Solar Wind Heating Near the Sun: A Radial Evolution Approach

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 (${\hat{I}}^2$), Alfvén Mach number ($M_A$), and magnetic field fluctuations ($\hat{I}´B/B$) for sub and super-Alfvénic regions. In super-Alfvénic 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-Alfvénic region, increasing just beyond the Alfvén 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.

15 pages, 8 figures, accepted for publication in The Astrophysical Journal