Asymmetric Tangential Velocity inside Switchbacks: Implication for Switchback Origin

Switchbacks are structures in the solar wind marked by large rotations in the magnetic field direction without significant changes in its magnitude, typically accompanied by enhanced bulk speed. Parker Solar Probe (PSP) measurements have established the switchback as a prominent feature of the near-Sun solar wind. Detailed observations have shed light on switchback properties, leading to various proposed mechanisms for their generation. However, one key aspect that remains unexplored is the distribution of the tangential bulk velocity within switchbacks. A particular challenge has been accurately determining the tangential component of the plasma flow in the near-Sun plasma. With the most recent Solar Probe Cup measurements, it has been possible to gather sufficient statistics of proton tangential speed data in near-Sun solar wind switchbacks. This is important because some switchback models, particularly those involving solar wind interactions, predict that the tangential velocity should exhibit a strong one-sided polarity inside switchbacks. In this study, we examine the tangential component of the bulk velocity inside switchbacks during PSP encounters. Our findings reveal that switchbacks preferentially exhibit a positive tangential bulk velocity, strongly supporting the hypothesis that the majority of switchbacks are generated by the interaction of slow and fast wind across a superspiral outside the Alfvén critical point.