124 research outputs found
Evidence of Critical Balance in Kinetic Alfven Wave Turbulence Simulations
A numerical simulation of kinetic plasma turbulence is performed to assess
the applicability of critical balance to kinetic, dissipation scale turbulence.
The analysis is performed in the frequency domain to obviate complications
inherent in performing a local analysis of turbulence. A theoretical model of
dissipation scale critical balance is constructed and compared to simulation
results, and excellent agreement is found. This result constitutes the first
evidence of critical balance in a kinetic turbulence simulation and provides
evidence of an anisotropic turbulence cascade extending into the dissipation
range. We also perform an Eulerian frequency analysis of the simulation data
and compare it to the results of a previous study of magnetohydrodynamic
turbulence simulations.Comment: 10 pages, 9 figures, accepted for publication in Physics of Plasma
Thermal disequilibration of ions and electrons by collisionless plasma turbulence
Does overall thermal equilibrium exist between ions and electrons in a weakly
collisional, magnetised, turbulent plasma---and, if not, how is thermal energy
partitioned between ions and electrons? This is a fundamental question in
plasma physics, the answer to which is also crucial for predicting the
properties of far-distant astronomical objects such as accretion discs around
black holes. In the context of discs, this question was posed nearly two
decades ago and has since generated a sizeable literature. Here we provide the
answer for the case in which energy is injected into the plasma via Alfv\'enic
turbulence: collisionless turbulent heating typically acts to disequilibrate
the ion and electron temperatures. Numerical simulations using a hybrid
fluid-gyrokinetic model indicate that the ion-electron heating-rate ratio is an
increasing function of the thermal-to-magnetic energy ratio,
: it ranges from at to at
least for . This energy partition is
approximately insensitive to the ion-to-electron temperature ratio
. Thus, in the absence of other equilibrating
mechanisms, a collisionless plasma system heated via Alfv\'enic turbulence will
tend towards a nonequilibrium state in which one of the species is
significantly hotter than the other, viz., hotter ions at high
, hotter electrons at low . Spectra of
electromagnetic fields and the ion distribution function in 5D phase space
exhibit an interesting new magnetically dominated regime at high and
a tendency for the ion heating to be mediated by nonlinear phase mixing
("entropy cascade") when and by linear phase mixing
(Landau damping) when $\beta_\mathrm{i}\gg1
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
Ion versus electron heating in compressively driven astrophysical gyrokinetic turbulence
The partition of irreversible heating between ions and electrons in
compressively driven (but subsonic) collisionless turbulence is investigated by
means of nonlinear hybrid gyrokinetic simulations. We derive a prescription for
the ion-to-electron heating ratio Q_\rmi/Q_\rme as a function of the
compressive-to-Alfv\'enic driving power ratio P_\compr/P_\AW, of the ratio of
ion thermal pressure to magnetic pressure \beta_\rmi, and of the ratio of
ion-to-electron background temperatures T_\rmi/T_\rme. It is shown that
Q_\rmi/Q_\rme is an increasing function of P_\compr/P_\AW. When the
compressive driving is sufficiently large, Q_\rmi/Q_\rme approaches \simeq
P_\compr/P_\AW. This indicates that, in turbulence with large compressive
fluctuations, the partition of heating is decided at the injection scales,
rather than at kinetic scales. Analysis of phase-space spectra shows that the
energy transfer from inertial-range compressive fluctuations to
sub-Larmor-scale kinetic Alfv\'en waves is absent for both low and high
\beta_\rmi, meaning that the compressive driving is directly connected to the
ion entropy fluctuations, which are converted into ion thermal energy. This
result suggests that preferential electron heating is a very special case
requiring low \beta_\rmi and no, or weak, compressive driving. Our heating
prescription has wide-ranging applications, including to the solar wind and to
hot accretion disks such as M87 and Sgr A*.Comment: Accepted for publication in Phys. Rev.
A Hybrid Gyrokinetic Ion and Isothermal Electron Fluid Code for Astrophysical Plasma
This paper describes a new code for simulating astrophysical plasmas that
solves a hybrid model composed of gyrokinetic ions (GKI) and an isothermal
electron fluid (ITEF) [A. Schekochihin et al., Astrophys. J. Suppl.
\textbf{182}, 310 (2009)]. This model captures ion kinetic effects that are
important near the ion gyro-radius scale while electron kinetic effects are
ordered out by an electron-ion mass ratio expansion. The code is developed by
incorporating the ITEF approximation into {\tt AstroGK}, an Eulerian
gyrokinetics code specialized to a slab geometry [R. Numata et al., J. Compute.
