Switchbacks and Associated Magnetic Holes Observed near the Alfvén Critical Surface

During recent solar encounters, the Parker Solar Probe (PSP) began its initial dips below the Alfvén critical surface to measure in situ the sub-Alfvénic coronal wind. While the near-Sun super-Alfvénic solar wind is shown to be dominated by impulsive magnetic switchbacks (short magnetic field reversals), these brief encounters with the sub-Alfvénic coronal wind show switchbacks and associated magnetic holes (MHs) to still be present but different in character. In this work, we compare and contrast specific features of the switchbacks, including the change in B _r and V _r and associated boundary B -field dropouts (MHs) at locations when PSP was both above and below the Alfvén critical surface. We use observations from the PSP perihelion Encounters 8 (E8) and 12 (E12) in the analysis. We first perform a superposed epoch analysis to identify common features in the switchback boundaries, including the formation of the associated ∣ B ∣ dropouts/MHs in slow and fast flows. We then examine the presence of B -field dropouts/MHs as a function of Alfvén Mach number, M _A . From E12, we find that the switchbacks have a systematic reduction in rotation (and reduction in B _r deflection) with decreasing M _A . Further, the ∣ B ∣ dropouts/MHs associated with the boundaries were also found to decrease in strength and occurrence with M _A (with no or few ∣ B ∣ dropouts at M _A < 0.7). The results suggest that the switchback rotation and boundary-associated MHs are connected, possibly consistent with diamagnetic effects at the boundary that require large rotations to be initiated.