全文获取类型
收费全文 | 643篇 |
免费 | 23篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 44篇 |
地球物理 | 155篇 |
地质学 | 252篇 |
海洋学 | 64篇 |
天文学 | 62篇 |
综合类 | 2篇 |
自然地理 | 75篇 |
出版年
2024年 | 2篇 |
2023年 | 3篇 |
2021年 | 2篇 |
2020年 | 12篇 |
2019年 | 10篇 |
2018年 | 21篇 |
2017年 | 16篇 |
2016年 | 21篇 |
2015年 | 18篇 |
2014年 | 20篇 |
2013年 | 27篇 |
2012年 | 24篇 |
2011年 | 33篇 |
2010年 | 37篇 |
2009年 | 49篇 |
2008年 | 42篇 |
2007年 | 35篇 |
2006年 | 19篇 |
2005年 | 33篇 |
2004年 | 25篇 |
2003年 | 23篇 |
2002年 | 21篇 |
2001年 | 14篇 |
2000年 | 7篇 |
1999年 | 9篇 |
1998年 | 17篇 |
1997年 | 4篇 |
1996年 | 8篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 7篇 |
1984年 | 15篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 4篇 |
排序方式: 共有670条查询结果,搜索用时 15 毫秒
451.
Barbara Lo Faro Pierluigi Monaco Eros Vanzella Fabio Fontanot Laura Silva Stefano Cristiani 《Monthly notices of the Royal Astronomical Society》2009,399(2):827-841
It has recently been shown that galaxy formation models within the Λ cold dark matter cosmology predict that, compared to the observed population, small galaxies (with stellar masses <1011 M⊙ ) form too early, are too passive since z ∼ 3 and host too old stellar populations at z = 0 . We then expect an overproduction of small galaxies at z ≳ 4 that should be visible as an excess of faint Lyman-break galaxies. To check whether this excess is present, we use the morgana galaxy formation model and grasil spectrophotometric + radiative transfer code to generate mock catalogues of deep fields observed with Hubble Space Telescope Advanced Camera for Surveys. We add observational noise and the effect of Lyman α emission, and perform colour–colour selections to identify Lyman-break galaxies. The resulting mock candidates have plausible properties that closely resemble those of observed galaxies. We are able to reproduce the evolution of the bright tail of the luminosity function of Lyman-break galaxies (with a possible underestimate of the number of the brightest i -dropouts), but uncertainties and degeneracies in dust absorption parameters do not allow to give strong constraints to the model. Besides, our model shows a clear excess with respect to observations of faint Lyman-break galaxies, especially of z 850 ∼ 27 V -dropouts at z ∼ 5 . We quantify the properties of these 'excess' galaxies and discuss the implications: these galaxies are hosted in dark matter haloes with circular velocities in excess of 100 km s−1 , and their suppression may require a deep rethinking of stellar feedback processes taking place in galaxy formation. 相似文献
452.
Nate Bastian Mark Gieles Barbara Ercolano Rob Gutermuth 《Monthly notices of the Royal Astronomical Society》2009,392(2):868-878
We present an analysis of the spatial distribution of various stellar populations within the Large Magellanic Cloud (LMC). We combine mid-infrared selected young stellar objects, optically selected samples with mean ages between ∼9 and ∼1000 Myr and existing stellar cluster catalogues to investigate how stellar structures form and evolve within the LMC. For the analysis we use Fractured Minimum Spanning Trees, the statistical Q parameter and the two-point correlation function. Restricting our analysis to young massive (OB) stars, we confirm our results obtained for M33, namely that the luminosity function of the groups is well described by a power law with index −2, and that there is no characteristic length-scale of star-forming regions. We find that stars in the LMC are born with a large amount of substructure, consistent with a two-dimensional fractal distribution with dimension and evolve towards a uniform distribution on a time-scale of ∼175 Myr. This is comparable to the crossing time of the galaxy, and we suggest that stellar structure, regardless of spatial scale, will be eliminated in a crossing time. This may explain the smooth distribution of stars in massive/dense young clusters in the Galaxy, while other, less massive, clusters still display large amounts of structure at similar ages. By comparing the stellar and star cluster distributions and evolving time-scales, we show that infant mortality of clusters (or 'popping clusters') has a negligible influence on the galactic structure. Finally, we quantify the influence of the elongation, differential extinction and contamination of a population on the measured Q value. 相似文献
453.
