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151.
Emilio J. Alfaro Federico Elias Jesús Cabrera-Caño 《Astrophysics and Space Science》2009,324(2-4):141-146
We perform a study into the spatial and kinematical distribution of young open clusters in the solar neighborhood, distinguishing between Gould Belt and local Galactic disk members. We use a previous estimate of the structural parameters of both systems obtained from a sample of O to B6 stars from Hipparcos. The two star-forming regions that dominate and give the Gould Belt its characteristic inclined shape show a striking difference in their content of star clusters: while Ori OB1 is richly populated by open clusters, not a single one can be found within the boundaries of Sco OB2. This is mirrored in velocity space, translating again into an abundance of clusters in the region of the kinematic space populated by the members of Ori OB1, and a marginal number of them associated with Sco OB2. We interpret all these differences by characterizing the Orion region as a cluster complex typically surrounded by a stellar halo, and the Sco-Cen region as an OB association in the outskirts of the complex. The different contents of star clusters, the different heights above the Galactic plane and the different residual velocities of Ori OB1 and Sco OB2 can be explained in terms of their relative position with respect to the density maximum of the Local Arm in the solar neighborhood. The origin of this feature could have been the interaction of a density wave with the local interstellar medium close to the Galactic co-rotation radius. 相似文献
152.
C. Sigismondi D. W. Dunham K. Guhl S. Andersson H. Bode O. Canales P. Colona O. Farago M. Fernández-Ocaña A. Gabel M. Haupt C. Herold R. Nugent P. Oliva M. Patel C. Perello W. Rothe J. Rovira T. Schaefer C. Schnabel D. Schwartz A. Selva W. Strickling A. Tegtmeier C. Tegtmeier B. Thome W. H. Warren 《Solar physics》2009,258(2):191-202
In the annular or total eclipses of 3 October 2005, 29 March 2006, 22 September 2006, and 1 August 2008, observational campaigns
were organized to record the phenomenon of Baily’s beads. These campaigns were internationally coordinated through the International
Occultation Timing Association (IOTA) at both its American and European sections. From the stations in the northern and southern
zones of grazing eclipse, the eclipses have been recorded on video. Afterward, as many beads as possible have been identified
by analyzing the video data of each observing station. The atlas presented in this paper includes 598 data points, obtained
by 23 observers operating at 28 different observing stations. The atlas lists the geographic positions of the observing stations
and the observed time instants of disappearance or reappearance of beads, identified by an angle measured relative to the
Moon’s axis of rotation. The atlas will serve as a basis for determining the solar diameter.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
153.
We use linear analysis to simulate the evolution of a coronal loop in response to a localized impulsive event. The disturbance is modeled by injecting a narrow Gaussian velocity pulse near one footpoint of a loop in equilibrium. Three different damping mechanisms, namely viscosity, thermal conduction, and optically thin radiation, are included in the loop calculations. We consider homogeneous and gravitationally stratified, isothermal loops of varying length (50≤L≤400 Mm) and temperature (2≤T≤10 MK). We find that a localized pulse can effectively excite slow magnetoacoustic waves that propagate up along the loop. The amplitudes of the oscillations increase with decreasing loop temperature and increasing loop length and size of the pulse width. At T≥4 MK, the waves are dissipated by the combined effects of viscosity and thermal conduction, whereas at temperatures of 2 MK, or lower, wave dissipation is governed by radiative cooling. We predict periods in the range of 4.6?–?41.6 minutes. The wave periods remain unaltered by variations of the pulse size, decrease with the loop temperature, and increase almost linearly with the loop length. In addition, gravitational stratification results in a small reduction of the periods and amplification of the waves as they propagate up along the loop. 相似文献
154.
