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441.
442.
L. A. Balona B. Bartlett † J. A. R. Caldwell J. Gaobakwe † G. Handler P. Kalebwe † K. Khoabane † C. Koen D. Laney T. Medupe J. Menzies M. Msikinya † M. Phillips † T. Sono † 《Monthly notices of the Royal Astronomical Society》2001,321(2):239-246
We present new multicolour photometry and simultaneous high-dispersion spectroscopy for the δ Scuti star 1 Mon. The two main periodicities in the star are still present, but the third known period is not directly detected in the new radial velocities or light variations. However, it is detected in the periodogram of the second moment of the line profile variations. We use the cross-correlation function as an approximation for the line profile variations. By computing theoretical profiles for a given mode and comparing them with phased cross-correlation profiles, we are able to determine a goodness-of-fit criterion and estimate the most probable spherical harmonic degree, the azimuthal order, m , of the pulsation and also the angle of inclination. We then compare the relative amplitudes and phases of the photometric variations in five wavebands and obtain the best estimates of for the two visible periodicities. We confirm the earlier determinations that the main periodicity is a radial mode and that the other periodicity is probably We show that the line profile variations and light variations give consistent results. We point out the importance of a long wavelength range when using the photometric mode identification technique. Finally, we attempt to match the two periods with unstable modes from linear, non-adiabatic calculations. We are able to show that the principal period is well matched by either the fundamental or first overtone radial mode, but could not find a satisfactory fit to the mode. We discuss implications for mode identification of δ Scuti stars based on what we have learned from this star. 相似文献
443.
William M. Phillips Adrian M. Hall Colin K. Ballantyne Steven Binnie Peter W. Kubik Stewart Freeman 《第四纪科学杂志》2008,23(2):101-107
The extent of the last British–Irish Ice Sheet (BIIS) in northern Scotland is disputed. A restricted ice sheet model holds that at the global Last Glacial Maximum (LGM; ca. 23–19 ka) the BIIS terminated on land in northern Scotland, leaving Buchan, Caithness and the Orkney Islands ice‐free. An alternative model implies that these three areas were ice‐covered at the LGM, with the BIIS extending offshore onto the adjacent shelves. We test the two models using cosmogenic 10Be surface exposure dating of erratic boulders and glacially eroded bedrock from the three areas. Our results indicate that the last BIIS covered all of northern Scotland during the LGM, but that widespread deglaciation of Caithness and Orkney occurred prior to rapid warming at ca. 14.5 ka. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
444.
Biver Nicolas Bockelée-Morvan Dominique Colom Pierre Crovisier Jacques Henry Florence Lellouch Emmanuel Winnberg Anders Johansson Lars E.B. Gunnarsson Marcus Rickman Hans Rantakyrö Fredrik Davies John K. Dent William R.F. Paubert Gabriel Moreno Raphaël Wink Jörn Despois Didier Benford Dominic J. Gardner Matt Lis Dariusz C. Mehringer David Phillips Thomas G. Rauer Heike 《Earth, Moon, and Planets》2002,90(1-4):5-14
The bright comet Hale–Bopp provided the first opportunity to follow the outgassing rates of a number of molecular species over a large range of heliocentric distances. We present the results of our observing campaign at radio wavelengths which began in August 1995 and ended in January 2002. The observations were carried out with the telescopes of Nançay, IRAM, JCMT, CSO and, since September 1997, SEST. The lines of nine molecules (OH, CO, HCN, CH3OH, H2CO, H2S, CS, CH3CN and HNC) were monitored. CS, H2S, H2CO, CH3CN were detected up to rh= 3–4 AU from the Sun, while HCN and CH3OH were detected up to 6 AU. CO, which is the main driver of cometary activity at heliocentric distances larger than 3–4 AU, was last detected in August 2001, at rh= 14 AU. The gas production rates obtained from this programme contain important information on the nature of cometary ices, their thermal properties and sublimation mechanisms.Line shapes allow to measure gas expansion velocities, which, at large heliocentric distances, might be directly connected to the temperature of the nucleus surface. Inferred expansion velocity of the gas varied as rh -0.4 within 7 AU from the Sun, but remained close to 0.4 km s-1 further away. The CO spectra obtained at large rhare strongly blueshifted and indicative of an important day-to-night asymmetry in outgassing and expansion velocity. The kinetic temperature of the coma, estimated from the relative intensities of the CH3OH and CO lines, increased with decreasing rh, from about 10 K at 7 AU to 110 K around perihelion. 相似文献
445.
