Polarization opposition effect for the Galilean satellites of Jupiter     

Vera K. Rosenbush  Nikolai N. Kiselev 《Icarus》2005,179(2):490-496

We present results of polarimetric observations of the Galilean satellites Io, Europa, Ganymede, and Callisto at phase angles ranging from 0.19° to 2.22°. The observations in the UBVR filters were performed using a one-channel photoelectric polarimeter attached to 70-cm telescope of the Chuguev Observation Station (Ukraine) on November 19-December 7, 2000. We have observed the polarization opposition effect for Io, Europa, and Ganymede to be a sharp secondary spike of negative polarization with an amplitude of about −0.4% centered at phase angles of 0.2°-0.7° and superimposed on the regular negative polarization branch. Although these minima for Io, Europa, and Ganymede show many similarities, they also exhibit a number of distinctions. The polarization opposition effect appears to be wavelength-dependent, at least for Europa and Ganymede. No polarization opposition effect was found for Callisto. The results obtained are discussed within the framework of different mechanisms of light scattering.  相似文献   

Modélisation glacio-hydrologique et gestion des ressources en eau dans les Andes équatoriennes : l’exemple de Quito     

Jean-Christophe Pouget  David Proaño  Andrea Vera  Marcos Villacís  Thomas Condom  Marisa Escobar 《水文科学杂志》2017,62(3):431-446

La forte croissance socio-économique de Quito a conduit à d’importants projets de transferts interbassins, intensifiant la mobilisation des ressources d’altitude situées dans des zones écologiques sensibles et connaissant une fonte accélérée des glaciers. Afin d’étudier divers scénarios d’évolutions, nous proposons une modélisation du continuum climat/glacier/hydrologie/gestion des ressources en eau. Utilisant l’outil Water Evaluation and Planning (WEAP), nous avons développé : (1) une modélisation hydro-climatologique semi-distribuée avec des données mensuelles homogénéisées par vectorisation régionale ; (2) une modélisation de la production en eau des glaciers et de leur évolution interannuelle ; (3) une modélisation en unités hydrologiques distinguant différentes couvertures de sols ; et (4) une modélisation de la gestion distinguant droits, allocation et usages de l’eau. Nous présentons les résultats du calage hydrologique mensuel (1963–2006), en étudiant particulièrement l’équifinalité de diverses paramétrisations. Nous montrons la souplesse, la robustesse et les limites de la modélisation proposée, contribuant à cerner différentes incertitudes dans l’évaluation de scénarios prospectifs.  相似文献   

A Budget Analysis of the Variances of Temperature and Moisture in Precipitating Shallow Cumulus Convection     

Vera Schemann  Axel Seifert 《Boundary-Layer Meteorology》2017,163(3):357-373

Large-eddy simulations of an evolving cloud field are used to investigate the contribution of microphysical processes to the evolution of the variance of total water and liquid water potential temperature in the boundary layer. While the first hours of such simulations show a transient behaviour and have to be analyzed with caution, the final portion of the simulation provides a quasi-equilibrium situation. This allows investigation of the budgets of the variances of total water and liquid water potential temperature and quantification of the contribution of several source and sink terms. Accretion is found to act as a strong sink for the variances, while the contributions from the processes of evaporation and autoconversion are small. A simple parametrization for the sink term connected to accretion is suggested and tested with a different set of simulations.  相似文献   

Quantitative methods in the earth sciences     

A. F?rster  D. F. Merriam  Vera Pawlowsky  D. F. Merriam  Andrea F?rster 《Mathematical Geology》1997,29(6):859

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The Group Evolution Multiwavelength Study (GEMS): bimodal luminosity functions in galaxy groups     

Trevor A. Miles  Somak Raychaudhury  Duncan A. Forbes  Paul Goudfrooij  Trevor J. Ponman  Vera Kozhurina-Platais 《Monthly notices of the Royal Astronomical Society》2004,355(3):785-793

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Individual and integrated pulse properties of PSR B0943+10 involved in the mode-changing phenomenon     

Svetlana A. Suleymanova  Vera A. Izvekova  Joanna M. Rankin  N. Rathnasree 《Journal of Astrophysics and Astronomy》1998,19(1-2):1-18

The characteristics of the “burst” (B) mode and “quiescent” (Q) mode pulse sequences–long known from studies at or below 103 MHz–are identified at 430 MHz for the first time. An 18-minute, Polarimetrie observation begins with a long Bmode sequence, which has a higher average intensity, regular drifting subpulses, and a preponderance of primary polarisationmode radiation. An abrupt transition to a Q-mode sequence is then marked by a) weaker average intensity, but occasional very bright individual subpulses, b) a complete cessation of drifting subpulses, with disorganized subpulses now being emitted over a much wider longitude interval, and c) near parity between the primary and secondary polarisation modes, resulting in pronounced depolarisation, both of individual pulses and the average profile. Careful study, however, of profile changes before and after this mode change reveals slower variations which both anticipate the abrupt transition and respond to it. A slow attenuation of the intensity level of the dominant component is observed throughout the duration of the B-mode sequence, which then accelerates with the onset of the Q-mode sequence. This slow variation appears to represent a “preswitching transition” process; and the combination of effects on slow and abrupt time scales are finally responsible for the formation of the characteristic B and Q-mode average profiles.  相似文献   

High-precision late Aptian Pb/U age for the Punta del Barco Formation (Baqueró Group), Santa Cruz Province,Argentina     

