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221.
Seb Oliver Robert G. Mann Ruth Carballo Alberto Franceschini Michael Rowan-Robinson Maria Kontizas Anastasios Dapergolas Evanghelos Kontizas Aprajita Verma David Elbaz Gian Luigi Granato Laura Silva Dimitra Rigopoulou J. Ignacio Gonzalez-Serrano Steve Serjeant reas Efstathiou Paul P. van der Werf 《Monthly notices of the Royal Astronomical Society》2002,332(3):536-548
We present results from a deep mid-infrared survey of the Hubble Deep Field South (HDF-S) region performed at 6.7 and 15 μm with the ISOCAM instrument on board the Infrared Space Observatory ( ISO ). The final map in each band was constructed by the co-addition of four independent rasters, registered using bright sources securely detected in all rasters, with the absolute astrometry being defined by a radio source detected at both 6.7 and 15 μm. We sought detections of bright sources in a circular region of radius 2.5 arcmin at the centre of each map, in a manner that simulations indicated would produce highly reliable and complete source catalogues using simple selection criteria. Merging source lists in the two bands yielded a catalogue of 35 distinct sources, which we calibrated photometrically using photospheric models of late-type stars detected in our data. We present extragalactic source count results in both bands, and discuss the constraints that they impose on models of galaxy evolution, given the volume of space sampled by this galaxy population. 相似文献
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Paolo Padovani †‡ Luigi Costamante Paolo Giommi Gabriele Ghisellini rea Comastri Anna Wolter Laura Maraschi Gianpiero Tagliaferri C. Megan Urry 《Monthly notices of the Royal Astronomical Society》2001,328(3):931-943
We present new BeppoSAX observations of seven BL Lacertae objects selected from the 1-Jy sample plus one additional source. The collected data cover the energy range (observer's frame), reaching ∼50 keV for one source (BL Lac). All sources characterized by a peak in their multifrequency spectra at infrared/optical energies (i.e., of the low-energy peaked BL Lac type, LBL) display a relatively flat X-ray spectrum, which we interpret as inverse Compton emission. Four objects (two-thirds of the LBLs) show some evidence for a low-energy steepening, which is probably due to the synchrotron tail merging into the inverse Compton component around ∼ . If this were generally the case with LBLs, it would explain why the ROSAT spectra of our sources are systematically steeper than the BeppoSAX ones . The broad-band spectral energy distributions fully confirm this picture, and a synchrotron inverse Compton model allows us to derive the physical parameters (intrinsic power, magnetic field, etc.) of our sources. Combining our results with those obtained by BeppoSAX on BL Lacs covering a wide range of synchrotron peak frequency, ν peak , we confirm and clarify the dependence of the X-ray spectral index on ν peak originally found in ROSAT data. 相似文献
224.
Luigi Guzzo 《New Astronomy》1997,2(6):517-532
I critically discuss in a pedagogical and phenomenological way a few crucial tests challenging the recent claims by Pietronero and collaborators that there is no evidence from available galaxy catalogues that the Universe is actually homogeneous above a certain scale. In a series of papers, these authors assert that observations are consistent with a fractal distribution of objects extending to the limit of the present data. I show that while galaxies are indeed clustered in a scale-free (fractal) way on small and intermediate scales, this behaviour does not continue indefinitely. Although the specific wavelength at which the galaxy distribution apparently turns to homogeneity is dangerously close to the size of the largest samples presently available, there are serious hints suggesting that this turnover is real and that its effects are detected in the behaviour of statistical estimators. The most recent claims of a continuing fractal hierarchy up to scales of several hundreds Megaparsecs seem to be ascribable to the use of incomplete samples or to an improper treatment of otherwise high-quality data sets. The fractal perspective, nevertheless, represents a fruitful way to look at the clustering properties of galaxies, when properly coupled to the traditional gravitational instability scenario. In the last part of this paper I will try to clarify, at a very simple level, some of the confusion existing on the actual scaling properties of the galaxy distribution, and discuss how these can provide hints on the evolution of the large-scale structure of the Universe. 相似文献
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D’Aniello Andrea Cozzolino Luca Cimorelli Luigi Della Morte Renata Pianese Domenico 《Natural Hazards》2015,75(2):1403-1433
Natural Hazards - In this paper, it is described the development and the assessment of a 1D numerical procedure for the simulation of debris flow phenomena. The procedure focuses on: (1) the... 相似文献
228.
Luigi Costamante 《Astrophysics and Space Science》2007,309(1-4):487-495
The study of the TeV emission from extragalactic sources is hindered by the uncertainties on the diffuse Extragalactic Background
Light (EBL). The recent H.E.S.S. results on the blazars 1ES 1101-232 and H 2356-309 represent a breakthrough on this issue.
Their unexpectedly hard spectra allow an upper limit to be derived on the EBL in the optical/near-infrared range, which is
very close to the lower limit given by the resolved galaxy counts. This result seems to exclude a large contribution to the
EBL from other sources (e.g. Population III stars) and indicates that the intergalactic space is more transparent to γ-rays than previously thought. A discussion of EBL absorption effects and further observational tests with Cherenkov telescopes
are presented.
For the H.E.S.S. collaboration. 相似文献
229.
230.
High‐frequency sampling of stable water isotopes in precipitation and stream water during winter and summer storm events was carried out in a 2·3 km2 lowland agricultural catchment. During peak flows of monitored events, the responses of δ2H and δ18O were comparable and inferred the dominance (ca 70%) of ‘old’ pre‐event water. Transit Time Distribution (TTD) inferred by a gamma function were fitted (Nash–Sutcliffe = 0·8) and were also similar for δ2H and δ18O. However, the shape (α) and scaling (β) parameters were markedly different for summer and winter events. Consequently, when antecedent wetness was high, mean transit times were in the order of days; when drier, they increased to months. Moreover, while the responses of δ2H and δ18O exhibited similar gradual recovery to pre‐event conditions during winter hydrograph recessions, they differed dramatically on summer recessions. Time series analysis showed that δ2H isotope content was correlated with the diurnal cycle of air temperature, suggesting an evaporative fractionation pattern which could be reproduced by a temperature‐based first‐order autoregressive model. The heavier δ18O isotope showed no evidence for such diurnal variability. The study highlights the utility of high‐frequency stable isotope sampling to explore the time‐variant nature of TTDs. Furthermore, it shows that the time of sampling in a diurnal cycle may have crucial significance for interpreting stream isotope signatures, particularly δ2H. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献