Residual Energy Spectrum of Solar Wind Turbulence

TL;DR

This study analyzes 5 years of solar wind data to understand the residual energy spectrum and its relation to turbulence, finding that kinetic normalization yields closer to equipartition and a steeper residual energy spectrum.

Contribution

It introduces a kinetic normalization approach for residual energy analysis and characterizes its spectral properties in solar wind turbulence.

Findings

Residual energy spectrum is steeper than velocity and magnetic spectra.

Kinetic normalization reduces residual energy, approaching equipartition.

Local patches of imbalance exist within globally balanced intervals.

Abstract

It has long been known that the energy in velocity and magnetic field fluctuations in the solar wind is not in equipartition. In this paper, we present an analysis of 5 years of Wind data at 1 AU to investigate the reason for this. The residual energy (difference between energy in velocity and magnetic field fluctuations) was calculated using both the standard magnetohydrodynamic (MHD) normalization for the magnetic field and a kinetic version, which includes temperature anisotropies and drifts between particle species. It was found that with the kinetic normalization, the fluctuations are closer to equipartition, with a mean normalized residual energy of sigma_r = -0.19 and mean Alfven ratio of r_A = 0.71. The spectrum of residual energy, in the kinetic normalization, was found to be steeper than both the velocity and magnetic field spectra, consistent with some recent MHD turbulence predictions and numerical simulations, having a spectral index close to -1.9. The local properties of residual energy and cross helicity were also investigated, showing that globally balanced intervals with small residual energy contain local patches of larger imbalance and larger residual energy at all scales, as expected for non-linear turbulent interactions.