全文获取类型
收费全文 | 72394篇 |
免费 | 1188篇 |
国内免费 | 519篇 |
专业分类
测绘学 | 1912篇 |
大气科学 | 5482篇 |
地球物理 | 13934篇 |
地质学 | 25094篇 |
海洋学 | 6205篇 |
天文学 | 16707篇 |
综合类 | 162篇 |
自然地理 | 4605篇 |
出版年
2021年 | 534篇 |
2020年 | 596篇 |
2019年 | 600篇 |
2018年 | 1433篇 |
2017年 | 1415篇 |
2016年 | 1737篇 |
2015年 | 1059篇 |
2014年 | 1819篇 |
2013年 | 3669篇 |
2012年 | 1867篇 |
2011年 | 2514篇 |
2010年 | 2303篇 |
2009年 | 3001篇 |
2008年 | 2718篇 |
2007年 | 2689篇 |
2006年 | 2544篇 |
2005年 | 2185篇 |
2004年 | 2143篇 |
2003年 | 1995篇 |
2002年 | 1916篇 |
2001年 | 1779篇 |
2000年 | 1671篇 |
1999年 | 1520篇 |
1998年 | 1442篇 |
1997年 | 1494篇 |
1996年 | 1202篇 |
1995年 | 1221篇 |
1994年 | 1164篇 |
1993年 | 1021篇 |
1992年 | 1005篇 |
1991年 | 947篇 |
1990年 | 1057篇 |
1989年 | 944篇 |
1988年 | 918篇 |
1987年 | 1020篇 |
1986年 | 869篇 |
1985年 | 1159篇 |
1984年 | 1264篇 |
1983年 | 1239篇 |
1982年 | 1184篇 |
1981年 | 1096篇 |
1980年 | 1035篇 |
1979年 | 949篇 |
1978年 | 932篇 |
1977年 | 858篇 |
1976年 | 806篇 |
1975年 | 730篇 |
1974年 | 798篇 |
1973年 | 787篇 |
1972年 | 495篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
172.
After briefly recalling basic facts about the metallicity gradients inspiral galaxies, we summarize two recent N-body gas-dynamical modelsthat implement a non-instantaneous gas recycling and point our their potential for an improved treatment of the chemical evolution ingalactic disks. 相似文献
173.
Knowledge of the optical constants of elemental sulfur has potential applications to Venus, Jupiter, Io, Amalthea, and the Earth. The real part, n, of the index of refraction of liquid sulfur (at 133°C) and of solid orthorhombic sulfur (at 25°C) for the wavelength range 0.4–2.0 μm were measured ellipsometrically. The imaginary part, k, of the refractive index of liquid sulfur was obtained by transmittance measurements at the same temperature and wavelength range. The reflectance of semi-infinite slabs of solid and liquid sulfur is calculated using the measured n and k values. We confirm that sulfur melts on Io would be classified as “black” by the Voyager imaging system. 相似文献
174.
175.
R.E. Hartle E.C. Sittler R.E. Johnson F. Crary D.T. Young D. Simpson D. Reisenfeld J.J. Berthelier J. Vilppola N. Andre 《Planetary and Space Science》2006,54(12):1211-1224
The Cassini plasma spectrometer (CAPS) instrument made measurements of Titan's plasma environment when the Cassini Orbiter flew through the moon's plasma wake October 26, 2004 (flyby TA). Initial CAPS ion and electron measurements from this encounter will be compared with measurements made by the Voyager 1 plasma science instrument (PLS). The comparisons will be used to evaluate previous interpretations and predictions of the Titan plasma environment that have been made using PLS measurements. The plasma wake trajectories of flyby TA and Voyager 1 are similar because they occurred when Titan was near Saturn's local noon. These similarities make possible direct, meaningful comparisons between the various plasma wake measurements. They lead to the following: (A) The light and heavy ions, H+and N+/O+, were observed by PLS in Saturn's magnetosphere in the vicinity of Titan while the higher mass resolution of CAPS yielded H+ and H2+as the light constituents and O+/CH4+ as the heavy ions. (B) Finite gyroradius effects were apparent in PLS and CAPS measurements of ambient O+ ions as a result of their absorption by Titan's extended atmosphere. (C) The principal pickup ions inferred from both PLS and CAPS measurements are H+, H2+, N+, CH4+ and N2+. (D) The inference that heavy pickup ions, observed by PLS, were in narrow beam distributions was empirically established by the CAPS measurements. (E) Slowing down of the ambient plasma due to pickup ion mass loading was observed by both instruments on the anti-Saturn side of Titan. (F) Strong mass loading just outside the ionotail by a heavy ion such as N2+ is apparent in PLS and CAPS measurements. (G) Except for the expected differences due to the differing trajectories, the magnitudes and structures of the electron densities and temperatures observed by both instruments are similar. The high-energy electron bite-out observed by PLS in the magnetotail is consistent with that observed by CAPS. 相似文献
176.
177.
Jack B. Zirker 《Solar physics》1985,100(1-2):281-287
This paper reviews research highlights of the past five years. Considerable progress has been made in observing and interpreting coronal mass ejections. The stability of coronal loops is much better understood and new observations of the onset of wind streams in coronal holes have been made. Observations from the Solar Maximum Mission should helpt to clarify the physics of the active corona.The mechanisms that heat the corona and accelerate the high-speed wind streams remain to be identified, however.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 相似文献
178.
Spectra of the central core and surrounding coma of Comet IRAS-Araki-Alcock (1983d) were obtained at 8–13 μm on 11 May and 2–4 μm on 12 May 1983. Spatially resolved measurements at 10 μm with a 4-arcsec beam showed that the central core was more than 100 times brighter than the inner coma only 8 arcsec away; for radially outflowing dust, the brightness ratio would be a factor of 8. The observations of the central core are consistent with direct detection of a nucleus having a radius of approximately 5 km. The temperature of the sunlit hemisphere was > 300 K. Spectra of the core are featureless, while spectra of the coma suggest weak silicate emission. The spectra show no evidence for icy grains. The dust producton rate on 11.4 May was ~ 105 g/sec, assuming that the gas flux from the dust-producing areas on the nucleus was ~ 10?5 g/cm2/sec. 相似文献
179.
Ralph B. Baldwin 《Icarus》1985,61(1):63-91
This paper contains a reasonably successful attempt to determine relative ages and then absolute ages of individual craters younger than Imbrium, and the rate of infalls onto the Moon as a function of time. After the tail of the massive premare bombardment became depleted before 3 aeons (1 aeon = 109 years) ago, there was a period of minimal numbers of infalls. The rate of infalls increased rather steadily from this minimum to the present. The rate in the geologically recent past (0.3 aeon) was about two times that found for the period immediately after the last of the major lave outpourings (3.2 aeons). Absolute ages were determined for large craters (?8 km) from crater counts on the surfaces within and on the rims of the large craters. Key dates were 0 and 0.3 aeon for terrestrial meteoritic craters, 3.2, 3.5, 3.8, and 3.82 aeons for the various mare surfaces according to the determinations of D.E. Wilhelms (1980, Geologic history of the Moon, U.S. Geol. Surv. Prof. Pap.) and 3.85 aeons from the formation of Imbrium. 相似文献
180.