PERPENDICULAR ION HEATING BY LOW-FREQUENCY ALFVÉN-WAVE TURBULENCE IN THE SOLAR WIND

By B. Chandran, Bo Li, B. Rogers, E. Quataert, K. Germaschewski. 2010

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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 = δvρ/v⊥, where v⊥ (v∥) is the component of the ion velocity perpendicular (parallel) to the background magnetic field B0, and δvρ (δBρ) is the rms amplitude of the velocity (magnetic-field) fluctuations at the gyroradius scale. In the case of thermal protons, when ε ≪ εcrit, 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 δvρ, 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 v2⊥ much more than v2∥ 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.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/1319274d733f9e8071ad504d530e5602b38866e0

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Citations in the corpus

INCORPORATING KINETIC PHYSICS INTO A TWO-FLUID SOLAR-WIND MODEL WITH TEMPERATURE ANISOTROPY AND LOW-FREQUENCY ALFVÉN-WAVE TURBULENCE (2011, methodology): “Chandran et al. (2010) found thatc2 = 0.34 for randomly phased AWs and KAWs, but conjectured thatc2 is smaller (and hence stochastic heating is more effective) in strong AW/KAW turbulence, because much of the dissipation in strong AW/KAW turbulence occurs within coherent structures in which the…”
Recent progress in astrophysical plasma turbulence from solar wind observations (2016, background): “ssibilities have been proposed, including ion cyclotron damping (e.g., Coleman 1968; Smith et al. 2012), Landau damping (e.g., Howes et al. 2008; TenBarge & Howes 2013), stochastic heating (e.g., Chandran et al. 2010), entropy cascade (Schekochihin et al. 2009), and reconnection associated mecha”
Integrated Science Investigation of the Sun (ISIS): Design of the Energetic Particle Investigation (2016, background): “Acceleration via reconnection can be complex, involving direct acceleration, trapping particles in collapsing magnetic islands (Drake et al. 2006), stochastic acceleration (Le Roux et al. 2002; Chandran et al. 2010), and perpendicular betatron acceleration (Dmitruk et al. 2004; Dalena et al. 2014).”
A prescription for the turbulent heating of astrophysical plasmas (2010, background): “…the solar corona (Velli 2003; Verdini et al. 2009; Chandran 2010), the solar wind (Saito et al. 2008; Valentini et al. 2008; Breech et al. 2009; Cranmer et al. 2009; Lehe et al. 2009; Chandran et al. 2010), and the magnetospheres of the Earth (Chaston et al. 2008) and of other planets (Saur 2004).”