全文获取类型
收费全文 | 37113篇 |
免费 | 732篇 |
国内免费 | 578篇 |
专业分类
测绘学 | 938篇 |
大气科学 | 3093篇 |
地球物理 | 7455篇 |
地质学 | 12283篇 |
海洋学 | 3393篇 |
天文学 | 8743篇 |
综合类 | 121篇 |
自然地理 | 2397篇 |
出版年
2021年 | 199篇 |
2020年 | 265篇 |
2019年 | 258篇 |
2018年 | 620篇 |
2017年 | 569篇 |
2016年 | 899篇 |
2015年 | 632篇 |
2014年 | 855篇 |
2013年 | 1901篇 |
2012年 | 1040篇 |
2011年 | 1470篇 |
2010年 | 1205篇 |
2009年 | 1791篇 |
2008年 | 1581篇 |
2007年 | 1506篇 |
2006年 | 1426篇 |
2005年 | 1292篇 |
2004年 | 1212篇 |
2003年 | 1176篇 |
2002年 | 1100篇 |
2001年 | 984篇 |
2000年 | 988篇 |
1999年 | 922篇 |
1998年 | 842篇 |
1997年 | 840篇 |
1996年 | 723篇 |
1995年 | 644篇 |
1994年 | 564篇 |
1993年 | 517篇 |
1992年 | 507篇 |
1991年 | 480篇 |
1990年 | 477篇 |
1989年 | 418篇 |
1988年 | 401篇 |
1987年 | 450篇 |
1986年 | 433篇 |
1985年 | 523篇 |
1984年 | 584篇 |
1983年 | 559篇 |
1982年 | 519篇 |
1981年 | 463篇 |
1980年 | 437篇 |
1979年 | 397篇 |
1978年 | 413篇 |
1977年 | 358篇 |
1976年 | 323篇 |
1975年 | 336篇 |
1974年 | 336篇 |
1973年 | 338篇 |
1972年 | 201篇 |
排序方式: 共有10000条查询结果,搜索用时 351 毫秒
811.
H. C. Bhatt T. Chandrasekhar N. M. Ashok J. N. Desai 《Astrophysics and Space Science》1984,104(2):293-296
Near-infrared photometric and low-resolution spectroscopic observations of Aurigae at two phases during the current eclipse are presented. The eclipse depths are found to be wavelength-independent in the infrared right up to 2.5 m. There is no infrared excess at wavelengths shorter than 2.5 m. The light absorbing particles in the eclipsing body must be larger than 10 m in size. 相似文献
812.
We present the main results obtained from the coordinated observations of solar radio noise storms organized by C.E.S.R.A. in May 1981. They concern the structure and polarization of radio sources, and the relation with the photospheric magnetic field. A model of coronal magnetic field, accounting for the observations, is briefly discussed. 相似文献
813.
In this paper, a theoretical investigation is undertaken of the group travel-time for the Sun's pulses to travel from the source of the pulse to the solar surface. For mathematical simplicity, we consider a simple ionized model of the Sun that includes the thermal effect and rotation with uniform angular velocity. The expression for the group travel time gives two terms: the term arising from the linear theory which varies inversely to (2-)1/2 and the term introduced by the thermal effect which is inversely proportional to (2-)7/2. The thermal effect variation has been shown and an estimation of the temperature of the medium has been made. Furthermore, the velocity distribution and the amplitude of the magnetic field of the wave, arising from the damping of ionized particles, have been calculated. 相似文献
814.
Close binaries can evolve through various ways of interaction into compact objects (white dwarfs, neutron stars, black holes). Massive binary systems (mass of the primaryM
1 larger than 14 to 15M
0) are expected to leave, after the first stage of mass transfer a compact component orbiting a massive star. These systems evolve during subsequent stages into massive X-ray binaries. Systems with initial large periode evolve into Be X-ray binaries.Low mass X-ray sources are probably descendants of lower mass stars, and various channels for their production are indicated. The evolution of massive close binaries is examined in detail and different X-ray stages are discussed. It is argued that a first X-ray stage is followed by a reverse extensive mass transfer, leading to systems like SS 433, Cir X1. During further evolution these systems would become Wolf-Rayet runaways. Due to spiral in these system would then further evolve into ultra short X-ray binaries like Cyg X-3.Finally the explosion of the secondary will in most cases disrupt the system. In an exceptional case the system remains bound, leading to binary pulsars like PSR 1913+16. In such systems the orbit will shrink due to gravitational radiation and finally the two neutron stars will coalesce. It is argued that the millisecond pulsar PSR 1937+214 could be formed in this way.A complete scheme starting from two massive ZAMS stars, ending with a millisecond pulsar is presented.Paper presented at the Lembang-Bamberg IAU Colloquium No. 80 on Double Stars: Physical Properties and Generic Relations, held at Bandung, Indonesia 3–7 June, 1983. 相似文献
815.
