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   244篇
  免费   14篇
  国内免费   7篇
大气科学   4篇
地球物理   61篇
地质学   89篇
海洋学   56篇
天文学   32篇
综合类   2篇
自然地理   21篇
  2024年   1篇
  2023年   1篇
  2021年   5篇
  2019年   2篇
  2018年   2篇
  2017年   9篇
  2016年   11篇
  2015年   12篇
  2014年   12篇
  2013年   13篇
  2012年   13篇
  2011年   20篇
  2010年   18篇
  2009年   12篇
  2008年   15篇
  2007年   9篇
  2006年   12篇
  2005年   14篇
  2004年   13篇
  2003年   9篇
  2002年   1篇
  2001年   6篇
  2000年   6篇
  1998年   8篇
  1997年   2篇
  1996年   1篇
  1995年   2篇
  1994年   4篇
  1993年   5篇
  1992年   2篇
  1989年   3篇
  1988年   3篇
  1987年   2篇
  1986年   1篇
  1984年   1篇
  1983年   2篇
  1982年   2篇
  1981年   2篇
  1980年   1篇
  1979年   2篇
  1978年   1篇
  1977年   1篇
  1976年   2篇
  1972年   1篇
  1969年   1篇
排序方式: 共有265条查询结果,搜索用时 0 毫秒
161.
野生动物友好型耕作(Wildlife-friendly Farming,WFF)实践是一种维持农田生态恢复的良好方式。本研究的目的在于探讨影响日本佐渡岛农民实施WFF的因素和提出影响这种实践传播的政策。对佐渡岛上把大米销售给日本农业合作社(Japan Agricultural Cooperatives,JA)的5010个农户进行了问卷调查,总共收到2231农户反馈问卷(反馈率45%)。通过与未经认证农民的态度和属性对比,我们确定了影响农民实施WFF的关键因素。与未经认证的农民相比,已认证的农民:(1)对生物多样性和WFF的经济效益表现出更高的兴趣;(2)拥有更多的已认证农民朋友;(3)感受到了来自消费者的更多的压力和期望;(4)当实施WFF时,不受恶劣劳动力或农田条件的限制。为了在佐渡岛进一步宣传WFF,我们建议把WFF实践对稻田生物多样性的有效性信息应用到公共教育中,并在非认证农民和已认证农民之间、农民与消费者之间创建交流的机会。  相似文献   
162.
We report a detailed examination about the relationship between the evolution of the Hα flare ribbons and the released magnetic energy during the April 10 2001 flare. In the Hα images, several bright kernels are observed in the flare ribbons. We identified the conjugated foot-points, by analyzing the lightcurves at each Hα kernels, and showed their connectivities during the flare. Then, based on the magnetic reconnection model, we calculated quantitatively the released energy by using the photospheric magnetic field strengths and separation speeds of the Hα flare ribbons. Finally, we examined the downward motions which are observed at the Hα kernels. We found that the stronger the red-asymmetry tends to be associated with the brighter the Hα kernel.  相似文献   
163.
Abstract— Mn‐Cr systematics in phosphates (sarcopside, graftonite, beusite, galileiite, and johnsomervilleite) in IIIAB iron meteorites were investigated by secondary ion mass spectrometry (SIMS). In most cases, excesses in 53Cr are found and δ53Cr is well correlated with Mn/Cr ratios, suggesting that 53Mn was alive at the time of IIIAB iron formation. The inferred Mn‐Cr “ages” are different for different phosphate minerals. This is presumably due to a combined effect of the slow cooling rates of IIIAB iron meteorites and the difference in the diffusion properties of Cr and Mn in the phosphates. The ages of sarcopside are the same for the IIIAB iron meteorites. Johnsomervilleite shows apparent old ages, probably because of a gain of Cr enriched in 53Cr during the closure process. Apparently, old Mn‐Cr ages reported in previous studies can also be explained in a similar way. Therefore, the IIIAB iron meteorites probably experienced identical thermal histories and thus derived from the core of a parent body. Thermal histories of the parent body of IIIAB iron meteorites that satisfy the Mn‐Cr chronology and metallographic cooling rates were constructed by computer simulation. The thermal history at an early stage (<10 Ma after CAI formation) is well determined, though later history may be more model‐dependent. It is suggested that relative timing of various events in the IIIAB parent body may be estimated with the aid of the thermal history. There is a systematic difference in Mn and Cr concentrations in various minerals (phosphates, sulfide, etc.) among the IIIAB iron meteorites, which seems to be mainly controlled by redox conditions.  相似文献   
164.
It is shown that the direct current dissipation is very unlikely to be the heat source of the coronal loop, because it accompanies unacceptably high heating rate in the chromospheric portion of the loop. This also suggests that a rather weak current density can supply the heat to a small (R < 107 cm) chromospheric magnetic features. A larger magnetic element may be heated by the direct current dissipation only if the current changes directions within a single element so that the generated magnetic field is sufficiently weak to insure MHD stability.  相似文献   
165.
