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991.
David J. Axon A. Marconi F.D. Macchetto A. Capetti Andrew Robinson 《Astrophysics and Space Science》1997,248(1-2):69-77
We have obtained HST FOC f/48 long-slit spectroscopy of the central 2 arcseconds of the Narrow Line Region of NGC 1068 between
3500-5400\OA with a spectral resolution of 1.78\OA/pixel. At a spatial scale of 0″.0287 per pixel these data provide an order
of magnitude improvement in resolution over previous ground based spectra and allow us to trace the interaction between the
radio jet and the gas in the NLR. Our results show that, within ±0″.5 of the radio-jet the emission lines are split into two
components whose velocity separation is 1500 km s-1. The emission line structure is reminiscent of that seen previously around
the jet of 3C120. Furthermore, this material enveloping the radio-jet is in a much higher ionization state than that of the
surrounding NLR gas. The highest excitation is coincident with the jet axis where emission in the coronal line of [FeVII]
λ3769\OA is detected but where [OII] λ3727 \OA is depressed. These results imply that we are witnessing a cocoon of hot gas
in expansion around the radio-jet created by its interaction with the gas, and that these shocks are sufficiently fast, at
least ± km s-1, that they are creating localized ionization effects.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
992.
Jean-Pierre Macquart Lucyna Kedziora-Chudczer David Jauncey David Rayner 《Astrophysics and Space Science》2001,277(1-2):135-138
It is found that velocity shear enables the extraction of kinetic energy from the background flow by Dust-Acoustic waves.
It is also shown that the velocity shear leads to the appearance of a new mode of the dust particles collective behaviour, called shear dust vortices.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
993.
David W. Mittlefehldt 《Meteoritics & planetary science》1994,29(2):214-221
Abstract— ALH84001, originally classified as a diogenite, is a coarse-grained, cataclastic, orthopyroxenite meteorite related to the martian (SNC) meteorites. The orthopyroxene is relatively uniform in composition, with a mean composition of Wo3.3En69.4Fs27.3. Minor phases are euhedral to subhedral chromite and interstitial maskelynite, An31.1Ab63.2Or5.7, with accessory augite, Wo42.2En45.1Fs12.7, apatite, pyrite and carbonates, Cc11.5Mg58.0Sd29.4Rd1.1. The pyroxenes and chromites in ALH84001 are similar in composition to these phases in EETA79001 lithology A megacrysts but are more homogeneous. Maskelynite is similar in composition to feldspars in the nakhlites and Chassigny. Two generations of carbonates are present, early (pre-shock) strongly zoned carbonates and late (post-shock) carbonates. The high Ca content of both types of carbonates indicates that they were formed at moderately high temperature, possibly ~700 °C. ALH84001 has a slightly LREE-depleted pattern with La 0.67x and Lu 1.85x CI abundances and with a negative Eu anomaly (Eu/Sm 0.56x CI). The uniform pyroxene composition is unusual for martian meteorites, and suggests that ALH84001 cooled more slowly than did the shergottites, nakhlites or Chassigny. The nearly monomineralic composition, coarse-grain size, homogenous orthopyroxene and chromite compositions, the interstitial maskelynite and apatite, and the REE pattern suggest that ALH84001 is a cumulate orthopyroxenite containing minor trapped, intercumulus material. 相似文献
994.
Abhijit BASU David S. McKAY Richard V. MORRIS Susan J. WENTWORTH 《Meteoritics & planetary science》1996,31(6):777-782
Abstract— We report results of our investigation of the relationship between values of Is/FeO (relative concentration of nanophase Fe0 divided by total FeO content), glass abundance, total Fe content, and degree of digestion of <20 μm clasts for 22 individual agglutinates (250–1000 μm) from the mature Apollo 16 soil 61181 (Is/FeO = 82 units in the <250 μm fraction). Agglutinates are important products of space weathering on the Moon, and they influence spectral observations at visible and near-IR wavelengths. Values of Is/FeO for individual agglutinates (250–1000 μm) within this single soil span a range from 3 to 262 units which is larger than the range observed for all Apollo 16 bulk soils (~0 to 110 units). No correlation was observed between Is/FeO and glass abundance and FeO concentrations for either agglutinitic glass or whole agglutinate particles under investigation. Our results suggest that the variation in Is/FeO for agglutinates from a single soil may be in part a consequence of natural mixing processes on the Moon that produce highly-variable environments (with respect to surface exposure) for agglutinate formation and in part to variable kinetics of reactions in an agglutinate melt, which are influenced by a variety of factors including melt composition, temperature, impactor velocity, and quench rate. We cannot exclude but do not see evidence for other processes including addition of exotic agglutinates, micrometeoritic bombardment into compositionally-diverse microtargets, recycling of agglutinates, preferential melting of very fine soil particles, and production of nanophase Fe0 in amorphous rims of very fine irradiated lunar grains contributing to the observed variation of Is/FeO. 相似文献
995.
