Inertial range magnetic fluctuation anisotropy observed from PSP first seven orbits

TL;DR

This study analyzes magnetic fluctuation anisotropy in the solar wind using Parker Solar Probe data, revealing how turbulence properties vary with distance from the Sun and confirming a 2D plus slab turbulence model.

Contribution

It demonstrates the consistency of the 2D plus slab turbulence model with in-situ measurements and quantifies the variation of turbulence anisotropy with solar distance.

Findings

The 2D plus slab model fits the observed data.

The power ratio of 2D to slab fluctuations decreases closer to the Sun.

Turbulence anisotropy varies with radial distance from the Sun.

Abstract

Solar wind turbulence is anisotropic with respect to the mean magnetic field. Anisotropy leads to ambiguity when interpreting in-situ turbulence observations in the solar wind because an apparent change in the measurements could be due either to the change of intrinsic turbulence properties or to a simple change of the spacecraft sampling direction. We demonstrate the ambiguity using the spectral index and magnetic compressibility in the inertial range observed by the Parker Solar Probe during its first seven orbits ranging from 0.1 to 0.6 AU. To unravel the effects of the sampling direction, we assess whether the wavevector anisotropy is consistent with a two-dimensional (2D) plus slab turbulence transport model and determine the fraction of power in the 2D versus slab component. Our results confirm that the 2D plus slab model is consistent with the data and the power ratio between 2D and slab components depends on radial distance, with the relative power in 2D fluctuations becoming smaller closer to the Sun.