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931.
Andrew Millington Salem al-Hussein Roderic Dutton 《Applied geography (Sevenoaks, England)》1999,19(4):1059
The paper reports on preliminary observations from northern Jordan aimed at testing the view that people migrate from areas of relatively high potential for cultivation to the marginal semi-arid/arid frontier because of social differentiation, political factors or environmental constraints. Cultivated areas have been mapped from multi-date remotely sensed imagery, a typology of fields in the area has been constructed, and their dynamics between 1972 and 1992 analysed. In addition, semi-structured interviews with farmers attempted to understand the reasoning behind village growth, changing farming systems and cultivation practices. The findings are discussed in the context of the area's demography and national and regional shifts in economic policy. 相似文献
932.
Using the new data from Hipparcos and the TD-1 UV data, we fix the positions of three objects, HD 104237, HD 100546 and ε Cha in the HR diagram and discuss their theoretical evolutionary status. 相似文献
933.
The Leonid meteor storms of 1833 and 1966 总被引:2,自引:0,他引:2
D. J. Asher 《Monthly notices of the Royal Astronomical Society》1999,307(4):919-924
The greatest Leonid meteor storms since the late eighteenth century are generally regarded as being those of 1833 and 1966. They were evidently due to dense meteoroid concentrations within the Leonid stream. At those times, the orbit of Comet 55P/Tempel–Tuttle was significantly nearer that of the Earth than at most perihelion returns, but still some tens of Earth radii away. Significantly reducing this miss distance can be critical for producing a storm. Evaluation of differential gravitational perturbations, comparing meteoroids with the comet, shows that, in 1833 and 1966 respectively, the Earth passed through meteoroid trails generated at the 1800 and 1899 returns. 相似文献
934.
根据Hipparcos的资料和TD-1对εCha和HD100546的紫外观测资料讨论了εCha、HD104237和HD100546在赫罗图上的位置以及它们的演化状态. 相似文献
935.
撞击事件诱发核反应热力学过程及其在事件地质学研究中的意义 总被引:1,自引:0,他引:1
摘要:从大规模撞击事件的基本动力学和热力学公式推导出发,讨论了白垩纪-第三纪界线(K/T界线)事件能量随时间的释放过程,并对发生热核聚变反应的条件进行了分析。最后,还对其可能的事件地质学意义进行了讨论. 相似文献
936.
本文从水产无脊椎动物与环境的关系论述其来克丁物质的功能;来克丁与微生物间的相互作用包括相互作用中有关来克丁、微生物种类,作用部位和作用物质等。从来克丁在水产无脊椎动物识别外来物质,包括抵御病原体的作用看,健康发展的水产业之一项关键任务便是认识和掌握来克丁在御防病害活动中的机理。 相似文献
937.
An intensive survey has been conducted of the distributions of some chemical properties (dissolved oxygen, nutrients and carbonate
properties) in the Kuroshio/Oyashio Interfrontal Zone. Many low-salinity water patches were found down to depths of 640 m.
Each chemical property also showed anomalies in these patches, but the degree of variation showed a low correlation with salinity.
This may be due to the high variability of biological processes in the surface waters where these patches are formed. Vertical
profiles of the chemical properties were also observed along the Kuroshio extension axis from 140.50°E to 146.75°E. The concentrations
of nutrients and total carbonate (TC) in the water having densities greater than σθ=26.60 can be regarded as being formed by the isopycnal mixing of the Kuroshio component water and Oyashio component water
and biological degradation within the density surfaces. This implies that the transport of chemical properties by the diapycnal
mixing is negligible in these density layers in the K/O zone. 相似文献
938.
Oceanic current data in the warm pool region of the western equatorial Pacific measured by upward-looking moored Acoustic
Doppler Current Profilers at two equatorial sites (147°E and 154°E) and two off-equatorial sites (2°N and 2°S, 156°E) during
TOGA/COARE Intensive Observing Period (IOP) from November 1992 to February 1993 are used to examine short-term variabilities
in the upper layer above 160–240 m. In time series of the zonal and meridional currents in many layers, spectral peaks are
found at periods around 2 days and 4 days in addition to high energies in a period range longer than 10 days. The signal with
the period of about 2 days has significantly high energies at all sites, and its magnitude is higher for the meridional current
than for the zonal one. This signal is especially active in the first half of IOP from November to December in 1992. In this
period, the quasi-2-day signal in the current field is coherent between northern (2°N) and southern (2°S) stations, but it
has no evident relationship with that in the surface wind field around the stations. The quasi-4-day signal with the period
of about 4 days has highest energies in layers above 160 m at the southern station, and is coherent between northern and southern
stations. Besides, the signal at the station of 2°S has a significantly high coherence with that in the wind at the southern
station, suggesting that it is a local phenomenon. 相似文献
939.
用数值模型来模拟和验证加卸载响应比(LURRLoad/Unload ResponseRatio)理论.编制非均匀各向异性材料宏细观损伤破坏程序,模拟固体材料在损伤破坏时的加卸载过程,证实了加卸载响应比理论的正确性.结果表明,LURR这一反映地球介质临近破坏的指标,也可以用来预测材料的破坏.同时讨论了一些参数(如均匀程度和脆性程序)对LURR的影响,发现材料越呈脆性,或越均匀,LURR值上升得越晚,曲线越陡.该结果对用LURR指标预测地震具有一定启发. 相似文献
940.
Carlo Laj Herv Guillou Nadia Szeremeta Robert Coe 《Earth and Planetary Science Letters》1999,170(4):191-376
We have sampled a sequence of 107 lava flows in the Waianae series on Oahu, Hawaii, in two separate stretches. The first (51 flows) at Kaena Point extends from sea level to an altitude of 190 m, and the second (56 flows) follows the Satellite Tracking Station road from 218 m to 360 m above sea level. Thermal and af demagnetization yield very similar results, and reveal only normal polarities. K/Ar dating at five different horizons combined with the paleomagnetic results indicate that the sequence was emplaced entirely in the normal polarity interval between the Upper Mammoth and the Lower Kaena polarity transitions (3.22–3.11 Ma). Some of the flows are serially correlated. Filtering does not, however, drastically affect the final results. Inclinations are shallower than those expected from a centered dipole field. Although large (13°), the inclination anomaly is consistent with results from other sites at the same latitude. The scatter of the VGPs about the geographic pole is consistent with available data from other regions at similar latitudes. The scatter of the directions is also consistent with the predictions of recent statistical models of paleosecular variation. Therefore, these results do not support the hypothesis of a Pacific dipole window. 相似文献