全文获取类型
收费全文 | 57559篇 |
免费 | 1009篇 |
国内免费 | 642篇 |
专业分类
测绘学 | 1607篇 |
大气科学 | 4084篇 |
地球物理 | 10891篇 |
地质学 | 20687篇 |
海洋学 | 5279篇 |
天文学 | 13567篇 |
综合类 | 207篇 |
自然地理 | 2888篇 |
出版年
2022年 | 391篇 |
2021年 | 664篇 |
2020年 | 703篇 |
2019年 | 767篇 |
2018年 | 1666篇 |
2017年 | 1601篇 |
2016年 | 1982篇 |
2015年 | 1094篇 |
2014年 | 1875篇 |
2013年 | 3112篇 |
2012年 | 1982篇 |
2011年 | 2577篇 |
2010年 | 2227篇 |
2009年 | 2888篇 |
2008年 | 2513篇 |
2007年 | 2540篇 |
2006年 | 2383篇 |
2005年 | 1768篇 |
2004年 | 1773篇 |
2003年 | 1698篇 |
2002年 | 1604篇 |
2001年 | 1382篇 |
2000年 | 1304篇 |
1999年 | 1055篇 |
1998年 | 1111篇 |
1997年 | 1003篇 |
1996年 | 880篇 |
1995年 | 827篇 |
1994年 | 735篇 |
1993年 | 646篇 |
1992年 | 638篇 |
1991年 | 632篇 |
1990年 | 661篇 |
1989年 | 532篇 |
1988年 | 544篇 |
1987年 | 570篇 |
1986年 | 533篇 |
1985年 | 669篇 |
1984年 | 721篇 |
1983年 | 641篇 |
1982年 | 617篇 |
1981年 | 558篇 |
1980年 | 517篇 |
1979年 | 520篇 |
1978年 | 499篇 |
1977年 | 394篇 |
1976年 | 371篇 |
1975年 | 377篇 |
1974年 | 333篇 |
1973年 | 376篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
111.
K. R. Subramanian R. Ramesh M. S. Sundara Rajan Ch. V. Sastry 《Astrophysics and Space Science》1996,243(1):259-262
Observations of the Sun at two frequencies (51 and 77 MHz) using the East-West arm of the Gauribidanur Radio heliograph are presented. 相似文献
112.
A numerical study of the pre-ejection, magnetically-sheared corona as a free boundary problem 总被引:1,自引:0,他引:1
A class of magnetostatic equilibria with axial symmetry outside a unit sphere in the presence of plasma pressure and an r
–2 gravitational field is constructed. The structure contains a localized current-carrying region confined by a background bipolar potential field, and the shape of the region changes subject to the variation of the electric current. The continuity requirement for the magnetic field and plasma pressures at the outer boundary of the cavity defines a free boundary problem, which is solved numerically using a spectral boundary scheme. The model is then used to study the expansion of the current-carrying region, caused by the buildup of magnetic shear, against the background confining field. The magnetic shear in our model is induced by the loading of an azimuthal field, accompanied by a depletion of plasma density.We show that due to the additional effect of confinement by the dense surrounding plasma, the energy of the magnetic field can exceed the energy of its associated open field, presumably a necessary condition for the onset of coronal mass ejections. (However, the plasma beta of the confining fluid is higher than that in the outer boundary of a realistic helmet-streamer structure.) Furthermore, under the assumption that coronal mass ejections are driven by magnetic buoyancy, the result from our model study lends further support to the notion of a suspended magnetic flux rope in the low-density cavity of a helmet-streamer as a promising pre-ejection configuration.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
113.
114.
