Measurement of the Electric Fluctuation Spectrum of Magnetohydrodynamic Turbulence

TLDR

Solar‑wind MHD turbulence displays a Kolmogorov‑like inertial‑range spectrum that breaks at short wavelengths where dissipation occurs, and kinetic Alfvén waves damp on ions and electrons, potentially isotropizing the plasma. The study aims to provide the first measurements of the electric field fluctuation spectrum across inertial and dissipative scales in a β ≥ 1 plasma. Measurements were obtained by sampling the electric field fluctuations over the inertial and dissipative wave‑number ranges in a β ≥ 1 plasma. The electric spectrum follows a k^–5/3 inertial‑range that matches the magnetic spectrum and Alfvénic phase speeds, while at krho_i ≥ 1 it is enhanced and consistent with kinetic Alfvén wave dispersion, suggesting that their damping may explain the solar‑wind’s fluidlike behavior.

Abstract

Magnetohydrodynamic (MHD) turbulence in the solar wind is observed to show the spectral behavior of classical Kolmogorov fluid turbulence over an inertial subrange and departures from this at short wavelengths, where energy should be dissipated. Here we present the first measurements of the electric field fluctuation spectrum over the inertial and dissipative wave number ranges in a Beta > or approximately = 1 plasma. The k(-5/3) inertial subrange is observed and agrees strikingly with the magnetic fluctuation spectrum; the wave phase speed in this regime is shown to be consistent with the Alfvén speed. At smaller wavelengths krho(i) > or = 1 the electric spectrum is enhanced and is consistent with the expected dispersion relation of short-wavelength kinetic Alfvén waves. Kinetic Alfvén waves damp on the solar wind ions and electrons and may act to isotropize them. This effect may explain the fluidlike nature of the solar wind.