 相似文献
959.
The Very Large Array (VLA) and the frequency agile interferometer at the Owens Valley Radio Observatory (OVRO) were used to observe the M8.1 flare of 23 June, 1988. The VLA obtained images prior to and during the flare at 333 MHz, and at 1.5 and 4.7 GHz. The frequency agile interferometer at Owens Valley obtained interferometer amplitude and total power spectra of the flare at 45 frequencies between 1 and 18 GHz. The observations were supplemented by radiometer measurements made by the USAF RSTN network site at Palehua, HI, by GOES soft X-ray observations, by USAF SOON H filtergrams, and by a KPNO photospheric magnetogram.The radio data reveal a wide variety of phenomena, including: (i) a multiply impulsive microwave burst that is essentially thermal in character; (ii) stationary discrete components at 1.5 GHz, associated temporally and spatially with distant brightenings in Ha; (iii) a dynamical component at 1.5 GHz associated with hot plasma moving subsonically into the corona; (iv) the appearance of intense, short-lived, decimetric burst activity near the lead sunspot in the active region at 1.5 GHz, indicative of a high degree of inhomogeneity in the source.The unusually complete radio coverage allows us to investigate the transport of energy from the initial site to sites of distant H brightenings. The transport of energy appears to be most consistent with slow, thermal processes, rather than rapid transport by nonthermal electron beams. 相似文献
960.
We study the center-limb (CL) variation of the average profiles of four Ca i lines near 6500 and compare these observations with synthetic data obtained from several line formation models having different thermal structures, line parameters, LTE and non-LTE conditions, and micro and macroturbulence values, to assess the formation characteristics of our Ca i lines in the solar photosphere.Comparison of numerical results with observations indicates that non-LTE is indispensable to fit the CL variation of the central residual intensity for the line 6493, and anisotropic microturbulence is indispensable to improve the CL behavior of the equivalent widths for all lines. The Ca i line analysis favors a cool photospheric model, but this cannot be disentangled clearly from the effects of non-LTE and small-scale velocity fields on the grounds of the present line formation models. 相似文献