Anisotropic Magnetic Turbulence in the Inner Heliosphere -- Radial Evolution of Distributions observed by Parker Solar Probe

TL;DR

This study analyzes Parker Solar Probe data to understand how magnetic turbulence evolves radially in the inner heliosphere, revealing anisotropic properties and their implications for solar wind dynamics.

Contribution

It provides new insights into the anisotropic nature and radial evolution of magnetic turbulence in the inner heliosphere based on in-situ observations.

Findings

Perpendicular fluctuations are Gaussian, parallel fluctuations show high kurtosis and skewness.

Variance anisotropy increases closer to the Sun.

Fluctuation strength relative to mean field decreases with decreasing distance, following a r^{1/4} law.

Abstract

Observations from Parker Solar Probe's first five orbits are used to investigate the helioradial evolution of probability density functions (PDFs) of fluctuations of magnetic field components, between \(\sim 28\) - 200 \(\rs\). Transformation of the magnetic field vector to a local mean-field coordinate system permits examination of anisotropy relative to the mean magnetic field direction. Attention is given to effects of averaging-interval size. It is found that PDFs of the perpendicular fluctuations are well approximated by a Gaussian function, with the parallel fluctuations less so: kurtoses of the latter are generally larger than 10, and their PDFs indicate increasing skewness with decreasing distance \(r\) from the Sun, with the latter observation possibly explained by the increasing Alfv\'enicity of the fluctuations. The ratio of perpendicular to parallel variances is greater than unity; this variance anisotropy becomes stronger with decreasing \(r\). The ratio of the total rms fluctuation strength to the mean field magnitude decreases with decreasing \(r\), with a value \(\sim 0.8\) near 1 AU and \(\sim 0.5\) at 0.14 AU; the ratio is well approximated by a \(r^{1/4}\) power law. These findings improve our understanding of the radial evolution of turbulence in the solar wind, and have implications for related phenomena such as energetic-particle transport in the inner heliosphere.