16,769 research outputs found
Turbulence model reduction by deep learning
A central problem of turbulence theory is to produce a predictive model for
turbulent fluxes. These have profound implications for virtually all aspects of
the turbulence dynamics. In magnetic confinement devices, drift-wave turbulence
produces anomalous fluxes via cross-correlations between fluctuations. In this
work, we introduce a new, data-driven method for parameterizing these fluxes.
The method uses deep supervised learning to infer a reduced mean-field model
from a set of numerical simulations. We apply the method to a simple drift-wave
turbulence system and find a significant new effect which couples the particle
flux to the local \emph{gradient} of vorticity. Notably, here, this effect is
much stronger than the oft-invoked shear suppression effect. We also recover
the result via a simple calculation. The vorticity gradient effect tends to
modulate the density profile. In addition, our method recovers a model for
spontaneous zonal flow generation by negative viscosity, stabilized by
nonlinear and hyperviscous terms. We highlight the important role of symmetry
to implementation of the new method.Comment: To be published in Phys. Rev. E Rap. Comm. 6 pages, 7 figure
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
Dynamics of a 1-D model for the emergence of the plasma edge shear flow layer with momentum conserving Reynolds stress
A one-dimensional version of the second-order transition model based on the
sheared flow amplification by Reynolds stress and turbulence supression by
shearing is presented. The model discussed in this paper includes a form of the
Reynolds stress which explicitly conserves momentum. A linear stability
analysis of the critical point is performed. Then, it is shown that the
dynamics of weakly unstable states is determined by a reduced equation for the
shear flow. In the case in which the flow damping term is diffusive, the
stationary solutions are those of the real Ginzburg-Landau equation.Comment: 21 pages, 8 figure
Plus Charge Prevalence in Cosmic Rays: Room for Dark Matter in the Positron Spectrum
The unexpected energy spectrum of the positron/electron ratio is interpreted
astrophysically, with a possible exception of the 100-300 GeV range. The data
indicate that this ratio, after a decline between GeV, rises steadily
with a trend towards saturation at 200-400GeV. These observations (except for
the trend) appear to be in conflict with the diffusive shock acceleration (DSA)
mechanism, operating in a \emph{single} supernova remnant (SNR) shock. We argue
that ratio can still be explained by the DSA if positrons are
accelerated in a \emph{subset} of SNR shocks which: (i) propagate in clumpy gas
media, and (ii) are modified by accelerated CR \emph{protons}. The protons
penetrate into the dense gas clumps upstream to produce positrons and,
\emph{charge the clumps positively}. The induced electric field expels
positrons into the upstream plasma where they are shock-accelerated. Since the
shock is modified, these positrons develop a harder spectrum than that of the
CR electrons accelerated in other SNRs. Mixing these populations explains the
increase in the ratio at GeV. It decreases at GeV
because of a subshock weakening which also results from the shock modification.
Contrary to the expelled positrons, most of the antiprotons, electrons, and
heavier nuclei, are left unaccelerated inside the clumps. Scenarios for the
100-300 GeV AMS-02 fraction exceeding the model prediction, including, but not
limited to, possible dark matter contribution, are also discussed.Comment: 36 pages, 6 figure
Modern theory of Fermi acceleration: a new challenge to plasma physics
One of the main features of astrophysical shocks is their ability to
accelerate particles to extremely high energies. The leading acceleration
mechanism, the diffusive shock acceleration is reviewed. It is demonstrated
that its efficiency critically depends on the injection of thermal plasma into
acceleration which takes place at the subshock of the collisionless shock
structure that, in turn, can be significantly smoothed by energetic particles.
Furthermore, their inhomogeneous distribution provides free energy for MHD
turbulence regulating the subshock strength and injection rate. Moreover, the
MHD turbulence confines particles to the shock front controlling their maximum
energy and bootstrapping acceleration. Therefore, the study of the MHD
turbulence in a compressive plasma flow near a shock is a key to understanding
of the entire process. The calculation of the injection rate became part of the
collisionless shock theory. It is argued that the further progress in diffusive
shock acceleration theory is impossible without a significant advance in these
two areas of plasma physics.Comment: 12 pages, 4 figures, invited talk at APS/ICPP, Quebec 2000, to appear
in Phys. of Plasma
Critical self-organization of astrophysical shocks
There are two distinct regimes of the first order Fermi acceleration at
shocks. The first is a linear (test particle) regime in which most of the shock
energy goes into thermal and bulk motion of the plasma. The second is an
efficient regime when it goes into accelerated particles. Although the
transition region between them is narrow, we identify the factors that drive
the system to a {\it self-organized critical state} between those two. Using an
analytic solution, we determine this critical state and calculate the spectra
and maximum energy of accelerated particles.Comment: To appear in ApJL, Sec.3 extensively rewritten, 4 pages, Latex,
emulateapj.sty, eps
Hadronic Gamma Rays from Supernova Remnants
A gas cloud near a supernova remnant (SNR) provides a target for
pp-collisions leading to subsequent gamma-ray emission through neutral pion
decay. The assumption of a power-law ambient spectrum of accelerated particles
with index near -2 is usually built into models predicting the spectra of
very-high energy (VHE) gamma-ray emission from SNRs. However, if the gas cloud
is located at some distance from the SNR shock, this assumption is not
necessarily correct. In this case, the particles which interact with the cloud
are those leaking from the shock and their spectrum is approximately
monoenergetic with the injection energy gradually decreasing as the SNR ages.
In the GLAST energy range the gamma-ray spectrum resulting from particle
interactions with the gas cloud will be flatter than expected, with the cutoff
defined by the pion momentum distribution in the laboratory frame. We evaluate
the flux of particles escaping from a SNR shock and apply the results to the
VHE diffuse emission detected by the HESS at the Galactic centre.Comment: 4 pages, 3 figures. Contribution to the 30th ICRC, Merida, Mexico,
2007 (final version
Polarization morphology of SiO masers in the circumstellar envelope of the AGB star R Cassiopeiae
Silicon monoxide maser emission has been detected in the circumstellar
envelopes of many evolved stars in various vibrationally-excited rotational
transitions. It is considered a good tracer of the wind dynamics close to the
photosphere of the star. We have investigated the polarization morphology in
the circumstellar envelope of an AGB star, R Cas. We mapped the linear and
circular polarization of SiO masers in the v=1, J=1-0 transition. The linear
polarization is typically a few tens of percent while the circular polarization
is a few percent. The fractional polarization tends to be higher for emission
of lower total intensity. We found that, in some isolated features the
fractional linear polarization appears to exceed 100%. We found the Faraday
rotation is not negligible but is ~15 deg., which could produce small scale
structure in polarized emission whilst total intensity is smoother and partly
resolved out. The polarization angles vary considerably from feature to feature
but there is a tendency to favour the directions parallel or perpendicular to
the radial direction with respect to the star. In some features, the
polarization angle abruptly flips 90 deg. We found that our data are in the
regime where the model of Goldreich et al (1973) can be applied and the
polarization angle flip is caused when the magnetic field is at close to 55
deg. to the line of sight. The polarization angle configuration is consistent
with a radial magnetic field although other configurations are not excluded.Comment: 14 pages, 15 figures. Accepted for publication in MNRA
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