  Parker Solar Probe (PSP) observed sub-Alfvenic solar wind intervals during
encounters 8 - 14, and low-frequency magnetohydrodynamic turbulence in these
regions may differ from that in super-Alfvenic wind. We apply a new
mode-decomposition analysis (Zank et al 2023) to the sub-Alfv\'enic flow
observed by PSP on 2021 April 28, identifying and characterizing entropy,
magnetic islands, forward and backward Alfv\'en waves, including
weakly/non-propagating Alfv\'en vortices, forward and backward fast and slow
magnetosonic modes. Density fluctuations are primarily and almost equally
entropy and backward propagating slow magnetosonic modes. The
mode-decomposition provides phase information (frequency and wavenumber k) for
each mode. Entropy-density fluctuations have a wavenumber anisotropy k_{||} >>
k_{perp} whereas slow mode density fluctuations have k_{perp} > k_{||}.
Magnetic field fluctuations are primarily magnetic island modes (delta B^i)
with an O(1) smaller contribution from uni-directionally propagating Alfven
waves (delta B^{A+}) giving a variance anisotropy of <{\delta B^i}^2> / <delta
B^A}^2> = 4.1. Incompressible magnetic fluctuations dominate compressible
contributions from fast and slow magnetosonic modes. The magnetic island
spectrum is Kolmogorov-like k_{perp}^{-1.6} in perpendicular wavenumber and the
uni-directional Alfven wave spectra are k_{||}^{-1.6} and k_{perp}^{-1.5}. Fast
magnetosonic modes propagate at essentially the Alfv\'en speed with
anti-correlated transverse velocity and magnetic field fluctuations and are
almost exclusively magnetic due to beta_p<<1. Transverse velocity fluctuations
are the dominant velocity component in fast magnetosonic modes and longitudinal
fluctuations dominate in slow modes. Mode-decomposition is an effective tool in
identifying the basic building blocks of MHD turbulence and provides detailed
phase information about each of the modes.
