10,448 research outputs found
Suppression of Cross-Field Transport of a Passive Scalar in Two-Dimensional Magnetohydrodynamic Turbulence
The theory of passive scalar transport in two dimensional turbulent fluids is
generalized to the case of 2D MHD. Invariance of the cross correlation of
scalar concentration and magnetic potential produces a novel contribution to
the concentration flux. This pinch effect is proportional to the mean potential
gradient, and is shown to drastically reduce transport of the passive scalar
across the mean magnetic field when . Transport parallel to the mean magnetic
field is unchanged. Implications for models of transport in turbulent
magnetofluids are discussed.
PAC NOS. 47.25.Jn, 47.65.+aComment: uuencoded compressed postscript fil
X-ray film holder permits single continuous picture of tubing joint
X ray technique produces a clear continuous picture of a welded brazed tubing joint on a single film with one exposure. A stationary X ray source located in the plane of the joint to be inspected, a means of rotating the tube, and a unique internal film holder and positioning fixture are used
Fluid Models for Kinetic Effects on Coherent Nonlinear Alfven Waves. II. Numerical Solutions
The influence of various kinetic effects (e.g. Landau damping, diffusive and
collisional dissipation, and finite Larmor radius terms) on the nonlinear
evolution of finite amplitude Alfvenic wave trains in a finite-beta environment
is systematically investigated using a novel, kinetic nonlinear Schrodinger
(KNLS) equation. The dynamics of Alfven waves is sensitive to the sense of
polarization as well as the angle of propagation with respect to the ambient
magnetic field. Numerical solution for the case with Landau damping reveals the
formation of dissipative structures, which are quasi-stationary, S-polarized
directional (and rotational) discontinuities which self-organize from parallel
propagating, linearly polarized waves. Parallel propagating circularly
polarized packets evolve to a few circularly polarized Alfven harmonics on
large scales. Stationary arc-polarized rotational discontinuities form from
obliquely propagating waves. Collisional dissipation, even if weak, introduces
enhanced wave damping when beta is very close to unity. Cyclotron motion
effects on resonant particle interactions introduce cyclotron resonance into
the nonlinear Alfven wave dynamics.Comment: 38 pages (including 23 figures and 1 table
Theory of the Spatio-Temporal Dynamics of Transport Bifurcations
The development and time evolution of a transport barrier in a magnetically
confined plasma with non-monotonic, nonlinear dependence of the anomalous flux
on mean gradients is analyzed. Upon consideration of both the spatial
inhomogeneity and the gradient nonlinearity of the transport coefficient, we
find that the transition develops as a bifurcation front with radially
propagating discontinuity in local gradient. The spatial location of the
transport barrier as a function of input flux is calculated. The analysis
indicates that for powers slightly above threshold, the barrier location
where is the local transition
power threshold and is the neoclassical diffusivity . This result
suggests a simple explanation of the high disruptivity observed in reversed
shear plasmas. The basic conclusions of this theory are insensitive to the
details of the local transport model.Comment: 21 page Tex file, 10 postscript file
Magnetic fields and cosmic rays in GRBs. A self-similar collisionless foreshock
Cosmic rays accelerated by a shock form a streaming distribution of outgoing
particles in the foreshock region. If the ambient fields are negligible
compared to the shock and cosmic ray energetics, a stronger magnetic field can
be generated in the shock upstream via the streaming (Weibel-type) instability.
Here we develop a self-similar model of the foreshock region and calculate its
structure, e.g., the magnetic field strength, its coherence scale, etc., as a
function of the distance from the shock. Our model indicates that the entire
foreshock region of thickness , being comparable
to the shock radius in the late afterglow phase when ,
can be populated with large-scale and rather strong magnetic fields (of
sub-gauss strengths with the coherence length of order )
compared to the typical interstellar medium magnetic fields. The presence of
such fields in the foreshock region is important for high efficiency of Fermi
acceleration at the shock. Radiation from accelerated electrons in the
foreshock fields can constitute a separate emission region radiating in the
UV/optical through radio band, depending on time and shock parameters. We also
speculate that these fields being eventually transported into the shock
downstream can greatly increase radiative efficiency of a gamma-ray burst
afterglow shock.Comment: 10 pages, 1 figure. Submitted to Ap
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