全文获取类型
收费全文 | 10811篇 |
免费 | 1150篇 |
国内免费 | 1654篇 |
专业分类
测绘学 | 386篇 |
大气科学 | 769篇 |
地球物理 | 1262篇 |
地质学 | 4248篇 |
海洋学 | 914篇 |
天文学 | 5068篇 |
综合类 | 405篇 |
自然地理 | 563篇 |
出版年
2024年 | 25篇 |
2023年 | 85篇 |
2022年 | 217篇 |
2021年 | 251篇 |
2020年 | 285篇 |
2019年 | 347篇 |
2018年 | 254篇 |
2017年 | 255篇 |
2016年 | 277篇 |
2015年 | 319篇 |
2014年 | 578篇 |
2013年 | 611篇 |
2012年 | 616篇 |
2011年 | 749篇 |
2010年 | 775篇 |
2009年 | 1083篇 |
2008年 | 1001篇 |
2007年 | 905篇 |
2006年 | 835篇 |
2005年 | 769篇 |
2004年 | 613篇 |
2003年 | 512篇 |
2002年 | 406篇 |
2001年 | 354篇 |
2000年 | 286篇 |
1999年 | 278篇 |
1998年 | 199篇 |
1997年 | 93篇 |
1996年 | 80篇 |
1995年 | 69篇 |
1994年 | 69篇 |
1993年 | 83篇 |
1992年 | 34篇 |
1991年 | 44篇 |
1990年 | 38篇 |
1989年 | 27篇 |
1988年 | 30篇 |
1987年 | 14篇 |
1986年 | 19篇 |
1985年 | 22篇 |
1984年 | 21篇 |
1983年 | 17篇 |
1982年 | 18篇 |
1981年 | 6篇 |
1980年 | 13篇 |
1979年 | 2篇 |
1978年 | 8篇 |
1977年 | 17篇 |
1877年 | 1篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
Peter J. Wheatley Christopher W. Mauche Janet A. Mattei 《Monthly notices of the Royal Astronomical Society》2003,345(1):49-61
We present the most complete multiwavelength coverage of any dwarf nova outburst: simultaneous optical, Extreme Ultraviolet Explorer and Rossi X-ray Timing Explorer observations of SS Cygni throughout a narrow asymmetric outburst. Our data show that the high-energy outburst begins in the X-ray waveband 0.9–1.4 d after the beginning of the optical rise and 0.6 d before the extreme-ultraviolet rise. The X-ray flux drops suddenly, immediately before the extreme-ultraviolet flux rise, supporting the view that both components arise in the boundary layer between the accretion disc and white dwarf surface. The early rise of the X-ray flux shows that the propagation time of the outburst heating wave may have been previously overestimated.
The transitions between X-ray and extreme-ultraviolet dominated emission are accompanied by intense variability in the X-ray flux, with time-scales of minutes. As detailed by Mauche & Robinson, dwarf nova oscillations are detected throughout the extreme-ultraviolet outburst, but we find they are absent from the X-ray light curve.
X-ray and extreme-ultraviolet luminosities imply accretion rates of 3 × 1015 g s−1 in quiescence, 1 × 1016 g s−1 when the boundary layer becomes optically thick, and ∼1018 g s−1 at the peak of the outburst. The quiescent accretion rate is two and a half orders of magnitude higher than predicted by the standard disc instability model, and we suggest this may be because the inner accretion disc in SS Cyg is in a permanent outburst state. 相似文献
The transitions between X-ray and extreme-ultraviolet dominated emission are accompanied by intense variability in the X-ray flux, with time-scales of minutes. As detailed by Mauche & Robinson, dwarf nova oscillations are detected throughout the extreme-ultraviolet outburst, but we find they are absent from the X-ray light curve.
X-ray and extreme-ultraviolet luminosities imply accretion rates of 3 × 10
995.
A Fundamental Plane of black hole activity 总被引:1,自引:0,他引:1
996.
997.
998.
Peter J. Wheatley Koji Mukai † Domitilla de Martino 《Monthly notices of the Royal Astronomical Society》2003,346(3):855-860
We present the first X-ray observations of the 4 Draconis system, consisting of an M3 III giant with a hot ultraviolet companion. It has been claimed that the companion is itself an AM Her-type binary system, an identification that places strong constraints on the evolution of cataclysmic variables. We find that the X-ray properties of 4 Draconis are consistent with the presence of an accreting white dwarf, but not consistent with the presence of an AM Her system. We conclude that 4 Draconis is therefore most likely a symbiotic binary containing a white dwarf accreting material from the wind of the red giant.
The X-ray spectrum of 4 Draconis is sometimes dominated by partially ionized photoelectric absorption, presumably due to the wind of the red giant. We note that X-ray monitoring of such systems would provide a powerful probe of the wind and mass-loss rate of the giant, and would allow a detailed test of wind accretion models. 相似文献
The X-ray spectrum of 4 Draconis is sometimes dominated by partially ionized photoelectric absorption, presumably due to the wind of the red giant. We note that X-ray monitoring of such systems would provide a powerful probe of the wind and mass-loss rate of the giant, and would allow a detailed test of wind accretion models. 相似文献
999.
The kinetics of the reactions of C2H radical with ethane (k1), propane (k2), and n-butane (k3) are studied over the temperature range of T = 96-296 K with a pulsed Laval nozzle apparatus that utilizes a pulsed laser photolysis-chemiluminescence technique. The C2H decay profiles in the presence of both the alkane reactant and O2 are monitored by the CH(A2Δ) chemiluminescence tracer method. The results, together with available literature data, yield the following Arrhenius expressions: k1(T) = (0.51 ± 0.06) × 10−10 exp[(−76 ± 30)K/T] cm3 molecule−1 s−1 (T = 96-800 K), k2(T) = (0.98 ± 0.32) × 10−10exp[(−71 ± 60)K/T] cm3 molecule−1 s−1 (T = 96-361 K), and k3(T) = (1.23 ± 0.26) × 10−10 cm3 molecule−1 s−1 (T = 96-297 K). At T = 296 K, k1 is measured as a function of total pressure and has little or no pressure dependence. The results from this work support a direct hydrogen abstraction mechanism for the title reactions. Implications to the atmospheric chemistry of Titan are discussed. 相似文献
1000.
Mars Global Surveyor accelerometer observations of the martian upper atmosphere revealed large variations in density with longitude during northern hemisphere spring at altitudes of 130-160 km, all latitudes, and mid-afternoon local solar times (LSTs). This zonal structure is due to tides from the surface. The zonal structure is stable on timescales of weeks, decays with increasing altitude above 130 km, and is dominated by wave-3 (average amplitude 22% of mean density) and wave-2 (18%) harmonics. The phases of these harmonics are constant with both altitude and latitude, though their amplitudes change significantly with latitude. Near the South Pole, the phase of the wave-2 harmonic changes by 90° with a change of half a martian solar day while the wave-3 phase stays constant, suggesting diurnal and semidiurnal behaviour, respectively. We use a simple application of classical tidal theory to identify the dominant tidal modes and obtain results consistent with those of General Circulation Models. Our method is less rigorous, but simpler, than the General Circulation Models and hence complements them. Topography has a strong influence on the zonal structure. 相似文献