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111.
112.
Birgitta NordstrÖm 《Astrophysics and Space Science》2003,284(2):779-782
New, precise abundance data for a large number of elements in a growing sample of extremely metal-poor stars are accumulating
from the new 8-m telescopes. Combined with theoretical models, these results advance our understanding of the first generations
of stars, whose nucleosynthesis products are fossilised in the oldest stars we see today and thus give clues to the earliest
phases of evolution in the Galaxy. In particular, the heaviest elements give us insight into the different neutron capture
mechanisms and the stellar sites where such elements could be produced. They also afford an independent way to determine the
age of the Galaxy, by radioactive chronology.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
113.
The sequence of events determining the initial stages of star formation is analyzed in framework of the self-enrichment scenario. The computations are based on a single-zone chemo-dynamical model. It is shown that the first episode of star formation was characterized by an initial mass function shifted toward massive stars (M ≥ 8M⊙). We argue that the transition to a star formation with a normal (Salpeter) initial mass function was due to more efficient radiative cooling of the proto-globular cluster gas after its enrichment to a metallicity of Z ~ 0.02 Z⊙ in agreement with those observed in globular clusters. 相似文献
114.
We study non-axisymmetric oscillations of thin prominence fibrils. A fibril is modeled by a straight thin magnetic tube with
the ends frozen in dense plasmas. The density inside and outside the tube varies only along the tube and it is discontinuous
at the tube boundary. Making a viable assumption that the tube radius is much smaller than its length, we show that the squares
of the frequencies of non-axisymmetric tube oscillations are given by the eigenvalues of the Sturm–Liouville problem for a
second-order ordinary differential equation on a finite interval with the zero boundary conditions. For an equilibrium density
that is constant outside the tube and piecewise constant inside we derived a simple dispersion equation determining the frequencies
of non-axisymmetric oscillations. We carry out a parametric study of this equation both analytically and numerically, restricting
our analysis to the first even mode and the first odd mode. In particular, we obtained a criterion that allows to find out
if each of these modes is a normal or leaky mode. 相似文献
115.
The Sample Catchment Basin (SCB) mapping technique extends the representativeness of the geochemical features of stream sediments to the surface of the whole upstream drainage basin. SCB boundaries clash with the water divides traced from each sampling point and are limited upstream by the presence of further SCBs. They are also assumed to represent the elementary map unit. The rank of SCBs can be defined counting the number of upstream SCBs along the branches of a fluvial network. The presence of low rank SCBs minimizes the statistical redundancy of measures. The SCB technique is particularly suitable for the geochemical mapping of mountainous or hilly areas and to correctly display the information into a morphological context. This approach can be also valuable in the case of low sampling density, inhomogeneous sampling schemes, and especially when very accurate evaluations of the spatial distribution of chemicals (with natural or anthropogenic origin) are required. 相似文献
116.
117.
Olivier?FéméniasEmail author Nicolas?Coussaert Julien?Berger Jean-Claude?C.?Mercier Daniel?Demaiffe 《Contributions to Mineralogy and Petrology》2004,148(1):13-28
Mantle xenoliths from Puy Beaunit (French Massif Central) are compositionally varied, ranging from relatively fertile spinel lherzolites to refractory spinel dunites. Fertile peridotites have registered a modal (amphibole-bearing lherzolites) and cryptic metasomatic event that took place before the last Permian (257 Ma) melting episode. Depletion processes have been constrained by chemical modelling: the depletion is related to different degrees of partial melting, but two major melt extraction episodes are needed to explain the range of major element composition. The second event was responsible for the local large-scale dunitification of former residues. The first melting event (F25%) and metasomatic enrichment are attributed to an ancient fluid and/or liquid infiltration that could be related to a pre-Variscan regional subduction (located to the north of the Beaunit area). Texture acquisition and major deformation of the mantle xenoliths were sub-contemporaneous of the subduction and would result from lithospheric delamination. The second melting event (F17%) produced high-Mg basalts with calc-alkaline trace element signature that gave rise to the Permian underplating episode recognised in western Europe. 相似文献
118.
