The distributions of the temperature excess of alphas to protons ( ε ) were studied using Parker Solar Probe measurements for Encounters 2 through 14. The distributions were mapped based on heliographic distance, Coulomb number, plasma β , and Alfvén Mach number ( M A ). The importance of collisional effects in the thermalization of solar wind is observed for a wide range of Coulomb numbers. The distributions correlate better with N β and NM A than just N . Furthermore, evidence was found for a narrow region immediately above the Alfvén surface (1 < M A < 2) where ε has values much higher than the mass ratio.
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Amaro et al. (2024) studied this question.
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