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 (d, V-d/V-A) for the first time, where (d) and V-d are the radial angle and magnitude of alpha-proton differential flow vector, respectively, V-A is the local Alfven speed. Results show that the temperatures significantly depend on (d) as well as V-d/V-A. In case of low proton parallel beta (beta(p parallel to) < 1), it is found that the proton perpendicular temperature is clearly enhanced when (d) is small (less than or similar to 45 degrees) and V-d/V-A greater than or similar to 0.5. On the contrary, the perpendicular temperature of alpha particles is considerably enhanced when (d) is large (greater than or similar to 90 degrees) or V-d/V-A is sufficiently small. The maximum of proton parallel temperature takes place at (d) similar to 90 degrees accompanied by higher beta(p parallel to) 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 ((d), V-d/V-A) spaces than cyclotron resonance heating.