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

Abstract Regions of magnetic fields with near-radial, Parker-spiral-like geometry known as quiescent regions have been observed in Parker Solar Probe data. These regions have very low δ B /〈∣ B ∣〉 compared to nonquiescent 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. Inside of 15 solar radii ( R ⊙ ) , 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">R</mml:mi> </mml:math> -versus- β ∥ p plots (where <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">R</mml:mi> </mml:math> is the proton temperature anisotropy), quiescent region solar wind is shown to be more stable to proton cyclotron and firehose instabilities than nonquiescent solar wind at the same heliocentric distances. It is shown that quiescent regions evolve similarly to the surrounding nonquiescent solar wind, but quiescent solar wind begins at a different location in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">R</mml:mi> </mml:math> -versus- β ∥ p parameter space, suggesting that these regions have separate origins from the more turbulent nonquiescent solar wind. Namely, enhanced temperature anisotropies and enhanced magnetic field strength may be consistent with magnetic field lines that have undergone less magnetic field expansion compared to nonquiescent wind at the same heliocentric distances.