Localized Magnetic Field Structures and Their Boundaries in the Near-Sun Solar Wind from Parker Solar Probe Measurements

International audienceWe present the analysis of magnetic structures observed during the first encounter of Parker Solar Probe, first reported by Bale et al. [2019]. They are characterized by magnetic field rotations up to 45 degrees and even more and a significant increase of the radial flow velocity inside. Their durations last from seconds, to tens of minutes and sometimes well beyond, and their boundaries are well defined. According to Dudok de Wit et al., [2019] they manifest long correlation time and are clustered, which suggests they may be slowly varying quasi-solid structures formed closer to the Sun (they may represent S-type structures, Kasper et al. [2019]). Our study focuses on determination of the parameters of their boundaries (geometry, plasma flow, etc.). We determine the normals of the leading and trailing edges, normal and tangential components of the magnetic field and of the flow velocity using SWEAP data (Kasper et al, [2016]). Large flow velocities with respect to the background solar wind inside finite spatial regions suggest that they may be elongated "spaghetti-like structures". They typically reveal a density increase at the boundaries. Hodograms indicate that magnetic field rotation across the boundaries have, in general, opposite directions on the leading and trailing edges. We evaluate the radial component of the MHD Poynting flux, which shows a strong enhancement at the boundaries. It is worth noting that the MHD-type wave activity (in the frequency range lower than low hybrid frequency) inside the structures is significantly larger than outside. In many cases the magnetic field manifests complex rotations that allow an interpretation in terms of MHD type surface waves. Surprisingly, in some structures the magnetic field deviates from the radial direction so strongly that it becomes perpendicular to the radial direction. Bale et al., The magnetic structure and electrodynamics of the emerging solar wind, submitted to Nature, 2019; Dudok de Wit, T., et al., Abstract AGU, this session, 2019; Kasper, J. C. et al. SolarWind Electrons Alphas and Protons (SWEAP) Investigation: Design of the Solar Wind and Coronal Plasma Instrument Suite for Solar Probe Plus. Space Sci. Rev. 204, 131-186 (2016). DOI 10.1007/s11214-015-0206-3; Kasper et al., J.C., Young Solar Wind in the Grip of the Sun's Corona, submitted to Nature, 201