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241.
P.C. ThomasJ. Joseph B. CarcichJ. Veverka B.E. ClarkJ.F. Bell III A.W. ByrdR. Chomko M. RobinsonS. Murchie L. ProckterA. Cheng N. IzenbergM. Malin C. ChapmanL.A. McFadden R. KirkM. Gaffey P.G. Lucey 《Icarus》2002,155(1):18-37
Stereogrammetric measurement of the shape of Eros using images obtained by NEAR's Multispectral Imager provides a survey of the major topographic features and slope processes on this asteroid. This curved asteroid has radii ranging from 3.1 to 17.7 km and a volume of 2535±20 km3. The center of figure is within 52 m of the center of mass provided by the Navigation team; this minimal difference suggests that there are only modest variations in density or porosity within the asteroid. Three large depressions 10, 8, and 5.3 km across represent different stages of degradation of large impact craters. Slopes on horizontal scales of ∼300 m are nearly all less than 35°, although locally scarps are much steeper. The area distribution of slopes is similar to those on Ida, Phobos, and Deimos. Regions that have slopes greater than 25° have distinct brighter markings and have fewer large ejecta blocks than do flatter areas. The albedo patterns that suggest downslope transport of regolith have sharper boundaries than those on Phobos, Deimos, and Gaspra. The morphology of the albedo patterns, their lack of discrete sources, and their concentration on steeper slopes suggest transport mechanisms different from those on the previously well-observed small bodies, perhaps due to a reduced relative effectiveness of impact gardening on Eros. Regolith is also transported in talus cones and in connected, sinuous paths extending as much as 2 km, with some evident as relatively darker material. Talus material in at least one area is a discrete superposed unit, a feature not resolved on other small bodies. Flat-floored craters that apparently contain ponded material also suggest discrete units that are not well mixed by impacts. 相似文献
242.
Alfred N. N. Muzuka 《Journal of Earth System Science》2000,109(4):481-489
The stable isotope compositions of sedimentary organic carbon and content of organic carbon for sediment cores recovered at
two sites (sites 724C and 725C) during Ocean Drilling Program (ODP) Leg. 117 on the Oman continental margin are used to document
variability of the monsoon winds for the past 350 ka. Although both sites have a mean δ13C value of -20.1‰, three zones depleted in13C are observable at site 724C during isotope stages 3, 8 and 10, while only one zone is recognizable at site 725C. Increased
coastal upwelling during isotope stage 3 owing to intense SW monsoon winds resulted in higher concentration of CO2 in the water column causing the formation of organic matter that was depleted in13C. The other two zones deposited during oxygen isotope stages 8 and 10, which are also characterized by low values of organic
carbon, nitrogen and C/N ratios, could be attributed to the dilution by terrestrial material derived from paleosol by transported
by northwester lies. Because of utilization of13C enriched dissolved CO2 during the last glacial maximum Holocene sedimentary organic materials are depleted in13C relative to the the fomer. The content of residues organic carbon (ROC) is higher at site 724C (with an average of 2.3 ±
1.2%) relative to site 725C, which averages to 0.9 ± 0.4% probably because of differences in the degree of preservation. Organic
material deposited at site 725C has undergone more degradation relative to site 724C as reflected by a systematic downcore
decrease in13C resulting from a loss of13C enriched organic compounds. Owing to lack of good chronology at site 725C, a zone that is characterized by low δ13C values it could not be correlated with the other three zones observed at Site 724C. 相似文献
243.
Received: 15 June 1999 · Accepted: 30 August 1999 相似文献
244.
Cochran Anita Veverka Joseph Bell James Belton Michael Benkhoff Johannes Benkhoff Andrew Clark Benton Feldman Paul Kissel Jochen Mahaffy Paul Malin Michael Murchie Scott Neimann Hasso Owen Tobias Robinson Mark Schwehm Gerhard Squyres Steve Thomas Peter Whipple Fred Yeomans Donald 《Earth, Moon, and Planets》2000,89(1-4):289-300
In 1997, the COmet Nucleus TOUR (CONTOUR) was selected byNASA for a new start as part of the Discovery line. In this paper, we review the status of the mission, the mission timeline and the instruments to be flown. Detail is given of the science goals and how they are to be accomplished. 相似文献
245.
Precise measurements of heliographic position of solar filaments were used for determination of the proper motion of solar
filaments on the time-scale of days. The filaments have a tendency to make a shaking or waving of the external structure and
to make a general movement of whole filament body, coinciding with the transport of the magnetic flux in the photosphere.
The velocity scatter of individual measured points is about one order higher than the accuracy of measurements. 相似文献
246.
