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Y. V. Bogdanova C. J. Owen G. Siscoe A. N. Fazakerley I. Dandouras O. Marghitu Z. Kaymaz H. Rème E. A. Lucek 《Solar physics》2007,244(1-2):201-232
We present a study of the magnetospheric cusp response to extreme external parameters during passage of the ICME over the
Earth on 10 November 2004, based on Cluster observations of the plasma properties inside the low-latitude boundary layer (LLBL)/cusp regions. Two separate events are
observed while Cluster is in the dawn sector, 07 – 08 h magnetic local time (MLT). First, a LLBL/cusp crossing occurs during a period of strong
southward IMF. During this time, the LLBL/cusp is very small, ∼0.8 – 1° invariant latitude (ILAT) and moves equatorward, down
to 67° ILAT. This can be explained by the occurrence of significant magnetopause erosion due to enhanced dayside sub-solar
reconnection. The energy of the plasma inside this region is higher than normal, and the low-energy cut-off often observed
in the ion data is also unusually high. This might be explained by the suggestion that the local magnetosheath Alfvén velocity
and deHoffmann – Teller velocity are also both extremely high. However, the plasma convection and parallel velocity inside
this region are not very high. The second event discussed in this paper is a LLBL/cusp crossing during strong equatorial IMF
(mostly due to the dominant dawn – dusk component). Under these conditions, occurring at the same time as pulses of solar
wind dynamic pressure, the observations are very complicated. However, we suggest that in the polar region of the southern
hemisphere, Cluster cross two LLBLs/cusps, spatially separated by polar cap plasma. The first LLBL/cusp is formed by anti-parallel reconnection
in the dusk sector of the southern hemisphere and the second is formed by anti-parallel reconnection in the dawn sector of
the northern hemisphere. The second LLBL/cusp is located at extremely low latitude, less than ∼66.3° ILAT. During all LLBL/cusp
crossings, strong ionospheric O+ ion outflow is detected in the form of a narrow beam with limited pitch-angle range. 相似文献
215.
Kenneth F. Rijsdijk Julian P. Hume Frans Bunnik F.B. Vincent Florens Claudia Baider Beth Shapiro Johannes van der Plicht Anwar Janoo Owen Griffiths Lars W. van den Hoek Ostende Holger Cremer Tamara Vernimmen Perry G.B. De Louw Assenjee Bholah Salem Saumtally Nicolas Porch James Haile Mike Buckley Matthew Collins Edmund Gittenberger 《Quaternary Science Reviews》2009,28(1-2):14-24
Although the recent history of human colonisation and impact on Mauritius is well documented, virtually no records of the pre-human native ecosystem exist, making it difficult to assess the magnitude of the changes brought about by human settlement. Here, we describe a 4000-year-old fossil bed at Mare aux Songes (MAS) in south-eastern Mauritius that contains both macrofossils (vertebrate fauna, gastropods, insects and flora) and microfossils (diatoms, pollen, spores and phytoliths). With >250 bone fragments/m2 and comprising 50% of all known extinct and extant vertebrate species (ns = 44) of Mauritius, MAS may constitute the first Holocene vertebrate bone Concentration-Lagerstätte identified on an oceanic volcanic island. Fossil remains are dominated by extinct giant tortoises Cylindraspis spp. (63%), passerines (~10%), small bats (7.8%) and dodo Raphus cucullatus (7.1%). Twelve radiocarbon ages [four of them duplicates] from bones and other material suggest that accumulation of fossils took place within several centuries. An exceptional combination of abiotic conditions led to preservation of bones, bone collagen, plant tissue and microfossils. Although bone collagen is well preserved, DNA from dodo and other Mauritian vertebrates has proved difficult. Our analysis suggests that from ca 4000 years ago (4 ka), rising sea levels created a freshwater lake at MAS, generating an oasis in an otherwise dry environment which attracted a diverse vertebrate fauna. Subsequent aridification in the south-west Indian Ocean region may have increased carcass accumulation during droughts, contributing to the exceptionally high fossil concentration. The abundance of floral and faunal remains in this Lagerstätte offers a unique opportunity to reconstruct a pre-human ecosystem on an oceanic island, providing a key foundation for assessing the vulnerability of island ecosystems to human impact. 相似文献
216.
