A Simple yet Correct Theory for the Formation of Magnetic Switchbacks Observed by Parker Solar Probe

Abstract

Magnetic switchbacks are rapid high amplitude reversals of the radialmagnetic field in the solar wind that do not involve a heliosphericcurrent sheet crossing. First seen sporadically in the seventies inMariner and Helios data, switchbacks were later observed by theUlysses spacecraft beyond 1 au and have been recently identified as atypical component of solar wind fluctuations in the inner heliosphereby the Parker Solar Probe spacecraft. We provide a simple yetpredictive theory for the formation of these magnetic reversals: theswitchbacks are produced by the shear of circularly polarized Alfvenwaves by a transversely varying radial wave propagation velocity.  Thewave speed can be modulated by variations in bulk velocity, radialmagnetic field, density or any combination of these.  We provide ananalytic expression for the magnetic field variation as a function ofthe wave velocity shear, establish the necessary and sufficientconditions for the formation of switchbacks and show that themechanism works in a realistic solar wind scenario. The suggestedmechanism is in full agreement with Parker Solar Probe observations,including the shape of the switchbacks, the correlations of thecomponents of the magnetic field, and the dependence of variousquantities on radial distance.  We show conclusively that this is thefundamental process that creates switchbacks.