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Data obtained from Parker Solar Probe (PSP) since 2021 April have shown thefirst in situ observation of the solar corona, where the solar wind is formedand accelerated. Here we investigate the alpha-proton differential flow and itscharacteristics across the critical Alfv\'en surface (CAS) using data from PSPduring encounters 8-10 and 12-13. We first show the positive correlationbetween the alpha-proton differential velocity and the bulk solar wind speed atPSP encounter distances. Then we explore how the characteristics of thedifferential flow vary across the CAS and how they are affected by Alfv\'enicfluctuations including switchbacks. We find that the differential velocitybelow the CAS is generally smaller than that above the CAS, and the localAlfv\'en speed well limits the differential speed both above and below the CAS.The deviations from the alignment between the differential velocity and thelocal magnetic field vector are accompanied by large-amplitude Alfv\'enicfluctuations and decreases in the differential speed. Moreover, we observe that$V_{\alpha p}$ increases from $M_A < 1$ to $M_A \simeq 2$ and then starts todecrease, which suggests that alphas may remain preferentially accelerated wellabove the CAS. Our results also reveal that in the sub-Alfv\'enic solar windboth protons and alphas show a strong correlation between their velocityfluctuations and magnetic field fluctuations, with a weaker correlation foralphas. In contrast, in the super-Alfv\'enic regime the correlation remainshigh for protons, but is reduced for alphas.

The Alpha-Proton Differential Flow in the Alfv\'enic Young Solar Wind: From Sub-Alfv\'enic to Super-Alfv\'enic