首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   405篇
  免费   27篇
  国内免费   1篇
测绘学   7篇
大气科学   31篇
地球物理   98篇
地质学   195篇
海洋学   41篇
天文学   39篇
自然地理   22篇
  2024年   1篇
  2022年   2篇
  2021年   8篇
  2020年   7篇
  2019年   6篇
  2018年   8篇
  2017年   14篇
  2016年   22篇
  2015年   24篇
  2014年   15篇
  2013年   27篇
  2012年   27篇
  2011年   30篇
  2010年   22篇
  2009年   34篇
  2008年   23篇
  2007年   31篇
  2006年   18篇
  2005年   25篇
  2004年   6篇
  2003年   10篇
  2002年   9篇
  2001年   8篇
  2000年   6篇
  1999年   5篇
  1998年   2篇
  1997年   2篇
  1996年   3篇
  1994年   4篇
  1993年   3篇
  1992年   2篇
  1991年   1篇
  1989年   1篇
  1988年   1篇
  1987年   1篇
  1986年   2篇
  1985年   2篇
  1984年   5篇
  1983年   1篇
  1982年   1篇
  1981年   2篇
  1977年   1篇
  1976年   2篇
  1975年   3篇
  1974年   2篇
  1973年   1篇
  1971年   1篇
  1970年   2篇
排序方式: 共有433条查询结果,搜索用时 109 毫秒
311.
There are remarkable similarities in the structure of loop prominences when observed in H and coronal lines, although the lines arise from extremely different excitation conditions. This leads to the consideration of a multi-component model, where different emission lines come from different elements of the structure. The late phases of a large west limb loop prominence system followed by a surge were recorded at Haleakala Observatory on March 6, 1970. Simultaneous filter-grams in H and 5303 were obtained, together with spectra at three heights in the prominence over the range 3850–5950 Å. The positions of bright knots of emission as seen in the green line are compared with associated H knots. With these relative positions determined, the filtergrams are superimposed to demonstrate the two dimensional spatial relationship between H and 5303 structures. The results support a model of cool loops within a closely associated hot loop system.  相似文献   
312.
Cores and outcrops from the southern shore of Lake Biel were studied to reconstruct the nearshore environment of the lake between ca. 12000 and 5000 yr BP. Core correlations were established by lithostratigraphical and pollen analytical correlations. From the Allerød to the Preboreal time quiet hydrodynamic conditions favoured the deposition of lake marl in the littoral zone and peat on the shore. Between the Preboreal(?) and the Atlantic the littoral zone shows a higher hydrodynamic environment with allochthonous material, whereas peat and clay layers are recorded from the shore. During the Older Atlantic severe erosional episodes caused the erosion of Boreal, Preboreal and Younger Atlantic layers. The previously described long hiatus between the Allerød and the Boreal time can now be connected with these erosional episodes. From Younger Atlantic to Subboreal time the littoral zone displays quiet conditions again with sedimentation of lake marl. On the basis of these results a lake level curve for Lake Biel is proposed: high lake level stands can be traced during the Allerød, Boreal, Older Atlantic and Younger Atlantic biozones; low lake level stands are found during the Allerød, Younger Dryas, Preboreal and Older Atlantic biozones.  相似文献   
313.
314.
The unsaturated zone (UZ) retains aqueous solutions against gravity by capillary forces. This suction state corresponds to a decreasing internal pressure of the water, which modifies its thermodynamic properties. Accordingly, the speciation of solutes and the solubility of solids and gases in such capillary solutions change. The volumetric capillary water content of the soil at high suction can be calculated extrapolating the water retention curves (WRC) with the Rossi–Nimmo model. Interestingly, several tens of liters per cubic meter of soil can be thus suctioned, a sufficiently large volume to support that: (1) capillary water is not restricted to nanosized pores, which means it disobeys the Young–Laplace law and is metastable with respect to vapor (superheating); and (2) the geochemistry of capillary solutions might significantly influence the subsurface mass transfer. Two field situations are here interpreted using the capillary thermodynamic properties: (1) the trapping of sand grains during the growth of desert roses (gypsum), and (2) the development of abnormal paragenetic sequences in some saprolites.The capillary approach is extended to the soil solids, so that the micro-mineralogy can be explicitly (though sketchily) integrated in the calculations. The key conclusion is that capillarity changes the saturation indexes (and so the reaction rates) at given solution composition, in a way consistent with the field observations. This perspective amounts to geochemically distinguishing the capillary and percolating solutions, which is interestingly analogous to the immobile and mobile water distinction already often integrated in UZ flow models.  相似文献   
315.