The Dergaon (H5) chondrite: Fall,classification, petrological and chemical characteristics,cosmogenic effects,and noble gas records 总被引:1,自引:0,他引:1
P. N. SHUKLA A. D. SHUKLA V. K. RAI S. V. S. MURTY N. BHANDARI J. N. GOSWAMI A. C. MAZUMDAR P. PHUKON K. DUORAH R. E. GREENWOOD I. A. FRANCHI 《Meteoritics & planetary science》2005,40(4):627-637
Abstract— A multiple fall of a stony meteorite occurred near the town of Dergaon in Assam, India, on March 2, 2001. Several fragments weighing <2 kg and a single large fragment weighing ~10 kg were recovered from the strewn field, which extended over several tens of square kilometers. Chemical, petrographic, and oxygen isotopic studies indicate it to be, in most aspects, a typical H5 chondrite, except the unusually low K content of ~340 ppm. A cosmic ray exposure of 9.7 Ma is inferred from the cosmogenic noble gas records. Activities of eleven cosmogenic radionuclides were measured. 26Al and 22Na activities as well as the 22Na/26Al activity ratio are close to the values expected on the basis of solar modulation of galactic cosmic rays. The low 60Co activity (<1 dpm/kg) is indicative of a small preatmospheric size of the meteorite. Cosmic ray heavy nuclei track densities in olivine grains range from ~106 cm?2 in samples from the largest fragment to approximately (4–9) × 105 cm?2 in one of the smaller fragments. The combined track, radionuclide, and noble gas data suggest a preatmospheric radius of ~20 cm for the Dergaon meteorite. 相似文献
115.
Two-dimensional (2D) compressible magnetohydrodynamic simulations are performed to explore the idea that the asymmetric reconnection between newly emerging intranetwork magnetic field flux and pre-existing network flux causes the explosive events in the solar atmosphere. The dependence of the reconnection rate as a function of time on the density and temperature of the emerging flux are investigated. For a Lundquist number of L
u= 5000 we find that the tearing mode instability can lead to the formation and growth of small magnetic islands. Depending on the temperature and density ratio of the emerging plasma, the magnetic island can be lifted upward and convected out of the top boundary, or is suppressed downward and convected out of the top boundary, or is suppressed downward nad submerged below the bottom boundary. The motions of the magnetic islands with different direction are accompanied respectively with upward or downward high velocity flow which might be associated with the red- and blue-shifted components detected in the explosive events. 相似文献
116.
After briefly recalling basic facts about the metallicity gradients inspiral galaxies, we summarize two recent N-body gas-dynamical modelsthat implement a non-instantaneous gas recycling and point our their potential for an improved treatment of the chemical evolution ingalactic disks. 相似文献
117.
The observed properties of the long-period comet system, and its periodic disturbance by galactic forces manifesting as terrestrial impact episodes, may be indicative of a comet capture/escape cycle as the Solar System orbits the Galaxy. A mean number density of comets in molecular clouds of ~10?1±1 AU?3 is implied. This is sufficient to deplete metals from the gaseous component of the interstellar medium, as observed, but leads to the problem of how stars are formed nevertheless with solar metal abundances. Formation of comets prior to stars in dense systems of near-zero energy may be indicated, and isotope signatures in cometary particles may be diagnostic of conditions in young spiral arm material. 相似文献
118.
119.
120.
Spectra of the central core and surrounding coma of Comet IRAS-Araki-Alcock (1983d) were obtained at 8–13 μm on 11 May and 2–4 μm on 12 May 1983. Spatially resolved measurements at 10 μm with a 4-arcsec beam showed that the central core was more than 100 times brighter than the inner coma only 8 arcsec away; for radially outflowing dust, the brightness ratio would be a factor of 8. The observations of the central core are consistent with direct detection of a nucleus having a radius of approximately 5 km. The temperature of the sunlit hemisphere was > 300 K. Spectra of the core are featureless, while spectra of the coma suggest weak silicate emission. The spectra show no evidence for icy grains. The dust producton rate on 11.4 May was ~ 105 g/sec, assuming that the gas flux from the dust-producing areas on the nucleus was ~ 10?5 g/cm2/sec. 相似文献