Guy R. Holdgate Brian McGowran Tom Fromhold Barbara E. Wagstaff Stephen J. Gallagher Malcolm W. Wallace Ian R.K. Sluiter Michael Whitelaw 《Global and Planetary Change》2009,65(1-2):89-103
The carbon-isotope and palynological record through 580 m thick almost continuous brown coal in southeast Australia's Gippsland Basin is a relatively comprehensive southern hemisphere Middle Eocene to Middle Miocene record for terrestrial change. The carbon isotope δ13Ccoal values of these coals range from ? 27.7‰ to ? 23.2. This isotopic variability follows gymnosperm/angiosperm fluctuations, where higher ratios coincide with heavier δ13C values. There is also long-term variability in carbon isotopes through time. From the Eocene greenhouse world of high gymnosperm-heavier δ13Ccoal values, there is a progressive shift to lighter δ13Ccoal values that follows the earliest (Oi1?) glacial events around 33 Ma (Early Oligocene). The overlying Oligocene–Early Miocene brown coals have lower gymnosperm abundance, associated with increased %Nothofagus (angiosperm), and lightening of isotopes during Oligocene cooler conditions.The Miocene palynological and carbon-isotope record supports a continuation to the Oligocene trends until around the late Early Miocene (circa 19 Ma) when a warming commenced, followed by an even stronger isotope shift around 16 Ma that peaked in the Middle Miocene when higher gymnosperm abundance and heavier isotopes prevailed. The cycle between the two major warm peaks of Middle Eocene and Middle Miocene was circa 30 Ma long. This change corresponds to a fall in inferred pCO2 levels for the same period. The Gippsland data suggest a link between gymnosperm abundance, long-term plant δ13C composition, climatic change, and atmospheric pCO2. Climatic deterioration in the Late Miocene terminated peat accumulation in the Gippsland Basin and no further significant coals formed in southeast Australia.The poor correspondence between this terrestrial isotope data and the marine isotope record is explained by the dominant control on δ13C by the gymnosperm/angiosperm abundance, although in turn this poor correspondence may reflect palaeoclimate control. From the brown coal seam dating, the coal appears to have accumulated during a considerable part of the allocated 30 Ma Cenozoic time period. These brown coal carbon isotope and palynological data appear to record a more gradual atmospheric carbon isotope change compared to the marine record. 相似文献
454.
455.
Mixed land use and obesity: an empirical comparison of alternative land use measures and geographic scales 总被引:1,自引:0,他引:1
Yamada I Brown BB Smith KR Zick CD Kowaleski-Jones L Fan JX 《The Professional geographer》2012,64(2):157-177
Obesity is a growing epidemic in the United States. Walkable neighborhoods, characterized as having the 3Ds of walkability (population Density, land use Diversity, and pedestrian-friendly Design), have been identified as a potentially promising factor to prevent obesity for their residents. Past studies examining the relationship between obesity and walkability vary in geographic scales of neighborhood definitions and methods of measuring the 3Ds. To better understand potential influences of these sometimes arbitrary choices, we test how four types of alternative measures of land use diversity measured at three geographic scales relate to body mass index for 4960 Salt Lake County adults. Generalized estimation equation models demonstrate that optimal diversity measures differed by gender and geographic scale and that integrating walkability measures at different scales improved the overall performance of models. 相似文献
456.
Sub-bottom profiling and coring were undertaken at eight sub-basins along the lower French River and at five small lakes near
North Bay, Ontario, to collect stratigraphical and chronological evidence to investigate whether lakes occupying the Huron–Georgian
basins during the early- to mid-Holocene became hydrologically closed. All of the coring sites are located within the route
of the North Bay outlet that carried outflow from the upper Great Lakes during this period. Sand beds containing organic detritus
are present within five cores from Muskrat, Crombie and Deep bays that otherwise are composed of glaciolacustrine rhythmites
or fine-grained lacustrine deposits. These sand beds are interpreted to represent intervals when water levels within the sub-basins
were lower than present, based on chronology, sediment texture, and macrofossil assemblages. It is inferred that the water
surface in the Huron–Georgian basins fell below the level of the Dalles Rapids sill isolating the lower French River sub-basins
from the large lake. A core from Depensier Lake, North Bay, contains an organic-rich sand interval within a thicker sand unit
barren of organic materials. Macrofossils within this organic-rich interval are interpreted to be evidence of substantially
diminished flow through the North Bay outlet channel. Radiocarbon dates of terrestrial macrofossils provide correlation of
the sand beds between the French River cores as well as with the organic-rich sand in the Depensier Lake core. The possibility
that the sand beds in the French River cores represent flood deposits rather than evidence of hydrologically closed conditions
is considered, but rejected, based on the occurrence of multiple peaty layers and the record of shallow water conditions inferred
from macrofossils within the upper sand bed of core MUS1, Muskrat Bay, in combination with the evidence of quiescent depositional
conditions from similarly aged macrofossils in the core from Depensier Lake. Eight radiocarbon dates from the French River
cores are incorporated into an elevation-age plot of paleo-indicators of water levels in the Huron–Georgian basins, using
additional data from the literature. This plot and stratigraphic evidence from the Muskrat Bay cores indicates that separate
closed-basin intervals occurred between 9.0 and 8.4, and 9.5 and 9.3 ka cal BP (~ 8.1 and 7.6, and ~ 8.5 and 8.3 ka BP).
