全文获取类型
收费全文 | 49931篇 |
免费 | 8468篇 |
国内免费 | 10913篇 |
专业分类
测绘学 | 4097篇 |
大气科学 | 5423篇 |
地球物理 | 9424篇 |
地质学 | 29676篇 |
海洋学 | 5079篇 |
天文学 | 4737篇 |
综合类 | 3399篇 |
自然地理 | 7477篇 |
出版年
2024年 | 166篇 |
2023年 | 444篇 |
2022年 | 1232篇 |
2021年 | 1426篇 |
2020年 | 1483篇 |
2019年 | 1834篇 |
2018年 | 1453篇 |
2017年 | 1666篇 |
2016年 | 1789篇 |
2015年 | 1989篇 |
2014年 | 2493篇 |
2013年 | 2493篇 |
2012年 | 2804篇 |
2011年 | 3064篇 |
2010年 | 2624篇 |
2009年 | 3351篇 |
2008年 | 3272篇 |
2007年 | 3698篇 |
2006年 | 3588篇 |
2005年 | 3186篇 |
2004年 | 2984篇 |
2003年 | 2876篇 |
2002年 | 2524篇 |
2001年 | 2201篇 |
2000年 | 2062篇 |
1999年 | 1910篇 |
1998年 | 1673篇 |
1997年 | 1440篇 |
1996年 | 1296篇 |
1995年 | 1107篇 |
1994年 | 1130篇 |
1993年 | 955篇 |
1992年 | 758篇 |
1991年 | 524篇 |
1990年 | 475篇 |
1989年 | 390篇 |
1988年 | 288篇 |
1987年 | 181篇 |
1986年 | 127篇 |
1985年 | 86篇 |
1984年 | 40篇 |
1983年 | 33篇 |
1982年 | 31篇 |
1981年 | 20篇 |
1980年 | 24篇 |
1979年 | 29篇 |
1978年 | 36篇 |
1977年 | 24篇 |
1975年 | 4篇 |
1954年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
In this paper, we idealize the actual solar atmosphere as a multi-isothermal-layer system so as to obtain the energy transmittance of the linear Alfvén wave that propagates through such a system in presence of a uniform oblique magnetic filed. The results indicate that the two-layer model is essentially different to the three-layer one. In the two-layer model, the temperature jump acts as a high pass filter. In the three-layer model, resonant transfer will take place and the transmittance undergoes oscillation as the trigonometric function terms dominate its behavior. For actual solar atmosphere, the result reveals that the lower parts of solar atmosphere are more suitable for those Alfvén waves with period of seconds to transfer their energy. 相似文献
102.
Kyle T. Alfriend Srinivas R. Vadali Hanspeter Schaub 《Celestial Mechanics and Dynamical Astronomy》2001,81(1-2):57-62
Satellites flying in formation is a concept being pursued by the Air Force and NASA. Potential periodic formation orbits have been identified using Hill's (or Clohessy Wiltshire) equations. Unfortunately the gravitational perturbations destroy the periodicity of the orbits and control will be required to maintain the desired orbits. Since fuel will be one of the major factors limiting the system lifetime it is imperative that fuel consumption be minimized. To maximize lifetime we not only need to find those orbits which require minimum fuel we also need for each satellite to have equal fuel consumption and this average amount needs to be minimized. Thus, control of the system has to be addressed, not just control of each satellite. In this paper control of the individual satellites as well as the constellation is addressed from an astrodynamics perspective. 相似文献
103.
矿床谱系是对成矿多样性的理论概括,而成矿多样性又是由不同级别、不同性质的致矿地质异常决定的.本文论述了山东省内生金矿矿床谱系及其致矿地质异常. 相似文献
104.
本文分析了国外用于测定度盘分划改正的几种方法,并分析了固定角距法的优点和不足之处;详细叙述了组合固定角距法测定对径改正的原理和计算公式;并且讨论了两对显微镜之间夹角变化以及显微镜比例尺变化对对径改正的影响,给出了相应的修正方法。用固定角距法所测的对径改正精度可达±0″.009。 相似文献
105.
106.
107.
Benedetta Ciardi † rea Ferrara 《Monthly notices of the Royal Astronomical Society》2001,324(3):648-652
We calculate the expected mid-infrared (MIR) molecular hydrogen line emission from the first objects in the Universe. As a result of their low masses, the stellar feedback from massive stars is able to blow away their gas content and collect it into a cooling shell where H2 rapidly forms and IR roto-vibrational (as for example the rest-frame 2.12 μm) lines carry away a large fraction (up to 10 per cent) of the explosion energy. The fluxes from these sources are in the range 10−21 –10−17 erg s−1 cm−2 . The highest number counts are expected in the 20-μm band, where about 105 sources deg−2 are predicted at the limiting flux of 3×10−18 erg s−1 cm−2 . Among the planned observational facilities, we find that the best detection perspectives are offered by the Next Generation Space Telescope ( NGST ), which should be able to reveal about 200 first objects in one hour observation time at its limiting flux in the above band. Therefore, mid-IR instruments appear to represent perfect tools to trace star formation and stellar feedback in the high ( z ≳5) redshift Universe. 相似文献
108.
Davide Lazzati Rosalba Perna Gabriele Ghisellini 《Monthly notices of the Royal Astronomical Society》2001,325(4):L19-L23
The recent detection of a transient absorption feature in the X-ray prompt emission of GRB 990705 showed the importance of such observations in the understanding of gamma-ray bursts and their progenitors. We investigate the time dependence of photoionization edges during the prompt emission of bursts in different environments. We show that their variability can be used to infer the density and geometry of the surrounding medium, giving important clues to unveil the nature of the burst progenitor. 相似文献
109.
Hong Lin Zhang Douglas H. Sampson 《Monthly notices of the Royal Astronomical Society》2001,322(3):433-437
Calculations are made of the resonance contribution to electron-impact excitation of H-like 13 C and Li-like 23 Na, 25 Mg, 27 Al and 29 Si to the upper hyperfine levels that produce millimetre (mm) lines of likely astrophysical interest. The resonance contribution is found to be very important for these Li-like ions, considerably more important than for Li-like 57 Fe considered previously. However, resonances are found to be rather unimportant for H-like 13 C. The effect of radiative decay on the resonance contribution is found to be insignificant in all of the present calculations. 相似文献
110.
G. M. Karataeva V. A. Yakovleva V. A. Hagen-Thorn O. V. Mikolaichuk 《Astronomy Letters》2001,27(2):74-80
We analyze our BVR c photometry for UGC 5600, a candidate polar ring galaxy, obtained with the 6-m telescope. We have confirmed the existence of an inner polar ring and show that the outer ring-shaped structure represents spiral arms; i.e., UGC 5600 belongs to the rare class of gas-rich spiral galaxies with inner polar rings. 相似文献