[Show abstract][Hide abstract] ABSTRACT: We use analytic estimates and numerical simulations of test particles
interacting with magnetohydrodynamic (MHD) turbulence to show that subsonic MHD
turbulence produces efficient second-order Fermi acceleration of relativistic
particles. This acceleration is not well-described by standard quasi-linear
theory but is a consequence of resonance broadening of wave-particle
interactions in MHD turbulence. We provide momentum diffusion coefficients that
can be used for astrophysical and heliospheric applications and discuss the
implications of our results for accretion flows onto black holes. In
particular, we show that particle acceleration by subsonic turbulence in
radiatively inefficient accretion flows can produce a non-thermal tail in the
electron distribution function that is likely important for modeling and
interpreting the emission from low luminosity systems such as Sgr A* and M87.
The Astrophysical Journal 03/2014; 791(1). DOI:10.1088/0004-637X/791/1/71 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate the effects of pitch-angle scattering on the efficiency of
particle heating and acceleration by MHD turbulence using phenomenological
estimates and simulations of non-relativistic test particles interacting with
strong, subsonic MHD turbulence. We include an imposed pitch-angle scattering
rate, which is meant to approximate the effects of high frequency plasma waves
and/or velocity space instabilities. We focus on plasma parameters similar to
those found in the near-Earth solar wind, though most of our results are more
broadly applicable. An important control parameter is the size of the particle
mean free path lambda_{mfp} relative to the scale of the turbulent fluctuations
L. For small scattering rates, particles interact quasi-resonantly with
turbulent fluctuations in magnetic field strength. Scattering increases the
long-term efficiency of this resonant heating by factors of a few-10, but the
distribution function does not develop a significant non-thermal power-law
tail. For higher scattering rates, the interaction between particles and
turbulent fluctuations becomes non-resonant, governed by particles heating and
cooling adiabatically as they encounter turbulent density fluctuations. Rapid
pitch-angle scattering can produce a power-law tail in the proton distribution
function but this requires fine-tuning of parameters. Moreover, in the
near-Earth solar wind, a significant power-law tail cannot develop by this
mechanism because the particle acceleration timescales are longer than the
adiabatic cooling timescale set by the expansion of the solar wind. Our results
thus imply that MHD-scale turbulent fluctuations are unlikely to be the origin
of the v^{-5} tail in the proton distribution function observed in the solar
wind.
The Astrophysical Journal 06/2013; 777(2). DOI:10.1088/0004-637X/777/2/128 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
The Astrophysical Journal 03/2012; 758(2). DOI:10.1088/0004-637X/758/2/78 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The solar wind exhibits sustained heating after escaping the solar
corona, and it is anticipated that the damping of turbulent plasma modes
contributes to this heating. To investigate this, we simulate the
interaction of charged test particles with fully dynamical simulations
of magnetohydrodynamic Alfvenic turbulence, in which particles may be
accelerated by the cyclotron resonance with Alfven modes (providing
perpendicular acceleration) or transit-time damping with compressive
modes via the μ ∇ B force (parallel acceleration). We
implement a novel time-correlated driving scheme for the turbulence,
which allows us to calculate heating rates over long times for initially
thermal distributions of high-gyrofrequency particles, and investigate
whether energy gain is primarily thermal or non-thermal. Additionally,
we calculate particle diffusion coefficients. Some agreement is found
with prior theories of turbulent acceleration, but discrepancies remain.
Specifically, we find that: 1) the heating rate of thermal distributions
with vth >> vAlfven, where vth is
the typical speed of the Maxwellian, is proportional to
vth-1, implying that protons in the solar wind are
heated much more effectively than electrons by turbulence above the
proton gyroscale, and that minor ions are heated more effectively still;
2) thermal distributions heated by transit-time damping (TTD) develop
only weak non-thermal tails, which are still exponential rather than
power-law; 3) D∥ , the parallel momentum diffusion
coefficient, is proportional to v⊥4,
consistent with parallel acceleration of particles via TTD, for
high-v⊥ particles (surprisingly, for lower
v⊥, D∥ approaches a constant); 4) the
resonances predicted by quasi-linear theory are observed to be highly
broadened.