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551.
Long-term geomagnetic activity presented by the aa index has been used to show that the heliospheric magnetic field has more than doubled during the last 100 years. However,
serious concern has been raised on the long-term consistency of the aa index and on the centennial rise of the solar magnetic field. Here we reanalyze geomagnetic activity during the last 100
years by calculating the recently suggested IHV (Inter-Hour Variability) index as a measure of local geomagnetic activity
for seven stations. We find that local geomagnetic activity at all stations follows the same qualitative long-term pattern:
an increase from early 1900 to 1960, a dramatic dropout in 1960s and a (mostly weaker) increase thereafter. Moreover, at all
stations, the activity at the end of the 20th century has a higher average level than at the beginning of the century. This
agrees with the result based on the aa index that global geomagnetic activity, and thereby, the open solar magnetic field has indeed increased during the last 100
years. However, quantitatively, the estimated centennial increase varies greatly from one station to another. We find that
the relative increase is higher at the high-latitude stations and lower at the low- and mid-latitude stations. These differences
may indicate that the fraction of solar wind disturbances leading to only moderate geomagnetic activity has increased during
the studied time interval. We also show that the IHV index needs to be corrected for the long-term change of the daily curve,
and calculate the corrected IHV values. Most dramatically, we find the centennial increase in global geomagnetic activity
was considerably smaller, only about one half of that depicted by the aa index. 相似文献
552.
In the five years from June, 1967, to June, 1972, a total of 99 meteorites were found in Roosevelt County, New Mexico and in adjoining Curry County. Of this number, 74 were found by one man. The finds include two achondrites, one pallasite, one carbonaceous chondrite, and 95 chondrites. They appear to represent more than 50 separate meteorite falls. The finding of a large number of meteorites in a small area provides data for an estimate of the probable quantity and average size of the meteorite specimens reaching the earth. The problems involved in allocating a total of 17 available local place names among more than 50 meteorite falls are discussed. 相似文献
553.
Deborah?Rutecki Ruth?H.?CarmichaelEmail author Ivan?Valiela 《Estuaries and Coasts》2004,27(2):179-187
Atlantic horseshoe crabs,Limulus polyphemus, are currently harvested for biomedical, scientific, and bait purposes. In recent years, changes in population abundance
and magnitude of harvesting have raised concerns about the status of this resource. We found horseshoe crab harvest in Pleasant
Bay, Massachusetts, was selective by sex and size. Biomedical harvest preferred larger individuals and females, the scientific
harvest preferred smaller individuals and males, and the bait harvest preferred females. Total 2001 harvest for all purposes
accounted for the mortality of ∼1–2% the adult population. Biomedical harvest accounted for the greatest loss of horseshoe
crabs from Pleasant Bay, ∼1–1.6% of the total population. Although biomedical harvest had the lowest associated mortality
rate (∼10–15%), many more crabs were harvested from Pleasant Bay for biomedical purposes than for other uses. The scientific
harvest accounted for the mortality of ∼0.4% of the population, and bait harvest accounted for the smallest mortality at ∼0.06%
of the population. Harvest mortality rate was estimated to be lower in Pleasant Bay than in other Cape Cod areas and may be
lower than natural mortality in the population. This study is the first qualitative investigation of commercial harvest on
horseshoe crab populations and emphasizes that harvest pressures on different populations need to be individually evaluated. 相似文献
554.
Ivan R. King 《Celestial Mechanics and Dynamical Astronomy》1974,9(3):349-357
This review attempts to place stellar dynamics in relation to other dynamical fields and to describe some of its important techniques and present-day problems. Stellar dynamics has some parallels, in increasing order of closeness, with celestial mechanics, statistical mechanics, kinetic theory, and plasma theory; but even in the last case the parallels are not very close. Stellar dynamics describes, usually through distribution functions, the motions of a large number of bodies as they all act on each other gravitationally. To a good approximation each star can be considered to move in the smoothed-out field of all the others, with random encounters between pairs of stars adding a slow statistical change to these smooth motions. Smooth-field dynamics has a well-developed theory, and the state of smooth stellar systems can be described in some detail. The ‘third integral’ presents an outstanding problem, however. Stellar encounters also have a well-developed theory, but close encounters and encounters of a single star with a binary pose serious problems for the statistical treatment. Star-cluster dynamics can be approached through a theory of smooth-field dynamics plus changes due to encounters, or alternatively through numerical simulations. The relation between the two methods is not yet close enough. The dynamical evolution of star clusters is still not fully understood. 相似文献
555.
Four stratospheric contaminants catalogued in the Cosmic Dust Catalogs published by the NASA Johnson Space Center (JSC) in Houston were reanalysed. The results are compatible with those obtained in JSC, however additional features have occurred in our chemical analyses and their interpretation is also discussed. 相似文献
556.
The Atlantic horsehoe crab,Limulus polyphemus, is harvested by the biomedical industry for Limulus Amoebocyte Lysate (LAL) a blood compound used to detect endotoxins,
for scientific research on vision, and as bait for the eel and conch fisheries. They are also subject to sources of natural
mortality such as consumption of eggs by shorebirds and stranding. Recently there has been growing concern over possible declines
inLimulus populations along the east coast of the United States. We have used stage-structured matrix modeling to improve our understanding
ofLimulus population dynamics in three Cape Cod estuaries (Stage Harbor, Pleasant Bay, and Barnstable Harbor) with differing harvest
pressures. The results suggest that a low level of harvest is sustainable, particularly if timing and size-specificity of
the harvest are taken into consideration. The model also suggests that efforts should be made to avoid harvesting crabs prior
to sexual maturity, as the population growth rate is more sensitive to changes in survival rate of older juveniles than of
other stages. The information the model provides could assist local and regional fisheries managers in establishing appropriate
regulations for the horseshoe crab fishery. 相似文献
557.
