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 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_∥) and perpendicular (T_⊥) temperatures, temperature anisotropy (T_⊥/T_∥), plasma beta (β), Alfvén Mach number (MA), and magnetic field fluctuations (δB/B) for sub and super-Alfvénic regions. In super-Alfvénic regions, power laws of |B|, N, V, and T as a function of the heliocentric distance are broadly consistent with previous Helios results at >0.3 au. The radial evolution of the components of the temperature tensor reveals distinct behavior: T_⊥decreases monotonically with distance, whereas T_∥ exhibits a nonmonotonic trend—decreasing in the sub-Alfvénic region, increasing just beyond the Alfvén surface. We interpret the increase in T_∥ 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 the sunward direction. These fluctuations may provide free energy for beam generation and particle heating via wave–particle interactions.