60,666 research outputs found
Euler equation of the optimal trajectory for the fastest magnetization reversal of nano-magnetic structures
Based on the modified Landau-Lifshitz-Gilbert equation for an arbitrary
Stoner particle under an external magnetic field and a spin-polarized electric
current, differential equations for the optimal reversal trajectory, along
which the magnetization reversal is the fastest one among all possible reversal
routes, are obtained. We show that this is a Euler-Lagrange problem with
constrains. The Euler equation of the optimal trajectory is useful in designing
a magnetic field pulse and/or a polarized electric current pulse in
magnetization reversal for two reasons. 1) It is straightforward to obtain the
solution of the Euler equation, at least numerically, for a given magnetic
nano-structure characterized by its magnetic anisotropy energy. 2) After
obtaining the optimal reversal trajectory for a given magnetic nano-structure,
finding a proper field/current pulse is an algebraic problem instead of the
original nonlinear differential equation
Impacts of climate change on lake fluctuations in the Hindu Kush-Himalaya-Tibetan Plateau
Lakes in the Hindu Kush-Himalaya-Tibetan (HKHT) regions are crucial indicators for
the combined impacts of regional climate change and resultant glacier retreat. However, they lack
long-term systematic monitoring and thus their responses to recent climatic change still remain only
partially understood. This study investigated lake extent fluctuations in the HKHT regions over the
past 40 years using Landsat (MSS/TM/ETM+/OLI) images obtained from the 1970s to 2014. Influenced
by dierent regional atmospheric circulation systems, our results show that lake changing patterns
are distinct from region to region, with the most intensive lake shrinking observed in northeastern
HKHT (HKHT Interior, Tarim, Yellow, Yangtze), while the most extensive expansion was observed in
the western and southwestern HKHT (Amu Darya, Ganges Indus and Brahmaputra), largely caused
by the proliferation of small lakes in high-altitude regions during 1970s\u20131995. In the past 20 years,
extensive lake expansions (~39.6% in area and ~119.1% in quantity) were observed in all HKHT
regions. Climate change, especially precipitation change, is the major driving force to the changing
dynamics of the lake fluctuations; however, eects from the glacier melting were also significant,
which contributed approximately 31.9\u201340.5%, 16.5\u201339.3%, 12.8\u201329.0%, and 3.3\u20136.1% of runo to lakes
in the headwaters of the Tarim, Amu Darya, Indus, and Ganges, respectively. We consider that
the findings in this paper could have both immediate and long-term implications for dealing with
water-related hazards, controlling glacial lake outburst floods, and securing water resources in the
HKHT regions, which contain the headwater sources for some of the largest rivers in Asia that sustain
1.3 billion people
Geomorphometric assessment of the impacts of dam construction on river disconnectivity and flow regulation in the Yangtze basin
open4Rivers are under increasing pressure from anthropogenic impacts with incremental dam construction, experiencing global and regional alteration due to river disconnectivity, flow regulation, and sediment reduction. Assessing the cumulative impacts of dams on river disconnectivity in large river basins can help us better understand how humans disintegrate river systems and change the natural flow regimes. Using the Yangtze basin as the study area, this study employed three modified metrics (river connectivity index, RCI; basin disconnectivity index, BDI; and the degree of regulation for each river section, DOR) to evaluate the cumulative impacts on river disconnectivity over the past 50 years. The results indicated that the Yangtze had experienced strong alterations, despite varying degrees and spatial patterns. Among the major tributaries, the greatest impact (lowest RCI value) happened in the Wu tributary basin due to the construction of cascade dams on the main stem of the tributary, while the lowest impact (highest RCI value) happened in the Fu tributary basin, which still has no dams on its main stem. Collectively, rivers in the upper Yangtze reaches experienced more serious disturbances than their counterparts in the middle and lower reaches. The BDI results displayed that a substantial part of the Yangtze River, especially the Wu, Min, Jialing, and Yuan tributaries, only maintain connectivity among one to three representative river systems. No part of the Yangtze connects all the 12 representative river systems. This study also revealed that small dams can also exert significant impacts in flow regulation on regional river systems through their sheer number and density. The study results can help promote more environmentally sustainable river management policies in the Yangtze basin.openYang X.; Lu X.; Ran L.; Tarolli P.Yang, X.; Lu, X.; Ran, L.; Tarolli, P
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