Alpha Particle Heating and Anisotropy in the Solar Wind Turbulence: Insights from PSP Observations and PIC Simulations

Alpha particles constitute the most energetic ion population in the solar wind and play an important role in turbulent energy conversion and ion-scale heating. Yet, the physical processes governing their temperature evolution, anisotropy development, and differential streaming remain incompletely understood. Using Parker Solar Probe observations and 2.5D particle-in-cell simulations, we investigate how the alpha–proton temperature ratio regulates the subsequent alpha heating efficiency and associated kinetic signatures. The observations reveal that alpha heating and anisotropy are strongly modulated by the local value of temperature ratio. The simulations reproduce these trends, showing that increasing temperature ratio lowers the growth of alpha thermal energy, anisotropy, and differential drift. These results demonstrate that the alpha heating pathway could be self-regulated by its initial thermodynamic state, with hotter alphas remaining farther from the instability threshold and experiencing less resonant energization. Our findings provide new constraints on ion-scale dissipation in the near-Sun solar wind and offer a unified interpretation of alpha-proton heating.