Radial evolution of the wave-vector anisotropy of solar wind turbulence between 0.3 and 1 AU

TL;DR

This study analyzes how the anisotropy of solar wind turbulence evolves between 0.3 and 1 AU, revealing a preferential cascade toward perpendicular wave vectors and a radial development of turbulence anisotropy.

Contribution

It introduces a novel method to derive the 2D power spectral density in wave-vector space from single-satellite measurements, revealing new insights into turbulence anisotropy.

Findings

Power spectral density mainly distributed along a ridge inclined toward the k_perp axis.

The ridge approaches the k_perp axis as the outer scale length increases with distance.

A minor quasi-parallel Alfvénic component decreases with radial distance.

Abstract

We present observations of the power spectral anisotropy in wave-vector space of solar wind turbulence, and study how it evolves in interplanetary space with increasing heliocentric distance. For this purpose we use magnetic field measurements made by the Helios-2 spacecraft at three positions between 0.29 and 0.9 AU. To derive the power spectral density (PSD) in $(k_\parallel, k_\bot)$-space based on single-satellite measurements is a challenging task not yet accomplished previously. Here we derive the spectrum $\rm{PSD}_{\rm{2D}}$($\rm{k}_\parallel$, $\rm{k}_\bot$) from the spatial correlation function $\rm{CF}_{\rm{2D}}(r_\parallel, r_\bot)$ by a transformation according to the projection-slice theorem. We find the so constructed PSDs to be distributed in k-space mainly along a ridge that is more inclined toward the $\rm{k}_\bot$ than $\rm{k}_\parallel$ axis, a new result which probably indicates preferential cascading of turbulent energy along the $\rm{k}_\bot$ direction. Furthermore, this ridge of the distribution is found to gradually get closer to the $\rm{k}_\bot$ axis, as the outer scale length of the turbulence becomes larger while the solar wind flows further away from the Sun. In the vicinity of the $\rm{k}_\parallel$ axis, there appears a minor spectral component that probably corresponds to quasi-parallel Alfv\'enic fluctuations. Their relative contribution to the total spectral density tends to decrease with radial distance. These findings suggest that solar wind turbulence undergoes an anisotropic cascade transporting most of its magnetic energy towards larger $\rm{k}_\bot$, and that the anisotropy in the inertial range is radially developing further at scales that are relatively far from the ever increasing outer scale.