20,938 research outputs found
On-off intermittency and amplitude-phase synchronization in Keplerian shear flows
We study the development of coherent structures in local simulations of the
magnetorotational instability in accretion discs in regimes of on-off
intermittency. In a previous paper [Chian et al., Phys. Rev. Lett. 104, 254102
(2010)], we have shown that the laminar and bursty states due to the on-off
spatiotemporal intermittency in a one-dimensional model of nonlinear waves
correspond, respectively, to nonattracting coherent structures with higher and
lower degrees of amplitude-phase synchronization. In this paper we extend these
results to a three-dimensional model of magnetized Keplerian shear flows.
Keeping the kinetic Reynolds number and the magnetic Prandtl number fixed, we
investigate two different intermittent regimes by varying the plasma beta
parameter. The first regime is characterized by turbulent patterns interrupted
by the recurrent emergence of a large-scale coherent structure known as
two-channel flow, where the state of the system can be described by a single
Fourier mode. The second regime is dominated by the turbulence with sporadic
emergence of coherent structures with shapes that are reminiscent of a
perturbed channel flow. By computing the Fourier power and phase spectral
entropies in three-dimensions, we show that the large-scale coherent structures
are characterized by a high degree of amplitude-phase synchronization.Comment: 17 pages, 10 figure
Self-modulation of nonlinear Alfven waves in a strongly magnetized relativistic electron-positron plasma
We study the self-modulation of a circularly polarized Alfven wave in a strongly magnetized relativistic electron-positron plasma with finite temperature. This nonlinear wave corresponds to an exact solution of the equations, with a dispersion relation that has two branches. For a large magnetic field, the Alfven branch has two different zones, which we call the normal dispersion zone (where d omega/dk > 0) and the anomalous dispersion zone (where d omega/dk < 0). A nonlinear Schrodinger equation is derived in the normal dispersion zone of the Alfven wave, where the wave envelope can evolve as a periodic wave train or as a solitary wave, depending on the initial condition. The maximum growth rate of the modulational instability decreases as the temperature is increased. We also study the Alfven wave propagation in the anomalous dispersion zone, where a nonlinear wave equation is obtained. However, in this zone the wave envelope can evolve only as a periodic wave train.CONICyT 21100839 74110049FONDECyT 1110135 1110729 1080658 1121144CNPqEuropean Commission for a Marie Curie International Incoming FellowshipInstitute for Fusion Studie
Self-modulation of nonlinear waves in a weakly magnetized relativistic electron-positron plasma with temperature
We develop a nonlinear theory for self-modulation of a circularly polarized electromagnetic wave in a relativistic hot weakly magnetized electron-positron plasma. The case of parallel propagation along an ambient magnetic field is considered. A nonlinear Schrodinger equation is derived for the complex wave amplitude of a self-modulated wave packet. We show that the maximum growth rate of the modulational instability decreases as the temperature of the pair plasma increases. Depending on the initial conditions, the unstable wave envelope can evolve nonlinearly to either periodic wave trains or solitary waves. This theory has application to high-energy astrophysics and high-power laser physics.CONICyTFONDECyT 1110135 1080658Brazilian agency CNPqBrazilian agency FAPESPMarie Curie International Incoming Fellowshiphospitality of Paris ObservatoryInstitute for Fusion Studie
The true nature of the alleged planetary nebula W16-185
We report the discovery of a small cluster of massive stars embedded in a NIR
nebula in the direction of the IRAS15411-5352 point source, which is related to
the alleged planetary nebula W16-185. The majority of the stars present large
NIR excess characteristic of young stellar objects and have bright counterparts
in the Spitzer IRAC images; the most luminous star (IRS1) is the NIR
counterpart of the IRAS source. We found very strong unresolved Brgamma
emission at the IRS1 position and more diluted and extended emission across the
continuum nebula. From the sizes and electron volume densities we concluded
that they represent ultra-compact and compact HII regions, respectively.
Comparing the Brgamma emission with the 7 mm free-free emission, we estimated
that the visual extinction ranges between 14 and 20 mag. We found that only one
star (IRS1) can provide the number of UV photons necessary to ionize the
nebula.Comment: 30 pages, 15 figures, 2 tables V3: minor grammatical changes. Figure
4 is available in pdf file. Accepted for publication in AJ, April / 200
Ferromagnetic resonance study of polycrystalline Fe_{1-x}V_x alloy thin films
Ferromagnetic resonance has been used to study the magnetic properties and
magnetization dynamics of polycrystalline FeV alloy films with
. Films were produced by co-sputtering from separate Fe and V
targets, leading to a composition gradient across a Si substrate. FMR studies
were conducted at room temperature with a broadband coplanar waveguide at
frequencies up to 50 GHz using the flip-chip method. The effective
demagnetization field and the Gilbert damping
parameter have been determined as a function of V concentration. The
results are compared to those of epitaxial FeV films
A kinetic model of radiating electrons
A kinetic theory is developed to describe radiating electrons whose motion is governed by the Lorentz-Dirac equation. This gives rise to a generalized Vlasov equation coupled to an equation for the evolution of the physical submanifold of phase space. The pathological solutions of the 1-particle theory may be removed by expanding the latter equation in powers of τ ≔ q 2/6πm. The radiation-induced change in entropy is explored and its physical origin is discussed. As a simple demonstration of the theory, the radiative damping rate of longitudinal plasma waves is calculated
Pastures for hogs
Caption title."This circular is a summary of Missouri Agricultural Experiment Station Bulletin 247, April, 1927." -- [P.1]Digitized 2006 AES MoU
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