Domains of Magnetic Pressure Balance in Parker Solar Probe Observations of the Solar Wind

TL;DR

This study analyzes magnetic pressure balance domains in the solar wind near the Sun using Parker Solar Probe data, revealing their properties, variability, and relation to plasma conditions, and challenging previous assumptions about their shape and alignment.

Contribution

It provides the first detailed analysis of magnetic pressure balance domains in the near-Sun solar wind, including their filling fraction, duration, and geometric properties, based on PSP observations.

Findings

Domain filling fraction decreases with distance from the Sun.

Inverse relationship between domain duration and plasma beta.

Domains are consistent with isotropic rather than elongated structures.

Abstract

The Parker Solar Probe (PSP) spacecraft is performing the first in situ exploration of the solar wind within 0.2 au of the Sun. Initial observations confirmed the Alfv\'enic nature of aligned fluctuations of the magnetic field B and velocity V in solar wind plasma close to the Sun, in domains of nearly constant magnetic field magnitude $|{\bf B}|$, i.e., approximate magnetic pressure balance. Such domains are interrupted by particularly strong fluctuations, including but not limited to radial field (polarity) reversals, known as switchbacks. It has been proposed that nonlinear Kelvin-Helmholtz instabilities form near magnetic boundaries in the nascent solar wind leading to extensive shear-driven dynamics, strong turbulent fluctuations including switchbacks, and mixing layers that involve domains of approximate magnetic pressure balance. In this work we identify and analyze various aspects of such domains using data from the first five PSP solar encounters. The filling fraction of domains, a measure of Alfv\'enicity, varies from median values of 90% within 0.2 au to 38% outside 0.9 au, with strong fluctuations. We find an inverse association between the mean domain duration and plasma $\beta$. We examine whether the mean domain duration is also related to the crossing time of spatial structures frozen into the solar wind flow for extreme cases of the aspect ratio. Our results are inconsistent with long, thin domains aligned along the radial or Parker spiral direction, and compatible with isotropic domains, which is consistent with prior observations of isotropic density fluctuations or ``flocculae'' in the solar wind.