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One of the discoveries made by Parker Solar Probe during first encounterswith the Sun is the ubiquitous presence of relatively small-scale structuresstanding out as sudden deflections of the magnetic field. They were calledswitchbacks as some of them show up the full reversal of the radial componentof the magnetic field and then return to regular conditions. Analyzing themagnetic field and plasma perturbations associated with switchbacks we identifythree types of structures with slightly different characteristics: 1. Alfvenicstructures, where the variations of the magnetic field components take placewhile the magnitude of the field remains constant; 2. Compressional, the fieldmagnitude varies together with changes of the components; 3. Structuresmanifesting full reversal of the magnetic field (extremal class of Alfvenicstructures). Processing of structures boundaries and plasma bulk velocityperturbations lead to the conclusion that they represent localized magneticfield tubes with enhanced parallel plasma velocity and ion beta moving togetherwith respect to surrounding plasma. The magnetic field deflections before andafter the switchbacks reveal the existence of total axial current. The electriccurrents are concentrated on the relatively narrow boundary layers on thesurface of the tubes and determine the magnetic field perturbations inside thetube. These currents are closed on the structure surface, and typically havecomparable azimuthal and the axial components. The surface of the structure mayalso accommodate an electromagnetic wave, that assists to particles in carryingcurrents. We suggest that the two types of structures we analyzed here mayrepresent the local manifestations of the tube deformations corresponding to asaturated stage of the Firehose instability development.

Localized magnetic field structures and their boundaries in the near-Sun solar wind from Parker Solar Probe measurements