Characterization of Turbulent Fluctuations in the Sub-Alfvénic Solar Wind

Parker Solar Probe (PSP) observed sub-Alfv & eacute;nic solar wind intervals during encounters 8-14, and low-frequency magnetohydrodynamic (MHD) turbulence in these regions may differ from that in super-Alfv & eacute;nic wind. We apply a new mode decomposition analysis to the sub-Alfv & eacute;nic flow observed by PSP on 2021 April 28, identifying and characterizing entropy, magnetic islands, forward and backward Alfv & eacute;n waves, including weakly/nonpropagating Alfv & eacute;n vortices, forward and backward fast and slow magnetosonic (MS) modes. Density fluctuations are primarily and almost equally entropy- and backward-propagating slow MS modes. The mode decomposition provides phase information (frequency and wavenumber k) for each mode. Entropy density fluctuations have a wavenumber anisotropy of k(perpendicular to) > k(parallel to), whereas slow-mode density fluctuations have k(perpendicular to) > k(parallel to). Magnetic field fluctuations are primarily magnetic island modes (delta B-i ) with an O(1) smaller contribution from unidirectionally propagating Alfv & eacute;n waves (delta B A+) giving a variance anisotropy of < delta B-i2 > / < delta B-A2 > = 4.1 . Incompressible magnetic fluctuations dominate compressible contributions from fast and slow MS modes. The magnetic island spectrum is Kolmogorov-like k(perpendicular to) - 1.6 in perpendicular wavenumber, and the unidirectional Alfv & eacute;n wave spectra are k parallel to - 1.6 and k(perpendicular to) - 1.5 . Fast MS modes propagate at essentially the Alfv & eacute;n speed with anticorrelated 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 MS 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.