Switchbacks: Statistical properties and deviations from Alfvénicity

Context. Parker Solar Probe’s first solar encounter has revealed the presence of sudden magnetic field deflections in the slow Alfvenic 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 Alfvenic 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 Alfvenic. One of the possible candidates, among others, manifesting the described characteristics is the firehose instability.