Alpha-Proton Differential Flow of A Coronal Mass Ejection at 15 Solar Radii

Alpha-proton differential flow ($V_{\alpha p}$) of coronal mass ejections (CMEs) and solar wind from the Sun to 1 au and beyond could influence the instantaneous correspondence of absolute abundances of alpha particles (He$^{2+}$/H$^{+}$) between solar corona and interplanetary space as the abundance of a coronal source can vary with time. Previous studies based on Ulysses and Helios showed that $V_{\alpha p}$ is negligible within CMEs from 5 to 0.3 au, similar to slow solar wind ($<$ 400 km s$^{-1}$). However, recent new observations using Parker Solar Probe (PSP) revealed that the $V_{\alpha p}$ of slow wind increases to $\sim$60 km s$^{-1}$ inside 0.1 au. It is significant to answer whether the $V_{\alpha p}$ of CMEs exhibits the similar behavior near the Sun. In this Letter, we report the $V_{\alpha p}$ of a CME measured by PSP at $\sim$15 $R_\odot$ for the first time, which demonstrates that the $V_{\alpha p}$ of CMEs is obvious and complex inside 0.1 au while keeps lower than the local Alfv\'{e}n speed. A very interesting point is that the same one CME duration can be divided into A and B intervals clearly with Coulomb number below and beyond 0.5, respectively. The means of $V_{\alpha p}$ and alpha-to-proton temperature ratios of interval A (B) is 96.52 (21.96) km s$^{-1}$ and 7.65 (2.23), respectively. This directly illustrates that Coulomb collisions play an important role in reducing the non-equilibrium features of CMEs. Our study indicates that the absolute elemental abundances of CMEs also might vary during their propagation.