Isotropization and Evolution of Energy-Containing Eddies in Solar Wind Turbulence: Parker Solar Probe, Helios 1, ACE, WIND, and Voyager 1

TL;DR

This study investigates how the anisotropy of energy-containing eddies in solar wind turbulence evolves with distance from the Sun, using data from multiple spacecraft to reveal isotropization within 1 au.

Contribution

It provides new insights into the radial evolution of turbulence anisotropy in the heliosphere, highlighting the importance of sampling direction in PSP measurements.

Findings

Correlation lengths are anisotropic within 0.40 au, with 5% ratio of parallel to perpendicular.

Correlation lengths tend to isotropize within 1 au, with a ratio of approximately 1.29.

Limited data beyond 1 au prevents definitive conclusions on anisotropy evolution.

Abstract

We examine the radial evolution of correlation lengths perpendicular (\(\lambda_C^{\perp}\)) and parallel (\(\lambda_C^{\parallel}\)) to the magnetic-field direction, computed from solar wind magnetic-field data measured by Parker Solar Probe (PSP) during its first eight orbits, Helios 1, Advanced Composition Explorer (ACE), WIND, and Voyager 1 spacecraft. Correlation lengths are grouped by an interval's alignment angle; the angle between the magnetic-field and solar wind velocity vectors (\(\Theta_{\rm BV}\)). Parallel and perpendicular angular channels correspond to angles \(0^{\circ}~<~\Theta_{\rm BV}~<~40^{\circ}\) and \(50^{\circ}~<~\Theta_{\rm BV}~<~90^{\circ}\), respectively. We observe an anisotropy in the inner heliosphere within 0.40~au, with \(\lambda_C^{\parallel} / \lambda_C^{\perp} \approx 0.75\) at 0.10~au. This anisotropy reduces with increasing heliocentric distance and the correlation lengths roughly isotropize within 1~au. Results from ACE and WIND support a reversal of the anisotropy, such that \(\lambda_C^{\parallel} /\lambda_C^{\perp} \approx 1.29\) at 1~au. The ratio does not appear to change significantly beyond 1~au, although the small number of parallel intervals in the Voyager dataset precludes unambiguous conclusions from being drawn. This study provides insights regarding the radial evolution of the large, most energetic interacting turbulent fluctuations in the heliosphere. We also emphasize the importance of tracking the changes in sampling direction in PSP measurements as the spacecraft approaches the Sun, when using these data to study the radial evolution of turbulence. This can prove to be vital in understanding the more complex dynamics of the solar wind in the inner heliosphere and can assist in improving related simulations.