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81.
Using a simplified form of the bremsstrahlung cross-section, we obtain an analytic expression for the intensity of electron-beam-produced hard X-ray emission with depth in solar flares. The results show that footpoint emission is more likely than previously thought, and we discuss these results in the light of recent observations.Presidential Young Investigator.NAS/NRC Research Associate, on leave from CNIE, San Miguel, Argentina. 相似文献
82.
83.
SHIRLEY MARIA LIMA SABINO RAQUEL FRANCO CASSINO MAKÊNIA OLIVEIRA SOARES GOMES ENEIDA MARIA ESKINAZI SANT'ANNA CRISTINA HELENA RIBEIRO ROCHA AUGUSTIN DIEGO ALVES DE OLIVEIRA 《第四纪科学杂志》2021,36(6):1028-1039
The Pandeiros wetland is a high biodiversity ecosystem located within a semiarid region of the Cerrado biome, a neotropical savanna. This large wetland is of key importance for ecological and hydrological balance in central Brazil and for the conservation of the Cerrado fauna and flora. In this study, we present the first palaeoecological investigation of the Pandeiros wetland based on pollen analysis of a palm swamp sediment core encompassing the Late Holocene. Our results show that the wetland was subject to multicentennial-scale oscillations in water availability during the Late Holocene; in particular, higher local humidity was documented between 4100 and 3100 cal a bp and from 2600 to 1000 cal a bp , and two events of drier local conditions occurred at approximately 2900 and 900 cal a bp. Our results also indicate a general decreasing trend in arboreal density in the Pandeiros River Basin from the beginning of the Late Holocene to the present, with the greatest expansion of dry forest occurring between 3600 and 3100 cal a bp. 相似文献
84.
Barros José Leandro Tavares Alexandre Oliveira Santos Pedro Pinto 《Natural Hazards》2021,106(1):757-784
Natural Hazards - The objective of the present study is to evaluate the relation between the spatial and temporal dynamics of Land Use and Land Cover (LULC) and the hydro-geomorphological processes... 相似文献
85.
Juske Horita António M. dos Santos Christopher A. Tulk Bryan C. Chakoumakos Veniamin B. Polyakov 《Physics and Chemistry of Minerals》2010,37(10):741-749
A neutron powder diffraction study of hydrogenated and deuterated brucite was conducted at ambient temperature and at pressures
up to 9 GPa, using a Paris–Edinburgh high-pressure cell at the WAND instrument of the ORNL High Flux Isotope Reactor. The
two materials were synthesized by the same method and companion measurements of neutron diffraction were conducted under the
same conditions. Our refinement results show that the lattice-parameters of the a axis, parallel to the sheets of Mg–O octahedra, decrease only slightly with pressure with no effect of H–D substitution.
However, the c axis of Mg(OD)2 is shorter and may exhibit greater compressibility with pressure than that of Mg(OH)2. Consequently, the unit-cell volume of deuterated brucite is slightly, but systematically smaller than that of hydrogenated
brucite. When fitted to a third-order Birch–Murnaghan equation in terms of the normalized unit-cell volume, values of the
bulk modulus for hydrogenated and deuterated brucite (K
0 = 39.0 ± 2.8 and 40.4 ± 1.3 GPa, respectively) are, however, indistinguishable from each other within the experimental errors.
The measured effect of H–D substitution on the unit-cell volume also demonstrates that brucite (and other hydrous minerals)
preferentially incorporate deuterium over hydrogen under pressure, suggesting that the distribution of hydrogen isotopes in
deep-earth conditions may differ significantly from that in near-surface environments. 相似文献
86.
The results of an observational campaign on the new δ Scuti pulsator HD 207331 are reported. The star was observed photometrically from August 26 to September 2, 2009 from the Observatorio San Pedro Mártir (0.84-m telescope, Mexico) and the Observatorio del Teide (0.80-m telescope, Spain). An overall run of 53.8 h of useful data was collected from the two sites. Four oscillation frequencies for HD 207331 have been found above a 99% confidence level. These results confirm the multiperiodic pulsation nature of the star suggested in previous observations with sparse data. Spectroscopic observations carried out in 2009 allowed us to derive its spectral type and luminosity class as well as to estimate its rotation rate. A simple comparison with models is performed. 相似文献
87.
