Switchbacks: statistical properties and deviations from Alfvénicity

¹ LPC2E, CNRS/University of Orléans/CNES, 3A avenue de la Recherche Scientifique, Orléans, France
e-mail: andrea.larosa@cnrs-orleans.fr
² Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA
³ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA
⁴ Physics Department, University of California, Berkeley, CA 94720-7300, USA
⁵ The Blackett Laboratory, Imperial College London, London, SW7 2AZ, UK
⁶ School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
⁷ Smithsonian Astrophysical Observatory, Cambridge, MA 02138, USA
⁸ School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
⁹ Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA
¹⁰ Solar System Exploration Division, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹¹ National Institute for Astrophysics-Institute for Space Astrophysics and Planetology, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
¹² BWX Technologies, Inc., Washington, DC 20002, USA
¹³ Department of Earth, Planetary, and Space Sciences, UCLA, Los Angeles, CA 90095, USA
¹⁴ Astrophysical and Planetary Sciences Department, University of Colorado, Boulder, CO 80303, USA

Received: 15 September 2020
Accepted: 15 January 2021

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.