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141.
It is known that stellar winds from late type stars are of mixed thermal and magnetic origin. The stellar wind model presented in this work uses the hydrodynamic equations of mass and momentum conservation and closes the system of equations with a detailed energy equation. Both momentum and energy equations have terms due to the effects of Alfvén waves. A smooth transition between the two regimes for Alfvén wave propagation, the undamped and the damped modes, is achieved by considering the geometrical mean of both wave amplitudes. It will be shown that the initial push on the plasma is provided by the mechanical heating input, and that further out the Alfvén waves take over energetically. 相似文献
142.
143.
Ground measurements of the geomagnetic field, during the total solar eclipse of July 11, 1991, were carried out at three sites in Costa Rica located within the eclipse's path of totality. Near totality, there was a significant change in the total intensity,F. Its normal decrease in the afternoon hours was interrupted during a period of about 45 min, in whichF varied very little. This was due mainly to a small variation in horizontal intensity,H, during such a period. The departure ofF from normal lasted about 100 min; its maximum departure was near 10 nT and occurred about 38 min after totality, The declination,D, also experienced a small change (about 2.3 min) during the total period of the eclipse. The vertical intensity,Z, and the inclination,I, did not show significant changes. 相似文献
144.
Alejandro Ismael Monterroso-Rivas Ana Cecilia Conde-Álvarez José Luís Pérez-Damian Jorge López-Blanco Marcos Gaytan-Dimas Jesús David Gómez-Díaz 《Climatic change》2018,147(3-4):457-473
Vulnerability to climate change was evaluated for three different time periods: 1990, 2000, and 2010. Our objective was to discuss the scope of a multi-temporal assessment of vulnerability. The method used 55 indicators—with emphasis on the agricultural sector in Mexico—of which 27 were updated for the year 2010 and 33 were retrospectively estimated for the year 1990. The results show that in the 20-year study period, the exposure of the municipalities (and inhabitants) has increased, and sensitivity and adaptive capacity have decreased. The number of municipalities vulnerable to climate change declined over the 20-year period. We found that calculating vulnerability by adding exposure and sensitivity and subtracting adaptive capacity (E?+?S???AC) can lead to unintentional underestimation of total vulnerability. When rating vulnerability, care must be taken in what is reported: the results differ for the number of inhabitants versus the number of municipalities. Our previous published vulnerability evaluation was for the year 2000, so we wanted to evaluate the sensitivity of some variables and the vulnerability formula itself we used in that moment. It is possible to evaluate the vulnerability multi-temporally, which allows to evaluate the sensibility and calibration of the variables and indicators used and the reconsideration of their application. 相似文献
145.
Claudia Sayo Valladares Nuno Machado Monica Heilbron Beatriz Paschoal Duarte Gilles Gauthier 《Gondwana Research》2008,13(4):516-526
207Pb/206Pb ages from 420 zircons from 9 upper amphibolite and granulite facies quartzites of the post-1.8 Ga metasedimentary cover from the Occidental and Oriental terranes from the Neoproterozoic central Ribeira belt, in eastern Brazil, are discussed. Ages from the Occidental terrane show that Proterozoic ages predominate with a modal class at 2.1–2.2 Ga and that Archean ages are less common and lie within the 2.5–3.6 Ga range. A wider dispersion on Proterozoic ages down to 1.1 Ga and youngest Neoproterozoic ages (550–700 Ma) were also observed and could be related to the Brasiliano/PanAfrican metamorphic events. The data show that detrital sediments of the Occidental terrane were derived predominantly from erosion of a 1.8–2.2 Ga Paleoproterozoic terrane with a small proportion of a 2.6–3.6 Ga Archean crust. Ages from the Oriental terrane are greatly dispersed, but show a significant proportion of ages between 1.7 and 2.1 Ga, although the great majority are between 0.5 and 1.6 Ga. Only 3 out of 144 grains yield Archean ages, between 2.5 and 2.9 Ga. Based on the greatest errors observed on the data from the Oriental Terrane (100–300 Ma), and a continuum of obtained ages between 2072 Ma (Paleoproterozoic) and 517 Ma (Cambrian), the more plausible interpretation is that the dispersion of ages results from partial resetting of the U–Pb system by Brasiliano metamorphism in Proterozoic–Archean zircons, although a Mesoproterozoic source could not be discarded. 相似文献
146.
