Formation of magnetic switchbacks via expanding Alfvén waves

Context. Large-amplitude inversions of the solar wind’s interplanetary magnetic field have long been documented; however, observations from the Parker Solar Probe (PSP) mission have renewed interest in this phenomenon as such features, often termed switchbacks, may constrain both the sources of the solar wind as well as in situ nonlinear dynamics and turbulent heating. Aims. We aim to show that magnetic field fluctuations in the solar wind are consistent with Alfvénic fluctuations that naturally form switchback inversions in the magnetic field through expansion effects. Methods. We examine PSP observations of the evolution of a single stream of solar wind in a radial scan from PSP’s tenth perihelion encounter from ≈15 − 50 R⊙. We study the growth and radial scaling of normalized fluctuation amplitudes in the magnetic field, δB/B, within the framework of spherical polarization. We compare heating rates computed via outer-scale decay from consideration of wave action to proton heating rates empirically observed through adiabatic expansion. Results. We find that the magnetic field fluctuations are largely spherically polarized and that the normalized amplitude of the magnetic field, δB/B, increases with amplitude. The growth of the magnetic field amplitude leads to switchback inversions in the magnetic field. While the amplitude does not grow as fast as is predicted by the conservation of wave action, the deviation from the expected scaling yields an effective heating rate that is close to the empirically observed proton heating rate. Conclusions. The observed scaling of fluctuation amplitudes is largely consistent with a picture of expanding Alfvén waves that seed turbulence, leading to dissipation. The expansion of the waves leads to the growth of wave amplitudes, resulting in the formation of switchbacks.