Phase mixing and the 1/f spectrum in the solar wind

Abstract

The origin and evolution of the 1/f power law observed in the energy spectrum of solar coronal and solar wind fluctuations at scales of around an hour is not entirely understood. Several existing theories aim at explaining it, involving both linear and nonlinear mechanisms. An often overlooked property of the solar corona and solar wind is their highly inhomogeneous nature. In this paper we investigate the linear evolution of pure Alfvén and surface Alfvén waves propagating through a plasma which is inhomogeneous across the magnetic field. The inhomogeneity is given by density, which we model to be two-dimensional colored noise, with power spectral slopes ranging from -2 to -1. Alfvén waves propagate independently on individual magnetic field lines, and eventually get completely out of phase through the process of phase mixing, leading to unrealistic spectra. When the coupling between the inhomogeneous background and the propagating waves is fully accounted for, transverse waves such as surface Alfvén waves (also referred to as kink or Alfvénic) appear, showing collective wave behavior of neighboring magnetic field lines with different Alfvén speeds. We show that the linear cascade of surface Alfvén wave energy, induced by phase mixing and resonant absorption, leads to a perpendicular wave energy spectrum which tends to the perpendicular power spectrum of the background density. Based on our model, we propose that a perpendicular density power spectrum of 1/f in the solar corona can induce, through linear processes, the 1/f spectrum of the fluctuations that is observed at the largest scales.

arXiv commentAccepted in ApJ

References in corpus (21)

• Array Programming with NumPy
• Comparing the statistics of interstellar turbulence in simulations and observations: Solenoidal versus compressive turbulence forcing
• Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence
• On the Generation, Propagation, and Reflection of Alfven Waves from the Solar Photosphere to the Distant Heliosphere
• Compressible MHD Turbulence in Interstellar Plasmas
• On the nature of kink MHD waves in magnetic flux tubes
• First in-situ Measurements of Electron Density and Temperature from Quasi-Thermal Noise Spectroscopy with Parker Solar Probe/FIELDS
• Alfven Wave Reflection and Turbulent Heating in the Solar Wind from 1 Solar Radius to 1 AU: an Analytical Treatment
• On the origin of the 1/f spectrum in the solar wind magnetic field
• Turbulence in the sub-Alfvénic solar wind driven by reflection of low-frequency Alfvén waves
• Magnetohydrodynamic kink waves in nonuniform solar flux tubes: phase mixing and energy cascade to small scales
• On the 1/f spectrum in the solar wind and its connection with magnetic compressibility
• On the probability distribution function of small scale interplanetary magnetic field fluctuations
• Parametric Instability, Inverse Cascade, and the $1/f$ Range of Solar-Wind Turbulence
• The role of compressibility in solar wind plasma turbulence
• The Solar Corona as probed by Comet Lovejoy (C/2011 W3)
• The low-frequency break observed in the slow solar wind magnetic spectra
• On the emergence of very long time fluctuations and 1/f noise in ideal flows
• Anisotropic magnetohydrodynamic turbulence driven by parametric decay instability: the onset of phase mixing and Alfvén wave turbulence
• Transition to turbulence in nonuniform coronal loops driven by torsional Alfven waves
• Three-dimensional simulations of the inhomogeneous Low Solar Wind

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• Two Types of $1/f$ Range in Solar Wind Turbulence
• Interplanetary magnetic correlation and low-frequency spectrum over many solar rotations