- . . , . , . 相似文献
816.
An approximate form of the Boltzmann equation has been used to obtain local ionization rates due to the absorption of galactic cosmic rays in the Jovian atmosphere. It is shown that the muon flux component of the cosmic ray-induced cascade may be especially importannt in ionizing the atmosphere at levels where the total number density exceeds 1019 cm?3 (well below the ionospheric layers produced by solar euv). A model containing both positive and negative ion reactions has been employed to compute equilibrium electron and ion number densities. Peak electron number densities on the order of 103 cm?3 may be expected even at relatively low magnetic latitudes. The dominant positive ions are NH4+ and CnHm+ cluster ions, with n ? 2; it is suggested that the absorption of galactic cosmic ray energy at such relatively high pressures in the Jovian atmosphere and the subsequent chemical reactions may be instrumental in the local formation of complex hydrocarbons. 相似文献
817.
Djermaia is a unique case among gas-rich chondrites. Twelve light xenoliths studied so far in the two available stones show unequivocal evidences of large preirradiation effects, which stand out most clearly when both cosmic-ray track and spallogenic rare-gas data are considered and compared to data from chondrites with a simple radiation history. Each of the two stones studied displays a specific xenolith population with distinct morphological and preirradiation characteristics. The first population consists of rounded and more- or less-shocked xenoliths which show evidences of short solar-flare irradiation (<1 my) and a longer residence under heavier shielding (~50 g/cm2). The second population shows neither clear indication of surface erosion nor evidence of solar-flare effects, and its preirradiation occured in deep-seated locations (~150 g/cm2).The size distribution of xenoliths is also different for the two populations, indicative of more active reworking processes in the top ~20 cm of the Djermaia regolith than at a depth of ~50 cm. The amount of 21Nesp accumulated during the preirradiation stage is substantial (25–60% of total 21Nesp), corresponding to a formal preexposure time of ~15 my for both stones, assuming static irradiation conditions. These results give an insight into the dynamics of the upper levels of the H-asteroid regolith, which appears to be in good agreement with the model of Housen, Wilkening, Chapman, and Greenberg (Icarus, 1979, 39, 317–351). 相似文献
818.
Data on interstellar extinction are interpreted to imply an identification of interstellar grains with naturally freeze-dried bacteria and algae. The total mass of such bacterial and algal cells in the galaxy is enormous, 1040 g. The identification is based on Mie scattering calculations for an experimentally determined size distribution of bacteria. Agreement between our model calculations and astronomical data is remarkably precise over the wavelength intervals µ–1 < ;–2 < 1.94µ–1 and 2.5µ–1 < ;–1 < 3.0 ;–1. Over the more restricted waveband 4000–5000 Å an excess interstellar absorption is found which is in uncannily close agreement with the absorption properties of phytoplankton pigments. The strongest of the diffuse interstellar bands are provisionally assingned to carotenoid-chlorophyll pigment complexes such as exist in algae and pigmented bacteria. The 2200 Å interstellar absorption feature could be due to degraded cellulose strands which form spherical graphitic particles, but could equally well be due to protein-lipid-nucleic acid complexes in bacteria and viruses. Interstellar extinction at wavelengths <1800 Å could be due to scattering by virus particles. 相似文献
819.
C. B. Cosmovici M. Inguscio F. Strafella F. Strumia 《Astrophysics and Space Science》1979,60(2):475-491
The search for extraterrestrial and, in particular, interstellar molecules has suddenly increased in the last ten years after the discovery of many complex organic molecules by radioastronomy. Since some very interesting lines are not detectable in the radio region, but only in the middle and far infrared, heterodyne techniques seem to be a powerful tool for interstellar line detection in the submillimetric region and the detection of CO at 870 in Orion (Philipps and Huggins, 1977) represents an encouraging success for further efforts in this new field of research. In this work the possibility of observing interstellar lines at heterodyne laser wavelengths already available in laboratory has been considered. We also computed relative intensities for some representative rotational lines of interstellar molecules at three typical temperatures (10, 20, 50 K) assuming thermodynamic equilibrium and optically thin regions. The results achieved with heterodyne laser techniques developed in the laboratory are discussed here in order to study the possibility for ground and space astronomical observations. 相似文献
820.
Pluto and the chaotic satellite system of Neptune may have originated from a single encounter of Neptune with a massive solar system body. A series of numerical experiments has been carried out to try to set limits on the circumstances of such an encounter. These experiments show that orbits very much like those of Pluto, Triton, and Nereid can result from a single close encounter of such a body with Neptune. The implied mass range and encounter velocities limit the source of the encountering body to a former trans-Neptunian planet in the 2- to 5-Earth-mass range. 相似文献