全文获取类型
收费全文 | 81032篇 |
免费 | 1363篇 |
国内免费 | 580篇 |
专业分类
测绘学 | 2121篇 |
大气科学 | 5994篇 |
地球物理 | 15634篇 |
地质学 | 27618篇 |
海洋学 | 7184篇 |
天文学 | 18961篇 |
综合类 | 169篇 |
自然地理 | 5294篇 |
出版年
2021年 | 626篇 |
2020年 | 685篇 |
2019年 | 726篇 |
2018年 | 1613篇 |
2017年 | 1509篇 |
2016年 | 1945篇 |
2015年 | 1179篇 |
2014年 | 1952篇 |
2013年 | 4092篇 |
2012年 | 2019篇 |
2011年 | 2817篇 |
2010年 | 2606篇 |
2009年 | 3462篇 |
2008年 | 3026篇 |
2007年 | 3070篇 |
2006年 | 2881篇 |
2005年 | 2500篇 |
2004年 | 2472篇 |
2003年 | 2327篇 |
2002年 | 2277篇 |
2001年 | 2015篇 |
2000年 | 1873篇 |
1999年 | 1694篇 |
1998年 | 1649篇 |
1997年 | 1647篇 |
1996年 | 1362篇 |
1995年 | 1326篇 |
1994年 | 1257篇 |
1993年 | 1150篇 |
1992年 | 1098篇 |
1991年 | 1048篇 |
1990年 | 1128篇 |
1989年 | 1059篇 |
1988年 | 1014篇 |
1987年 | 1156篇 |
1986年 | 1006篇 |
1985年 | 1272篇 |
1984年 | 1423篇 |
1983年 | 1339篇 |
1982年 | 1262篇 |
1981年 | 1244篇 |
1980年 | 1092篇 |
1979年 | 1051篇 |
1978年 | 1032篇 |
1977年 | 969篇 |
1976年 | 893篇 |
1975年 | 799篇 |
1974年 | 879篇 |
1973年 | 901篇 |
1972年 | 538篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
381.
382.
K. S. Stankevich A. M. Aslanyan V. P. Ivanov R. M. Martirosyan Ye. Terzian 《Astrophysics》2003,46(4):429-433
Radio emission of the historical supernovae remnants Tycho (SNR1572) and Kepler (SNR1604) and evolution of their luminosity are considered. Measurement data of secular luminosity decrease rate, obtained earlier by the authors, were corrected with account of variation in time of the flux density of the reference sources. As a result, it is found that the SNR1604 luminosity at 1667 MHz is weakening with an annual mean rate equal to (0.2 ± 0.07)%. The corresponding rate for SNR1572 is (0.47 ± 0.05)%. Since the radio luminosity evolution, as well as energy densities of magnetic field and relativistic electrons inside SNR1604 and SNR1572 are essentially different, these remnants should be considered as different types of supernovae. Bandiera classified SN1604 as type SNIb or SNII. 相似文献
383.
Dr M. J. Wooller D. L. Swain K. J. Ficken A. D. Q. Agnew F. A. Street-Perrott G. Eglinton 《第四纪科学杂志》2003,18(1):3-15
Woody, subalpine shrubs and grasses currently surround Lake Rutundu, Mount Kenya. Multiple proxies, including carbon isotopes, pollen and grass cuticles, from a 755‐cm‐long core were used to reconstruct the vegetation over the past 38 300 calendar years. Stable carbon‐isotope ratios of total organic carbon and terrestrial biomarkers from the lake sediments imply that the proportion of terrestrial plants using the C4 photosynthetic pathway was greater during the Late Pleistocene than in the Holocene. Pollen data show that grasses were a major constituent of the vegetation throughout the Late Pleistocene and Holocene. The proportion of grass pollen relative to the pollen from other plants was greatest at the last glacial maximum (LGM). Grass cuticles confirm evidence that C4 grass taxa were present at the LGM and that the majority followed the cold‐tolerant NADP‐MEC4 subpathway. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
384.
