全文获取类型
收费全文 | 86266篇 |
免费 | 987篇 |
国内免费 | 490篇 |
专业分类
测绘学 | 2191篇 |
大气科学 | 6111篇 |
地球物理 | 15823篇 |
地质学 | 32793篇 |
海洋学 | 7122篇 |
天文学 | 19028篇 |
综合类 | 272篇 |
自然地理 | 4403篇 |
出版年
2021年 | 656篇 |
2020年 | 698篇 |
2019年 | 747篇 |
2018年 | 3983篇 |
2017年 | 3717篇 |
2016年 | 3094篇 |
2015年 | 1104篇 |
2014年 | 1863篇 |
2013年 | 3557篇 |
2012年 | 2766篇 |
2011年 | 4688篇 |
2010年 | 4328篇 |
2009年 | 4988篇 |
2008年 | 4228篇 |
2007年 | 4844篇 |
2006年 | 2614篇 |
2005年 | 2330篇 |
2004年 | 2208篇 |
2003年 | 2202篇 |
2002年 | 2010篇 |
2001年 | 1652篇 |
2000年 | 1510篇 |
1999年 | 1369篇 |
1998年 | 1300篇 |
1997年 | 1313篇 |
1996年 | 1051篇 |
1995年 | 1052篇 |
1994年 | 990篇 |
1993年 | 882篇 |
1992年 | 877篇 |
1991年 | 830篇 |
1990年 | 929篇 |
1989年 | 823篇 |
1988年 | 777篇 |
1987年 | 885篇 |
1986年 | 764篇 |
1985年 | 995篇 |
1984年 | 1099篇 |
1983年 | 1062篇 |
1982年 | 1013篇 |
1981年 | 963篇 |
1980年 | 863篇 |
1979年 | 847篇 |
1978年 | 823篇 |
1977年 | 743篇 |
1976年 | 709篇 |
1975年 | 630篇 |
1974年 | 700篇 |
1973年 | 720篇 |
1972年 | 438篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
New determination of the Earth orientation parameters (EOP), based on optical astrometry observations since the beginning of the century, is now under preparation by the Working group established by Commission 19 of the IAU. The Hipparcos catalog is to define the celestial reference frame in which the new series of EOP are to be described, The novelties of the prepared solution are the higher resolution (5 days) and more parameters estimated from the solution (celestial pole offsets, rheological parameters of the Earth, certain instrumental constants). The mathematical model of the solution is described, and the results based on the observations made with 46 instruments at 29 observatories and a preliminary Hipparcos catalog are presented. 相似文献
33.
Parametric transduction offers valuable advantages for underwater acoustic communications. Perhaps the most significant benefit is the fact that high directivity is achieved by means of a physically small transmit transducer. This feature may, ultimately, be employed to permit long-range, low-frequency communication using a compact source. The high directivity is desirable to combat multipath propagation and to achieve data communications in water which is shallow by comparison with range. A real-time, high data-rate “model” differential phase shift keying (DPSK) communication system has been constructed and demonstrated. This system uses parametric transduction, with a 300-kHz primary frequency and a 50-kHz secondary frequency. Experimental results show that the system can be employed to combat multipath propagation in shallow water and can achieve high data-rate text and color image transmission at 10 and 20 kb s-1 for 2-DPSK and 4-DPSK, respectively, through a transmission bandwidth of 10 kHz. The “model” system was developed to confirm performance predictions for a future, operational long-range link employing a 50-kHz primary frequency and a 5-kHz secondary frequency 相似文献
34.
In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques. 相似文献
35.
36.
The 362.156 GHz absorption spectrum of H2O2 in the Mars atmosphere was observed on September 4 of 2003, employing the James Clerk Maxwell Telescope (JCMT) sub-millimeter facility on Mauna Kea, Hawaii. Radiative transfer analysis of this line absorption yields an average volume mixing ratio of 18±0.4 ppbv within the lower (0-30 km) Mars atmosphere, in general accordance with standard photochemical models (e.g., Nair et al., 1994, Icarus 111, 124-150). Our derived H2O2 abundance is roughly three times greater than the upper limit retrieved by Encrenaz et al. (2002, Astron. Astrophys. 396, 1037-1044) from infrared spectroscopy, although part of this discrepancy may result from the different solar longitudes (Ls) of observation. Aphelion-to-perihelion thermal forcing of the global Mars hygropause generates substantial (>200%) increases in HOx abundances above ∼10 km altitudes between the Ls=112° period of the Encrenaz et al. upper limit measurement and the current Ls=250° period of detection (Clancy and Nair, 1996, J. Geophys. Res. 101, 12785-12590). The observed H2O2 line absorption weakens arguments for non-standard homogeneous (Encrenaz et al., 2002, Astron. Astrophys. 396, 1037-1044) or heterogeneous (Krasnopolsky, 2003a, J. Geophys. Res. 108; 2003b, Icarus 165, 315-325) chemistry, which have been advocated partly on the basis of infrared (8 μm) non-detections for Mars H2O2. Observation of Mars H2O2 also represents the first measurement of a key catalytic specie in a planetary atmosphere other than our own. 相似文献
37.
38.
Michail I. PETAEV 《Meteoritics & planetary science》1996,31(6):807-815
Abstract— Olivine grains from selected meteorites (the Springwater pallasite, the Lowicz mesosiderite, the ALH 84025 brachinite, the Krymka LL3 chondrite, and the Calcalong Creek lunar meteorite) and terrestrial rocks (San Carlos forsterite and Rockport fayalite) were studied by optical microscopy and high-precision electron microprobe analysis. Detailed microprobe traverses revealed regular igneous zoning in the Krymka and Calcalong Creek olivines. Traverses across the San Carlos forsterite grain are flat and display no chemical variations larger than the 2σ range of counting error (±0.2 mol% Fa). Traverses across olivine grains in the ALH 84025, Lowicz, and Springwater meteorites show regular patterns of periodic or wavy chemical variations well exceeding the 2σ uncertainty range. However, no lamellar structure was seen in backscattered electron images. It is suggested that the periodic chemical variations may be due to spinodal decomposition of primary, more or less homogeneous grains. I conclude that the absence of earlier reports of such variations simply means that olivine grains in equilibrated meteorites have not been examined closely enough to detect them. 相似文献
39.
Long-term changes in growth of Norwegian spring-spawning herring 总被引:3,自引:1,他引:2
40.
We discuss long-time changes of polar activity of the Sun using the new observational data sets in the optical range during
1872–2001. A study of the secular and cycle variations of the magnetic activity at the high-latitude regions is the main goal
that includes polar magnetic field reversals during 1872–2001 and secular changes of the duration of polar activity cycles.
The secular increase of the area of polar zones during the minimum activity in the last 120 years and as consequence a decrease
of coronal temperature of the Sun in the high-latitude zones during the last 50 years. Correlation between the polar cycles
of Caii-K bright points with the Wolf sunspot numbers cycles, W(t), and the 22-year polar magnetic cycles of Caii-K bright points at the high latitudes during 1905–1995 is discussed. 相似文献