Dependence of ion temperatures on alpha-proton differential flow vector and heating mechanisms in the solar wind

According to \emph{Wind} observations between June 2004 and May 2019, this Letter investigates the proton and alpha particle temperatures in the space of ($\theta_d$, $V_d/V_A$) for the first time, where $\theta_d$ and $V_d$ are the radial angle and magnitude of alpha$-$proton differential flow vector respectively, $V_A$ is the local Alfv\'en speed. Results show that the temperatures significantly depend on $\theta_d$ as well as $V_d/V_A$. In case of low proton parallel beta ($\beta_{p{\parallel}} < 1$), it is found that the proton perpendicular temperature is clearly enhanced when $\theta_d$ is small ($\lesssim 45^\circ$) and $V_d/V_A \gtrsim 0.5$. On the contrary, the perpendicular temperature of alpha particles is considerably enhanced when $\theta_d$ is large ($\gtrsim 90^\circ$) or $V_d/V_A$ is sufficiently small. The maximum of proton parallel temperature takes place at $\theta_d \sim 90^\circ$ accompanied by higher $\beta_{p{\parallel}}$ 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 ($\theta_d$, $V_d/V_A$) spaces than cyclotron resonance heating.