Heating and Acceleration of α Particles and Protons Across the Alfvén Critical Surface in Solar Cycle 25

Abstract Minor ions are preferentially heated and accelerated in the solar coronal holes (CHs). It remains unclear how ion energization varies across the Alfvén critical surface (ACS). This work reports the variation in the temperature and the flow speed of protons and α particles from 10 solar radii ( R ⊙ ) to 0.2 au in solar cycle 25. The α –proton differential speed normalized to the local Alfvén speed ( v αp / v A ) increases significantly beyond the ACS. Since v A decreases, v αp decreases with distance. The protons in the CH wind speed up to no less than 350 km s −1 beyond 17 R ⊙ , though some of them are as slow as in streamer wind within 17 R ⊙ . Beyond 34 R ⊙ , CH wind can be identified from v αp / v A and proton speed. Near and within the ACS, α particles receive heating more than the α particle to proton mass ratio and show simultaneous heating in parallel and perpendicular directions to the local magnetic field. The temperature anisotropy of α particles begins to decrease when the Coulomb collisional age A C ( α , p ) is larger than 1 within the ACS. The temperature anisotropy of protons begins to decrease when A C ( α , p ) is in the range of 0.02 and 1 within the ACS. Both species show isotropic temperature beyond the ACS. The results support that preferential α particle heating compared to protons takes place within the ACS, and candidate processes heat the α particles more than the protons below and near the ACS, where v αp / v A is small.