Dependence of Ion Temperatures on Alpha–Proton Differential Flow Vector and Heating Mechanisms in the Solar Wind

According to Wind observations between 2004 June and 2019 May, this Letter investigates the proton and alpha particle temperatures in the space of (θd, Vd/VA) for the first time, where θd and Vd are the radial angle and magnitude of alpha−proton differential flow vector ${{\boldsymbol{V}}}_{d}$, respectively, VA is the local Alfvén speed. Results show that the temperatures significantly depend on θd as well as Vd/VA. In case of low proton parallel beta (βp∥ < 1), it is found that the proton perpendicular temperature is clearly enhanced when θd is small (≲45°) and Vd/VA ≳ 0.5. On the contrary, the perpendicular temperature of alpha particles is considerably enhanced when θd is large (≳90°) or Vd/VA is sufficiently small. The maximum of proton parallel temperature takes place at θd ∼ 90° accompanied by higher βp∥ and by larger turbulence amplitude of magnetic fluctuations in inertial range. This study should present strong evidence for cyclotron resonance heating of protons and alpha particles in the solar wind. Other mechanisms including Landau resonance and stochastic heating are also proposed, which tend to have different (θd, Vd/VA) spaces than cyclotron resonance heating.