Directional discontinuities in the inner heliosphere from Parker Solar Probe and Solar Orbiter observations

Abstract

Context. Directional discontinuities (DDs) are common structures in the solar wind plasma and are among the most important discontinuities besides shock waves. The Parker Solar Probe and Solar Orbiter spacecraft currently provide whole new insight into the inner heliosphere in spatial coverage and timescales. Aims. We search for DDs and classify them into subgroups of tangential (TD) and rotational (RD) discontinuities. The analysis of the DD thicknesses allows us to test different theories about their origin and evolution. Methods. We applied an automatic algorithm to select discontinuities between heliocentric distances of 0.06–1.01 AU. The method uses the spatial rotation of the magnetic field to identify the DDs and minimum variance analysis to determine the normal vector of the discontinuity surface. A classification into TDs and RDs was carried out using the magnetic field data and the Walén test in both the spacecraft and the deHoffmann–Teller frame. Results. With strict conditions, we found more than 140 000 DDs in the time intervals. We find that the spatial density of DDs decreases with increasing radial distance from the Sun in the innermost heliosphere. The comprehensive analysis revealed that most of the DD, for which the normal component of the magnetic field is small are in fact TDs, regardless of the jump in field magnitude. After the classification, we were able to determine the radial thickness evolution for the TDs and RDs separately. We found that the thickness of RDs decreases from 0.06 to 0.30 AU, and beyond this (0.30–1.01 AU) it increases with the local ion inertial length. This characteristic scaling is present for TDs throughout between 0.06 and 1.01 AU. Conclusions. Our results give us a simple classification tool for future studies of DDs, that is based only on magnetic field measurements. After we analyzed the DD thickness, we observationally confirmed that RDs are produced by Alfvén-wave steepening, while the TDs are most likely the boundaries of flux tubes.