Cross Helicity Reversals In Magnetic Switchbacks

TL;DR

This paper analyzes magnetic switchbacks observed by Parker Solar Probe, revealing their impact on cross helicity measurements and showing that they are local magnetic field kinks rather than surface polarity regions.

Contribution

It provides the first detailed analysis of switchbacks' effect on MHD invariants and their role as local magnetic field kinks during Parker Solar Probe's first encounter.

Findings

Switchbacks are associated with negative cross helicity populations.

MHD waves follow the local magnetic field through switchbacks.

Switchbacks are likely local magnetic field kinks, not surface polarity regions.

Abstract

We consider 2D joint distributions of normalised residual energy $\sigma_r(s,t)$ and cross helicity $\sigma_c(s,t)$ during one day of Parker Solar Probe's (PSP's) first encounter as a function of wavelet scale $s$. The broad features of the distributions are similar to previous observations made by HELIOS in slow solar wind, namely well correlated and fairly Alfv\'enic, except for a population with negative cross helicity which is seen at shorter wavelet scales. We show that this population is due to the presence of magnetic switchbacks, brief periods where the magnetic field polarity reverses. Such switchbacks have been observed before, both in HELIOS data and in Ulysses data in the polar solar wind. Their abundance and short timescales as seen by PSP in its first encounter is a new observation, and their precise origin is still unknown. By analysing these MHD invariants as a function of wavelet scale we show that MHD waves do indeed follow the local mean magnetic field through switchbacks, with net Elsasser flux propagating inward during the field reversal, and that they therefore must be local kinks in the magnetic field and not due to small regions of opposite polarity on the surface of the Sun. Such observations are important to keep in mind as computing cross helicity without taking into account the effect of switchbacks may result in spurious underestimation of $\sigma_c$ as PSP gets closer to the Sun in later orbits.