Pays. \textbf{229}, 9347 (2010)]. The new code treats the linear terms in the
ITEF equations implicitly while the nonlinear terms are treated explicitly. We
show linear and nonlinear benchmark tests to prove the validity and
applicability of the simulation code. Since the fast electron timescale is
eliminated by the mass ratio expansion, the Courant--Friedrichs--Lewy condition
is much less restrictive than in full gyrokinetic codes; the present hybrid
code runs times faster than
{\tt AstroGK}\ with a single ion species and kinetic electrons where
is the ion-electron mass ratio. The improvement of
the computational time makes it feasible to execute ion scale gyrokinetic
simulations with a high velocity space resolution and to run multiple
simulations to determine the dependence of turbulent dynamics on parameters
such as electron--ion temperature ratio and plasma beta
Temporal Properties of the Compressible Magnetohydrodynamic Turbulence
The temporal property of the compressible magneto-hydrodynamic (MHD)
turbulence remains a fundamental unsolved question. Recent studies based on the
spatial-temporal analysis in the global frame of reference suggest that the
majority of fluctuation power in turbulence does not follow any of the MHD wave
dispersion relations but has very low temporal frequency with finite
wavenumbers. Here, we demonstrate that the Lorentzian broadening of the
dispersion relations of the three MHD modes where the nonlinear effects act
like the damping of a harmonic oscillator can explain many salient features of
frequency spectra for all MHD modes. The low frequency fluctuations are
dominated by modes with the low parallel wavenumbers that have been broadened
by the nonlinear processes. The Lorentzian broadening widths of the three MHD
modes exhibit scaling relations to the global frame wavenumbers and are
intrinsically related to energy cascade of each mode. Our results provide a new
window to investigate the temporal properties of turbulence which offers
insights for building a comprehensive understanding of the compressible MHD
turbulence.Comment: 15 pages, 4 figures, submitte
Particle acceleration at magnetized, relativistic turbulent shock fronts
The efficiency of particle acceleration at shock waves in relativistic,
magnetized astrophysical outflows is a debated topic with far-reaching
implications. Here, for the first time, we study the impact of turbulence in
the pre-shock plasma. Our simulations demonstrate that, for a mildly
relativistic, magnetized pair shock (Lorentz factor , magnetization level ), strong turbulence can revive
particle acceleration in a superluminal configuration that otherwise prohibits
it. Depending on the initial plasma temperature and magnetization,
stochastic-shock-drift or diffusive-type acceleration governs particle
energization, producing powerlaw spectra with . At larger magnetization levels, stochastic
acceleration within the pre-shock turbulence becomes competitive and can even
take over shock acceleration
Viriato: a Fourier-Hermite spectral code for strongly magnetised fluid-kinetic plasma dynamics
We report on the algorithms and numerical methods used in Viriato, a novel
fluid-kinetic code that solves two distinct sets of equations: (i) the Kinetic
Reduced Electron Heating Model (KREHM) equations [Zocco & Schekochihin, Phys.
Plasmas 18, 102309 (2011)] (which reduce to the standard Reduced-MHD equations
in the appropriate limit) and (ii) the kinetic reduced MHD (KRMHD) equations
[Schekochihin et al., Astrophys. J. Suppl. 182:310 (2009)]. Two main
applications of these equations are magnetised (Alfvenic) plasma turbulence and
magnetic reconnection. Viriato uses operator splitting (Strang or Godunov) to
separate the dynamics parallel and perpendicular to the ambient magnetic field
(assumed strong). Along the magnetic field, Viriato allows for either a
second-order accurate MacCormack method or, for higher accuracy, a
spectral-like scheme composed of the combination of a total variation
diminishing (TVD) third order Runge-Kutta method for the time derivative with a
7th order upwind scheme for the fluxes. Perpendicular to the field Viriato is
pseudo-spectral, and the time integration is performed by means of an iterative
predictor-corrector scheme. In addition, a distinctive feature of Viriato is
its spectral representation of the parallel velocity-space dependence, achieved
by means of a Hermite representation of the perturbed distribution function. A
series of linear and nonlinear benchmarks and tests are presented, including a
detailed analysis of 2D and 3D Orszag-Tang-type decaying turbulence, both in
fluid and kinetic regimes.Comment: 42 pages, 15 figures, submitted to J. Comp. Phy
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