1,721,017 research outputs found
Low-frequency 1f fluctuations in hydrodynamic and magnetohydrodynamic turbulence
Full text linkWe investigate the occurrence of 1f spectra of low-frequency fluctuations in numerical simulations of three-dimensional hydrodynamic and magnetohydrodynamic turbulence driven by a random forcing with a controlled correlation time. A range of one decade of 1f spectrum is observed when a strong background magnetic field is present. The frequency spectra of individual Fourier modes is also analyzed and it is observed that the 1f range is present in the largest available wavelength mode for the magnetohydrodynamic simulations with and without a background magnetic field and it is not observed (or is less clear) for the hydrodynamic case. The presence of 1f spectra of low-frequency fluctuations is also analyzed for two-dimensional magnetohydrodynamic and hydrodynamic turbulence simulations and it is observed in both cases. The origin of these long period fluctuations is discussed. © 2007 The American Physical Society.Fil: Dmitruk, Pablo Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Matthaeus, W.H.. Bartol Research Institute; Reino Unid
Waiting-time distributions of magnetic discontinuities: Clustering or Poisson process?
Full text linkUsing solar wind data from the Advanced Composition Explorer spacecraft, with the support of Hall magnetohydrodynamic simulations, the waiting-time distributions of magnetic discontinuities have been analyzed. A possible phenomenon of clusterization of these discontinuities is studied in detail. We perform a local Poisson's analysis in order to establish if these intermittent events are randomly distributed or not. Possible implications about the nature of solar wind discontinuities are discussed. © 2009 The American Physical Society.Fil: Greco, A.. Universita Della Calabria; EspañaFil: Matthaeus, W.H.. Bartol Research Institute; Estados UnidosFil: Servidio, S.. Universita Della Calabria; España. Bartol Research Institute; Estados UnidosFil: Dmitruk, Pablo Ariel. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentin
Time decorrelation in isotropic magnetohydrodynamic turbulence
Full text linkTime decorrelation in isotropic incompressible magnetohydrodynamic (MHD) turbulence is studied using three-dimensional numerical simulation. Eulerian and wavenumber-dependent correlation functions are obtained. Scaling with wavenumber separates effects of nonlinear distortion and effects of random sweeping and Alfvénic wave propagation. Results show that the dominant effect is the combined influence of convective sweeping and Alfvénic propagation, both being nonlocal effects on small-scale fluctuations by large-scale fluctuations. This conclusion finds applications in space plasma observation, particle scattering, predictability, and turbulence theory. © Europhysics Letters Association.Fil: Servidio, S.. Universita Della Calabria; ItaliaFil: Carbone, V.. Universita Della Calabria; ItaliaFil: Dmitruk, Pablo Ariel. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Matthaeus, W.H.. University Of Delaware; Estados Unido
On the compressibility effect in test particle acceleration by magnetohydrodynamic turbulence
Full text linkThe effect of compressibility in a charged particle energization by magnetohydrodynamic (MHD) fields is studied in the context of test particle simulations. This problem is relevant to the solar wind and the solar corona due to the compressible nature of the flow in those astrophysical scenarios. We consider turbulent electromagnetic fields obtained from direct numerical simulations of the MHD equations with a strong background magnetic field. In order to explore the flow compressibility effect over the particle dynamics, we performed different numerical experiments: an incompressible case and two weak compressible cases with Mach number M = 0.1 and M = 0.25. We analyze the behavior of protons and electrons in those turbulent fields, which are well known to form aligned current sheets in the direction of the guide magnetic field. What we call protons and electrons are test particles with scales comparable to (for protons) and much smaller than (for electrons) the dissipative scale of MHD turbulence, maintaining the correct mass ratio me/mi" role="presentation">me/mi. For these test particles, we show that compressibility enhances the efficiency of proton acceleration, and that the energization is caused by perpendicular electric fields generated between currents sheets. On the other hand, electrons remain magnetized and display an almost adiabatic motion, with no effect of compressibility observed. Another set of numerical experiments takes into account two fluid modifications, namely, electric field due to Hall effect and electron pressure gradient. We show that the electron pressure has an important contribution to electron acceleration allowing highly parallel energization. In contrast, no significant effect of these additional terms is observed for the protons.Fil: González, C.A.. Universidad de Buenos Aires; ArgentinaFil: Dmitruk, Pablo Ariel. Universidad de Buenos Aires; ArgentinaFil: Mininni, Pablo Daniel. Universidad de Buenos Aires; ArgentinaFil: Matthaeus, W.H.. Bartol Research Institute
