TY - JOUR
T1 - Perpendicular ion heating by low-frequency Alfvén-wave turbulence in the solar wind
AU - Chandran, Benjamin D.G.
AU - Li, Bo
AU - Rogers, Barrett N.
AU - Quataert, Eliot
AU - Germaschewski, Kai
N1 - Copyright:
Copyright 2015 Elsevier B.V., All rights reserved.
PY - 2010/9/1
Y1 - 2010/9/1
N2 - We consider ion heating by turbulent Alfvén waves (AWs) and kinetic Alfvén waves (KAWs) with wavelengths (measured perpendicular to the magnetic field) that are comparable to the ion gyroradius and frequencies ω smaller than the ion cyclotron frequency Ω. We focus on plasmas in which β ≲ 1, where β is the ratio of plasma pressure to magnetic pressure. As in previous studies, we find that when the turbulence amplitude exceeds a certain threshold, an ion's orbit becomes chaotic. The ion then interacts stochastically with the time-varying electrostatic potential, and the ion's energy undergoes a random walk. Using phenomenological arguments, we derive an analytic expression for the rates at which different ion species are heated, which we test by simulating test particles interacting with a spectrum of randomly phased AWs and KAWs. We find that the stochastic heating rate depends sensitively on the quantity ε = δυρ/ ψ1, where υ1 (υ∥) is the component of the ion velocity perpendicular (parallel) to the background magnetic field B0, and δυρ (δB ρ) is the rms amplitude of the velocity (magnetic-field) fluctuations at the gyroradius scale. In the case of thermal protons, when ε ε crit, where εcrit is a constant, a proton's magnetic moment is nearly conserved and stochastic heating is extremely weak. However, when ε > εcrit, the proton heating rate exceeds half the cascade power that would be present in strong balanced KAW turbulence with the same value of δυρ, and magnetic-moment conservation is violated even when ω Ω. For the random-phase waves in our test-particle simulations, εcrit = 0.19. For protons in low-β plasmas, ε ≃ β -1/2δBρ/B0, and ε can exceed εcrit even when δBρ/B0 εcrit. The heating is anisotropic, increasing υ12 much more than υ∥2 when β « 1. (In contrast, at β >≳ 1 Landau damping and transit-time damping of KAWs lead to strong parallel heating of protons.) At comparable temperatures, alpha particles and minor ions have larger values of ε than protons and are heated more efficiently as a result. We discuss the implications of our results for ion heating in coronal holes and the solar wind.
AB - We consider ion heating by turbulent Alfvén waves (AWs) and kinetic Alfvén waves (KAWs) with wavelengths (measured perpendicular to the magnetic field) that are comparable to the ion gyroradius and frequencies ω smaller than the ion cyclotron frequency Ω. We focus on plasmas in which β ≲ 1, where β is the ratio of plasma pressure to magnetic pressure. As in previous studies, we find that when the turbulence amplitude exceeds a certain threshold, an ion's orbit becomes chaotic. The ion then interacts stochastically with the time-varying electrostatic potential, and the ion's energy undergoes a random walk. Using phenomenological arguments, we derive an analytic expression for the rates at which different ion species are heated, which we test by simulating test particles interacting with a spectrum of randomly phased AWs and KAWs. We find that the stochastic heating rate depends sensitively on the quantity ε = δυρ/ ψ1, where υ1 (υ∥) is the component of the ion velocity perpendicular (parallel) to the background magnetic field B0, and δυρ (δB ρ) is the rms amplitude of the velocity (magnetic-field) fluctuations at the gyroradius scale. In the case of thermal protons, when ε ε crit, where εcrit is a constant, a proton's magnetic moment is nearly conserved and stochastic heating is extremely weak. However, when ε > εcrit, the proton heating rate exceeds half the cascade power that would be present in strong balanced KAW turbulence with the same value of δυρ, and magnetic-moment conservation is violated even when ω Ω. For the random-phase waves in our test-particle simulations, εcrit = 0.19. For protons in low-β plasmas, ε ≃ β -1/2δBρ/B0, and ε can exceed εcrit even when δBρ/B0 εcrit. The heating is anisotropic, increasing υ12 much more than υ∥2 when β « 1. (In contrast, at β >≳ 1 Landau damping and transit-time damping of KAWs lead to strong parallel heating of protons.) At comparable temperatures, alpha particles and minor ions have larger values of ε than protons and are heated more efficiently as a result. We discuss the implications of our results for ion heating in coronal holes and the solar wind.
KW - Magnetohydrodynamics (MHD)
KW - Solar wind
KW - Sun: corona
KW - Turbulence
KW - Waves
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U2 - 10.1088/0004-637X/720/1/503
DO - 10.1088/0004-637X/720/1/503
M3 - Article
AN - SCOPUS:78149240942
VL - 720
SP - 503
EP - 515
JO - Astrophysical Journal
JF - Astrophysical Journal
SN - 0004-637X
IS - 1
ER -