Magnetic reconnection as the cause of a photospheric canceling feature and mass flows in a filament     

Litvinenko  Yuri E.  Martin  Sara F. 《Solar physics》1999,190(1-2):45-58

Magnetic reconnection in the temperature minimum region of the solar photosphere can account for the canceling magnetic features on the Sun. Litvinenko (1999a) showed that a reconnection model explains the quiet-Sun features with the magnetic flux cancelation rate of order 10¹⁷ Mx hr⁻¹. In this paper the model is applied to cancelation in solar active regions, which is characterized by a much larger rate of cancelation ∖ ge10¹⁹ Mx hr⁻¹. In particular, the evolution of a photospheric canceling feature observed in an active region on July 2, 1994 is studied. The theoretical predictions are demonstrated to be in reasonable agreement with the measured speed of approaching magnetic fragments, the magnetic field in the fragments, and the flux cancelation rate, deduced from the combined Big Bear Hα time-lapse images and videomagnetograms calibrated against the daily NSO/Kitt Peak magnetogram. Of particular interest is the prediction that photospheric reconnection should lead to a significant upward mass flux and the formation of a solar filament. Hα observations indeed showed a filament that had one of its ends spatially superposed with the canceling feature. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005284116353  相似文献   

Geomorphic process from topographic form: automating the interpretation of repeat survey data in river valleys          下载免费PDF全文

Alan Kasprak  Joshua Caster  Sara G. Bangen  Joel B. Sankey 《地球表面变化过程与地形》2017,42(12):1872-1883

The ability to quantify the processes driving geomorphic change in river valley margins is vital to geomorphologists seeking to understand the relative role of transport mechanisms (e.g. fluvial, aeolian, and hillslope processes) in landscape dynamics. High‐resolution, repeat topographic data are becoming readily available to geomorphologists. By contrasting digital elevation models derived from repeat surveys, the transport processes driving topographic changes can be inferred, a method termed ‘mechanistic segregation.’ Unfortunately, mechanistic segregation largely relies on subjective and time consuming manual classification, which has implications both for its reproducibility and the practical scale of its application. Here we present a novel computational workflow for the mechanistic segregation of geomorphic transport processes in geospatial datasets. We apply the workflow to seven sites along the Colorado River in the Grand Canyon, where geomorphic transport is driven by a diverse suite of mechanisms. The workflow performs well when compared to field observations, with an overall predictive accuracy of 84% across 113 validation points. The approach most accurately predicts changes due to fluvial processes (100% accuracy) and aeolian processes (96%), with reduced accuracy in predictions of alluvial and colluvial processes (64% and 73%, respectively). Our workflow is designed to be applicable to a diversity of river systems and will likely provide a rapid and objective understanding of the processes driving geomorphic change at the reach and network scales. We anticipate that such an understanding will allow insight into the response of geomorphic transport processes to external forcings, such as shifts in climate, land use, or river regulation, with implications for process‐based river management and restoration. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Reliability based assessment of axial pile bearing capacity: static analysis,SPT and CPT-based methods     

Sara Heidarie Golafzani  Abolfazl Eslami 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2020,14(3):216-230

ABSTRACT

Since piles are one of the major geotechnical foundation systems, estimation of their axial bearing capacity is of great importance. Employing different design methods, resulting in a wide range of bearing capacity estimations, complicates the selection of an appropriate design scheme and confirms the existence of model error along with the inherent soil variability in bearing capacity prediction. This paper tends to evaluate different predictive methods in Reliability-Based Design (RBD) framework. In this regard, different static analyses, SPT and CPT-based methods are considered to evaluate which approaches collectively and which method individually, have more reliable predictions for compiled data bank. In order to assess reliability indices and resistance factors, two approaches have been considered, i.e. First Order Second Moment method (FOSM) and First Order Reliability Method (FORM). To investigate the reliability indices for different methods in both RBD approaches, various safety factors and loading ratios have been considered. Also, the Load and Resistance Factor Design (LRFD) resistance factors are calibrated for different target reliability indices and loading ratios. Results show that CPT-based methods are more reliable among other methods. Furthermore, the estimated efficiency ratio, i.e. the ratio of resistance factor to resistance bias factor, confirms this agreement.  相似文献   

Debris-flow Indicator for an early warning system in the Aosta valley region     

Ponziani  Michel  Pogliotti  Paolo  Stevenin  Hervé  Ratto  Sara Maria 《Natural Hazards》2020,104(2):1819-1839

Natural Hazards - Aosta Valley, an Alpine region in north-western Italy, has an early warning system (EWS) that issues hydrogeological alerts based on hydrological modelling and rainfall thresholds...  相似文献   

The fusion crust of the Winchcombe meteorite: A preserved record of atmospheric entry processes     

