27,114 research outputs found
Inertial range turbulence in kinetic plasmas
The transfer of turbulent energy through an inertial range from the driving
scale to dissipative scales in a kinetic plasma followed by the conversion of
this energy into heat is a fundamental plasma physics process. A theoretical
foundation for the study of this process is constructed, but the details of the
kinetic cascade are not well understood. Several important properties are
identified: (a) the conservation of a generalized energy by the cascade; (b)
the need for collisions to increase entropy and realize irreversible plasma
heating; and (c) the key role played by the entropy cascade--a dual cascade of
energy to small scales in both physical and velocity space--to convert
ultimately the turbulent energy into heat. A strategy for nonlinear numerical
simulations of kinetic turbulence is outlined. Initial numerical results are
consistent with the operation of the entropy cascade. Inertial range turbulence
arises in a broad range of space and astrophysical plasmas and may play an
important role in the thermalization of fusion energy in burning plasmas.Comment: 11 pages, 2 figures, submitted to Physics of Plasmas, DPP Meeting
Special Issu
Three Dimensional Pseudo-Spectral Compressible Magnetohydrodynamic GPU Code for Astrophysical Plasma Simulation
This paper presents the benchmarking and scaling studies of a GPU accelerated
three dimensional compressible magnetohydrodynamic code. The code is developed
keeping an eye to explain the large and intermediate scale magnetic field
generation is cosmos as well as in nuclear fusion reactors in the light of the
theory given by Eugene Newman Parker. The spatial derivatives of the code are
pseudo-spectral method based and the time solvers are explicit. GPU
acceleration is achieved with minimal code changes through OpenACC
parallelization and use of NVIDIA CUDA Fast Fourier Transform library (cuFFT).
NVIDIAs unified memory is leveraged to enable over-subscription of the GPU
device memory for seamless out-of-core processing of large grids. Our
experimental results indicate that the GPU accelerated code is able to achieve
upto two orders of magnitude speedup over a corresponding OpenMP parallel, FFTW
library based code, on a NVIDIA Tesla P100 GPU. For large grids that require
out-of-core processing on the GPU, we see a 7x speedup over the OpenMP, FFTW
based code, on the Tesla P100 GPU. We also present performance analysis of the
GPU accelerated code on different GPU architectures - Kepler, Pascal and Volta
Spatio-temporal evolution of the nonresonant instability in shock precursors of young supernova remnants
A nonresonant cosmic-ray-current-driven instability may operate in the shock
precursors of young supernova remnants and be responsible for magnetic-field
amplification, plasma heating and turbulence. Earlier simulations demonstrated
magnetic-field amplification, and in kinetic studies a reduction of the
relative drift between cosmic rays and thermal plasma was observed as
backreaction. However, all published simulations used periodic boundary
conditions, which do not account for mass conservation in decelerating flows
and only allow the temporal development to be studied. Here we report results
of fully kinetic Particle-In-Cell simulations with open boundaries that permit
inflow of plasma on one side of the simulation box and outflow at the other
end, hence allowing an investigation of both the temporal and the spatial
development of the instability. Magnetic-field amplification proceeds as in
studies with periodic boundaries and, observed here for the first time, the
reduction of relative drifts causes the formation of a shock-like compression
structure at which a fraction of the plasma ions are reflected. Turbulent
electric field generated by the nonresonant instability inelastically scatters
cosmic rays, modifying and anisotropizing their energy distribution. Spatial CR
scattering is compatible with Bohm diffusion. Electromagnetic turbulence leads
to significant nonadiabatic heating of the background plasma maintaining bulk
equipartition between ions and electrons. The highest temperatures are reached
at sites of large-amplitude electrostatic fields. Ion spectra show
supra-thermal tails resulting from stochastic scattering in the turbulent
electric field. Together, these modifications in the plasma flow will affect
the properties of the shock and particle acceleration there.Comment: Accepted for publication in MNRAS. 16 pages, 15 figure
Acoustic wave propagation in the solar sub-photosphere with localised magnetic field concentration: effect of magnetic tension
Aims: We analyse numerically the propagation and dispersion of acoustic waves in the solar-like sub-photosphere with localised non-uniform magnetic field concentrations, mimicking sunspots with various representative magnetic field configurations.
Methods: Numerical simulations of wave propagation through the solar sub-photosphere with a localised magnetic field concentration are carried out using SAC, which solves the MHD equations for gravitationally stratified plasma. The initial equilibrium density and pressure stratifications are derived from a standard solar model. Acoustic waves are generated by a source located at the height corresponding approximately to the visible surface of the Sun. By means of local helioseismology we analyse the response of vertical velocity at the level corresponding to the visible solar surface to changes induced by magnetic field in the interior.
Results: The results of numerical simulations of acoustic wave propagation and dispersion in the solar sub-photosphere with localised magnetic field concentrations of various types are presented. Time-distance diagrams of the vertical velocity perturbation at the level corresponding to the visible solar surface show that the magnetic field perturbs and scatters acoustic waves and absorbs the acoustic power of the wave packet. For the weakly magnetised case, the effect of magnetic field is mainly thermodynamic, since the magnetic field changes the temperature stratification. However, we observe the signature of slow magnetoacoustic mode, propagating downwards, for the strong magnetic field cases
Simulating AIA observations of a flux rope ejection
Extreme ultraviolet (EUV) images from the Atmospheric Imaging Assembly (AIA)
on board the Solar Dynamic Observatory (SDO) are providing new insights into
the early phase of CME evolution. Observations now show the ejection of
magnetic flux ropes from the solar corona and how they evolve into CMEs. These
observations are difficult to interpret in terms of basic physical mechanisms
and quantities. To fully understand CMEs we need to compare equivalent
quantities derived from both observations and theoretical models. To this end
we aim to produce synthesised AIA observations from simluations of a flux rope
ejection. To carry this out we include the role of thermal conduction and
radiative losses, both of which are important for determining the temperature
distribution of the solar corona during a CME. We perform a simulation where a
flux rope is ejected from the solar corona. From the density and temperature of
the plasma in the simulation we synthesise AIA observations. The emission is
then integrated along the line of sight using the instrumental response
function of AIA. We sythesise observations of AIA in the channels at 304 A, 171
A, 335 A, and 94 A. The synthesised observations show a number of features
similar to actual observations and in particular reproduce the general
development of CMEs in the low corona as observed by AIA. In particular we
reproduce an erupting and expanding arcade in the 304 A and 171 A channels with
a high density core. The ejection of a flux rope reproduces many of the
features found in the AIA observations. This work is therefore a step forward
in bridging the gap between observations and models, and can lead to more
direct interpretations of EUV observations in terms of flux rope ejections. We
plan to improve the model in future studies in order to perform a more
quantitative comparison
Polarized Light from the Transportation of a Matter-Antimatter Beam in a Plasma
A relativistic electron-positron beam propagating through a magnetized electron-ion plasma is shown to generate both circularly and linearly polarized synchrotron radiation. The degrees of circular and linear polarizations depend both on the density ratio of pair beam to background plasma and initial magnetization, and a maximum degree of circular polarization is found to occur for a tenuous pair beam. We demonstrate that the generation of circularly polarized radiation is intrinsically linked to asymmetric energy dissipation of the pair beam during the filamentation instability dynamics in the electron-ion plasma. These results can help in understanding the recent observations of circularly polarized radiation from gamma-ray-bursts
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