Properties of Steady Sub-Alfvénic Solar Wind in Comparison with Super-Alfvénic Wind from Parker Solar Probe Measurements

We identify more than 10 steady sub-Alfvenic solar wind intervals from the measurements of the Parker Solar Probe (PSP) from encounter 8 to encounter 14. An analysis of these sub-Alfvenic intervals reveals similar properties and similar origins. In situ measurements show that these intervals feature a decreased radial Alfven Mach number resulting from a reduced density and a relatively low velocity, and that switchbacks are suppressed in these intervals. Magnetic source tracing indicates that these sub-Alfvenic streams generally originate from the boundaries inside coronal holes or narrow/small regions of open magnetic fields. Such properties and origins suggest that these streams are mostly low Mach-number boundary layers (LMBLs), which is a special component of the pristine solar wind proposed by Liu et al. We find that the LMBL wind, the fast wind from deep inside coronal holes, and the slow streamer wind constitute three typical components of the young solar wind near the Sun. In these sub-Alfvenic intervals, the Alfven radius varies between 15 and 25 solar radii, in contrast with a typical 12 radii for the Alfven radius of the super-Alfvenic wind. These results give a self-consistent picture interpreting the PSP measurements in the vicinity of the Sun.