Matthew J. Genge  Luke Alesbrook  Natasha V. Almeida  Helena C. Bates  Phil A. Bland  Mark R. Boyd  Mark J. Burchell  Gareth S. Collins  Luke T. Cornwell  Luke Daly  Hadrien A. R. Devillepoix  Matthias van Ginneken  Ansgar Greshake  Daniel Hallatt  Christopher Hamann  Lutz Hecht  Laura E. Jenkins  Diane Johnson  Rosie Jones  Ashley J. King  Haithem Mansour  Sarah McMullan  Jennifer T. Mitchell  Gavyn Rollinson  Sara S. Russell  Christian Schröder  Natasha R. Stephen  Martin D. Suttle  Jon D. Tandy  Patrick Trimby  Eleanor K. Sansom  Vassilia Spathis  Francesca M. Willcocks  Penelope J. Wozniakiewicz 《Meteoritics & planetary science》2024,59(5):948-972

Fusion crusts form during the atmospheric entry heating of meteorites and preserve a record of the conditions that occurred during deceleration in the atmosphere. The fusion crust of the Winchcombe meteorite closely resembles that of other stony meteorites, and in particular CM2 chondrites, since it is dominated by olivine phenocrysts set in a glassy mesostasis with magnetite, and is highly vesicular. Dehydration cracks are unusually abundant in Winchcombe. Failure of this weak layer is an additional ablation mechanism to produce large numbers of particles during deceleration, consistent with the observation of pulses of plasma in videos of the Winchcombe fireball. Calving events might provide an observable phenomenon related to meteorites that are particularly susceptible to dehydration. Oscillatory zoning is observed within olivine phenocrysts in the fusion crust, in contrast to other meteorites, perhaps owing to temperature fluctuations resulting from calving events. Magnetite monolayers are found in the crust, and have also not been previously reported, and form discontinuous strata. These features grade into magnetite rims formed on the external surface of the crust and suggest the trapping of surface magnetite by collapse of melt. Magnetite monolayers may be a feature of meteorites that undergo significant degassing. Silicate warts with dendritic textures were observed and are suggested to be droplets ablated from another stone in the shower. They, therefore, represent the first evidence for intershower transfer of ablation materials and are consistent with the other evidence in the Winchcombe meteorite for unusually intense gas loss and ablation, despite its low entry velocity.  相似文献   

The impact of submersed aquatic vegetation on the development of river mouth bars     

Sara Lera  William Nardin  Lawrence Sanford  Cindy Palinkas  Roberto Guercio 《地球表面变化过程与地形》2019,44(7):1494-1506

This work is inspired by the sudden resurgence of the submersed aquatic vegetation (SAV) bed in the Chesapeake Bay (USA). Because the SAV bed occurs at the mouth of the Bay's main tributary (Susquehanna River), it plays a significant role in modulating sediment and nutrient inputs from the Susquehanna to the Bay. Previous model studies on the impact of submersed aquatic vegetation on the development of river mouth bars lacked a complete mechanistic understanding. This study takes advantage of new advances in 3D computational models that include explicit physical-sedimentological feedbacks to obtain this understanding. Specifically, we used Delft3D, a state-of-the-art hydrodynamic model that provides fine-scale computations of three-dimensional flow velocity and bed shear stress, which can be linked to sediment deposition and erosion. Vegetation is modeled using a parameterization of hydraulic roughness that depends on vegetation height, stem density, diameter, and drag coefficient. We evaluate the hydrodynamics, bed shear stresses, and sediment dynamics for different vegetation scenarios under conditions of low and high river discharge. Model runs vary the vegetation height, density, river discharge, and suspended-sediment concentration. Numerical results from the idealized model show that dense SAV on river mouth bars substantially diverts river discharge into adjacent channels and promotes sediment deposition at ridge margins, as well as upstream bar migration. Increasing vegetation height and density forms sandier bars closer to the river mouth and alteration of the bar shape. Thus, this study highlights the important role of SAV in shaping estuarine geomorphology, which is especially relevant for coastal management. © 2019 John Wiley & Sons, Ltd.  相似文献   

Mapping of groundwater prospective zones integrating remote sensing,geographic information systems and geophysical techniques in El-Qaà Plain area,Egypt     

Sara M. Abuzied  Hamed A. Alrefaee 《Hydrogeology Journal》2017,25(7):2067-2088

The geospatial mapping of groundwater prospective zones is essential to support the needs of local inhabitants and agricultural activities in arid regions such as El-Qaà area, Sinai Peninsula, Egypt. The study aims to locate new wells that can serve to cope with water scarcity. The integration of remote sensing, geographic information systems (GIS) and geophysical techniques is a breakthrough for groundwater prospecting. Based on these techniques, several factors contributing to groundwater potential in El-Qaà Plain were determined. Geophysical data were supported by information derived from a digital elevation model, and from geologic, geomorphologic and hydrologic data, to reveal the promising sites. All the spatial data that represent the contributing factors were integrated and analyzed in a GIS framework to develop a groundwater prospective model. An appropriate weightage was specified to each factor based on its relative contribution towards groundwater potential, and the resulting map delineates the study area into five classes, from very poor to very good potential. The very good potential zones are located in the Quaternary deposits, with flat to gentle topography, dense lineaments and structurally controlled drainage channels. The groundwater potential map was tested against the distribution of groundwater wells and cultivated land. The integrated methodology provides a powerful tool to design a suitable groundwater management plan in arid regions.  相似文献   

