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991.
C. Gontikakis A. Anastasiadis C. Efthymiopoulos 《Monthly notices of the Royal Astronomical Society》2007,378(3):1019-1030
The acceleration of charged particles in a site of magnetic reconnection is analysed by detailed numerical simulations. Single or multiple encounters of the particles with Harris-type reconnecting current sheets (RCSs) are modelled as an overall stochastic process taking place within an active region. RCS physical parameters are selected in a parameter space relevant to solar flares. Initially, the charged particles form a thermal (Maxwellian) distribution corresponding to coronal temperature ≃2 × 106 K . Our main goal is to investigate how the acceleration process changes the shape of the particles' kinetic energy distribution. The evolution of the kinetic energy distribution, calculated numerically after one encounter of the particles with a single RCS, is found to be in good agreement with our previously published analytical formulae. In the case of consecutive encounters, we find that the kinetic distribution tends to converge to a practically invariant form after a relatively small number of encounters. We construct a discrete stochastic process that reproduces the numerical distributions and we provide a theoretical interpretation of the asymptotic convergence of the energy distribution. We finally compute the theoretical X-ray spectra that would be emitted by the simulated particles in a thick target model of radiation. 相似文献
992.
993.
Andrés E. Piatti Ata Sarajedini Doug Geisler Carme Gallart Marina Wischnjewsky† 《Monthly notices of the Royal Astronomical Society》2007,382(3):1203-1212
Colour–magnitude diagrams in the Washington system are presented for the first time for five star clusters projected on to the outer region of the Small Magellanic Cloud (SMC). The clusters are found to have ages in the range 0.1–1.0 Gyr, as derived from the fit of isochrones with Z = 0.004 . This sample increases substantially the number of young clusters in the outer SMC – particularly in the south-east quadrant – with well-derived parameters. We combine our results with those for other clusters in the literature to derive as large and homogeneous a data base as possible (totalling 49 clusters) in order to study global effects. We find no conclusive evidence for a dispersion in the cluster ages and metallicities as a function of their distance from the galaxy centre, in the SMC outer region. L 114 and 115, although very distant, are very young clusters, lying in the bridge of the SMC and therefore most likely formed during the interaction which formed this feature. We also find very good agreement between the cluster age–metallicity relation (AMR) and the prediction from a bursting model from Pagel & Tautvaišienė with a burst that occurred 3 Gyr ago. Comparing the present cluster AMR with that derived by Harris & Zaritsky for field stars in the main body of the SMC, we find that field stars and clusters underwent similar chemical enrichment histories during approximately the last couple of Gyr, but their chemical evolution was clearly different between 4 and 10 Gyr ago. 相似文献
994.
Y. F. Yurovsky 《Solar physics》2007,246(2):415-418
The calculation of the Fourier transform of noise storm (NS) fluctuations showed that the power spectrum was adequately described
by the expression G(F)∼1/F. Our results rule out the possibility that NS radiation is formed from random, short-term bursts (so-called type I bursts),
since the spectrum of the sum of random short fluctuations is flat, but the real NS has a hyperbolic spectrum. This spectrum
is monotonic and does not contain any components that exceed the level of the statistical fluctuations (i.e., the results of observations do not reveal the presence of periodic or resonant properties of the emission source). The hyperbolic
shape of the spectrum shows that the main energy of a NS is contained in the slower temporal fluctuations. 相似文献
995.
996.
Zhi-Jian Luo Cheng-Gang Shu 《中国天文和天体物理学报》2007,7(4):590-594
According to the new preheating mechanism of galaxy formation suggested by Mo et al., we construct a simple model of formation of disk galaxies within the current paradigm of galaxy formation. It incorporates preheating, gas cooling, bulge formation and star for-mation. The predicted stellar and HI mass functions of galaxies are discussed and compared with the observations. It is found that our model can roughly match both the observed galaxy luminosity function and the observed HI-mass function. 相似文献
997.
S. Mazevet M. Challacombe P. M. Kowalski D. Saumon 《Astrophysics and Space Science》2007,307(1-3):273-277
We investigate the conductivity of warm dense helium under conditions found in the atmospheres of cool white dwarfs using
ab initio simulations. The calculations performed consist of quantum molecular dynamics simulations where the electronic wavefunction
at each time step is obtained using density functional theory, while the ion trajectories are calculated using the resulting
quantum mechanical forces. We use both conventional DFT (PW91) and hybrid (PBE0) functionals to calculate the conductivities
that provide an estimate of the ionization fraction. While the calculations are in good agreement with the measurements for
the equation of state, a significant discrepancy exists with the recently measured conductivity. 相似文献
998.
Axel D. Schwope Valeri Hambaryan Frank Haberl Christian Motch 《Astrophysics and Space Science》2007,308(1-4):619-623
We present a first analysis of a deep X-ray spectrum of the isolated neutron star RBS1223 obtained with XMM-Newton. Spectral
data from four new monitoring observations in 2005/2006 were combined with archival observations obtained in 2003 and 2004
to form a spin-phase averaged spectrum containing 290 000 EPIC-pn photons. This spectrum shows higher complexity than its
predecessors, and can be parameterised with two Gaussian absorption lines superimposed on a blackbody. The line centers, E
2≃2E
1, could be regarded as supporting the cyclotron interpretation of the absorption features in a field B∼4×1013 G. The flux ratio of those lines does not support this interpretation. Hence, either feature might be of truly atomic origin.
相似文献
999.
Cepheid parallaxes and the Hubble constant 总被引:1,自引:0,他引:1
Floor van Leeuwen Michael W. Feast Patricia A. Whitelock Clifton D. Laney 《Monthly notices of the Royal Astronomical Society》2007,379(2):723-737
Revised Hipparcos parallaxes for classical Cepheids are analysed together with 10 Hubble Space Telescope ( HST )-based parallaxes. In a reddening-free V , I relation we find that the coefficient of log P is the same within the uncertainties in our Galaxy as in the Large Magellanic Cloud (LMC), contrary to some previous suggestions. Cepheids in the inner region of NGC 4258 with near solar metallicities confirm this result. We obtain a zero-point for the reddening-free relation and apply it to the Cepheids in galaxies used by Sandage et al. to calibrate the absolute magnitudes of Type Ia supernova (SNIa) and to derive the Hubble constant. We revise their result for H 0 from 62 to 70 ± 5 km s−1 Mpc−1 . The Freedman et al. value is revised from 72 to 76 ± 8 km s−1 Mpc−1 . These results are insensitive to Cepheid metallicity corrections. The Cepheids in the inner region of NGC 4258 yield a modulus of 29.22 ± 0.03 (int.) compared with a maser-based modulus of 29.29 ± 0.15. Distance moduli for the LMC, uncorrected for any metallicity effects, are 18.52 ± 0.03 from a reddening-free relation in V , I ; 18.47 ± 0.03 from a period–luminosity relation at K ; 18.45 ± 0.04 from a period–luminosity–colour relation in J , K . Adopting a metallicity correction in V , I from Macri et al. leads to a true LMC modulus of 18.39 ± 0.05. 相似文献
1000.