The metamorphic evolution of dolomitic marbles and associated calc-silicate rocks from Punta Tota (NE Tandilia belt, Buenos Aires province, Argentina) has been evaluated through petrographic, geothermobarometric, and fluid inclusion studies. Thin beds of dolomitic marble are intercalated in amphibolites and constitute the upper part of a stratified basement sequence, which starts at the base with garnet migmatites showing a great abundance of pegmatitic segregates, overlain by biotite–garnet gneisses. Peak metamorphic conditions are estimated at 750–800 °C and 5–6 kb, followed by near isobaric cooling to about 500–450 °C and 5.5–6.5 kb. Anhydrous progressive metamorphic assemblages in both marbles (Fo + Cal + Dol + Cpx + Spl) and adjacent calc-silicate rocks (Cpx + An + Cal + Qtz) strongly retrogressed to hydrous minerals (Tr, Tlc, Grs, Czo, Srp) with decreasing temperatures and increasing water activities. The intense rehydration of the rocks relates to the emplacement of volatile-rich pegmatitic bodies (Qtz + Pl + Kfs + Bt + Grt), which also resulted in the crystallization of clinochlore + phlogopite in the marble and biotite + muscovite in the adjacent calc-silicate rocks. Metamorphic reactions based on textural relations and evaluated on a suitable petrogenetic grid, combined with geothermobarometric results and fluid inclusion isochores, indicate a metamorphic evolution along a counterclockwise P–T path. Two probable geotectonic settings for the determined P–T trajectory are proposed: (1) thinning of the crust and overlying supracrustal basin in an ensialic intraplate tectonic setting and (2) development of a marginal back-arc basin, associated with an oceanic–continental convergent plate margin. In both models, the initial extensional regime is followed by a compressional stage, with overthickening of the basement and supracrustal rocks, during the climax of the Transamazonian cycle at approximately 1800 Ma ago. Continuous convergence and blockage of structures produce transition to transcurrent tectonics (transpression) with a consequent moderate uplift. 相似文献
147.
Carlos J. P. Rosa Jocelyn McPhie Jorge M. R. S. Relvas Zélia Pereira Tomás Oliveira Nelson Pacheco 《Mineralium Deposita》2008,43(4):449-466
In the Iberian Pyrite Belt, volcanic rocks are relatively scarce, accounting for approximately only 25% of the geologic record,
with the remaining 75% consisting of sedimentary units. This association is very clear in the host succession to the Neves
Corvo massive sulfide deposit in Portugal. The Neves Corvo host succession comprises the products of explosive and effusive
rhyolitic eruptions intercalated with mudstone that records a submarine below-wave-base environment and provides precise biostratigraphic
age constraints. The first and second volcanic events involved eruptions at local intrabasinal vents. The first event generated
thick beds of fiamme breccia that are late Famennian in age. The fiamme were originally pumice clasts produced by explosive
eruptions and were subsequently compacted. The second event was the late Strunian (latest Famennian) effusion of rhyolitic
lava that was pervasively quench-fragmented. The third and final event is younger than the massive sulfide deposits poorly
represented in the mine area and minor compared with the two other events. The integration of biostratigraphic data with the
volcanic facies architecture indicates that the Neves Corvo ore deposits are similar in age to the late Strunian rhyolitic
lava. Although regionally the Iberian Pyrite Belt is essentially a sedimentary succession, ore formation at Neves Corvo can
be closely linked to discrete volcanic events that produced a relatively narrow range of volcanic facies. 相似文献
148.
Among the various components of the water balance, measurement of evapotranspiration has probably been the most difficult
component to quantify and measure experimentally. Some attempts for direct measurement of evapotranspiration have included
the use of weighing lysimeters. However, quantification of evapotranspiration has been typically conducted using energy balance
approaches or indirect water balance methods that rely on quantification of other water balance components. This paper initially
presents the fundamental aspects of evapotranspiration as well as of its evaporation and transpiration components. Typical
methods used for prediction of evapotranspiration based on meteorological information are also discussed. The current trend
of using evapotranspirative cover systems for closure of waste containment facilities located in arid climates has brought
renewed needs for quantification of evapotranspiration. Finally, case histories where direct or indirect measurements of evapotranspiration
have been conducted are described and analyzed. 相似文献
149.
Gary L Rowe Jr Shinji Ohsawa Bokuichiro Takano Susan L Brantley Jose F Fernandez Jorge Barquero 《Bulletin of Volcanology》1992,54(6):494-503
Monitoring of crater lake chemistry during the recent decline and disappearance of the crater lake of Poás Volcano revealed that large variations in SO4/Cl, F/Cl, and Mg/Cl ratios were caused by the enhanced release of HCl vapor from the lake surface due to increasing lake temperature and solution acidity. Variation in the concentration of polythionic acids (H2SxO6, x=4–6) was the most reliable predictor of renewed phreatic eruptive activity at the volcano, exhibiting sharp decreases three months prior to the initiation of phreatic eruptions in June 1987. Polythionic acids may offer a direct indicator of changing subsurface magmatic activity whereas chloride-based element ratios may be influenced by surface volatilization of HCl and subsequent recycling of acidic fluids in crater lake volcanoes. 相似文献
150.
Jorge A. Jiménez 《Estuaries and Coasts》1990,13(2):182-192
Mangrove forests along the Pacific Coast of Central America cover around 4,000 km2. Most of this coast is occupied with tropical dry forest mangroves where basal areas range between 6 and 20 m2 ha?1 and canopy heights rarely exceed 20 m. Rainfall and runoff alter structure and floristic composition from site to site. Reproductive phenology and mortality appear to be related to soil water availability.Avicennia bicolor forests reach a density of 4,350 plants that are taller than 0.50 m ha?1, with 769 trees above 5 cm diameter at breast height A total basal area of 41 m2 ha?1 together with a canopy height of about 23 m place these forests among the most developed in the western hemisphere. Growth rate (0.38 m2 ha?1 yr?1) is surprisingly high for mangrove forests under a seasonal dry climate. 相似文献