297 research outputs found
The Effect of Coherent Structures on Stochastic Acceleration in MHD Turbulence
We investigate the influence of coherent structures on particle acceleration
in the strongly turbulent solar corona. By randomizing the Fourier phases of a
pseudo-spectral simulation of isotropic MHD turbulence (Re ), and
tracing collisionless test protons in both the exact-MHD and phase-randomized
fields, it is found that the phase correlations enhance the acceleration
efficiency during the first adiabatic stage of the acceleration process. The
underlying physical mechanism is identified as the dynamical MHD alignment of
the magnetic field with the electric current, which favours parallel
(resistive) electric fields responsible for initial injection. Conversely, the
alignment of the magnetic field with the bulk velocity weakens the acceleration
by convective electric fields - \bfu \times \bfb at a non-adiabatic stage of
the acceleration process. We point out that non-physical parallel electric
fields in random-phase turbulence proxies lead to artificial acceleration, and
that the dynamical MHD alignment can be taken into account on the level of the
joint two-point function of the magnetic and electric fields, and is therefore
amenable to Fokker-Planck descriptions of stochastic acceleration.Comment: accepted for publication in Ap
Electron Acceleration and Efficiency in Nonthermal Gamma-Ray Sources
In energetic nonthermal sources such as gamma-ray bursts, AGN or galactic jet
sources, etc., one expects both relativistic and transrelativistic shocks
acompanied by violent motions of moderately relativistic plasma. We present
general considerations indicating that these sites are electron and positron
accelerators leading to a modified power law spectrum. The electron (or
) energy index is very hard, or flatter up to a
comoving frame break energy , and becomes steeper above that. In
the example of gamma-ray bursts the Lorentz factor reaches for accelerated by the internal shock ensemble on
subhydrodynamical time scales. For pairs accelerated on hydrodynamical
timescales in the external shocks similarly hard spectra are obtained, and the
break Lorentz factor can be as high as \gamma_\star \siml 10^5. Radiation
from the nonthermal electrons produces photon spectra with shape and
characteristic energies in qualitative agreement with observed generic
gamma-ray burst and blazar spectra. The scenario described here provides a
plausible way to solve one of the crucial problems of nonthermal high energy
sources, namely the efficient transfer of energy from the proton flow to an
apropriate nonthermal lepton component.Comment: Ap.J. (Letters) in press, uuencoded latex file (uses AAS macro
aaspp4), 10 page
High Lundquist Number Simulations of Parker\u27s Model of Coronal Heating: Scaling and Current Sheet Statistics Using Heterogeneous Computing Architectures
Parker\u27s model [Parker, Astrophys. J., 174, 499 (1972)] is one of the most discussed mechanisms for coronal heating and has generated much debate. We have recently obtained new scaling results for a 2D version of this problem suggesting that the heating rate becomes independent of resistivity in a statistical steady state [Ng and Bhattacharjee, Astrophys. J., 675, 899 (2008)]. Our numerical work has now been extended to 3D using high resolution MHD numerical simulations. Random photospheric footpoint motion is applied for a time much longer than the correlation time of the motion to obtain converged average coronal heating rates. Simulations are done for different values of the Lundquist number to determine scaling. In the high-Lundquist number limit (S \u3e 1000), the coronal heating rate obtained is consistent with a trend that is independent of the Lundquist number, as predicted by previous analysis and 2D simulations. We will present scaling analysis showing that when the dissipation time is comparable or larger than the correlation time of the random footpoint motion, the heating rate tends to become independent of Lundquist number, and that the magnetic energy production is also reduced significantly. We also present a comprehensive reprogramming of our simulation code to run on NVidia graphics processing units using the Compute Unified Device Architecture (CUDA) and report code performance on several large scale heterogenous machines
Mini-Conference on Hamiltonian and Lagrangian Methods in Fluid and Plasma Physics
A mini-conference on Hamiltonian and Lagrangian methods in fluid and plasma
physics was held on November 14, 2002, as part of the 44th meeting of the
Division of Plasma Physics of the American Physical Society. This paper
summarizes the material presented during the talks scheduled during the
Mini-Conference, which was held to honor Allan Kaufman on the occasion of his
75th birthday.Comment: 14 pages, conference summar
Gyrokinetic electron acceleration in the force-free corona with anomalous resistivity
We numerically explore electron acceleration and coronal heating by
dissipative electric fields. Electrons are traced in linear force-free magnetic
fields extrapolated from SOHO/MDI magnetograms, endowed with anomalous
resistivity () in localized dissipation regions where the magnetic twist
\nabla \times \bhat exceeds a given threshold. Associated with is
a parallel electric field which can accelerate runaway
electrons. In order to gain observational predictions we inject electrons
inside the dissipation regions and follow them for several seconds in real
time. Precipitating electrons which leave the simulation system at height =
0 are associated with hard X rays, and electrons which escape at height
3 km are associated with normal-drifting type IIIs at the
local plasma frequency. A third, trapped, population is related to
gyrosynchrotron emission. Time profiles and spectra of all three emissions are
calculated, and their dependence on the geometric model parameters and on
is explored. It is found that precipitation generally preceeds escape by
fractions of a second, and that the electrons perform many visits to the
dissipation regions before leaving the simulation system. The electrons
impacting = 0 reach higher energies than the escaping ones, and
non-Maxwellian tails are observed at energies above the largest potential drop
across a single dissipation region. Impact maps at = 0 show a tendency of
the electrons to arrive at the borders of sunspots of one polarity. Although
the magnetograms used here belong to non-flaring times, so that the simulations
refer to nanoflares and `quiescent' coronal heating, it is conjectured that the
same process, on a larger scale, is responsible for solar flares
Research briefing on contemporary problems in plasma science
An overview is presented of the broad perspective of all plasma science. Detailed discussions are given of scientific opportunities in various subdisciplines of plasma science. The first subdiscipline to be discussed is the area where the contemporary applications of plasma science are the most widespread, low temperature plasma science. Opportunities for new research and technology development that have emerged as byproducts of research in magnetic and inertial fusion are then highlighted. Then follows a discussion of new opportunities in ultrafast plasma science opened up by recent developments in laser and particle beam technology. Next, research that uses smaller scale facilities is discussed, first discussing non-neutral plasmas, and then the area of basic plasma experiments. Discussions of analytic theory and computational plasma physics and of space and astrophysical plasma physics are then presented
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