Abstract
Abstract 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.
Highlights
The Parker Solar Probe (PSP) mission (Fox et al 2015) addresses two fundamental opened questions in space physics, which are coronal plasma heating and the acceleration of solar wind plasmas
Neubauer & Barnstorf (1981) and Burlaga et al (1977) classified these boundaries as tangential discontinuities (TD) when the normal component of the magnetic field is small with respect to its magnitude, and rotational discontinuities (RD) when the normal component is of the same order of magnitude as the field
In the following we shall focus on three examples that correspond to three types of structures: first, an event that manifests pure Alfvenic properties; second, one that is compressional because the total magnetic field changes inside the structure; a switchback with a full inversion of the magnetic field
Summary
The Parker Solar Probe (PSP) mission (Fox et al 2015) addresses two fundamental opened questions in space physics, which are coronal plasma heating and the acceleration of solar wind plasmas. Initial studies of its magnetic field measurements focused on the presence of different types of discontinuities that were observed both in fast and slow solar winds. Marsch et al (1982a) reported that at perihelion, during periods of low solar activity, the solar wind velocity was typically 300 km/s to 400 km/s with occasional bursts up to 600 km/s that could last for a few hours The correlation between their velocity and magnetic fluctuations, which is usually regarded as a condition for of Alfvenic type fluctuations, was shown to be qualitatively verified for the discontinuities that were observed by Helios. Switchbacks show up as sudden increases in the radial component Their better time resolution; we shall rely on proton data to study fast transients.
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