Near-infrared observations of the nightside of Venus reveal regions of high brightness temperatures. These regions of high brightness temperatures are caused by the localized evaporation of the middle and lower cloud decks, which are about 50 to 60 km above the surface of the planet. We simulate the Venus condensational middle and lower cloud deck with the University of Colorado/NASA Ames Community Aerosol and Radiation Model for Atmospheres (CARMA). Our simulated clouds have similar characteristics to the observed Venus clouds. Our radiative transfer model reproduces the observed temperature structure and atmospheric stability structure within the middle cloud region. A radiative-dynamical feedback occurs which generates mixing due to increased absorption of upwelling infrared radiation within the lower cloud region, as previously suggested by others. We find that localized variations in temperature structure or in sub-grid scale mixing cannot directly explain the longevity and optical depth of the clouds. However, vertical motions are capable of altering the cloud optical depth by a sufficient magnitude in a short enough timescale to be responsible for the observed clearings. 相似文献
217.
Determining the cause of sediment mobilization is a major problem; possible triggers include earthquakes, sediment loading and wave action. A detailed sedimentological and palaeoenvironmental analysis of soft‐sediment deformation in Upper Carboniferous deltaic deposits in SW Wales, UK, shows that two styles of deformation occur. Type A (syndepositional convolute stratification) affects most sandstone beds and was generated by rapid sedimentation. Type B (localized sand‐in‐sand pseudonodules) incorporates beds that already contained Type A deformation, and developed when the substrate was liquefied by disturbance due to movement on a near‐surface gravity slide. Neither type of deformation was triggered by seismic events. 相似文献
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Jason M. Dortch Lewis A. Owen Marc W. Caffee Dewen Li Thomas V. Lowell 《第四纪科学杂志》2010,25(8):1259-1269
Glacial landforms and outwash terraces in the Nenana River valley, Reindeer Hills and Monahan Flat in the central Alaska Range were dated with 60 10Be exposure ages to determine the timing of Late Pleistocene glaciation. In the Nenana River valley, glaciation occurred at 104–180 ka (Lignite Creek glaciation), ca. 55 ka (Healy glaciation), and ca. 16 ka (Carlo Creek phase); glaciers retreated in the Reindeer Hills and Monahan Flat by ca. 14 ka and ca. 13 ka, respectively. The Carlo Creek moraine is similar in age to at least six other moraines in the Alaska Range, Ahklun Mountains and Brooks Range. The new data suggest that post‐depositional geological processes limit the usefulness of 10Be methods to the latter part (≤60 ka) of the late Quaternary in central Alaska. Ages on Healy and younger landforms cluster well, with the exception of Riley Creek moraines and Monahan Flat‐west sites, where boulders were likely affected by post‐depositional processes. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
220.
The factors controlling the chemistry of 69 low-order streams in the Blue Ridge and Valley and Ridge physiographic provinces of Virginia and Maryland were studied over a 13-month period. Principal component analysis was used to examine regional patterns in stream chemistry and to examine the degree to which the chemistry of low-order streams is controlled by the bedrock upon which they flow. Streams clustered into regionally isolated groups, strongly related to bedrock type, with SO2?4 and HCO?3 the chemical variables of most importance. Sulphate concentrations appear to be strongly controlled by climate and hydrology, and sorption in the soils within the watershed. Much of the atmospherically derived SO2?4 accumulates in watersheds during the growing season and is later flushed out. Weathering reactions were found to be particularly important in the production of HCO?3, accounting for 91 per cent on an annual basis, and export of divalent cations from these watersheds, accounting for 48–50 per cent on an annual basis. About half of non-anthropogenic Na+ was derived from weathering of silicates, whereas nearly all K+ was identified with leaching by SO2?4. Water chemistry was strongly related to the rock type in the watershed and the weatherability of the component minerals. Rock type is not a randomly distributed function; instead, it is controlled by geologic factors that result in clusters of similar rock types in a given region. When planning large synoptic studies, it is extremely important to consider that a sampling scheme based on random sampling of a non-randomly distributed function May, not provide the most accurate representation of the variables of interest. Instead, a hierarchical sampling scheme May, be indicated. Our results also suggest that, although one sample in time May, be sufficient to characterize the primary geochemical factors controlling stream chemistry throughout the year, it May, not be sufficient to detect subtle, flow-related alterations in chemistry. 相似文献