How Alfvén waves energize the solar wind: heat vs work

Abstract

A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfvén-wave (AW) energy flux. AWs energize the solar wind via two mechanisms: heating and work. We use high-resolution direct numerical simulations of reflection-driven AW turbulence (RDAWT) in a fast-solar-wind stream emanating from a coronal hole to investigate both mechanisms. In particular, we compute the fraction of the AW power at the coronal base ($P_{\mathrm{AWb}}$) that is transferred to solar-wind particles via heating between the coronal base and heliocentric distance $r$, which we denote ${\chi }_{\mathrm{H}}(r)$, and the fraction that is transferred via work, which we denote ${\chi }_{\mathrm{W}}(r)$. We find that ${\chi }_{\mathrm{W}}(r_{\mathrm{A}})$ ranges from 0.15 to 0.3, where $r_{\mathrm{A}}$ is the Alfvén critical point. This value is small compared to~one because the Alfvén speed $v_{\mathrm{A}}$ exceeds the outflow velocity $U$ at $r<r_{\mathrm{A}}$, so the AWs race through the plasma without doing much work. At $r>r_{\mathrm{A}}$, where $v_{\mathrm{A}}<U$, the AWs are in an approximate sense "stuck to the plasma", which helps them do pressure work as the plasma expands. However, much of the AW power has dissipated by the time the AWs reach $r=r_{\mathrm{A}}$, so the total rate at which AWs do work on the plasma at $r>r_{\mathrm{A}}$ is a modest fraction of $P_{\mathrm{AWb}}$. We find that heating is more effective than work at $r<r_{\mathrm{A}}$, with ${\chi }_{\mathrm{H}}(r_{\mathrm{A}})$ ranging from 0.5 to 0.7. The reason that ${\chi }_{\mathrm{H}}\ge 0.5$ in our simulations is that an appreciable fraction of the local AW power dissipates within each Alfvén-speed scale height in RDAWT, and there are a few Alfvén-speed scale heights between the coronal base and $r_{\mathrm{A}}$.

arXiv comment15 pages, 2 figures, accepted for publication in the Journal of Plasma Physics

References in corpus (25)

• Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence
• Alfven Wave Solar Model (AWSoM): Coronal Heating
• On the Generation, Propagation, and Reflection of Alfven Waves from the Solar Photosphere to the Distant Heliosphere
• Alfven Waves and Turbulence in the Solar Atmosphere and Solar Wind
• A turbulence-driven model for heating and acceleration of the fast wind in coronal holes
• Spectral energy dynamics in magnetohydrodynamic turbulence
• Incorporating Kinetic Physics into a Two-Fluid Solar-Wind Model with Temperature Anisotropy and Low-Frequency Alfven-Wave Turbulence
• Effect of Adiabatic Deceleration on the Focused Transport of Solar Cosmic Rays
• Shearing Box Simulations of the MRI in a Collisionless Plasma
• Coronal heating distribution due to low-frequency wave-driven turbulence
• Solar Winds Driven by Nonlinear Low-Frequency Alfven Waves from the Photosphere : Parametric Study for Fast/Slow Winds and Disappearance of Solar Winds
• Alfven Wave Reflection and Turbulent Heating in the Solar Wind from 1 Solar Radius to 1 AU: an Analytical Treatment
• Spectral scaling laws in MHD turbulence simulations and in the solar wind
• Heating and Acceleration of the Fast Solar Wind by Alfvén Wave Turbulence
• Forecasting Solar Wind Speeds
• On the origin of the 1/f spectrum in the solar wind magnetic field
• Three-dimensional simulation of the fast solar wind driven by compressible magnetohydrodynamic turbulence
• Reflection-driven MHD turbulence in the solar atmosphere and solar wind
• Parametric Instability, Inverse Cascade, and the $1/f$ Range of Solar-Wind Turbulence
• Direct and Inverse Cascades in the Acceleration Region of the Fast Solar Wind
• Observation of an Alfvén Wave Parametric Instability in a Laboratory Plasma
• The parametric instability of Alfvén waves: effects of temperature anisotropy
• Residual Energy in Magnetohydrodynamic Turbulence
• On the Conservation of Cross Helicity and Wave Action in Solar-Wind Models with Non-WKB Alfven Wave Reflection
• Direct Numerical Simulations of Reflection-Driven, Reduced MHD Turbulence from the Sun to the Alfven Critical Point

Cited by in corpus (7)

• New Observations of Solar Wind 1/f Turbulence Spectrum from Parker Solar Probe
• Quantifying the Energy Budget in the Solar Wind from 13.3-100 Solar Radii
• Magnetic field intermittency in the solar wind: PSP and SolO observations ranging from the Alfven region out to 1 AU
• Statistical analysis of intermittency and its association with proton heating in the near Sun environment
• An approximate analytic solution to the coupled problems of coronal heating and solar-wind acceleration
• Characterizing the Impact of Alfvén Wave Forcing in Interplanetary Space on the Distribution of near-Earth Solar Wind Speeds
• The influence of Parker spiral on the reflection-driven turbulence