In situ formation of switchbacks from finite-amplitude Alfvén waves

Abstract:

Finite-amplitude transverse Alfvén waves are characterised by constant pressure, density, and magnetic field strength. However, the existence of this wave is subject to a geometrical constraint, namely that its amplitude compared to the background field be below a certain critical threshold which depends on the angle between the wavevector and the magnetic field. As the solar wind travels away from the sun and expands, in a background radial field, Alfvénic fluctuations grow in relative amplitude and have their wavevector rotated towards the field-parallel direction, both of which drive the fluctuations toward the critical threshold. Once the critical threshold is reached, rotational discontinuities are forced to develop as a natural part of the evolution. This provides a natural and parsimonious explanation for the ubiquitous “switchbacks” or magnetic-field reversals observed by Parker Solar Probe. We perform an analytic calculation of the evolution of finite-amplitude Alfvén waves in an expanding medium, and compare our results to the observed statistical properties of both switchbacks and the large-amplitude, constant-magnetic-field fluctuations observed at large scales in the near-Sun solar wind.