全文获取类型
收费全文 | 2709篇 |
免费 | 125篇 |
国内免费 | 33篇 |
专业分类
测绘学 | 112篇 |
大气科学 | 230篇 |
地球物理 | 620篇 |
地质学 | 995篇 |
海洋学 | 227篇 |
天文学 | 407篇 |
综合类 | 14篇 |
自然地理 | 262篇 |
出版年
2023年 | 15篇 |
2022年 | 23篇 |
2021年 | 66篇 |
2020年 | 89篇 |
2019年 | 79篇 |
2018年 | 91篇 |
2017年 | 108篇 |
2016年 | 130篇 |
2015年 | 94篇 |
2014年 | 116篇 |
2013年 | 165篇 |
2012年 | 128篇 |
2011年 | 173篇 |
2010年 | 150篇 |
2009年 | 154篇 |
2008年 | 138篇 |
2007年 | 101篇 |
2006年 | 96篇 |
2005年 | 97篇 |
2004年 | 82篇 |
2003年 | 80篇 |
2002年 | 66篇 |
2001年 | 51篇 |
2000年 | 45篇 |
1999年 | 33篇 |
1998年 | 27篇 |
1997年 | 33篇 |
1996年 | 30篇 |
1995年 | 28篇 |
1994年 | 14篇 |
1993年 | 16篇 |
1992年 | 22篇 |
1991年 | 18篇 |
1990年 | 22篇 |
1989年 | 12篇 |
1988年 | 15篇 |
1987年 | 19篇 |
1986年 | 8篇 |
1985年 | 21篇 |
1984年 | 26篇 |
1983年 | 20篇 |
1982年 | 19篇 |
1981年 | 22篇 |
1980年 | 14篇 |
1979年 | 9篇 |
1978年 | 12篇 |
1977年 | 14篇 |
1976年 | 10篇 |
1975年 | 14篇 |
1974年 | 17篇 |
排序方式: 共有2867条查询结果,搜索用时 0 毫秒
91.
D. Heath Jones Will Saunders Matthew Colless Mike A. Read Quentin A. Parker Fred G. Watson Lachlan A. Campbell Daniel Burkey Thomas Mauch Lesa Moore Malcolm Hartley Paul Cass Dionne James Ken Russell Kristin Fiegert John Dawe John Huchra Tom Jarrett Ofer Lahav John Lucey Gary A. Mamon Dominique Proust Elaine M. Sadler Ken-ichi Wakamatsu 《Monthly notices of the Royal Astronomical Society》2004,355(3):747-763
92.
Mininni Pablo D. López Fuentes Marcelo Mandrini Cristina H. Gómez Daniel O. 《Solar physics》2004,219(2):367-378
We present a bi-orthogonal decomposition of the temporal and latitudinal distribution of solar magnetic fields from synoptic
magnetograms. Results are compared with a similar decomposition of the distribution of sunspots since 1874. We show that the
butterfly diagrams can be interpreted as the result of approximately constant amplitudes and phases of two oscillations with
periods close to 22 years. A clear periodicity of 7 years can also be identified in the most energetic modes of both spatio-temporal
series. These results can be used to obtain relevant information concerning the physics of the solar dynamo. 相似文献
93.
94.
Ionization fronts, the sharp radiation fronts behind which H/He ionizing photons from massive stars and galaxies propagate
through space, were ubiquitous in the universe from its earliest times. The cosmic dark ages ended with the formation of the
first primeval stars and galaxies a few hundred Myr after the Big Bang. Numerical simulations suggest that stars in this era
were very massive, 25–500 solar masses, with H(II) regions of up to 30,000 light-years in diameter. We present three-dimensional
radiation hydrodynamical calculations that reveal that the I-fronts of the first stars and galaxies were prone to violent
instabilities, enhancing the escape of UV photons into the early intergalactic medium (IGM) and forming clumpy media in which
supernovae later exploded. The enrichment of such clumps with metals by the first supernovae may have led to the prompt formation
of a second generation of low-mass stars, profoundly transforming the nature of the first protogalaxies. Cosmological radiation
hydrodynamics is unique because ionizing photons coupled strongly to both gas flows and primordial chemistry at early epochs,
introducing a hierarchy of disparate characteristic timescales whose relative magnitudes can vary greatly throughout a given
calculation. We describe the adaptive multistep integration scheme we have developed for the self-consistent transport of
both cosmological and galactic ionization fronts. 相似文献
95.