The occurrence of these two closed-basin intervals between 9.6 and 8.4 ka cal BP (~ 8.7 and 7.6 ka BP) implies that run-off
derived exclusively from precipitation within the non-glaciated portions of the upper Great Lakes drainage basins was likely
insufficient at this time to support an open-basin lake hydrology during the contemporary climate, which was colder and drier
than present, without being supplemented from glacial Lake Agassiz overflow and/or Laurentide Ice Sheet meltwater. 相似文献
457.
Spatial interpolation methods for nonstationary plume data 总被引:1,自引:0,他引:1
Plume interpolation consists of estimating contaminant concentrations at unsampled locations using the available contaminant data surrounding those locations. The goal of ground water plume interpolation is to maximize the accuracy in estimating the spatial distribution of the contaminant plume given the data limitations associated with sparse monitoring networks with irregular geometries. Beyond data limitations, contaminant plume interpolation is a difficult task because contaminant concentration fields are highly heterogeneous, anisotropic, and nonstationary phenomena. This study provides a comprehensive performance analysis of six interpolation methods for scatter-point concentration data, ranging in complexity from intrinsic kriging based on intrinsic random function theory to a traditional implementation of inverse-distance weighting. High resolution simulation data of perchloroethylene (PCE) contamination in a highly heterogeneous alluvial aquifer were used to generate three test cases, which vary in the size and complexity of their contaminant plumes as well as the number of data available to support interpolation. Overall, the variability of PCE samples and preferential sampling controlled how well each of the interpolation schemes performed. Quantile kriging was the most robust of the interpolation methods, showing the least bias from both of these factors. This study provides guidance to practitioners balancing opposing theoretical perspectives, ease-of-implementation, and effectiveness when choosing a plume interpolation method. 相似文献
458.
Torsti Jarmo Kocharov Leon Laivola Jarno Chertok Ilya Thompson Barbara J. 《Solar physics》2003,214(1):177-193
Energetic particle observations of the ERNE instrument on board SOHO enable measurements of 3He and 4He fluxes beyond 15 MeV nucleon–1 with good statistical resolution. We report results of a survey of the ERNE observations covering the period from 8 February 1999 to 6 December 2000. We find 10 and 5 days during which the 3He-to-4He ratio exceeds the levels of 20% and 50%, respectively. Those periods include, in particular, four 3He-rich events that are sufficiently strong for a reasonably accurate estimate of the time-intensity profiles. We analyze the history of solar and interplanetary phenomena associated with these high-energy 3He-rich events. Basic properties of such events and significant solar and interplanetary factors are formulated. The significant factors comprise, in particular, a strong, impulsive flare, typically observed about
day before the 3He onset, and an interplanetary shock wave or magnetic field enhancement arriving at 1 AU about frac43 day after the 3He onset. The high-energy 3He-rich events make up a new kind of hybrid events, possessing the impulsive-type composition and the gradual-type time-profiles. We emphasize a dependence of the resultant particle event on the history of the particular solar eruption comprising coronal mass ejection (CME) and the flare associated with the CME. 相似文献
459.
460.
This paper examines historical evidence on the occurrence and status of an important landscape element in the Central Tablelands of New South Wales: the swampy meadow/chain of ponds landform. Our findings reject the notion that it is possible to reconstruct this landform as it was at the time of early European settlement, based solely on early colonial documents. Furthermore the analysis of such documents should be used with appropriate caution when benchmarking this landform. These conclusions have been reached by analysing 826 nineteenth-century portion plans, 18 early explorers and settlers’ journals, letters and maps, and nine Surveyor General's maps and plans. We suggest that our findings have significant implications for developing management and restoration strategies, better informing conservation initiatives and the development of policies aimed at protecting, conserving and restoring the swampy meadow/chain of ponds landform. Our experience suggests that researchers involved in historical analyses of documents for similar purposes should be wary about factors such as the limitations and reliability of source data, unwarranted interpretations and the imprecise and/or contradictory use of terms. We also suggest that the possible subjective views of some observers, particularly those with limitations in interpreting such a landform, are further reasons to be cautious. If these factors are not taken into account it is very likely to result in a flawed interpretation. We conclude that both landform awareness and other perceptions of an observer at the time of early European settlement may present a hitherto unrecognised subjective element in this and other analyses which could limit the precision of historical reconstruction, without resort to other complementary methods. 相似文献