51,864 research outputs found
How hole defects modify vortex dynamics in ferromagnetic nanodisks
Defects introduced in ferromagnetic nanodisks may deeply affect the structure
and dynamics of stable vortex-like magnetization. Here, analytical techniques
are used for studying, among other dynamical aspects, how a small cylindrical
cavity modify the oscillatory modes of the vortex. For instance, we have
realized that if the vortex is nucleated out from the hole its gyrotropic
frequencies are shifted below. Modifications become even more pronounced when
the vortex core is partially or completely captured by the hole. In these
cases, the gyrovector can be partially or completely suppressed, so that the
associated frequencies increase considerably, say, from some times to several
powers. Possible relevance of our results for understanding other aspects of
vortex dynamics in the presence of cavities and/or structural defects are also
discussed.Comment: 9 pages, 4 page
Global Alfven Wave Heating of the Magnetosphere of Young Stars
Excitation of a Global Alfven wave (GAW) is proposed as a viable mechanism to
explain plasma heating in the magnetosphere of young stars. The wave and basic
plasma parameters are compatible with the requirement that the dissipation
length of GAWs be comparable to the distance between the shocked region at the
star's surface and the truncation region in the accretion disk. A two-fluid
magnetohydrodynamic plasma model is used in the analysis. A current carrying
filament along magnetic field lines acts as a waveguide for the GAW. The
current in the filament is driven by plasma waves along the magnetic field
lines and/or by plasma crossing magnetic field lines in the truncated region of
the disk of the accreting plasma. The conversion of a small fraction of the
kinetic energy into GAW energy is sufficient to heat the plasma filament to
observed temperatures.Comment: Submitted to ApJ, aheatf.tex, 2 figure
Instantons and Fluctuations in a Lagrangian Model of Turbulence
We perform a detailed analytical study of the Recent Fluid Deformation (RFD)
model for the onset of Lagrangian intermittency, within the context of the
Martin-Siggia-Rose-Janssen-de Dominicis (MSRJD) path integral formalism. The
model is based, as a key point, upon local closures for the pressure Hessian
and the viscous dissipation terms in the stochastic dynamical equations for the
velocity gradient tensor. We carry out a power counting hierarchical
classification of the several perturbative contributions associated to
fluctuations around the instanton-evaluated MSRJD action, along the lines of
the cumulant expansion. The most relevant Feynman diagrams are then integrated
out into the renormalized effective action, for the computation of velocity
gradient probability distribution functions (vgPDFs). While the subleading
perturbative corrections do not affect the global shape of the vgPDFs in an
appreciable qualitative way, it turns out that they have a significant role in
the accurate description of their non-Gaussian cores.Comment: 32 pages, 9 figure
The Onset of Intermittency in Stochastic Burgers Hydrodynamics
We study the onset of intermittency in stochastic Burgers hydrodynamics, as
characterized by the statistical behavior of negative velocity gradient
fluctuations. The analysis is based on the response functional formalism, where
specific velocity configurations - the viscous instantons - are assumed to play
a dominant role in modeling the left tails of velocity gradient probability
distribution functions. We find, as expected on general grounds, that the field
theoretical approach becomes meaningful in practice only if the effects of
fluctuations around instantons are taken into account. Working with a
systematic cumulant expansion, it turns out that the integration of
fluctuations yields, in leading perturbative order, to an effective description
of the Burgers stochastic dynamics given by the renormalization of its
associated heat kernel propagator and the external force-force correlation
function.Comment: 10 pages, 6 figure
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