Eduardo Luquin Miguel A. Campo-Bescós Rafael Muñoz-Carpena Ronald L. Bingner Richard M. Cruse Henrique G. Momm Robert R. Wells Javier Casalí 《地球表面变化过程与地形》2021,46(10):1909-1925
Ephemeral gully (EG) erosion has an important impact on agricultural soil losses and increases field surface hydrology connectivity and transport of pollutants to nearby water bodies. Watershed models including an EG component are scarce and not yet properly evaluated. The objective of this study is to evaluate the capacity of one such tool, AnnAGNPS, to simulate the evolution of two EG formed in a conservation tillage system. The dataset for model testing included runoff measurements and EG morphological characteristics during 3 years. Model evaluation focused on EG evolution of volume, width, and length model outputs, and included calibration and testing phases and a global sensitivity analysis (GSA). While the model did not fully reproduce width and length, the model efficiency to simulate EG volume was satisfactory for both calibration and testing phases, supporting the watershed management objectives of the model. GSA revealed that the most sensitive factors were EG depth, critical shear stress, headcut detachment exponent coefficient b, and headcut detachment leading coefficient a. For EG outputs the model was additive, showing low sensitivity to interactions between the inputs. Prediction of EG spatial evolution on conservation tillage systems requires improved development of gully erosion components, since many of the processes were developed originally for traditional tillage practices or larger channel systems. Our results identify the need for future research when EG form within conservation tillage systems, in particular to study gully headcut, soil erodibility, and width functions specific to these practices. 相似文献
155.
We investigate the damping of longitudinal (i.e., slow or acoustic) waves in nonisothermal, hot (T≥ 5.0 MK), gravitationally stratified coronal loops. Motivated by SOHO/SUMER and Yohkoh/SXT observations, and by taking into account a range of dissipative mechanisms such as thermal conduction, compressive viscosity, radiative cooling, and heating, the nonlinear governing equations of one-dimensional hydrodynamics are solved numerically for standing-wave oscillations along a magnetic field line. A semicircular shape is chosen to represent the geometry of the coronal loop. It was found that the decay time of standing waves decreases with the increase of the initial temperature, and the periods of oscillations are affected by the different initial footpoint temperatures and loop lengths studied by the numerical experiments. In general, the period of oscillation of standing waves increases and the damping time decreases when the parameter that characterises the temperature at the apex of the loop increases for a fixed footpoint temperature and loop length. A relatively simple second-order scaling polynomial between the damping time and the parameter determining the apex temperature is found. This scaling relation is proposed to be tested observationally. Because of the lack of a larger, statistically relevant number of observational studies of the damping of longitudinal (slow) standing oscillations, it can only be concluded that the numerically predicted decay times are well within the range of values inferred from Doppler shifts observed by SUMER in hot coronal loops. 相似文献
156.
Devonian sediments of the Malaguide Complex potentially could include the Frasnian–Famennian boundary, one of the five greatest Phanerozoic biotic crises. Conodont biofacies and microfacies of carbonate clasts from a pebbly mudstone underlying Tournaisian radiolarites allows identification, for the first time in the Malaguide Complex, of Devonian shallow marine environments laterally grading to deeper realms. The clasts yielded Frasnian conodont associations of the falsiovalis to rhenana biozones, with six biofacies that reveal different environmental conditions in their source areas. Source sediments were dismantled and redeposited within the pebbly mudstone, whose origin is tentatively related to one of the events that are associated worldwide with the Frasnian–Famennian crisis. The latter is recorded, in two equivalent Malaguide pelagic successions, by stratigraphic discontinuities, and it was, probably, tectonically and/or eustatically controlled, as in other Alpine‐Mediterranean Paleotethyan margins. 相似文献
157.
Jesus A. Uriarte Laura Damas Mollá Maialen Sagarna Arantza Aranburu Francisco García Iñaki Antigüedad Tomás Morales 《水文研究》2020,34(4):1004-1015
Locating and quantifying groundwater flow in many built-up areas are a priority with regard to its complete restoration. In this work, a hydrogeological survey of the surroundings of the Punta Begoña Galleries (Getxo, Bizkaia), built on a coastal cliff, was completed by using ground penetrating radar (GPR) testing. Thus, the preliminary characterization of soils and rocks in accessible areas of the cliff was first improved by hydrogeological information gathered from a single survey borehole, including permeability measurements by low pressure injection tests (LPTs) and continuous water level monitoring. As a complementary method, the non-destructive GPR technique was performed during both dry and wet hydrological periods and in tandem with the injection tests, providing more complete spatial and temporal images of water flows. Specifically, GPR allows mapping of flow paths in soils and assessing the continuity of fractures in rock masses. Altogether, this complementary approach provides greater knowledge of complex underground flow dynamics in built environments, thus making it easier to make decisions for their management. 相似文献
158.
