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 ( M A ), 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.