Eric F. Bell † W. J. G. de Blok ‡ 《Monthly notices of the Royal Astronomical Society》2000,311(4):668-672
We have assessed the significance of Tully and Verheijen's bimodal Ursa Major Cluster spiral galaxy near-infrared surface brightness distribution, focusing on whether this bimodality is simply an artefact of small number statistics. A Kolmogorov–Smirnov style of significance test shows that the total distribution is fairly represented by a single-peaked distribution, but that their isolated galaxy subsample (with no significant neighbours within a projected distance of ∼80 kpc) is bimodal at the 96 per cent level. We have also investigated the assumptions underlying the isolated galaxy surface brightness distribution, finding that the (often large) inclination corrections used in the construction of this distribution reduce the significance of the bimodality. We conclude that the Ursa Major Cluster data set is insufficient to establish the presence of a bimodal near-infrared surface brightness distribution: an independent sample of ∼100 isolated, low-inclination galaxies is required to establish bimodality at the 99 per cent level. 相似文献
247.
One dimensional numerical results of the non-linear interaction between cosmic rays and a magnetic field are presented. These
show that cosmic ray streaming drives large amplitude Alfvénic waves. The cosmic ray streaming energy is very efficiently
transfered to the perturbed magnetic field of the Alfvén waves. Thus a magnetic field of interstellar values, assumed in models
of supernova remnant blast wave acceleration, would not be appropriate in the region of the shock. The increased magnetic
field reduces the acceleration time and so increases the maximum cosmic ray energy, which may provide a simple and elegant
resolution to the highest energy galactic cosmic ray problem were the cosmic rays themselves provide the fields necessary
for their acceleration.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
248.
Eric F. Bell Richard G. Bower 《Monthly notices of the Royal Astronomical Society》2000,319(1):235-254
We have constructed a family of simple models for spiral galaxy evolution to allow us to investigate observational trends in star formation history with galaxy parameters. The models are used to generate broad-band colours from which ages and metallicities are derived in the same way as the data. We generate a grid of model galaxies and select only those that lie in regions of parameter space covered by the sample. The data are consistent with the proposition that the star formation history of a region within a galaxy depends primarily on the local surface density of the gas but that one or two additional ingredients are required to explain the observational data fully. The observed age gradients appear steeper than those produced by the density dependent star formation law, indicating that the star formation law or infall history must vary with galactocentric radius. Furthermore, the metallicity–magnitude and age–magnitude correlations are not reproduced by a local density dependence alone. These correlations require one or both of the following: (i) a combination of mass dependent infall and metal enriched outflow, or (ii) a mass dependent galaxy formation epoch. Distinguishing these possibilities on the basis of current data is extremely difficult. 相似文献
249.
E. Charro K. L. Bell I. Martin A. Hibbert 《Monthly notices of the Royal Astronomical Society》2000,313(2):247-251
An 11-state R -matrix calculation is performed to obtain cross-sections for the photoionization of the 1s2 2s 2 S ground states of N v and O vi . For both ions at low photon energies, corresponding solely to the 2s photoionization cross-section, excellent agreement is found with previous calculations. For higher photon energies, permitting inner-shell photoionization of the 1s electron, excellent accord is found with the recent calculation of Nahar for O vi , whilst for N v resonance structure is found which previous calculations omitted. 相似文献
250.
Charles I. Scaife Jonathan M. Duncan Laurence Lin Christina Tague Colin D. Bell Lawrence Band 《水文研究》2021,35(9):e14313
The spatial variation of soil moisture over very small areas (<100 m2) can have nonlinear impacts on cycling and flux rates resulting in bias if it is not considered, but measuring this variation is difficult over extensive temporal and spatial scales. Most studies examining spatial variation of soil moisture were conducted at hillslope (0.01 km2) to multi-catchment spatial scales (1000 km2). They found the greatest variation at mid wetness levels and the smallest variation at wet and dry wetness levels forming a concave down relationship. There is growing evidence that concave down relationships formed between spatial variation of soil moisture and average soil moisture are consistent across spatial scales spanning several orders of magnitude, but more research is needed at very small, plot scales (<100 m2). The goal of this study was to characterise spatial variation in shallow soil moisture at the plot scale by relating the mean of measurements collected in a plot to the standard deviation (SD). We combined data from a previous study with thousands of new soil moisture measurements from 212 plots in eight catchments distributed across the US Mid-Atlantic Region to (1) test for a generalisable mean–SD relationship at plot scales, (2) characterise how landcover, land use, season, and hillslope position contribute to differences in mean–SD relationships, and (3) use these generalised mean–SD relationships to quantify their impacts on catchment scale nitrification and denitrification potential. Our study found that 98% of all measurements formed a generalised mean–SD relationship like those observed at hillslope and catchment spatial scales. The remaining 2% of data comprised a mean–SD relationship with greater spatial variation that originated from two riparian plots reported in a previous study. Incorporating the generalised mean–SD relationship into estimates of nitrification and denitrification potential revealed strong bias that was even greater when incorporating mean–SD observations from the two riparian plots with significantly greater spatial variation. 相似文献