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We identify more than ten steady sub-Alfv\'enic solar wind intervals from themeasurements of the Parker Solar Probe (PSP) from encounter 8 to encounter 14.An analysis of these sub-Alfv\'enic intervals reveals similar properties andsimilar origins. In situ measurements show that these intervals feature adecreased radial Alfv\'en Mach number resulting from a reduced density and arelatively low velocity, and that switchbacks are suppressed in theseintervals. Magnetic source tracing indicates that these sub-Alfv\'enic streamsgenerally originate from the boundaries inside coronal holes, or narrow/smallregions of open magnetic fields. Such properties and origins suggest that thesestreams are low Mach-number boundary layers (LMBLs), which is a specialcomponent of the pristine solar wind proposed by Liu et al. We find that theLMBL wind, the fast wind from deep inside coronal holes, and the slow streamerwind constitute three typical components of the young solar wind near the Sun.In these sub-Alfv\'enic intervals, the Alfv\'en radius varies between 15 and 25solar radii, in contrast with a typical 12 radii for the Alfv\'en radius of thesuper-Alfv\'enic wind. These results give a self-consistent pictureinterpreting the PSP measurements in the vicinity of the Sun.

Properties of Steady Sub-Alfv\'enic Solar Wind in Comparison with Super-Alfv\'enic Wind from Measurements of Parker Solar Probe