The liquefaction of clayey soils under cyclic loading 总被引:4,自引:0,他引:4
This paper seeks to investigate the liquefaction of clayey soils, a phenomenon that has been the trigger for many natural disasters in the last few decades, including landslides. Research was conducted on artificial clay-sand mixtures and natural clayey soils collected from the sliding surfaces of earthquake-induced landslides. The undrained response of normally consolidated clayey soils to cyclic loading was studied by means of a ring-shear apparatus. For the artificial clay-sand mixtures, it was found that the presence of a small amount of bentonite (≤ 7%) would cause rapid liquefaction, while a further increase in bentonite content (≥ 11%) produced the opposite effect of raising soil resistance to liquefaction by a significant degree. It was demonstrated that the bentonite-sand mixture was considerably more resistant to liquefaction than the kaolin-, and illite-mixtures, given the same clay content. The test results of plastic soils revealed the significant influence of plasticity on the liquefaction resistance of soil. The microfabric of clayey soil was investigated by means of a scanning electron microscope. The analysis showed that the liquefaction potential of soil was strongly related to certain particle arrangements. For example, soil vulnerable to liquefaction had an open microfabric in which clay aggregations generally gathered at the sand particle contact points, forming low-strength “clay bridges” that were destroyed easily during cyclic loading. On the other hand, the microfabric of soil that was resistant to liquefaction appeared to be more compact, with the clay producing a matrix that prevented sand grains from liquefying. In the case of the natural soils, the obtained results indicated that their cyclic behavior was similarly influenced by factors such as clay content, clay mineralogy and plasticity. The relation between the liquefaction potential of natural soil and its microfabric was thus also established. On the basis of the obtained results, the authors posited an explanation on the mechanism of liquefaction for clayey soil. 相似文献
558.
559.
Akimiski Island contains good examples of emergent coastal landscapes of a cold mesotidal inland sea. The Paleozoic reefal trend of the Attawapiskat Formation dictated the overall shape and the main structure of the island. Differential erosion and deposition by Pleistocene glaciers have fluted the island in a north-south direction. That surface was later modified by emergent landforms developed in the last 3500–4000 yrs.The modern steeper southern area reaches an elevation of 60 m approximately 3 km inland from the southern shoreline, and contains well-developed sandy and gravelly longitudinal beach ridges and spits, now inactive and covered by a lichen-rich taiga (boreal forest). The flatter, northern part of the island shows a wide transition between the primarily erosional, sand-starved, coastal marshland and the inland organic-rich fens. Partially paludified longitudinal and transversal beach ridges subdivide those northern flat wetlands forcing a straight course to the north-flowing streams.The vegetation zonations of the marshes are as varied as the coasts, facing different oceanographic conditions. Longshore and tidal currents affect the western and southern coasts greatly. Tides, waves and sea ice affect the others more. The marshes resemble those of the mainland in having well-defined Puccinellia phryganodes lower marsh, Carex subspathacea upper marsh, and peat-forming coastal fens. Some components of the marshes of the island, such as isolated mounds with vegetation minisequences, incipient permafrost features generated by seasonal frozen ground conditions, and intense grazing by large populations of geese, are typical of cold settings, more commonly found along mainland coasts farther to the north in James Bay and Hudson Bay. 相似文献
560.
Akimiski Strait is a wide (17–20 km), shallow, emergent (0.70 cm per century) waterway in James Bay. It is localized in a saddle of a Paleozoic reef track, which has been enhanced and molded by Pleistocene glaciers. Drumlinoid ridges form the till cores of shoals and islets of the strait. The boundary conditions of the strait change throughout the year, as it is covered by ice for six months. During spring break-up the strait remains clogged with ice at its northern approach for several weeks, and acts as a large tidal inlet. It is during this period that most of the fluvial sediments are carried to sea. Other sediments are obtained by erosion of the Pleistocene tills and Holocene subtidal clays and silts exposed in nearshore areas. Resuspension of nearshore material is achieved through the action of wind-driven, short choppy waves and ice scour. Tides are the most important process for the redistribution of sediments along the coast, both flooding onshore and flooding and ebbing into and out from the strait generating locally powerful (2 m s?1) reversing currents. Ice rafting and ice pushing are important processes in this frigid environment, particularly in upwind sides of shaols, and at/or near river mouths.Different intertidal sedimentary sequences develop as functions of sediment supply and exposure of the environments to ice, currents and waves. The eastern shores and the southern shoals of the strait develop pebble lags over till, covered by thin (5–20 cm) drapes of silty sand trapped and protected from erosion by algae. In these shores and in emerging small islands significant sedimentation (1–1.5 m thick) occurs in the marshes where the suspended load of tidal waters is trapped by vegetation. The western shores of the strait receive considerable amounts of sediment from large rivers and are affected by strong tidal longshore currents. Thick (3–4 m) and narrow tidal flats and marshes develop on the maincoast. The shoals of the northern part of the strait have characteristic sediments. Those near the western shore have thin (up to 80 cm) tidal silty sand deposits, locally heavily burrowed by Macoma balthica. Those strung across the northern approach to the strait have well-developed, thin, coarse sand dune fields, indicating a prevalent ebb flow out of the strait. 相似文献