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Search Results: 1 - 10 of 6883 matches for " Julio Gratton "
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Convergent flow in a two-layer system and mountain building
Carlos Alberto Perazzo,Julio Gratton
Physics , 2009, DOI: 10.1063/1.3431740
Abstract: With the purpose of modelling the process of mountain building, we investigate the evolution of the ridge produced by the convergent motion of a system consisting of two layers of liquids that differ in density and viscosity to simulate the crust and the upper mantle that form a lithospheric plate. We assume that the motion is driven by basal traction. Assuming isostasy, we derive a nonlinear differential equation for the evolution of the thickness of the crust. We solve this equation numerically to obtain the profile of the range. We find an approximate self-similar solution that describes reasonably well the process and predicts simple scaling laws for the height and width of the range as well as the shape of the transversal profile. We compare the theoretical results with the profiles of real mountain belts and find and excellent agreement.
Compressible Kelvin-Helmholtz instability at the terrestrial magnetopause
González, Alejandro G.;Gratton, Julio;Gratton, Fausto T.;Farrugia, Charles J.;
Brazilian Journal of Physics , 2002, DOI: 10.1590/S0103-97332002000500021
Abstract: the compressible magnetohydrodynamic kelvin-helmholtz instability occurs in two varieties, one that can be called incompressible as it exists in the limit of vanishing compressibility (primary instability), while the other exists only when compressibility is included in the model (secondary instability). in previous work we developed techniques to investigate the stability of a surface of discontinuity between two different uniform ows. our treatment includes arbitrary jumps of the velocity and magnetic fields as well as of density and temperature, with no restriction on the wave vector of the modes. then it allows stability analyses of complex configurations not previously studied in detail. here we apply our methods to investigate the stability of various typical situations occurring at different regions of the front side, and the near anks of the magnetopause. the physical conditions of the vector and scalar fields that characterize the equilibrium interface at the positions considered are obtained both from experimental data and from results of simulation codes of the magnetosheath available in the literature. we give particular attention to the compressible modes in configurations in which the incompressible modes are stabilized by the magnetic shear. for configurations of the front of the magnetopause, which have small relative velocities, we find that the incompressible mhd model gives reliable estimates of their stability, and compressibility effects do not introduce significant changes. however, at the anks of the magnetopause the occurrence of the secondary instability and the shift of the boundary of the primary instability play an important role. consequently, configurations that are stable if compressibility is neglected turn out to be unstable when it is considered and the stability properties are quite sensitive on the values of the parameters. then compressibility should be taken into account when assessing the stability properties of these configura
Compressible Kelvin-Helmholtz instability at the terrestrial magnetopause
González Alejandro G.,Gratton Julio,Gratton Fausto T.,Farrugia Charles J.
Brazilian Journal of Physics , 2002,
Abstract: The compressible magnetohydrodynamic Kelvin-Helmholtz instability occurs in two varieties, one that can be called incompressible as it exists in the limit of vanishing compressibility (primary instability), while the other exists only when compressibility is included in the model (secondary instability). In previous work we developed techniques to investigate the stability of a surface of discontinuity between two different uniform ows. Our treatment includes arbitrary jumps of the velocity and magnetic fields as well as of density and temperature, with no restriction on the wave vector of the modes. Then it allows stability analyses of complex configurations not previously studied in detail. Here we apply our methods to investigate the stability of various typical situations occurring at different regions of the front side, and the near anks of the magnetopause. The physical conditions of the vector and scalar fields that characterize the equilibrium interface at the positions considered are obtained both from experimental data and from results of simulation codes of the magnetosheath available in the literature. We give particular attention to the compressible modes in configurations in which the incompressible modes are stabilized by the magnetic shear. For configurations of the front of the magnetopause, which have small relative velocities, we find that the incompressible MHD model gives reliable estimates of their stability, and compressibility effects do not introduce significant changes. However, at the anks of the magnetopause the occurrence of the secondary instability and the shift of the boundary of the primary instability play an important role. Consequently, configurations that are stable if compressibility is neglected turn out to be unstable when it is considered and the stability properties are quite sensitive on the values of the parameters. Then compressibility should be taken into account when assessing the stability properties of these configurations, since the estimates based on incompressible MHD may be misleading. A careful analysis is required in each case, since no simple rule of thumb can be given.
Path Integral for Stochastic Inflation: Non-Perturbative Volume Weighting, Complex Histories, Initial Conditions and the End of Inflation
Steven Gratton
Physics , 2010, DOI: 10.1103/PhysRevD.84.063525
Abstract: In this paper we present a path integral formulation of stochastic inflation, in which volume weighting can easily be implemented. With an in-depth study of inflation in a quartic potential, we investigate how the inflaton evolves and how inflation typically ends both with and without volume weighting. Perhaps unexpectedly, complex histories sometimes emerge with volume weighting. The reward for this excursion into the complex plane is an insight into how volume-weighted inflation both loses memory of initial conditions and ends via slow-roll. The slow-roll end of inflation mitigates certain "Youngness Paradox"-type criticisms of the volume-weighted paradigm. Thus it is perhaps time to rehabilitate proper time volume weighting as a viable measure for answering at least some interesting cosmological questions.
Evaluation of Italian astronomical production: 2010-2012
Raffaele Gratton
Physics , 2014,
Abstract: The purpose of this document is to present a few statistics about the role of Italian astronomy, focusing on the production by INAF (Istituto Nazionale di Astrofisica). Data are presented but very few comments are given. We did not use lists of members of different scientific INAF macro-areas because a few trials showed that they are largely incomplete and result in an underestimate of the Italian astronomical production by more than a factor of two. The structure of this document is as follows: first I give some detail about the methods used; I then present data about the role of Italy in astronomical research worldwide; finally, I give some statistics about the h-factor of astronomers that are members of scientific INAF macro-areas (both INAF staff and associate).
