Characterization of Turbulent Fluctuations in the Sub-Alfvenic Solar Wind

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 to the sub-Alfvenic flow observed by PSP on 2021 April 28, identifying and characterizing entropy, magnetic islands, forward and backward Alfven waves, including weakly/non-propagating Alfven vortices, forward and backward fast and slow magnetosonic modes. Density fluctuations are primarily and almost equally entropy and backward propagating slow magnetosonic/ion acoustic modes. The mode-decomposition provides phase information (frequency and wavenumber) for each mode. Magnetic field fluctuations are primarily magnetic island modes with a smaller contribution from uni-directionally propagating Alfven waves with a variance anisotropy between magnetic island and Alfvenic modes of 4.1. Incompressible magnetic fluctuations dominate compressible contributions from fast and slow magnetosonic modes. Spectra of the incompressible and compressible modes are presented. Mode-decomposition is effective in identifying the basic building blocks of MHD turbulence and provides detailed phase information about each of the modes.