Speaker: Dr. Nikolaos Sioulas (Imperial)
Title: Generation and Expansion-Driven Growth of Switchbacks in the Outer Solar Corona and Solar Wind
Abstract: We analyze Parker Solar Probe and Solar Orbiter observations to examine the emergence and evolution of magnetic-field reversals (switchbacks) in the context of finite-amplitude Alfvénic wavepackets advected through an inhomogeneous, accelerating solar wind. Wave-action conservation provides the baseline scaling of fluctuation amplitudes with the Alfvénic Mach number, Ma = U/Va; deviations quantify the net effect of amplification versus dissipation. The measurements separate across the Alfvén surface (Ma = 1): below it (Ma < 1) amplitudes evolve in a WKB-like, wave-action–consistent manner and inferred damping lengths are large, implying weak dissipation and conditions favorable for expansion-driven growth toward large rotations, while above it (Ma > 1) the evolution departs from wave-action scaling with clear scale dependence, consistent with turbulent processing. We further show that the commonly reported sub-Alfvénic “dropout” of switchbacks is methodological—driven by conditioning on instantaneous Ma inflated during strong deflections and by short-window background fields that suppress measured angles—and disappears when Ma and reference fields are defined in a stream-scale, event-independent way. Overall, the observations are consistent with a formation pathway in which coronal fluctuations are amplified by large-scale expansion through the sub-Alfvénic regime, with subsequent propagation into the super-Alfvénic wind where turbulent decay modifies their scale-dependent properties.