In Vivo Imaging of Single-Molecule Translocation Through Nuclear Pore Complexes by Pair Correlation Functions
Francesco Cardarelli,Enrico Gratton
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0010475
Abstract: Nuclear pore complexes (NPCs) mediate bidirectional transport of proteins, RNAs, and ribonucleoproteins across the double-membrane nuclear envelope. Although there are many studies that look at the traffic in the nucleus and through the nuclear envelope we propose a method to detect the nucleocytoplasmic transport kinetics in an unperturbed cell, with no requirement for specific labeling of isolated molecules and, most important, in the presence of the cell milieu.
Fast optical imaging of human brain function
Gabriele Gratton,Monica Fabiani
Frontiers in Human Neuroscience , 2010, DOI: 10.3389/fnhum.2010.00052
Abstract: Great advancements in brain imaging during the last few decades have opened a large number of new possibilities for neuroscientists. The most dominant methodologies (electrophysiological and magnetic resonance-based methods) emphasize temporal and spatial information, respectively. However, theorizing about brain function has recently emphasized the importance of rapid (within 100 ms or so) interactions between different elements of complex neuronal networks. Fast optical imaging, and in particular the event-related optical signal (EROS, a technology that has emerged over the last 15 years) may provide descriptions of localized (to sub-cm level) brain activity with a temporal resolution of less than 100 ms. The main limitations of EROS are its limited penetration, which allows us to image cortical structures not deeper than 3 cm from the surface of the head, and its low signal-to-noise ratio. Advantages include the fact that EROS is compatible with most other imaging methods, including electrophysiological, magnetic resonance, and trans-cranial magnetic stimulation techniques, with which can be recorded concurrently. In this paper we present a summary of the research that has been conducted so far on fast optical imaging, including evidence for the possibility of recording neuronal signals with this method, the properties of the signals, and various examples of applications to the study of human cognitive neuroscience. Extant issues, controversies, and possible future developments are also discussed.
Inflation without a beginning: a null boundary proposal
Anthony Aguirre,Steven Gratton
Physics , 2003, DOI: 10.1103/PhysRevD.67.083515
Abstract: We develop our recent suggestion that inflation may be made past eternal, so that there is no initial cosmological singularity or "beginning of time". Inflation with multiple vacua generically approaches a steady-state statistical distribution of regions at these vacua, and our model follows directly from making this distribution hold at all times. We find that this corresponds (at the semi-classical level) to particularly simple cosmological boundary conditions on an infinite null surface near which the spacetime looks de Sitter. The model admits an interesting arrow of time that is well-defined and consistent for all physical observers that can communicate, even while the statistical description of the entire universe admits a symmetry that includes time-reversal. Our model suggests, but does not require, the identification of antipodal points on the manifold. The resulting "elliptic" de Sitter spacetime has interesting classical and quantum properties. The proposal may be generalized to other inflationary potentials, or to boundary conditions that give semi-eternal but non-singular cosmologies.
The absolute magnitude of field metal-poor horizontal branch stars
Raffaele G. Gratton
Physics , 1997, DOI: 10.1046/j.1365-8711.1998.01434.x
Abstract: HIPPARCOS satellite parallaxes for 22 metal-poor field horizontal branch stars with Vo<9 are used to derive their absolute magnitude. The weighted mean value is Mv=+0.69+/-0.10 for an average metallicity of [Fe/H]=-1.41; a somewhat brighter average magnitude of Mv=+0.60+/-0.12 for an average metallicity of [Fe/H]=-1.51 is obtained eliminating HD17072, that might be on the first ascent of the giant branch rather than on the horizontal branch. The present values agree with determinations based on proper motions and application of the Baade-Wesselink method to field RR Lyraes; they are from 0.1 to 0.2 mag fainter than those based on calibration of cluster distances obtained by using local subdwarfs, and on alternative distance calibrators for the LMC. The possibility that there is a real difference between the luminosity of the horizontal branch for clusters and the field is briefly commented.
Women in Italian astronomy
Francesca Matteucci,Raffaele Gratton
Physics , 2014,
Abstract: This document gives some quantitative facts about the role of women in Italian astronomy. More than 26% of Italian IAU members are women: this is the largest fraction among the world leading countries in astronomy. Most of this high fraction is due to their presence in INAF, where women make up 32% of the research staff (289 out of 908) and 40% of the technical/administrative staff (173 out of 433); the percentage is slightly lower among permanent research staff (180 out of 599, about 30%). The presence of women is lower in the Universities (27 out of 161, about 17%, among staff). In spite of these (mildly) positive facts, we notice that similarly to other countries (e.g. USA and Germany) career prospects for Italian astronomers are clearly worse for women than for men. Within INAF, the fraction of women is about 35-40% among non-permanent position, 36% for Investigators, 17% for Associato/Primo Ricercatore, and only 13% among Ordinario/Dirigente di Ricerca. The situation is even worse at University (only 6% of Professore Ordinario are women). We found that similar trends are also present if researchers are ordered according to citation rather than position: for instance, women make up only 15% among the 100 most cited astronomers working in Italy, a percentage which is however twice that over all Europe. A similar fraction is found among first authors of most influential papers, which cannot be explained as a residual of a lower female presence in the past. We conclude that implicit sex discrimination factors probably dominate over explicit ones and are still strongly at work. Finally, we discuss the possible connection between the typical career pattern and these factors.
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