Density and magnetic field signatures of interplanetary 1/f noise
Full text linkWe investigate the occurrence of 1/f noise in the interplanetary density and the magnetic field at varying heliocentric latitudes. The characteristic spectral amplitudes can be found in Ulysses density and magnetic data in the expected frequency ranges at all available latitudes, ranging from the ecliptic plane to more than 80°. Average spectra indicate a latitudinal variation, with a 1/f density signal becoming more pronounced in higher latitude bands. Azimuthal spectral analysis of solar magnetogram data using the SOHO Michelson Doppler Interferometer also shows 1/f noise in the photospheric magnetic field, most clearly at high latitude. Accordingly, we discuss possibilities that the 1/f signal arises at varying altitudes, possibly surviving coronal dynamics. This raises questions that may be addressed in future studies using spectroscopic, white light, and radio scintillation data. © 2007. The American Astronomical Society. All rights reserved.Fil: Matthaeus, W.H.. Bartol Research Institute; Reino UnidoFil: Breech, B.. Bartol Research Institute; Reino UnidoFil: Dmitruk, Pablo Ariel. Bartol Research Institute; Reino Unido. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Bemporad, A.. Osservatorio Astrofisico Di Arcetri; ItaliaFil: Poletto, German Ariel. Osservatorio Astrofisico Di Arcetri; ItaliaFil: Velli, M.. Università degli Studi di Firenze; ItaliaFil: Romoli, M.. Università degli Studi di Firenze; Itali
Partial Variance of Increments Method in Solar Wind Observations and Plasma Simulations
Full text linkThe method called “PVI” (Partial Variance of Increments) has been increasingly used in analysis of spacecraft and numerical simulation data since its inception in 2008. The purpose of the method is to study the kinematics and formation of coherent structures in space plasmas, a topic that has gained considerable attention, leading the development of identification methods, observations, and associated theoretical research based on numerical simulations. This review paper will summarize key features of the method and provide a synopsis of the main results obtained by various groups using the method. This will enable new users or those considering methods of this type to find details and background collected in one place.Fil: Greco, A.. Università della Calabria; ItaliaFil: Matthaeus, W.H.. University of Delaware; Estados UnidosFil: Perri, S.. Università della Calabria; ItaliaFil: Osman, K.T.. University of Warwick; Reino UnidoFil: Servidio, S.. Università della Calabria; ItaliaFil: Wan, M.. South University of Science and Technology of China; República de ChinaFil: Dmitruk, Pablo Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentin
Intermittent MHD structures and classical discontinuities
Full text linkWe examine statistics of rapid spatial variations of the magnetic field in simulations of magnetohydrodynamic (MHD) turbulence, by analyzing intermittency properties. and by using classical methods for identifying discontinuities. The methods identify similar structures, and give very similar event distribution functions. When the results are scaled to the correlation length, the average waiting times agree with typically reported waiting times between solar wind discontinuities. Thus discontinuities may be related to flux tube boundaries and intermittent structures that appear spontaneously in MHD turbulence. Copyright 2008 by the American Geophysical Union.Fil: Greco, Andres Francisco. Universita Della Calabria; EspañaFil: Chuychai, P.. Bartol Research Institute; Estados UnidosFil: Matthaeus, W.H.. Bartol Research Institute; Estados UnidosFil: Servidio, S.. Bartol Research Institute; Estados UnidosFil: Dmitruk, Pablo Ariel. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentin
On the spatio-temporal behavior of magnetohydrodynamic turbulence in a magnetized plasma
Full text linkUsing direct numerical simulations of three-dimensional magnetohydrodynamic turbulence, the spatio-temporal behavior of magnetic field fluctuations is analyzed. Cases with relatively small, medium, and large values of a mean background magnetic field are considered. The (wavenumber) scale dependent time correlation function is directly computed for different simulations, varying the mean magnetic field value. From this correlation function, the time decorrelation is computed and compared with different theoretical times, namely, the local non-linear time, the random sweeping time, and the Alfvénic time, the latter being a wave effect. It is observed that time decorrelations are dominated by sweeping effects, and only at large values of the mean magnetic field and for wave vectors mainly aligned with this field time decorrelations are controlled by Alfvénic effects.Fil: Lugones, Rodrigo. Universidad de Buenos Aires; ArgentinaFil: Dmitruk, Pablo Ariel. Universidad de Buenos Aires; ArgentinaFil: Mininni, Pablo Daniel. Universidad de Buenos Aires; ArgentinaFil: Wang, Minmin. Bartol Research Institute; Reino UnidoFil: Matthaeus, W.H.. Bartol Research Institute; Reino Unid
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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