Characteristics of Interplanetary Discontinuities in the Inner Heliosphere Revealed by Parker Solar Probe

We present a statistical analysis for the characteristics and spatial evolution of the interplanetary discontinuities (IDs) in the solar wind, from 0.13-0.9 au, by using the Parker Solar Probe measurements on Orbits 4 and 5. We collected 3948 IDs, including 2511 rotational discontinuities (RDs) and 557 tangential discontinuities (TDs), with the remnant unidentified. The statistical results show that (1) the ID occurrence rate decreases from 200 events per day at 0.13 au to 1 event per day at 0.9 au, following a spatial scaling of r -2.00, (2) the RD to TD ratio decreases quickly with the heliocentric distance, from 8 at r < 0.3 au to 1 at r > 0.4 au, (3) the magnetic field tends to rotate across the IDs, 45° for TDs and 30° for RDs, in the pristine solar wind within 0.3 au, (4) a special subgroup of RDs exists within 0.3 au, characterized by small field rotation angles and parallel or antiparallel propagations to the background magnetic fields, (5) the TD thicknesses normalized by local ion inertial lengths (d i ) show no clear spatial scaling and generally range from 5-35 d i and the normalized RD thicknesses follow an r -1.09 spatial scaling, and (6) the outward (antisunward) propagating RDs predominate in all RDs, with the propagation speeds in the plasma rest frame proportional to r -1.03. This work could improve our understanding of the ID characteristics and evolution and shed light on the study of the turbulent environment in the pristine solar wind.