Journal of Paleolimnology - Stephanodiscus niagarae Ehrenberg is currently restricted to specific regions of central Mexico, however, during the late Pleistocene, it had a wider distribution in the... 相似文献
159.
V. Rull M. C. Trapote E. Safont N. Cañellas-Boltà N. Pérez-Zanón J. Sigró T. Buchaca T. Vegas-Vilarrúbia 《Journal of Paleolimnology》2017,57(1):95-108
Lakes with varved sediments are especially well suited for paleoecological study, from annual to even seasonal resolution. The interpretative power of such high-resolution paleoenvironmental reconstructions relies on the availability of modern analogs with the same temporal resolution. We studied seasonal pollen sedimentation in varved Lake Montcortès, Central Pyrenees (Spain), as a modern analog for high-resolution reconstruction of Late Holocene vegetation and landscape dynamics. Seasonal samples were obtained from sediment traps that were submerged near the maximum water depth for a 2-year period (fall 2013 to fall 2015). Seasonal pollen sedimentation was compared with meteorological variables from a nearby weather station. Bulk pollen sedimentation, dominated by Pinus (pine) and Quercus (oak), followed a clear seasonal pattern that peaked during the spring/summer, coinciding with maximum temperature and precipitation, minimum relative humidity and moderate winds from the SSE. Pollen sedimentation lags (PSL) were observed for most pollen types, as substantial amounts of pollen were found in the traps outside of their respective flowering seasons. Two pollen assemblages were clearly differentiated by their taxonomic composition, corresponding to spring/summer and fall/winter. This pattern is consistent with existing interpretation of the sediment varves, specifically, that varves are formed by two-layer couplets that represent the same seasonality as pollen. We concluded that pollen sedimentation in Lake Montcortès exhibits a strong seasonal signal in the quantity of pollen, the taxonomic composition of the pollen assembalges, and relationships between the pollen and meteorological variables. Thus, varved sediments provide a potentially powerful tool for paleoecological reconstruction at seasonal resolution. This method could be used not only to identify paleoenvironmental trends, but also to identify annual layers and therefore date sediments, even in the absence of evident sediment laminations. A satisfactory explanation of PSL will require further studies that examine internal lake dynamics and pollen production/dispersal patterns. 相似文献
160.
Rev. A. Romañá 《Pure and Applied Geophysics》1950,18(1):148-154
Summary A comparative study of the geomagnetic bays at the Observatories of Cuajimalpa and Teoloyucan (Mexico) and Tortosa, shows: 1) that the grouping of the bays into a certain number of fundamental types, and that the tendency of their frequency maxima to recur in a cyclical order during the day, are general features of geomagnetic bays, at least for all middle geomagnetic latitudes; 2) that the relative importance of the different types and the hour of their maxima varies according to the geomagnetic latitude of the Observatory where the bays are recorded.
Résumé En comparant les baies géomagnétiques enregistrées aux Observatoires de Cuajimalpa et Teoloyucan (Mexique) avec celles observées à Tortosa, on examine la validité générale de la classification des baies établie à l'Observatoire de l'Ebre et de la loi de succession des différents types le long du jour. L'on arrive aux conclusions suivantes: 1o La classification des baies géomagnétiques en un certain nombre de types fondamentaux, de même que la tendence à l'apparition de leurs maxima de fréquence dans un ordre cyclique pendant la journée, a un caractère général, tout au moins pour les latitudes géomagnétiques moyennes; 2o L'importance relative des différents types et l'heure d'apparition de leurs maxima varie d'un Observatoire à l'autre, d'après leur latitude géomagnétique.相似文献