This paper reports results from an experiment designed to measure the nascent rovibrational population of H2 molecules that have formed through the heterogeneous recombination of H atoms on the surface of cosmic dust analogues under
conditions approaching those of the interstellar medium (ISM). H2 that has formed on a highly oriented pyrolytic graphite (HOPG) surface has been detected, using laser induced resonance-enhanced
multi-photon ionization (REMPI), in the v = 1 (J= 0–3) rovibrational states at surface temperatures of 30 K and 50 K. These excited product molecules display rotational temperatures
significantly higher than the target surface temperature. These first results suggest that a considerable proportion of the
binding energy released on formation of the H2 is deposited in the surface, in addition to internal excitation of the product molecules.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
385.
D. H. Morgan Q. A. Parker Martin Cohen 《Monthly notices of the Royal Astronomical Society》2003,346(3):719-730
We report the discovery of the first probable Galactic [WN] central star of a planetary nebula (CSPN). The planetary nebula candidate was found during our systematic scans of the AAO/UKST Hα Survey of the Milky Way. Subsequent confirmatory spectroscopy of the nebula and central star reveals the remarkable nature of this object. The nebular spectrum shows emission lines with large expansion velocities exceeding 150 km s−1 , suggesting that perhaps the object is not a conventional planetary nebula. The central star itself is very red and is identified as being of the [WN] class, which makes it unique in the Galaxy. A large body of supplementary observational data supports the hypothesis that this object is indeed a planetary nebula and not a Population I Wolf–Rayet star with a ring nebula. 相似文献
386.
387.
388.
M. R. Bate S. H. Lubow G. I. Ogilvie K. A. Miller 《Monthly notices of the Royal Astronomical Society》2003,341(1):213-229
We analyse the non-linear, three-dimensional response of a gaseous, viscous protoplanetary disc to the presence of a planet of mass ranging from 1 Earth mass (1 M⊕ ) to 1 Jupiter mass (1 MJ ) by using the zeus hydrodynamics code. We determine the gas flow pattern, and the accretion and migration rates of the planet. The planet is assumed to be in a fixed circular orbit about the central star. It is also assumed to be able to accrete gas without expansion on the scale of its Roche radius. Only planets with masses M p ≳ 0.1 MJ produce significant perturbations in the surface density of the disc. The flow within the Roche lobe of the planet is fully three-dimensional. Gas streams generally enter the Roche lobe close to the disc mid-plane, but produce much weaker shocks than the streams in two-dimensional models. The streams supply material to a circumplanetary disc that rotates in the same sense as the orbit of the planet. Much of the mass supply to the circumplanetary disc comes from non-coplanar flow. The accretion rate peaks with a planet mass of approximately 0.1 MJ and is highly efficient, occurring at the local viscous rate. The migration time-scales for planets of mass less than 0.1 MJ , based on torques from disc material outside the Roche lobes of the planets, are in excellent agreement with the linear theory of type I (non-gap) migration for three-dimensional discs. The transition from type I to type II (gap) migration is smooth, with changes in migration times of about a factor of 2. Starting with a core which can undergo runaway growth, a planet can gain up to a few MJ with little migration. Planets with final masses of the order of 10 MJ would undergo large migration, which makes formation and survival difficult. 相似文献
389.
We present observations of a sample of Herbig AeBe stars, as well as the FU Orionis object V1057 Cygni. Our K-band (2.2μm)
observations from the Palomar Testbed Interferometer (PTI) used baselines of 110 m and 85 m, resulting in fringe spacings
of ∼4 mas and 5 mas, respectively. Fringes were obtained for the first time on V1057 Cygnias well as V594 Cas. Additional
measurements were made of MWC147, while upper limits to visibility-squared are obtained for MWC297, HD190073, and MWC614.
These measurements are sensitive to the distribution of warm, circumstellar dust in these sources. If the circumstellar infrared
emission comes from warm dust in a disk, the inclination of the disk to the line of sight implies that the observed interferometric
visibilities should depend upon hour angle. Surprisingly, the observations of Millan-Gabet, Schloerb and Traub (2001)(hereafter
MST) did not show significant variation with hour angle. However, limited sampling of angular frequencies on the sky was possible
with the IOTA interferometer, motivating us to study a subset of their objects to further constrain these systems.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
390.