3,001 research outputs found
Thermal conduction and particle transport in strong MHD turbulence, with application to galaxy-cluster plasmas
We investigate field-line separation in strong MHD turbulence analytically
and with direct numerical simulations. We find that in the
static-magnetic-field approximation the thermal conductivity in galaxy clusters
is reduced by a factor of about 5-10 relative to the Spitzer thermal
conductivity of a non-magnetized plasma. We also estimate how the thermal
conductivity would be affected by efficient turbulent resistivity.Comment: Major revision: higher resolution simulations lead to significantly
different conclusions. 26 pages, 10 figure
Acceleration of energetic particles by large-scale compressible magnetohydrodynamic turbulence
Fast particles diffusing along magnetic field lines in a turbulent plasma can
diffuse through and then return to the same eddy many times before the eddy is
randomized in the turbulent flow. This leads to an enhancement of particle
acceleration by large-scale compressible turbulence relative to previous
estimates in which isotropic particle diffusion is assumed.Comment: 13 pages, 3 figures, accepted for publication in Ap
Pulse to pulse flux density modulation from pulsars at 8.35 GHz
Aims. To investigate the flux density modulation from pulsars and the
existence of specific behaviour of modulation index versus frequency. Methods.
Several pulsars have been observed with the Effelsberg radio telescope at 8.35
GHz. Their flux density time series have been corrected for interstellar
scintillation effects. Results. We present the measurement of modulation
indices for 8 pulsars. We confirm the presence of a critical frequency at ~1
GHz for these pulsars (including 3 new ones from this study). We derived
intrinsic modulation indices for the resulting flux density time series. Our
data analysis revealed strong single pulses detected from 5 pulsars.Comment: accepted for publication in A&
Cardiac Screening of Young Athletes: a Practical Approach to Sudden Cardiac Death Prevention.
PURPOSE OF REVIEW: We aim to report on the current status of cardiovascular screening of athletes worldwide and review the up-to-date evidence for its efficacy in reducing sudden cardiac death in young athletes. RECENT FINDINGS: A large proportion of sudden cardiac death in young individuals and athletes occurs during rest with sudden arrhythmic death syndrome being recognised as the leading cause. The international recommendations for ECG interpretation have reduced the false-positive ECG rate to 3% and reduced the cost of screening by 25% without compromising the sensitivity to identify serious disease. There are some quality control issues that have been recently identified including the necessity for further training to guide physicians involved in screening young athletes. Improvements in our understanding of young sudden cardiac death and ECG interpretation guideline modification to further differentiate physiological ECG patterns from those that may represent underlying disease have significantly improved the efficacy of screening to levels that may make screening more attractive and feasible to sporting organisations as a complementary strategy to increased availability of automated external defibrillators to reduce the overall burden of young sudden cardiac death
The onset of a small-scale turbulent dynamo at low magnetic Prandtl numbers
We study numerically the dependence of the critical magnetic Reynolds number
Rmc for the turbulent small-scale dynamo on the hydrodynamic Reynolds number
Re. The turbulence is statistically homogeneous, isotropic, and
mirror--symmetric. We are interested in the regime of low magnetic Prandtl
number Pm=Rm/Re<1, which is relevant for stellar convective zones, protostellar
disks, and laboratory liquid-metal experiments. The two asymptotic
possibilities are Rmc->const as Re->infinity (a small-scale dynamo exists at
low Pm) or Rmc/Re=Pmc->const as Re->infinity (no small-scale dynamo exists at
low Pm). Results obtained in two independent sets of simulations of MHD
turbulence using grid and spectral codes are brought together and found to be
in quantitative agreement. We find that at currently accessible resolutions,
Rmc grows with Re with no sign of approaching a constant limit. We reach the
maximum values of Rmc~500 for Re~3000. By comparing simulations with Laplacian
viscosity, fourth-, sixth-, and eighth-order hyperviscosity and Smagorinsky
large-eddy viscosity, we find that Rmc is not sensitive to the particular form
of the viscous cutoff. This work represents a significant extension of the
studies previously published by Schekochihin et al. 2004, PRL 92, 054502 and
Haugen et al. 2004, PRE, 70, 016308 and the first detailed scan of the
numerically accessible part of the stability curve Rmc(Re).Comment: 4 pages, emulateapj aastex, 2 figures; final version as published in
ApJL (but with colour figures
Compressible Sub-Alfvenic MHD turbulence in Low-beta Plasmas