David Eichler 《Monthly notices of the Royal Astronomical Society》2002,335(4):883-886
996.
David W. Hughes 《Monthly notices of the Royal Astronomical Society》2001,326(2):515-523
The mass distribution and perihelion distribution of long-period comets are re-assessed. The mass distribution index is found to be 1.598±0.016 , indicating that the distribution is somewhat steeper than was obtained by previous analyses of an amalgam of all the available historical data. The number of long-period comets that have orbital perihelion distances, q , that fall in a specific q to q +d q range is found to be independent of q . It is also noted that the flux of long-period comets to the inner Solar system has remained constant throughout recorded history.
The number of long-period comets, , per 1-au interval of perihelion distance, per year, brighter than H , entering the inner Solar system is found to be given by log10 =−2.607+0.359 H . It is therefore estimated that, for example, about 0.5, 30 and 2000 long-period comets with absolute magnitudes brighter than 0, 5 and 10 respectively pass the Sun on orbits with perihelion distances less than 2.0 au, every century. 相似文献
The number of long-period comets, , per 1-au interval of perihelion distance, per year, brighter than H , entering the inner Solar system is found to be given by log
997.
Relative to those at sub‐bankfull flow, hydraulic conditions at overbank flow, whether in the channel or on the floodplain, are poorly understood. Here, velocity conditions are analysed over an unusually wide range of flows in the arid zone river of Cooper Creek with its complex system of anastomosing channels and large fluctuations in floodplain width. At‐a‐station hydraulic geometry relationships reveal sharp discontinuities in velocity at the inbank–overbank transition, the nature of the discontinuity varying with the degree of flow confinement and the level of channel–floodplain interaction. However, despite inter‐sectional differences, velocities remain modest throughout the flow range in this low‐gradient river, and the large increases in at‐a‐station discharge are principally accommodated by changes in cross‐sectional area. Velocity distribution plots suggest that within‐channel conditions during overbank flow are characterized by a central band of high velocity which penetrates far toward the bed, helping to maintain already deep cross‐sections. Floodplain resistance along Cooper Creek is concentrated at channel bank tops where vegetation density is highest, and the subsequent flow retardation is transmitted across the surface of the channels over distances as large as 50–70 m. The rough floodplain surface affects flood wave transmission, producing significant decreases in wave speeds downstream. The character of the wave‐speed–discharge relationship also changes longitudinally, from log–linear in the upper reaches to nonlinear where the floodplain broadens appreciably. The nonlinear form is similar in several respects to relationships proposed for more humid rivers, with flood wave speed reaching an intermediate maximum at about four‐fifths bankfull discharge before decreasing to a minimum at approximately Q2·33. It does not regain the value at the intermediate maximum until the 10 year flood, by which time floodplain depths have become relatively large and broad floodways more active. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
998.
David G. Frey 《Journal of Paleolimnology》1991,6(3):193-197
Silty pond sediments from Wulff Land in northernmost Greenland, radiocarbon dated 10480 BP, yielded many headshields, shells, and ephippia of Daphnia pulex, and headshields, shells, ephippia, and one postabdomen of Alona guttata. This is the first documentation of recognizable headshields and shells of Daphnia in the fossil record. 相似文献
999.
US regional and state migration data from the 1940s–80s, when members of the baby boom generation aged into their years of peak labor force mobility, suggest ways in which changing age composition regulates geographical mobility and interregional migration. Labor supply pressure plays a key role in the dynamics of the national migration system. A “delayed mobility” effect in the 1980s similar to the delayed fertility of the baby boom cohorts appears to be a result of the depressed rates of mobility experienced by members of this generation when they flooded regional labor markets with record numbers of entrants in the 1970s. Recent temporal shifts in age-specific volumes of interregional migration help predict the future pace of migration based upon the projected age distribution of the nation. 相似文献
1000.
The temporary capture of the dust grains in the exterior resonances with planets is studied in the frames of the planar circular three-body problem with Poynting-Robertson (PR) drag. For the Earth and particles ~ 10 Μm the resonances 4/5, 5/6, 6/7, 7/8 are shown to be most effective. The capture is only temporary (of order 105 years) and the position of resonance may be calculated from semi-analytical model using averaged disturbing function. These semi-analytical results are confirmed by numerical integration. For various planet this picture changes as with increasing planetary mass the more exterior resonances become more important. We showed that for Jupiter (at least in the space between Jupiter and Saturn) the resonance 1/2 plays the dominant role. The capture time is here several myr but again eccentricity is evolving to eccentricity e 0 ~ 0.48 of libration point for this resonance. 相似文献