Evaluation of an ensemble of Arctic regional climate models: spatiotemporal fields during the SHEBA year 总被引:1,自引:1,他引:0
A.?RinkeEmail author K.?Dethloff J.?J.?Cassano J.?H.?Christensen J.?A.?Curry P.?Du E.?Girard J.-E.?Haugen D.?Jacob C.?G.?Jones M.?K?ltzow R.?Laprise A.H.?Lynch S.?Pfeifer M.?C.?Serreze M.?J.?Shaw M.?Tjernstr?m K.?Wyser M.??agar 《Climate Dynamics》2006,26(5):459-472
Simulations of eight different regional climate models (RCMs) have been performed for the period September 1997–September
1998, which coincides with the Surface Heat Budget of the Arctic Ocean (SHEBA) project period. Each of the models employed
approximately the same domain covering the western Arctic, the same horizontal resolution of 50 km, and the same boundary
forcing. The models differ in their vertical resolution as well as in the treatments of dynamics and physical parameterizations.
Both the common features and differences of the simulated spatiotemporal patterns of geopotential, temperature, cloud cover,
and long-/shortwave downward radiation between the individual model simulations are investigated. With this work, we quantify
the scatter among the models and therefore the magnitude of disagreement and unreliability of current Arctic RCM simulations.
Even with the relatively constrained experimental design we notice a considerable scatter among the different RCMs. We found
the largest across-model scatter in the 2 m temperature over land, in the surface radiation fluxes, and in the cloud cover
which implies a reduced confidence level for these variables.
An erratum to this article can be found at 相似文献
119.
The importance of the landscape surrounding a protected area for sustaining its values is frequently discussed in conservation literature. Studies on the interactions of urbanisation and nature conservation at the global scale suggest that protected nature attracts urbanisation, and that this in turn might negatively impact the area. However, studies specifically addressing such land use dynamics at city scale are largely missing. In this study we examine the change in proportion of built up area in two zones (500 m and 1000 m) surrounding 15 urban nature reserves in southern Sweden. By using comprehensive maps from the last 50 years, we compared the zones to the overall urbanisation in the cities to reveal discrepancies in land use surrounding the nature reserves. We found that the amount of built up area in the buffer zones surrounding nature reserves followed the same trend as the corresponding cities and this relation was stable over time, although the positive relationship was not significant. The establishment of nature reserves had no detectable effect on surrounding land use, however two distinguished groups of reserves were identified with either more or less built up area in buffers zones compared to cities. These differences were related to specific local drivers such as land ownership, land use history and nature reserve location. In contrast to earlier studies at global scale, our study did not show that nature reserves attract urbanisation. Instead, we stress that the great variety of contextual factors at the city scale makes quantitative analysis of this kind extremely difficult. However, a general neglect from planning and nature conservation agencies to recognise nature reserves’ dependence on the surrounding landscape configuration could be detrimental to sustain their values in the long-term. Hence we suggest that zones surrounding nature-protected areas should be planned and managed according to local land use history and current landscape conditions to enable and enhance necessary cross-boundary interactions. 相似文献
120.
Analyses of possible changes in intense and extreme wind speeds over northern Europe under climate change scenarios 总被引:1,自引:0,他引:1
S. C. Pryor R. J. Barthelmie N. E. Clausen M. Drews N. MacKellar E. Kjellstr?m 《Climate Dynamics》2012,38(1-2):189-208
Dynamical downscaling of ECHAM5 using HIRHAM5 and RCA3 for a northern European domain focused on Scandinavia indicates sustained extreme wind speeds with long recurrence intervals (50?years) and intense winds are not likely to evolve out of the historical envelope of variability until the end of C21st. Even then, significant changes are indicated only in the SW of the domain and across the central Baltic Sea where there is some evidence for relatively small magnitude increases in the 50?year return period wind speed (of up to 15%). There are marked differences in results based on the two Regional Climate Models. Additionally, internal (inherent) variability and initial conditions exert a strong impact on projected wind climates throughout the twenty-first century. Simulations of wind gusts by one of the RCMs (RCA3) indicate some evidence for increased magnitudes (of up to +10%) in the southwest of the domain and across the central Baltic Sea by the end of the current century. As in prior downscaling of ECHAM4, dynamical downscaling of ECHAM5 indicates a tendency towards increased energy density and thus wind power generation potential over the course of the C21st. However, caution should be used in interpreting this inference given the high degree of wind climate projection spread that derives from the specific AOGCM and RCM used in the downscaling. 相似文献