446.
J.P. Phillips 《Astrophysics and Space Science》2003,288(3):341-351
The sizes of planetary nebulae are poorly known, principally because the distances to these sources are not very well established.
We point out in the following, however, that if one is careful in selecting nebulae having particular ranges of galactic latitude,
and narrow ranges of surface brightness, then it should be possible to establish mean trends in nebular diameter. We have,
in this way, been able to determine that where brightness temperatures are the same, then circular and elliptical sources,
and the nuclei of bipolar nebulae (BPNe) all have closely similar mean dimensions. It is also likely that circular and elliptical
sources have closely similar masses (the case of the BPNe is less clear). Finally, we note that halos are typically ∼3 times
larger than circular and elliptical PNe, and the lobes of BPNe are ∼5 times as large, making the latter envelopes the largest
of all nebular structures.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
447.
F.P. Keenan K.M. Aggarwal D.R. Williams M. Mathioudakis K.J.H. Phillips 《Monthly notices of the Royal Astronomical Society》2001,326(4):1387-1390
Recent R -matrix calculations of electron impact excitation rates for transitions among the 2s2 2p2 , 2s2p3 and 2p4 levels of Fe xxi are used to derive theoretical electron density ( N e ) sensitive emission-line ratios involving 2s2 2p2 –2s2p3 transitions in the ∼98–146Å wavelength range. A comparison of these with observations from the PLT tokamak plasma, for which the electron density has been independently determined, reveals generally very good agreement between theory and experiment, and in some instances removes discrepancies found previously. The observed Fe xxi ratios for a solar flare, obtained with the OSO–5 satellite, imply electron densities which are consistent, with discrepancies that do not exceed 0.2 dex. In addition, the derived values of N e are similar to those estimated for the high-temperature regions of other solar flares. The good agreement between theory and observation, in particular for the tokamak spectra, provides experimental support for the accuracy of the present line-ratio calculations, and hence for the atomic data on which they are based. 相似文献
448.
P. M. Phillips I. W. A. Browne P. N. Wilkinson 《Monthly notices of the Royal Astronomical Society》2001,321(2):187-198
We present the results of an unbiased radio search for gravitational lensing events with image separations between 15 and 60 arcsec, which would be associated with clusters of galaxies with masses >1013–14 M⊙ . A parent population of 1023 extended radio sources stronger than 35 mJy with stellar optical identifications was selected using the FIRST radio catalogue at 1.4 GHz and the APM optical catalogue. The FIRST catalogue was then searched for companions to the parent sources stronger than 7 mJy and with separation in the range 15 to 60 arcsec. Higher-resolution observations of the resulting 38 lens candidates were made with the VLA at 1.4 and 5 GHz, and with MERLIN at 5 GHz in order to test the lens hypothesis in each case. None of our targets was found to be a gravitational lens system. These results provide the best current constraint on the lensing rate for this angular scale, but improved calculations of lensing rates from realistic simulations of the clustering of matter on the relevant scales are required before cosmologically significant constraints can be derived from this null result. We now have an efficient, tested observational strategy with which it will be possible to make an order-of-magnitude larger unbiased search in the near future. 相似文献
449.
The European VLBI Network (EVN) has been used to make phase referenced, wide-field (several arcminute) spectral line observations of the 6.7-GHz methanol maser emission towards W51. In the W51 Main region, the bulk of the methanol is offset from an UCHii region. This probably indicates the methanol emission arises at the interface of the expanding UCHii region and not from an edge-on circumstellar disc, as previously suggested. Near the W51 IRS2 region, the methanol emission is associated with a very compact, extremely embedded source supporting the hypothesis that methanol masers trace the earliest stages of massive star formation. As well as these two well-studied sites of star formation, many previously unknown regions star formation are detected, demonstrating that methanol masers are powerful means of detecting young massive stars. 相似文献
450.