TY - JOUR
T1 - Resonance broadening and heating of charged particles in magnetohydrodynamic turbulence
AU - Lynn, Jacob W.
AU - Parrish, Ian J.
AU - Quataert, Eliot
AU - Chandran, Benjamin D.G.
N1 - Copyright:
Copyright 2015 Elsevier B.V., All rights reserved.
PY - 2012/10/20
Y1 - 2012/10/20
N2 - The heating, acceleration, and pitch-angle scattering of charged particles by magnetohydrodynamic (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 β ∼ 1, where β 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 sliding along moving wires. We show that test particle heating rates are consistent with a TTD resonance that is broadened according to a decorrelation prescription that is Gaussian in time (but inconsistent with Lorentzian broadening due to an exponential decorrelation function, a prescription widely used in the literature). TTD dominates the heating for vs ≫ vA (e.g., electrons), where vs is the thermal speed of species s and vA is the Alfvén speed, while FTB dominates for vs ≪ vA (e.g., minor ions). Proton heating rates for β ∼ 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.
AB - The heating, acceleration, and pitch-angle scattering of charged particles by magnetohydrodynamic (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 β ∼ 1, where β 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 sliding along moving wires. We show that test particle heating rates are consistent with a TTD resonance that is broadened according to a decorrelation prescription that is Gaussian in time (but inconsistent with Lorentzian broadening due to an exponential decorrelation function, a prescription widely used in the literature). TTD dominates the heating for vs ≫ vA (e.g., electrons), where vs is the thermal speed of species s and vA is the Alfvén speed, while FTB dominates for vs ≪ vA (e.g., minor ions). Proton heating rates for β ∼ 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.
KW - acceleration of particles
KW - magnetohydrodynamics (MHD)
KW - plasmas
KW - solar wind
KW - turbulence
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U2 - 10.1088/0004-637X/758/2/78
DO - 10.1088/0004-637X/758/2/78
M3 - Article
AN - SCOPUS:84867305527
VL - 758
JO - Astrophysical Journal
JF - Astrophysical Journal
SN - 0004-637X
IS - 2
M1 - 78
ER -