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911.
Spectropolarimetry of core collapse supernovae has shown that they are asymmetric and often bi-polar. This raises the issue
of whether some jet-like phenomena are at work involving rotation and magnetic fields. We summarize the importance of the
magnetorotational instability for the core collapse problem and sketch some of the effects that large magnetic fields, ~1015 G, may have on the physics of the supernova explosion. 相似文献
912.
Recent measurements of the high-energy, omni-directional electron environment by the Galileo spacecraft Energetic Particle Detector (EPD) have been analyzed in the range from 7 to 28 Jupiter radii. 10-min averages of these data between Jupiter orbit insertion in 1995 to the end of the mission have been analyzed to provide estimates of the electron differential fluxes at 1.5, 2, and 11 MeV in the jovian equatorial plane as a function of radial distance. These data provide a long term picture of the variations in the high-energy electron environment over the ∼8 years of the Galileo mission. This paper reviews those measurements and the statistics associated with them for the 8 year period. In general, the data variations are well behaved with variations being within a factor of ∼2 of a median value at a given distance from Jupiter. These results are analyzed in detail and the orbit variations discussed in the context of the overall data set. The results of this analysis of the long-term statistical variations in high-energy electron fluxes are directly applicable to models that estimate the effects of the radiation environment on Jupiter's moons and their atmospheres as they permit estimates of the possible range of radiation effects that might be expected. 相似文献
913.
914.
Arch M. Reid Richard J. Williams Everett K. Gibson Kurt Fredriksson 《Meteoritics & planetary science》1974,9(1):35-45
Vigarano, a type 3 carbonaceous chondrite, contains a chondrule composed of highly refractory Ca and Al rich glass with minor spinel. The chondrule formed from material similar to the Ca, Al, Ti-rich aggregates that are common in Vigarano and other type 3 chondrites and formation of these refractory aggregates must predate formation of some Vigarano chondrules. Experiments with synthetic analogues and a comparison with studies in the system CaO-MgO-Al2O3-SiO2 indicate a temperature for formation of the chondrule at or above 1700 °C followed by very rapid cooling. 相似文献
915.
P. Kenneth Seidelmann B. A. Archinal M. F. A’hearn A. Conrad G. J. Consolmagno D. Hestroffer J. L. Hilton G. A. Krasinsky G. Neumann J. Oberst P. Stooke E. F. Tedesco D. J. Tholen P. C. Thomas I. P. Williams 《Celestial Mechanics and Dynamical Astronomy》2007,98(3):155-180
Every three years the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements revises tables giving the
directions of the poles of rotation and the prime meridians of the planets, satellites, minor planets, and comets. This report
introduces improved values for the pole and rotation rate of Pluto, Charon, and Phoebe, the pole of Jupiter, the sizes and
shapes of Saturn satellites and Charon, and the poles, rotation rates, and sizes of some minor planets and comets. A high
precision realization for the pole and rotation rate of the Moon is provided. The expression for the Sun’s rotation has been
changed to be consistent with the planets and to account for light travel time 相似文献
916.
917.
918.
The effects of the production on dust grain surfaces of molecular hydrogen in excited states have been investigated. On the
assumption that all of the H2 formed on the surface of grains has a sufficient level of excitation too vercome the energy barriers in the formation reactions
for the important OH and CH+ radicals, we consider the likely abundances of excited H2 (H2
*), OH and CH+ in various situations. Two different models are employed; the first links the H2
* abundance directly to that of H2 using a steady-state approximation, whilst the second considers the time-dependence of H2
*. The second model is applied to gas that has been subjected to a strong isothermal shock (specifically, the shock-induced
collapse of a diffuse cloud), which results in an extreme (high density, high atomic hydrogen abundance) environment. In general,
it is found that the presence of the excited H2 has only marginal effects on the chemistry of interstellar clouds. However, in the isothermal shock model, the abundances
of CH+ are significantly enhanced, but only on short timescales, whilst the effects on the OH abundances are smaller, but last longer.
We conclude that other than in such exceptional environments there are no obvious chemical signatures of the formation of
H2
*.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
919.
920.
Julia?A.?BaldwinEmail author Samuel?A.?Bowring Michael?L.?Williams Ian?S.?Williams 《Contributions to Mineralogy and Petrology》2004,147(5):528-548
Eclogite occurs within the southern domain of the East Athabasca mylonite triangle in northern Saskatchewan. Situated at the boundary between the Archean Rae and Hearne Provinces of the western Canadian Shield, the East Athabasca mylonite triangle is a fundamental exposure of the ~3,000-km-long Snowbird tectonic zone. The eclogite occurs in association with a variety of lower crustal high-pressure granulites that record a complex metamorphic history from 2.6 to 1.9 Ga. Temperatures of the eclogite facies metamorphism are constrained by garnet-clinopyroxene exchange thermometry at 920–1,000 °C. Minimum pressure conditions are recorded by the jadeite+quartz=albite geobarometer at 1.8–2.0 GPa. A near-isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene-plagioclase and pargasite-plagioclase. U-Pb IDTIMS zircon geochronology of the eclogite yields a weighted mean 207Pb/206Pb date of 1,904.0±0.3 Ma, which we interpret as the time of peak eclogite facies metamorphism. SHRIMP in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207Pb/206Pb date of 1,905±19 Ma. Inclusion suites of high-pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism. Zircon from one eclogite sample has older cores that are 2.54 Ga, which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith. U-Pb rutile data indicate slow cooling at ~1°C/Ma below ~500 °C from 1.88 to 1.85 Ga. The formation and exhumation of the eclogites at ca.1.9 Ga has important implications for the tectonic significance of the Snowbird tectonic zone during the Paleoproterozoic. The eclogites described here are consistent with transport of continental crust to mantle depths during the Paleoproterozoic, followed by rapid buoyancy-driven exhumation to normal lower crustal depths.Editorial responsibility: T.L. Grove 相似文献