Spectral features of solar wind turbulent plasma

TL;DR

This study uses 3D Hall MHD simulations to analyze solar wind turbulence, revealing a spectral break at small scales, anisotropic cascades, and the influence of nonlinear Hall interactions on turbulence characteristics.

Contribution

It provides the first detailed 3D simulation analysis of spectral properties in compressible Hall MHD plasma relevant to solar wind turbulence, highlighting nonlinear Hall effects.

Findings

Spectral break occurs at scales smaller than ion gyroradius.

Turbulent spectra follow a $k^{-7/3}$ inertial range.

Fluctuations are highly anisotropic and compressible at small scales.

Abstract

Spectral properties of a fully compressible solar wind Hall Magnetohydrodynamic plasma are investigated by means of time dependent three dimensional Hall MHD simulations. Our simulations, in agreement with spacecraft data, identify a spectral break in turbulence spectra at characteristic length-scales associated with electromagnetic fluctuations that are smaller than the ion gyroradius. In this regime, our 3D simulations show that turbulent spectral cascades in the presence of a mean magnetic field follow an omnidirectional anisotropic inertial range spectrum close to $k^{-7/3}$. The onset of the spectral break in our simulations can be ascribed to the presence of nonlinear Hall interactions that modify the spectral cascades. Our simulations further show that the underlying charachteristic turbulent fluctuations are spectrally anisotropic, the extent of which depends critically on the local wavenumber. The fluctuations associated with length scales smaller than the ion gyroradius are highly compressible and tend to exhibit a near equipartition in the velocity and magnetic fields. Finally, we find that the orientation of velocity and magnetic field fluctuations critically determine the character of nonlinear interactions that predominantly govern a Hall MHD plasma, like the solar wind.