Response surface-based robust geotechnical design of supported excavation – spreadsheet-based solution     

Sara Khoshnevisan  Lei Wang  C. Hsein Juang 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2017,11(1):90-102

ABSTRACT

The robust geotechnical design (RGD) approach which involves optimization to obtain a design that is safe, cost-efficient, and robust in the face of uncertainties, can be computationally challenging for complex geotechnical structures. In this study, the RGD approach has become practical by introducing a response surface as a surrogate to finite element- or finite difference-based computer code that is used for analyzing the system, and developing a fast algorithm for the optimization process. For demonstration purposes, a real-world supported excavation project is designed using this modified RGD approach and it is compared with the one designed by a local expert.  相似文献   

Glacial and post-glacial sedimentary processes in the Irish Rockall Trough from an integrated acoustic analysis of near-seabed sediments     

Fabio Sacchetti  Sara Benetti  Rory Quinn  Colm Ó Cofaigh 《Geo-Marine Letters》2013,33(1):49-66

Near-seabed (<50 m) sediments were studied throughout the Irish sector of the Rockall Trough (ca. 123,000 km²) based on a combined analysis of shallow seismic (3.5 kHz) and multibeam swath data acquired by the Irish National Seabed Survey and reprocessed here at higher resolution. The detailed identification of seven acoustic facies served to classify the Rockall Trough into six main sedimentary provinces, incorporating the well-known Feni Drift, Donegal-Barra Fan and Rockall Bank mass flow. In the northern part of the study area, extensive mass transport deposits from both flanks of the trough are the dominant depositional features. Debris flow deposits formed by ice streaming of the British-Irish ice sheet characterise most of the Donegal-Barra Fan, whereas turbidite deposition occurs towards the toe of the fan. On the western margin of the trough, the post-glacial Rockall Bank mass flow deposit displays a rough topography with no acoustic penetration. Several failure scarps are visible on the flank of the bank where the mass flow originated, and pass downslope into large sediment lobes and smaller debris flow deposits. Smaller-scale mass transport deposits were also discovered close to some canyons indenting the eastern slope. High seismic penetration characterises the Feni contourite drift deposit, and precise mapping of its geographical extent shows that it is considerably wider than previously reported. The sediment waves that drape this drift are interpreted as predominantly relict features, and their varied geometry suggests a complex oceanographic regime. In the deeper part of the trough, the data reveal novel evidence of the widespread occurrence (about 12,000 km²) of distinct seismic and backscatter signatures indicating the possible presence of fluid escape structures within fine-grained sediments of mixed contouritic, hemipelagic and turbiditic origin. Sediment overloading and increased pore pressure resulting from extensive mass wasting to the north of the area is a likely cause of dewatering-rooted fluid migration towards the seabed, but further investigations are required to confirm the nature and origin of such fluids in the Rockall Trough.  相似文献   

Species loss in the brown world: are heterotrophic systems inherently stable?     

Michael J. Rubbo  Lisa K. Belden  Sara I. Storrs-Mendez  Jonathan J. Cole  Joseph M. Kiesecker 《Aquatic Sciences - Research Across Boundaries》2012,74(3):397-404

Determining the effects of species loss on ecosystems has received considerable attention given the current threats many ecosystems are facing. A significant body of research has yielded many insights to this question, but this work has been limited by a focus on ecosystems where primary production plays a significant role in energy transfer. As many ecosystems rely on energy sources that are not derived from in situ production, there is a need to better understand how species loss will affect ecosystems of varying trophic states. To examine the effects of species loss on an ecosystem that is not reliant on in situ primary production, we manipulated the larval amphibian community of temporary forest ponds. These ponds are heterotrophic systems that rely on allochthonous inputs of detritus as a basal energy source. The larvae of two amphibian species that are prone to local extinction, wood frogs (Lithobates sylvatica) and spotted salamanders (Ambystoma maculatum), were removed from ponds and net ecosystem production was monitored. We found no effects of the removal of these top consumers on ecosystem functioning or on lower trophic groups (i.e., zooplankton, algae, bacteria). While amphibians can influence food web dynamics in other systems, their influence on system processes in temporary forest ponds appears to be limited. We hypothesize that the lack of any effects is due to the microbial degradation of detritus ??swamping?? the system, providing more than enough energy to maintain the food web, and/or due to omnivory dampening species interactions. These data indicate that the functioning of heterotrophic systems may be inherently stable due to internal dynamics that minimize interaction strengths among trophic groups.  相似文献