全文获取类型
收费全文 | 49265篇 |
免费 | 794篇 |
国内免费 | 324篇 |
专业分类
测绘学 | 1335篇 |
大气科学 | 3832篇 |
地球物理 | 9290篇 |
地质学 | 16273篇 |
海洋学 | 4172篇 |
天文学 | 12164篇 |
综合类 | 108篇 |
自然地理 | 3209篇 |
出版年
2020年 | 339篇 |
2019年 | 337篇 |
2018年 | 828篇 |
2017年 | 810篇 |
2016年 | 1052篇 |
2015年 | 697篇 |
2014年 | 1170篇 |
2013年 | 2444篇 |
2012年 | 1110篇 |
2011年 | 1543篇 |
2010年 | 1436篇 |
2009年 | 1934篇 |
2008年 | 1775篇 |
2007年 | 1776篇 |
2006年 | 1679篇 |
2005年 | 1546篇 |
2004年 | 1509篇 |
2003年 | 1413篇 |
2002年 | 1358篇 |
2001年 | 1231篇 |
2000年 | 1155篇 |
1999年 | 1122篇 |
1998年 | 1056篇 |
1997年 | 1064篇 |
1996年 | 844篇 |
1995年 | 836篇 |
1994年 | 794篇 |
1993年 | 739篇 |
1992年 | 709篇 |
1991年 | 681篇 |
1990年 | 777篇 |
1989年 | 674篇 |
1988年 | 649篇 |
1987年 | 741篇 |
1986年 | 622篇 |
1985年 | 824篇 |
1984年 | 947篇 |
1983年 | 912篇 |
1982年 | 861篇 |
1981年 | 820篇 |
1980年 | 722篇 |
1979年 | 699篇 |
1978年 | 692篇 |
1977年 | 627篇 |
1976年 | 596篇 |
1975年 | 517篇 |
1974年 | 597篇 |
1973年 | 586篇 |
1972年 | 362篇 |
1971年 | 334篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
A. R. King 《Astrophysics and Space Science》1996,237(1-2):169-184
The discovery of X-ray binary systems in the 1960's opened up stellar evolution theory by revealing further endpoints in addition to white dwarfs. This review summarises recent progress in studies of stellar-evolutionary processes that lead to X-ray binaries themselves, the mass transfer rates that power them, and the accretion processes which convert this into electromagnetic radiation. Particular attention is paid to the topics of mass transfer fluctuations and of the accretion by magnetic compact stars. 相似文献
22.
The main properties of the first- and second-order moments of polarized hydrogen lines, forming in the presence of stationary electric and magnetic fields, are reviewed. The analytical results presented here apply directly to the case of optically-thin emission lines in the LTE regime. Some applications of such results to electric- and magnetic-field diagnostics in (solar) plasmas are then briefly considered.On leave from the Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Largo E. Fermi 5, I-50125 Firenze, Italy 相似文献
23.
24.
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
25.
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
26.
27.
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
28.
29.
Long-term changes in growth of Norwegian spring-spawning herring 总被引:3,自引:1,他引:2
30.
W. H. Jefferys G. F. Benedict P. D. Hemenway P. J. Shelus R. L. Duncombe 《Celestial Mechanics and Dynamical Astronomy》1985,37(3):299-305
The Hubble Space Telescope (HST), a large optical telescope having an aperture of 2.4 meters and a length of 8.8 meters, is being developed by the National Aeronautics and Space Administration. This telescope will be placed into earth orbit by the space shuttle. Astrometric observations with the HST are made using a Fine Guidance Sensor which is capable of measuring the position of one object relative to another with an accuracy of ±0.002 arcseconds. The astrometric user of HST will be provided with an Astrometric Data Reduction Software package (ADRS). The variety of astrometric problems to be investigated with HST is discussed. 相似文献