The preferential orientation of magnetic switchbacks and its implications for solar magnetic flux transport

¹ Institut de Recherche en Astrophysique et Planétologie, CNRS, UPS, CNES, Toulouse, France
e-mail: nais.fargette@irap.omp.eu
² Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, Pessac, France
³ Space Sciences Laboratory, University of California, Berkeley, CA, USA
⁴ Physics Department, University of California, Berkeley, CA, USA
⁵ Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA

Received: 14 March 2022
Accepted: 5 May 2022

Abstract

Context. Magnetic switchbacks in the solar wind are large deflections of the magnetic field vector, which often reverse their radial component, and are associated with a velocity spike consistent with their Alfvénic nature. The Parker Solar Probe (PSP) mission revealed them to be a dominant feature of the near-Sun solar wind. Where and how they are formed remains unclear and subject to discussion.

Aims. We investigate the orientation of the magnetic field deflections in switchbacks to determine if they are characterized by a possible preferential orientation.

Methods. We compute the deflection angles, ψ = [ϕ, θ]^T, of the magnetic field relative to the theoretical Parker spiral direction for encounters 1 to 9 of the PSP mission. We first characterize the distribution of these deflection angles for quiet solar wind intervals and assess the precision of the Parker model as a function of distance from the Sun. We then assume that the solar wind is composed of two populations, the background quiet solar wind and the population of switchbacks, the latter of which is characterized by larger fluctuations. We model the total distribution of deflection angles we observe in the solar wind as a weighed sum of two distinct normal distributions, each corresponding to one of the populations. We fit the observed data with our model using a Monte Carlo Markov chain algorithm and retrieve the most probable mean vector and covariance matrix coefficients of the two Gaussian functions, as well as the population proportion. This method allows us to quantify the properties of both the quiet solar wind and the switchback populations without setting an arbitrary threshold on the magnetic field deflection angles.

Results. We first confirm that the Parker spiral is a valid model for quiet solar wind intervals at PSP distances. We observe that the accuracy of the spiral direction in the ecliptic is a function of radial distance, in a manner that is consistent with PSP being near the solar wind acceleration region. We then find that the fitted switchback population presents a systematic bias in its deflections, with a mean vector consistently shifted toward lower values of ϕ (−5.52° on average) and θ (−2.15° on average) compared to the quiet solar wind population. This results holds for all encounters but encounter 6, and regardless of the magnetic field main polarity. This implies a marked preferential orientation of switchbacks in the clockwise direction in the ecliptic plane, and we discuss this result and its implications in the context of the existing switchback formation theories. Finally, we report the observation of a 12-hour patch of switchbacks that systematically deflect in the same direction, such that the magnetic field vector tip within the patch deflects and returns to the Parker spiral within a given plane.