316.
The article examines an alternative food network evolution in the post-communist context in Czechia in general and farm shop developments in particular. The study is based on semi-structured interviews with farm shopkeepers operating in Czechia. The results revealed that the development of the farm shop network was significantly shaped by the communist past of the country. While consumers’ demand for quality food contributed to the initiation of developments in local food movements such as farm shops in Czechia, the inconvenient structure of the farming sector, comprising a limited number of small-scale farmers, hindered the development. Although the farm shops usually operate as middlemen by reselling goods produced by a range of farmers, farm shopkeepers expressed significant awareness of the sustainability principles of the local food concept and dissociated themselves from unethical practices of large retailers. Nevertheless, the author finds that the local concept is used in a flexible manner to meet customers’ demands. Despite obstacles faced by farmers when developing their own farm shops, there is significant potential in ‘Czech-type’ farm shops operating solely as retailers. However, the author concludes that the threat of the diversion from the sustainability principles of the local food concept should not be neglected.  相似文献   
317.
As a result of burgeoning social and environmental issues, localisation movements such as Transition Towns are emerging. However, tension exists as to the best scale and method to create and maintain this community activism. In this study, we explore the activities of two Transition Towns in Aotearoa New Zealand through a qualitative mixed methodology, and argue that relational notions of scale and the notion of progressive localisms enable an appreciation of the locally rooted but outward looking nature of Transition activities. In turn, this creates space to acknowledge the diffuse nature of change, redefine success and sustain momentum.  相似文献   
318.
319.
The Tacquet Formation in southwestern Mare Serenitatis has unusually low visible albedos for the Al/Si intensity ratios measured by the Apollo 15 X-ray fluorescence experiment. This is a contradiction of the demonstrated good correlation between Al/Si and visible albedo data. To understand why this situation exists, a correlation study has been undertaken. This study is based on Al/Si intensity ratios and includes such other remote sensing data as visible and near IR spectral reflectance, color-difference and color-ratio photography, and broad spectrum visible albedo measurements. The Tacquet unit appears to have: (1) an average Al/Si intensity which is intermediate between mare and highland values (=0.21); (2) visible albedos equivalent to the two darkest mare units in the Tranquillitatis-Serenitatis area; (3) a low titanium content; and (4) a relatively high glass-to-crystal ratio. The evidence indicates that the discrepancy between visible albedos and Al/Si intensities in the vicinity of the Tacquet Formation might be due primarily to the addition of highland-type ejecta from the crater, Menelaus, onto a low albedo mare unit. The possibility that the formation consists of dark, high Al material such as spinel, perhaps extruded from the Menelaus Rilles, cannot be excluded, although no such material has been sampled by the lunar missions.  相似文献   
320.
A nonequilibrium stochastic coalescence model for terrestrial planetary accretion is developed by using an approximation to the Safronov-Golovin solution for the scalar transport equation with linear kernel. According to this model, formation of comparatively massive objects occurs quite rapidly during the early stages of accretive evolution in a given terrestrial planetesimal population, while during late growth stages, an increasingly substantial fraction of total population mass becomes incorporated into progressively fewer, relatively very large bodies. The model also implies that the (conservative) growth rate of the population's largest member varies directly as its mass, and further suggests that this growth rate may not decline significantly until very nearly final planetary mass is attained.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号