Quiescent Solar Wind Regions in the Near-Sun Environment: Properties and Radial Evolution

TL;DR

This paper investigates quiescent solar wind regions near the Sun, revealing their distinct thermal properties, stability, and potential different origins compared to more turbulent solar wind, based on Parker Solar Probe data.

Contribution

It characterizes the properties and evolution of quiescent solar wind regions, highlighting their distinct thermal and stability features and suggesting different origins from non-quiescent wind.

Findings

Quiescent regions have lower magnetic fluctuations and bulk speeds.

They exhibit higher proton temperature anisotropies near the Sun.

Quiescent wind is more stable to certain plasma instabilities.

Abstract

Regions of magnetic field with near-radial, Parker Spiral-like geometry known as quiescent regions have been observed in Parker Solar Probe data. These regions have very low $\delta B / \langle |B| \rangle$ compared to non-quiescent solar wind at the same heliocentric distances. Quiescent regions are observed to have lower solar wind bulk speeds, lower proton temperatures, and lower proton density, consistent with properties of the slow solar wind. Inside of 15 Rs, identified quiescent regions show distinct thermal properties, having higher proton temperature anisotropies and lower parallel plasma betas compared to switchback patches observed at the same heliocentric distances. When placed on $\mathcal{R}$ vs $\beta_{\parallel p}$ plots (where $\mathcal{R}$ is the proton temperature anisotropy), quiescent region solar wind is shown to be more stable to proton cyclotron and firehose instabilities than non-quiescent solar wind at the same heliocentric distances. It is shown that quiescent regions evolve similarly to the surrounding non-quiescent solar wind, but quiescent solar wind begins at a different location in the $\mathcal{R}$ vs $\beta_{\parallel p}$ parameter space, suggesting that these regions have separate origins than the more turbulent non-quiescent solar wind. Namely, enhanced temperature anisotropies and enhanced magnetic field strength may be consistent with magnetic field lines which have undergone less magnetic field expansion compared to non-quiescent wind at the same heliocentric distances.