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 (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₏₄ₑ₏ whereas slow mode density fluctuations have k₏₄ₑ₏ > k||. Magnetic field fluctuations are primarily magnetic island modes (delta Bⁱ) with an O (1) smaller contribution from uni-directionally propagating Alfven waves (delta B^A+) giving a variance anisotropy of / = 4. 1. Incompressible magnetic fluctuations dominate compressible contributions from fast and slow magnetosonic modes. The magnetic island spectrum is Kolmogorov-like k₏₄ₑ₏^-1. 6 in perpendicular wavenumber and the uni-directional Alfven wave spectra are k||^-1. 6 and k₏₄ₑ₏^-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 betaₚ<<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.