We present a model for compressible sub-Alfvenic isothermal
magnetohydrodynamic (MHD) turbulence in low-beta plasmas and numerically test
it. We separate MHD fluctuations into 3 distinct families - Alfven, slow, and
fast modes. We find that, production of slow and fast modes by Alfvenic
turbulence is suppressed. As a result, Alfven modes in compressible regime
exhibit scalings and anisotropy similar to those in incompressible regime. Slow
modes passively mimic Alfven modes. However, fast modes show isotropy and a
scaling similar to acoustic turbulence.Comment: 4 pages, 8 figures, Phys. Rev. Lett., in pres
On the connection between gamma and radio radiation spectra in pulsars
The model of pulsar radio emission is discussed in which a coherent radio
emis-sion is excited in a vacuum gap above polar cap of neutron star. Pulsar X
and gamma radiation are considered as the result of low-frequency radio
emission inverse Comp-ton scattering on ultra relativistic electrons
accelerated in the gap. The influence of the pulsar magnetic field on Compton
scattering is taken into account. The relation of radio and gamma radiation
spectra has been found in the framework of the model.Comment: 15 pages, 3 figures, Russian version accepted to JETP, partly
published in JETP Letters, Vol. 85, #6 (2007
Using Synthetic Spacecraft Data to Interpret Compressible Fluctuations in Solar Wind Turbulence
Kinetic plasma theory is used to generate synthetic spacecraft data to
analyze and interpret the compressible fluctuations in the inertial range of
solar wind turbulence. The kinetic counterparts of the three familiar linear
MHD wave modes---the fast, Alfven, and slow waves---are identified and the
properties of the density-parallel magnetic field correlation for these kinetic
wave modes is presented. The construction of synthetic spacecraft data, based
on the quasi-linear premise---that some characteristics of magnetized plasma
turbulence can be usefully modeled as a collection of randomly phased, linear
wave modes---is described in detail. Theoretical predictions of the
density-parallel magnetic field correlation based on MHD and Vlasov-Maxwell
linear eigenfunctions are presented and compared to the observational
determination of this correlation based on 10 years of Wind spacecraft data. It
is demonstrated that MHD theory is inadequate to describe the compressible
turbulent fluctuations and that the observed density-parallel magnetic field
correlation is consistent with a statistically negligible kinetic fast wave
energy contribution for the large sample used in this study. A model of the
solar wind inertial range fluctuations is proposed comprised of a mixture of a
critically balanced distribution of incompressible Alfvenic fluctuations and a
critically balanced or more anisotropic than critical balance distribution of
compressible slow wave fluctuations. These results imply that there is little
or no transfer of large scale turbulent energy through the inertial range down
to whistler waves at small scales.Comment: Accepted to Astrophysical Journal. 28 pages, 7 figure
Resonance Broadening and Heating of Charged Particles in Magnetohydrodynamic Turbulence
The heating, acceleration, and pitch-angle scattering of charged particles by
MHD turbulence are important in a wide range of astrophysical environments,
including the solar wind, accreting black holes, and galaxy clusters. We
simulate the interaction of high-gyrofrequency test particles with fully
dynamical simulations of subsonic MHD turbulence, focusing on the parameter
regime with beta ~ 1, where beta is the ratio of gas to magnetic pressure. We
use the simulation results to calibrate analytical expressions for test
particle velocity-space diffusion coefficients and provide simple fits that can
be used in other work.
The test particle velocity diffusion in our simulations is due to a
combination of two processes: interactions between particles and magnetic
compressions in the turbulence (as in linear transit-time damping; TTD) and
what we refer to as Fermi Type-B (FTB) interactions, in which charged particles
moving on field lines may be thought of as beads spiralling around moving
wires. We show that test particle heating rates are consistent with a TTD
resonance which is broadened according to a decorrelation prescription that is
Gaussian in time. TTD dominates the heating for v_s >> v_A (e.g. electrons),
where v_s is the thermal speed of species s and v_A is the Alfven speed, while
FTB dominates for v_s << v_A (e.g. minor ions). Proton heating rates for beta ~
1 are comparable to the turbulent cascade rate. Finally, we show that velocity
diffusion of collisionless, large gyrofrequency particles due to large-scale
MHD turbulence does not produce a power-law distribution function.Comment: 20 pages, 15 figures; accepted by The Astrophysical Journal; added
clarifying appendices, but no major changes to result
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