Switchbacks: statistical properties and deviations from Alfvénicity

A Larosa,D E Larson,V K Jagarlamudi,M L Stevens,J C Kasper,V Krasnoselskikh,M Velli,D Malaspina,M Pulupa,K E Korreck,C Froment,A W Case,R J Macdowall,T Dudok De Wit,O Agapitov,C Revillet,K Goetz,S D Bale,P Harvey

doi:10.1051/0004-6361/202039442

Abstract

Context.Parker Solar Probe’s first solar encounter has revealed the presence of sudden magnetic field deflections in the slow Alfvénic solar wind. These structures, which are often called switchbacks, are associated with proton velocity enhancements.Aims.We study their statistical properties with a special focus on their boundaries.Methods.Using data from SWEAP and FIELDS, we investigate particle and wavefield properties. The magnetic boundaries are analyzed with the minimum variance technique.Results.Switchbacks are found to be Alfvénic in 73% of cases and compressible in 27%. The correlations between magnetic field magnitude and density fluctuations reveal the existence of both positive and negative correlations, and the absence of perturbations in the magnetic field magnitude. Switchbacks do not lead to a magnetic shear in the ambient field. Their boundaries can be interpreted in terms of rotational or tangential discontinuities. The former are more frequent.Conclusions.Our findings provide constraints on the possible generation mechanisms of switchbacks, which have to be able to also account for structures that are not purely Alfvénic. One of the possible candidates, among others, manifesting the described characteristics is the firehose instability.

Highlights

The Parker Solar Probe (PSP) mission (Fox et al 2016), which was launched in 2018, offers a unique possibility to shed light on the main questions of solar physics: the heating of the solar corona and the acceleration of particles in the solar wind, by making in situ measurements in the extended solar corona
The currents we find are between one and two orders of magnitude larger than those observed in solar wind discontinuities at 1 AU (Artemyev et al 2018), even though the sizes of the discontinuities are comparable
We found that 32% of the switchback boundaries are rotational discontinuities (RDs), 17% are tangential discontinuities (TDs), 42% are in the “Either” area, and 9% are in the Current density [nA/m2]

Summary

Introduction

The Parker Solar Probe (PSP) mission (Fox et al 2016), which was launched in 2018, offers a unique possibility to shed light on the main questions of solar physics: the heating of the solar corona and the acceleration of particles in the solar wind, by making in situ measurements in the extended solar corona. Switchbacks had been observed before in fast streams of the polar solar wind by the Ulysses spacecraft (Balogh et al 1999). They were recognized as folds in the field, which were distinguished from other in situ structures (Yamauchi et al 2004b). Switchbacks were observed in fast wind streams by Helios (Horbury et al 2018) and were associated with one-sided radial jets (Gosling et al 2009). The first solar encounters of PSP show that they are ubiquitous features of the young slow solar wind Their investigation should help us shed light on the dynamics and evolution of the young solar wind

Objectives

Findings

Discussion

Conclusion