53 research outputs found
Stability of a relativistic rotating electron-positron jet: nonaxisymmetric perturbations
We investigate the linear stability of a hydrodynamic relativistic flow of
magnetized plasma in the force--free approximation. We considered the case of
light cylindrical jet in cold and dense environment, so the jet boundary
remains at rest. Continuous and discrete spectra of frequencies are
investigated analytically. An infinite sequence of eigenfrequencies is found
near the edge of Alfv\'en continuum. Numerical calculations showed that modes
are stable and have attenuation increment small. The dispersion curves
have a minimum for ( is the jet radius ). This results in accumulation of perturbations
inside the jet with wavelength of the order of the jet radius. The wave crests
of the perturbation pattern formed in such a way move along the jet with the
velocity exceeding light speed. If one has relativistic electrons emitting
synchrotron radiation inside the jet, than this pattern will be visible. This
provide us with the new type of superluminal source. If the jet is oriented
close to the line of sight, than the observer will see knots moving backward to
the core.Comment: 22 LaTeX pages, standard LaTeX article style, accepted by Mon. Not.
Roy. Astron. So
Relativistic parsec-scale jets: I. Particle acceleration
We develop a theory of particle acceleration inside relativistic rotating
electron-positron force-free jets with spiral magnetic fields. We considered
perturbation of the stationary magnetic field structure and found that
acceleration takes place in the regions where the Alfven resonant condition
with the eigenmodes in the jet is fulfilled, i.e. where the local Alfven speed
is equal to the phase speed of an eigenmode. The acceleration mechanism is
regular acceleration by the electric field of the electromagnetic wave, which
is the eigenmode of the force-free cylindrical jet: particles drift out of the
region of the large wave amplitude near the Alfven resonant surface and gain
energy. Acceleration in the strong electric field near the Alfven resonance and
synchrotron losses combine to form a power-law energy spectrum of
ultra-relativistic electrons and positrons with index between 2 and 3 depending
upon the initial energy of the injected particles. The power law distribution
ranges from 10 MeV to 1000 MeV.Comment: 14 pages, 4 figures; Astron. Astrophys. in pres
Magnetorotational Instability in Liquid Metal Couette Flow
Despite the importance of the magnetorotational instability (MRI) as a
fundamental mechanism for angular momentum transport in magnetized accretion
disks, it has yet to be demonstrated in the laboratory. A liquid sodium
alpha-omega dynamo experiment at the New Mexico Institute of Mining and
Technology provides an ideal environment to study the MRI in a rotating metal
annulus (Couette flow). A local stability analysis is performed as a function
of shear, magnetic field strength, magnetic Reynolds number, and turbulent
Prandtl number. The later takes into account the minimum turbulence induced by
the formation of an Ekman layer against the rigidly rotating end walls of a
cylindrical vessel. Stability conditions are presented and unstable conditions
for the sodium experiment are compared with another proposed MRI experiment
with liquid gallium. Due to the relatively large magnetic Reynolds number
achievable in the sodium experiment, it should be possible to observe the
excitation of the MRI for a wide range of wavenumbers and further to observe
the transition to the turbulent state.Comment: 12 pages, 22 figures, 1 table. To appear in the Astrophysical Journa
Magnetized Ekman Layer and Stewartson Layer in a Magnetized Taylor-Couette Flow
In this paper we present axisymmetric nonlinear simulations of magnetized
Ekman and Stewartson layers in a magnetized Taylor-Couette flow with a
centrifugally stable angular-momemtum profile and with a magnetic Reynolds
number below the threshold of magnetorotational instability. The magnetic field
is found to inhibit the Ekman suction. The width of the Ekman layer is reduced
with increased magnetic field normal to the end plate. A uniformly-rotating
region forms near the outer cylinder. A strong magnetic field leads to a steady
Stewartson layer emanating from the junction between differentially rotating
rings at the endcaps. The Stewartson layer becomes thinner with larger Reynolds
number and penetrates deeper into the bulk flow with stronger magnetic field
and larger Reynolds number. However, at Reynolds number larger than a critical
value , axisymmetric, and perhaps also nonaxisymmetric, instabilities
occur and result in a less prominent Stewartson layer that extends less far
from the boundary.Comment: 24 pages, 12 figures, accepted by PRE, revision according to referee
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