Preferential Energization of Solar Wind Ions Below the Alfvénic Surface

Abstract Understanding how solar wind ions are heated and accelerated remains a central question in heliophysics. Observations consistently show that alpha particles are hotter and faster than protons, particularly close to the Sun. This suggests that kinetic processes play a dominant role in shaping ion distributions. Theoretical models have proposed that much of this preferential energization occurs near the Alfvén critical surface, a spatially varying boundary where the solar wind transitions from magnetically dominated to thermally dominated flow. With in situ measurements from Parker Solar Probe (PSP), it is possible to explore this question near the Sun directly. In this study, we analyze PSP observations from near-perihelion encounters to statistically compare the thermodynamic properties of protons and alpha particles across the Alfvén surface. We find that ion temperature anisotropies, differential flows, and heating signatures are coherently organized by the local Alfvén Mach number ( M A ). In the sub-Alfvénic regime, alpha particles exhibit strong perpendicular temperature heating and elevated alpha-to-proton temperature ratio, which decline across the Alfvénic transition. Meanwhile, proton heating peaks near M A ∼ 1, consistent with anisotropic wave–particle energization. These findings highlight the Alfvén surface as a key boundary for ion energization and momentum exchange, and establish M A as a fundamental organizing parameter in the kinetic evolution of solar wind ions in the inner heliosphere.