全文获取类型
收费全文 | 56952篇 |
免费 | 1018篇 |
国内免费 | 684篇 |
专业分类
测绘学 | 1859篇 |
大气科学 | 4458篇 |
地球物理 | 11651篇 |
地质学 | 20133篇 |
海洋学 | 4877篇 |
天文学 | 12376篇 |
综合类 | 216篇 |
自然地理 | 3084篇 |
出版年
2021年 | 416篇 |
2020年 | 444篇 |
2019年 | 492篇 |
2018年 | 1207篇 |
2017年 | 1117篇 |
2016年 | 1497篇 |
2015年 | 927篇 |
2014年 | 1485篇 |
2013年 | 2949篇 |
2012年 | 1589篇 |
2011年 | 2134篇 |
2010年 | 1777篇 |
2009年 | 2327篇 |
2008年 | 2062篇 |
2007年 | 1987篇 |
2006年 | 2000篇 |
2005年 | 1683篇 |
2004年 | 1604篇 |
2003年 | 1543篇 |
2002年 | 1530篇 |
2001年 | 1324篇 |
2000年 | 1328篇 |
1999年 | 1195篇 |
1998年 | 1128篇 |
1997年 | 1142篇 |
1996年 | 991篇 |
1995年 | 967篇 |
1994年 | 905篇 |
1993年 | 805篇 |
1992年 | 736篇 |
1991年 | 718篇 |
1990年 | 759篇 |
1989年 | 649篇 |
1988年 | 671篇 |
1987年 | 793篇 |
1986年 | 667篇 |
1985年 | 898篇 |
1984年 | 978篇 |
1983年 | 969篇 |
1982年 | 848篇 |
1981年 | 756篇 |
1980年 | 779篇 |
1979年 | 677篇 |
1978年 | 676篇 |
1977年 | 628篇 |
1976年 | 613篇 |
1975年 | 587篇 |
1974年 | 635篇 |
1973年 | 648篇 |
1972年 | 409篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
M. J. Freyberg H. Bräuninger W. Burkert G. D. Hartner O. Citterio F. Mazzoleni G. Pareschi D. Spiga S. Romaine P. Gorenstein B. D. Ramsey 《Experimental Astronomy》2005,20(1-3):405-412
The Max-Planck-Institut für extraterrestrische Physik (MPE) in Garching, Germany, uses its large X-ray beam line facility PANTER for testing X-ray astronomical instrumentation. A number of telescopes, gratings, filters, and detectors, e.g. for astronomical satellite missions like Exosat, ROSAT, Chandra (LETG), BeppoSAX, SOHO (CDS), XMM-Newton, ABRIXAS, Swift (XRT), have been successfully calibrated in the soft X-ray energy range (< 15keV). Moreover, measurements with mirror test samples for new missions like ROSITA and XEUS have been carried out at PANTER. Here we report on an extension of the energy range, enabling calibrations of hard X-ray optics over the energy range 15–50 keV. Several future X-ray astronomy missions (e.g., Simbol-X, Constellation-X, XEUS) have been proposed, which make use of hard X-ray optics based on multilayer coatings. Such optics are currently being developed by the Osservatorio Astronomico di Brera (OAB), Milano, Italy, and the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, MA, USA. These optics have been tested at the PANTER facility with a broad energy band beam (up to 50 keV) using the XMM-Newton EPIC-pn flight spare CCD camera with its good intrinsic energy resolution, and also with monochromatic X-rays between C-K (0.277 keV) and Cu-Kα (8.04 keV).
PACS: 95.55.Ka, 95.55.Aq, 41 50.+h, 07.85.Fv 相似文献
22.
Mallory S. E. Roberts Crystal L. Brogan Bryan M. Gaensler Jason W. T. Hessels C.-Y. Ng Roger W. Romani 《Astrophysics and Space Science》2005,297(1-4):93-100
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be
associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the
pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies
seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM. 相似文献
23.
24.
25.
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
26.
Daniela Lazzaro Marcos A. Florczak Alberto Betzler Othon C. Winter Silvia M. Giuliatti-Winter Claudia A. Angeli Dietmar W. Foryta 《Planetary and Space Science》1996,44(12):1547-1550
The results of photometric observations of comet/asteroid 2060 Chiron at the Observatório do Pico dos Dias (Brazil-OPD) and the Observatoire de Haute-Provence (France-OHP) during 1994 and 1995 are presented. The analysis of the data shows a decrease of 2060 Chiron brightness from its peak values of 1988–1991. The absolute magnitude, Hv, varies from a maximum of 6.6 in February 1994 up to a minimum of 6.8 in June 1995. Therefore 2060 Chiron is back to a minimum of activity close to that of 1983–1985. The slope parameter G is found to be G = 0.71 ± 0.15. It is suggested that the H-G magnitude system, generally adopted to present 2060 Chiron brightness, is not the most appropriate due to the cometary activity of this object. 相似文献
27.
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
28.
29.
30.
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