Power and spectral index anisotropy of the entire inertial range of turbulence in the fast solar wind

TL;DR

This study analyzes the power and spectral index anisotropy of high-speed solar wind turbulence across the entire inertial range, revealing isotropic conditions at large scales and anisotropic scaling consistent with critically balanced Alfvénic turbulence at smaller scales.

Contribution

It provides the first comprehensive measurement of turbulence anisotropy from scales larger than the outer scale down to the ion gyroscale in the solar wind.

Findings

Power and spectral indices are isotropic at the outer scale.

Spectral scaling near k^{-5/3} across and k^{-2} along magnetic field lines.

The inertial range width remains constant with heliocentric distance.

Abstract

We measure the power and spectral index anisotropy of high speed solar wind turbulence from scales larger than the outer scale down to the ion gyroscale, thus covering the entire inertial range. We show that the power and spectral indices at the outer scale of turbulence are approximately isotropic. The turbulent cascade causes the power anisotropy at smaller scales manifested by anisotropic scalings of the spectrum: close to k^{-5/3} across and k^{-2} along the local magnetic field, consistent with a critically balanced Alfvenic turbulence. By using data at different radial distances from the Sun, we show that the width of the inertial range does not change with heliocentric distance and explain this by calculating the radial dependence of the ratio of the outer scale to the ion gyroscale. At the smallest scales of the inertial range, close to the ion gyroscale, we find an enhancement of power parallel to the magnetic field direction coincident with a decrease in the perpendicular power. This is most likely related to energy injection by ion kinetic modes such as the firehose instability and also marks the beginning of the dissipation range of solar wind turbulence.