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

Magnetic switchbacks are rapid high amplitude reversals of the radial
magnetic field in the solar wind that do not involve a heliospheric
current sheet crossing. First seen sporadically in the seventies in
Mariner and Helios data, switchbacks were later observed by the
Ulysses spacecraft beyond 1 au and have been recently identified as a
typical component of solar wind fluctuations in the inner heliosphere
by the Parker Solar Probe spacecraft. We provide a simple yet
predictive theory for the formation of these magnetic reversals: the
switchbacks are produced by the shear of circularly polarized Alfven
waves by a transversely varying radial wave propagation velocity.  The
wave speed can be modulated by variations in bulk velocity, radial
magnetic field, density or any combination of these.  We provide an
analytic expression for the magnetic field variation as a function of
the wave velocity shear, establish the necessary and sufficient
conditions for the formation of switchbacks and show that the
mechanism works in a realistic solar wind scenario. The suggested
mechanism is in full agreement with Parker Solar Probe observations,
including the shape of the switchbacks, the correlations of the
components of the magnetic field, and the dependence of various
quantities on radial distance.  We show conclusively that this is the
fundamental process that creates switchbacks.