Solar Wind Heating Near the Sun: A Radial Evolution Approach

TL;DR

This study uses Parker Solar Probe data to analyze how solar wind plasma and magnetic field properties evolve with distance from the Sun, revealing distinct temperature behaviors and magnetic fluctuations that inform solar wind heating mechanisms.

Contribution

It provides detailed radial profiles of plasma parameters near the Sun, highlighting the different behaviors of temperature components and magnetic fluctuations, advancing understanding of solar wind heating.

Findings

T_perp decreases monotonically with distance.

T_parallel shows non-monotonic behavior, increasing beyond the Alfvén surface.

Magnetic fluctuations vary with direction, influencing particle heating.

Abstract

Characterizing the plasma state in the near-Sun environment is essential to constrain the mechanisms that heat and accelerate the solar wind. In this study, we use Parker Solar Probe (PSP) observations from Encounters 1 through 24 to investigate the radial evolution of solar wind plasma and magnetic field properties in this region. Using intervals with high field-of-view ($>85\%$) coverage, we derive the radial profiles of magnetic field strength ($|B|$), proton density ($N$), bulk speed ($V$), total proton temperature ($T$), parallel ($T_\parallel$) and perpendicular ($T_\perp$) temperatures, temperature anisotropy ($T_\perp/T_\parallel$), plasma beta ($\beta$), Alfv\'{e}n Mach number ($M_A$), and magnetic field fluctuations ($\delta B/B$) for sub and super-Alfv\'{e}nic regions. In super-Alfv\'{e}nic regions, power-law of $|B|$, $N$, $V$, and $T$ as a function of heliocentric distance are broadly consistent with previous \textit{Helios} results at $>0.3$ AU. The radial evolution of the components of the temperature tensor reveals distinct behavior: $T_\perp$ decreases monotonically with distance, whereas $T_\parallel$ exhibits a non-monotonic trend -- decreasing in the sub-Alfv\'{e}nic region, increasing just beyond the Alfv\'{e}n surface. We interpret the increase in $T_\parallel$ as a proxy for proton beam occurrence. We further examine the evolution of magnetic field fluctuations, finding decreasing radial/parallel fluctuations but enhanced tangential/normal/perpendicular fluctuations in sunward direction. These fluctuations may provide free energy for beam generation and particle heating via wave-particle interactions.