Silvia N. Césari  Carlos O. Limarino  Magdalena Llorens  Mauro G. Passalia  Valeria Pérez Loinaze  Ezequiel I. Vera 《Journal of South American Earth Sciences》2011,31(4):426-431

The Baqueró Group holds a rich fossil flora including macrofloristic and palynological remains that characterize the Early Cretaceous vegetation of Argentina. The age of the upper part of the Group, known as Punta del Barco Formation, has been a topic of several discussions in the past. The type locality of this stratigraphic unit exposes fossiliferous tuffs together with muddy tuffs and tuffaceous sandstones beds. However, there have been no reliable age constraints for the fossil assemblages. ²⁰⁶Pb/²³⁸U analyses of zircon crystals yielded an age of 114.67 ± 0.18 Ma. This age control makes possible an improved analysis and comparison of the floras in the southern region.  相似文献   

Functional Methods for Classification of Different Petrographic Varieties by Means of Reflectance Spectra     

Vera?HoferEmail author 《Mathematical Geosciences》2011,43(2):165-181

The need for improved product quality in the aggregates industry is driving the search for greater automation in rock type identification. In practice, reflectance spectra in visible and near-infrared light may reliably be used for the classification of rock classes and their variants. Previous studies introduced statistical classification of six rock variants by means of infrared spectra. The present investigation extends these studies to cover twelve rock types and variants of worldwide economic importance. These were measured by visible and near-infrared light. Statistical classification of these spectra is highly challenging due to the high number of groups and the high dimensionality of the data. In functional data analysis, spectra are regarded as curves instead of vectors of characteristics. To obtain a compact form that is more susceptible to further analysis, the spectra are represented by a B-spline basis. Two functional versions of linear support vector machines and penalized functional discriminant analysis are considered for classification. The multiclass problem is addressed by margin trees and by considering all one-against-one classifications combined with a voting strategy for testing. Since classification error estimated by 5-fold cross-validation is very low, in particular for penalized discriminant analysis, we conclude that the rock types can be classified reliably.  相似文献   

40Ar‐39Ar chronology of lunar meteorites Northwest Africa 032 and 773     

Vera A. Fernandes  Ray Burgess  Grenville Turner 《Meteoritics & planetary science》2003,38(4):555-564

Abstract— The ⁴⁰Ar‐³⁹Ar dating technique has been applied to the lunar meteorites Northwest Africa 032 (NWA 032), an unbrecciated mare basalt, and Northwest Africa 773 (NWA 773), (composed of cumulate and breccia lithologies), to determine the crystallization age and timing of shock events these meteorites may have experienced. Stepped heating analyses of several different samples of NWA 032 gave complex age spectra but indistinguishable total ages with a mean of 2.779 ± 0.014 Gyr. Possible causes of the complex age spectra obtained from NWA 032 include recoil of ³⁹Ar, or the presence of pre‐shock ⁴⁰Ar incorporated into shock‐melt veins. The effects of shock veins were investigated by laser fusion of 20 small samples expected to contain varying proportions of the shock veins. The laser ages show a narrow age distribution between 2.61–2.86 Gyr and a mean of 2.73 ± 0.03 Gyr, identical to the total age of ?2.80 Gyr obtained for the bulk sample. Diffusion calculations based on the stepped heating data indicate that Ar release can be reconciled by release from feldspar (and possibly shock veins) at low temperatures followed by pyroxene at higher temperatures. The exposure age of NWA 032 is 212 ± 11 Myr, and it contains low trapped solar Ar. Stepped heating of cumulate and breccia portions of NWA 773 also give a relatively young age of 2.91 Gyr. The presence of trapped Ar in the breccia makes the age determination of this component less precise, but release of Ar appears to be from the same mineral phase, assumed to be plagioclase, in both lithologies. A marked difference in exposure age between the 2 lithologies also exists, with the breccia having spent 81 Myr longer at the lunar surface; this finding is consistent with the higher trapped Ar content of this lithology. Assuming that 2.80 Gyr and 2.91 Gyr are the crystallization ages of NWA 032 and NWA 773 respectively, these two meteorites are the youngest lunar mare basalts available for study.  相似文献   

polarimetric and photometric observations of split comet C/2001 A2 (LINEAR)     

Rosenbush  Vera  Kiselev  Nikolai  Velichko  Sergey 《Earth, Moon, and Planets》2002,90(1-4):423-433

We present the results of polarimetric and photometricobservations of split Comet C/2001 A2 (LINEAR), which wereobtained at the 70-cm telescope of the Astronomical Observatoryof Kharkiv National University between 30 June and 31 July 2001.The brightest fragment of the comet, nucleus B, was observed.Eight narrowband cometary filters in the continuum and in emissionbands and a wideband red filter have been used. The comet wasobserved through apertures of 88, 33, and 19 arcsec. Polarizationphase curves were obtained for the continuum and for the firsttime, for NH₂(0, 7, 0) emission. The degree of polarization ofthe light scattered by the dust decreases with the increase ofaperture size. An important temporal variation of the polarizationwith a rotation of the polarization plane was observed at twophase angles (26.5° and 36.2°). Molecular column densities and production rates of CN, C₂,C₃, and NH₂ species are calculated in the framework of theHaser model. A comparative analysis of the temporal variations ofthe visual magnitudes, gas and dust production rates, dust colorand polarization are presented.  相似文献