Satoshi Hinata 《Solar physics》1987,109(2):321-333
As the electrical conductivity along the magnetic field in solar atmosphere is large, parallel electric fields have been neglected in most investigations. We will first demonstrate their importance for post-flare loops, and then introduce a model for them which takes into account the effect of parallel electric fields. The electric field calculated from the model is consistent with the electric field observed by Foukal et al. (1983).  相似文献   
166.
167.
We have conducted geochemical and mineralogical investigations of the rare earth and yttrium (REY)‐rich mud from the Minami‐Torishima area in the Pacific in order to clarify the concentration of REY and their host‐phase in the mud. X‐ray diffraction analysis shows that the mud is mainly composed of phillipsite, fluorapatite, quartz, albite, illite and montmorillonite. Whole‐rock CaO, P2O5 and total REY contents of the mud are positively correlated. Relative abundance of apatite is also positively correlated to P2O5 and total REY contents. These correlations suggest that apatite is the main host of the P2O5 and REY in the mud. We make in situ compositional analyses of constituent minerals in the REY mud. The results show that the apatite is abundant in REY (9300–32,000 ppm) and is characterized by a negative Ce anomaly and enrichment in heavy rare‐earth elements. This abundance and composition of REY of the mud is similar those of fish debris apatites. In contrast, phillipsite is less abundant in REY (60–170 ppm). Therefore we conclude that the main REY host phase of the mud is apatite.  相似文献   
168.
Coupled hydro‐mechanical processes in granular media represent the interaction between solid particles movement and fluid flow during external and/or internal loading. The processes attract attention in geotechnical engineering since they cause many disasters such as landslide, slope collapse, boiling/quicksand and soil liquefaction, etc. This study presents a numerical method combining discontinuous deformation analysis (DDA) for mechanical calculation and finite element method for fluid flow simulation to model the interaction between solid particles' movement and fluid flow from microscopic point of view. The term of hydraulic pressure was newly formulated and introduced to the original DDA. The simplified examples are given to verify the new method, and the computational results correlate well with the theoretical calculations, further development is also considered. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   
169.
Hengjian  Lu  Kohiyama  Masayuki  Horie  Kei  Maki  Norio  Hayashi  Haruo  Tanaka  Satoshi 《Natural Hazards》2003,29(3):387-403
The relationship between building damage patterns and human casualties in Nishinomiya City – one of the most heavily damaged cities in the 1995 Hanshin-Awaji Earthquake Disaster – was investigated using photographs of damaged buildings. First, the photographs of buildings in which casualties occurred were identified, and the building damage patterns were judged based on the photographs considering the existence of survival space. Then the relationship between the building damage pattern and casualty occurrence, and the characteristics of casualty distribution, were investigated. The main findings were as follows: Most casualties occurred in relatively old two-story wooden buildings in which the ground floor completely collapsed without survival space; casualties occurred at all building damage levels including ``no damage', and it can be seen that building damage is the major, but not the sole cause, of casualties in an earthquake; in Nishinomiya City, the regional distributions of casualties due to the collapse of buildings that left no survival space is similar to that of casualties due to other types of building damage.  相似文献   
170.
We review the post-glacial climate variability along the East Antarctic coastline using terrestrial and shallow marine geological records and compare these reconstructions with data from elsewhere. Nearly all East Antarctic records show a near-synchronous Early Holocene climate optimum (11.5–9 ka BP), coinciding with the deglaciation of currently ice-free regions and the optimum recorded in Antarctic ice and marine sediment cores. Shallow marine and coastal terrestrial climate anomalies appear to be out of phase after the Early Holocene warm period, and show complex regional patterns, but an overall trend of cooling in the terrestrial records. A Mid to Late Holocene warm period is present in many East Antarctic lake and shallow coastal marine records. Although there are some differences in the regional timing of this warm period, it typically occurs somewhere between 4.7 and 1 ka BP, which overlaps with a similar optimum found in Antarctic Peninsula terrestrial records. The differences in the timing of these sometimes abrupt warm events in different records and regions points to a number of mechanisms that we have yet to identify. Nearly all records show a neoglacial cooling from 2 ka BP onwards. There is no evidence along the East Antarctic coastline for an equivalent to the Northern Hemisphere Medieval Warm Period and there is only weak circumstantial evidence in a few places for a cool event crudely equivalent in time to the Northern Hemisphere's Little Ice Age. There is a need for well-dated, high resolution climate records in coastal East Antarctica and particularly in Terre Adélie, Dronning Maud Land and Enderby Land to fully understand the regional climate anomalies, the disparity between marine and terrestrial records, and to determine the significance of the heterogeneous temperature trends being measured in the Antarctic today.  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