Generation and Expansion-Driven Growth of Switchbacks in the Outer Solar Corona and Solar Wind

Abstract

We analyze \emph{Parker Solar Probe} and \emph{Solar Orbiter} measurements of magnetic-field reversals (``switchbacks'') across the Alfv\'en surface ($M_a\simeq 1$), where $M_a$ is the Alfv\'en Mach number. The reported ``sub-Alfv\'enic switchback dropout'' follows from two diagnostic biases: conditioning on an instantaneous $M_a$, which is transiently elevated above unity by radial-velocity enhancements during large-amplitude Alfv\'enic rotations, and short-window local-mean backgrounds that partially track these rotations and suppress deflection angles. Treating $M_a$ as a bulk-stream property via rolling medians and referencing deflections to event-independent backgrounds -- a Parker-spiral direction or a sufficiently long rolling median -- recovers sub-Alfv\'enic switchbacks systematically. The mean deflection $\langle \theta \rangle$ separates into two regimes with $M_a$. For $M_a \lesssim 1$, $\langle \theta \rangle$ rises rapidly with weak dependence on the background window, consistent with expansion-driven amplification of Alfv\'enic fluctuations. For $M_a \gtrsim 1$, the evolution becomes scale dependent: large-scale $\langle \theta \rangle$ continues to grow with $M_a$ at reduced rate, while small-scale growth saturates, consistent with turbulent decay and dissipation. Collectively, these results indicate that switchbacks need not originate only in the super-Alfv\'enic solar wind. Instead, they are consistent with a formation pathway in which coronal fluctuations are amplified by large-scale expansion through the sub-Alfv\'enic regime, with subsequent propagation into the super-Alfv\'enic wind where turbulent decay modifies their scale-dependent properties.