One of the discoveries of the Parker Solar Probe during its first encounters with the Sun is ubiquitous presence of relatively small-scale structures standing out as sudden deflections of the magnetic field. They were named "switchbacks" since some of them show a full reversal of the radial component of the magnetic field and then return to "regular" conditions. We carried out an analysis of three typical switchback structures having different characteristics: I. Alfvénic structure, where the variations of the magnetic field components take place while conserving the magnitude of the magnetic field; II. Compressional structure, where the magnitude of the field varies together with changes of its components; and III. Structure manifesting full reversal of the magnetic field, presumably Alfvén, which is an extremal example of a switchback. We analyzed the properties of the magnetic fields of these structures and of their boundaries. Observations and analyses lead to the conclusion that they represent localized twisted magnetic tubes moving with respect to surrounding plasma. An important feature is the existence of a relatively narrow boundary layer at the surface of the tube that accommodates flowing currents. These currents are closed on the surface of the structure and typically have comparable azimuthal and tube-axis-aligned components. They are supported by the presence of an effective electric field due to strong gradients of the density and ion plasma pressure. The ion beta is typically larger inside the structure than outside. The surface of the structure may also accommodate electromagnetic waves that assist particles in carrying currents.

NASA's Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products.