Michael P. Callahan Aaron S. Burton Jamie E. Elsila Eleni M. Baker Karen E. Smith Daniel P. Glavin Jason P. Dworkin 《Meteoritics & planetary science》2013,48(5):786-795
The investigation into whether Mars contains signatures of past or present life is of great interest to science and society. Amino acids and nucleobases are compounds that are essential for all known life on Earth and are excellent target molecules in the search for potential Martian biomarkers or prebiotic chemistry. Martian meteorites represent the only samples from Mars that can be studied directly in the laboratory on Earth. Here, we analyzed the amino acid and nucleobase content of the shergottite Roberts Massif (RBT) 04262 using liquid chromatography‐mass spectrometry. We did not detect any nucleobases above our detection limit in formic acid extracts; however, we did measure a suite of protein and nonprotein amino acids in hot‐water extracts with high relative abundances of β‐alanine and γ‐amino‐n‐butyric acid. The presence of only low (to absent) levels of several proteinogenic amino acids and a lack of nucleobases suggest that this meteorite fragment is fairly uncontaminated with respect to these common biological compounds. The distribution of straight‐chained amine‐terminal n‐ω‐amino acids in RBT 04262 resembled those previously measured in thermally altered carbonaceous meteorites (Burton et al. 2012; Chan et al. 2012). A carbon isotope ratio of ?24‰ ± 6‰ for β‐alanine in RBT 04262 is in the range of reduced organic carbon previously measured in Martian meteorites (Steele et al. 2012). The presence of n‐ω‐amino acids may be due to a high temperature Fischer‐Tropsch‐type synthesis during igneous processing on Mars or impact ejection of the meteorites from Mars, but more experimental data are needed to support these hypotheses. 相似文献
96.
We present first results of our simulations of magnetic fields in the formation of single and binary stars using a recently
developed method for incorporating Magnetohydrodynamics (MHD) into the Smoothed Particle Hydrodynamics (SPH) method. An overview
of the method is presented before discussing the effect of magnetic fields on the formation of circumstellar discs around
young stars. We find that the presence of magnetic fields during the disc formation process can lead to significantly smaller
and less massive discs which are much less prone to gravitational instability. Similarly in the case of binary star formation
we find that magnetic fields, overall, suppress fragmentation. However these effects are found to be largely driven by magnetic
pressure. The relative importance of magnetic tension is dependent on the orientation of the field with respect to the rotation
axis, but can, with the right orientation, lead to a dilution of the magnetic pressure-driven suppression of fragmentation. 相似文献
97.
Pasachoff Jay M. Babcock Bryce A. Russell Kevin D. Seaton Daniel B. 《Solar physics》2002,207(2):241-257
As a part of a study of the cause of solar coronal heating, we searched for high-frequency (1 Hz) intensity oscillations in coronal loops in the [Fexiv] coronal green line. We summarize results from observations made at the 11 August 1999 total solar eclipse from Râmnicu-Vâlcea, Romania, through clear skies. We discuss the image reduction and analysis through two simultaneous series of coronal CCD images digitized at 10 Hz for a total time of about 140 s. One series of images was taken through a 3.6 Å filter isolating the 5303 Å[Fexiv] coronal green line and the other through a 100 Å filter in the nearby K-corona continuum. Previous observations, described in Pasachoff et al. (2000), showed no evidence for oscillations in the [Fexiv] green line at a level greater than 2% of coronal intensity. We describe several improvements made over the 1998 eclipse that led to increased image clarity and sensitivity. The corona was brighter in 1999 with the solar maximum, further improving the data. We use Fourier analysis to search in the [Fexiv] channel for intensity oscillations in loops at the base of the corona. Such oscillations in the 1-Hz range are predicted as a result of density fluctuations from the resonant absorption of MHD waves. The dissipation of a significant amount of mechanical energy from the photosphere into the corona through this mechanism could provide sufficient energy to heat the corona. A Monte Carlo model of the data suggests the presence of enhanced power, particularly in the 0.75–1.0 Hz range, and we conclude that MHD waves remain a viable method for coronal heating. 相似文献
98.
In addition to the detection of an asteroid moon or a binary asteroid, the knowledge of the satellite’s true orbit is of high importance to derive fundamental physical parameters of the binary system such as its mass and to shed light on its possible formation history and dynamical evolution (prograde/retrograde orbit, large/small eccentricity or inclination, etc.). A new methodology for preliminary orbit determination of binary asteroids – and visual binaries in general – is proposed. It is based on Thiele–Innes method combined with a ‘trial and error’ Monte-Carlo technique. This method provides the full set of solutions (bundle of orbits, with the 7 orbital elements) even for a reduced number of observations. The mass is a direct by-product of this orbit determination, from which one can next infer the bulk-density and porosity. In addition to the bundle of orbits, the method provides the marginal probability densities of the foreseen parameters. Such error analysis – since it avoids linear approximation – can be of importance for the prediction of the satellite’s position in the plane-of-sky during future stellar occultations or subsequent observations, but also for the analysis of the orbit’s secular evolution. After briefly describing the method, we present the algorithm and its application to some practical cases, with particular emphasis on asteroids binaries and applications on orbital evolution. 相似文献
99.
100.
Using the continuation method we prove that the circular and the elliptic symmetric periodic orbits of the planar rotating
Kepler problem can be continued into periodic orbits of the planar collision restricted 3-body problem. Additionally, we also
continue to this restricted problem the so called “comet orbits”.
An erratum to this article can be found at 相似文献