Solar Wind Heating Near the Sun: A Radial Evolution Approach
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Yogesh, Leon Ofman, Kristopher G. Klein, et al. “Solar Wind Heating near the Sun: A Radial Evolution Approach.” The Astrophysical Journal 999, no. 2 (2026): 225. https://doi.org/10.3847/1538-4357/ae4582.
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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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<sub>∥</sub>) and perpendicular (T<sub>⊥</sub>) temperatures, temperature anisotropy (T<sub>⊥</sub>/T<sub>∥</sub>), 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<sub>⊥</sub>decreases monotonically with distance, whereas T<sub>∥</sub> exhibits a nonmonotonic trend—decreasing in the sub-Alfvénic region, increasing just beyond the Alfvén surface. We interpret the increase in T<sub>∥</sub> 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.