2024/03/21 by G. P. Zank, Zank, Gary P., Lingling Zhao +19 · 1 citation
Computer Science · Physics and Astronomy · #Solar Radiation and Photovoltaics #Solar and Space Plasma Dynamics
paper · pdf · doi:10.48550/arxiv.2403.14861
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-Alfvénic flow observed by PSP on 2021 April 28, identifying and characterizing entropy, magnetic islands, forward and backward Alfvén waves, including weakly/non-propagating Alfvén 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|| >> kperp whereas slow mode density fluctuations have kperp > k||. Magnetic field fluctuations are primarily magnetic island modes (delta Bi) with an O(1) smaller contribution from uni-directionally propagating Alfven waves (delta BA+) giving a variance anisotropy of <δBi2> / = 4.1. Incompressible magnetic fluctuations dominate compressible contributions from fast and slow magnetosonic modes. The magnetic island spectrum is Kolmogorov-like kperp-1.6 in perpendicular wavenumber and the uni-directional Alfven wave spectra are k||-1.6 and kperp-1.5. Fast magnetosonic modes propagate at essentially the Alfvén 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.