Oderson Antônio de Souza Filho Adalene Moreira Silva Armando Zaupa Remacre Sérgio Sacani Sancevero Anne Elizabeth McCafferty Mônica Mazzini Perrotta 《Hydrogeology Journal》2010,18(4):905-916
Geostatistical modeling, using airborne and borehole electromagnetic data, was used to estimate electrical conductivity in groundwater within fractured paragneisses and migmatites in a semi-arid climate in northeastern Brazil. Despite the geologic heterogeneity of crystalline aquifers, the use of high resolution helicopter electromagnetic (HEM) data enabled the characterization of groundwater electrical conductivity where data from drilled wells were insufficient. The tacit assumption is that HEM measurements can be used to relate rock electrical conductivity to groundwater electrical conductivity. In this study, the HEM data were used as an external drift variable in non-stationary estimation and stochastic simulation to identify the variability of groundwater electrical conductivity. Validation tests, comparing predicted values for groundwater conductivity with measurements in new wells, confirmed the success of these models in locating fresh groundwater sources in crystalline bedrock. 相似文献
88.
Evaluation of climate change effects in a coastal aquifer in Morocco using a density-dependent numerical model 总被引:2,自引:1,他引:1
Júlio Ferreira Carneiro Mimoun Boughriba António Correia Yassine Zarhloule Abdelkrim Rimi Boubker El Houadi 《Environmental Earth Sciences》2010,61(2):241-252
A density-dependent numerical groundwater model was applied to study the climate change impact in a shallow aquifer in the Mediterranean coast of Morocco, the Saïdia aquifer. The stresses imposed to the model were derived from the IPCC emission scenarios and included recharge variation and sea level rise. The main effect of the climate change in the Saïdia aquifer will be a decrease in renewable resources, which in the worst-case scenario may decrease to 50–60% of present-day values, due to the decline in recharge and to a reduced inflow from the adjacent Triffa aquifer. The water quality will be affected mostly in the area immediately adjacent to the seashore, where salinity may increase up to 30 g/l. Localised areas may see a decrease in salinity due to the induced freshwater recharge from Oued Moulouya River and diminished inflow from high-salinity springs. 相似文献
89.
Augusto César Coelho Dias da Silva Jaqueline Comin de Castilhos Erik Allan Pinheiro dos Santos Luciana Sonnewend Brondízio Leandro Bugoni 《Ocean & Coastal Management》2010,53(9):570-576
The neritic waters of the state of Sergipe in Northeastern Brazil is adjacent to the main nesting area of the olive ridley sea turtles (Lepidochelys olivacea) in the Western Atlantic Ocean and an important area for shrimp trawl fishery. To address the problem of incidental mortality of sea turtles captured during trawling and reduce the risk of overexploitation of shrimp stocks, Projeto TAMAR/ICMBio, a Brazilian sea turtle conservation program, has adopted two main strategies: (1) the implementation of a marine monitoring program and (2) active participation in local forums. This paper describes the conflicts among stakeholders, the arrangements and established mechanisms of negotiation aimed to protect sea turtles and shrimp grounds, and strategies to reduce conflicts between user groups. The analysis of this co-management process highlights the importance of stakeholder participation in resource management decision-making through a cooperative process, the role assumed by non-governmental organizations as mediators, and the factors that influence this system. The key factors and actions learnt from the current study include clear identification of the conflicts, identification of stakeholders – both local and external, and local leaders, encouragement of actors and leaders to participate, support and strengthening of local groups, legitimization of the discussion forums through involvement with government, formalization of decisions taken through legislation, and monitoring of the management efficacy. 相似文献
90.
A fault plane model and a finite element hydrodynamic model are applied to the simulation of the Hokkaido Nansei-Oki tsunami of July 12, 1993. The joint performance of the models is assessed based on the overall ability to reproduce observed tsunami waveforms and to preserve mass and energy during tsunami propagation. While a number of observed characteristics of the waveforms are satisfactorily reproduced (in particular, amplitudes and arrival times at tidal gauges relatively close to the source, and general patterns of energy concentration), others are only marginally so (notably, wave periods at the same gauges, and wave heights along Okushiri); differences between observations and simulations are traceable to both the fault plane and the hydrodynamic models. Nonnegligible losses of energy occur throughout the simulated tsunami propagation. These losses seem to be due to a combination of factors, including numerical damping and possible deficiencies of the shallow water equations in preserving energy. 相似文献