Effects of Alpha–Proton Differential Flow on Ion-scale Waves’ Generation in the Near-Sun Solar Wind: Parker Solar Probe Observations

The formation and evolution of abundant ion-scale waves and alpha–proton differential flow are important research objects in the topic of solar wind formation and evolution. Based on observations from Parker Solar Probe perihelia of Encounters 4–11, we statistically studied how alpha–proton differential flow affects the generation of ion-scale waves within 0.3 au. Our results show that the dependence of left-handed (LH) and right-handed (RH) ion-scale waves on alpha–proton differential flows is quite different. The generation of the LH wave prefers an alpha–proton differential flow that is along the solar wind and quasi-parallel (or antiparallel) to the background magnetic field. Meanwhile, the increase in the velocity of the alpha–proton differential flow also promotes the generation of the LH waves. However, RH waves’ high occurrence rate regions prefer analpha–proton differential flow facing the Sun, and the concentration area of the RH wave data set, which is in the range of 2 0 ∘ < θ B V α p < 12 0 ∘ and ∣ V αp / V A ∣ ≃ 0, is different from its high-occurrence-rate area. Besides, the LH waves’ high-occurrence-rate areas correspond to relatively higher ion temperature, larger T α / T p , larger N α / N p , larger proton temperature anisotropy ( T p ⊥ / T p ∥ > 1), and smaller alpha temperature anisotropy ( T α ⊥ / T α ∥ < 1), while the RH waves’ high-occurrence-rate areas are the opposite. Our results suggest that the alpha–proton differential flow plays an significant role in the generation of the ion-scale waves in the solar wind.