Characterization of Turbulent Fluctuations in the Sub-Alfvénic Solar Wind

Abstract

Parker Solar Probe (PSP) observed sub-Alfvénic solar wind intervals during encounters 8–14, and low-frequency magnetohydrodynamic (MHD) turbulence in these regions may differ from that in super-Alfvénic wind. We apply a new mode decomposition analysis 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/nonpropagating Alfvén 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 ∥ ≫ k ⊥, whereas slow-mode density fluctuations have k ⊥ > k ∥. Magnetic field fluctuations are primarily magnetic island modes (δ B i ) with an O(1) smaller contribution from unidirectionally propagating Alfvén waves (δ B A+) giving a variance anisotropy of 〈 δ B i 2 〉 / 〈 δ B A 2 〉 = 4.1 . Incompressible magnetic fluctuations dominate compressible contributions from fast and slow MS modes. The magnetic island spectrum is Kolmogorov-like k ⊥ − 1.6 in perpendicular wavenumber, and the unidirectional Alfvén wave spectra are k ∥ − 1.6 and k ⊥ − 1.5 . Fast MS modes propagate at essentially the Alfvén speed with anticorrelated transverse velocity and magnetic field fluctuations and are almost exclusively magnetic due to β 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.

Solar Cycle Dependence of the Turbulence Cascade Rate at 1 au

“…Figure 7(c) shows the ratio of 2D and slab total turbulent-energy cascade rates ranging between 1 and 1.6. The 2D and slab cascade rates corresponding to E b and 〈u 2 〉 also exhibit solar cycle dependence as shown in Figure 7 Furthermore, the 2D magnetic energy density and turbulent kinetic energy cascade rates are consistently larger than the corresponding slab cascade rates throughout the solar cycle because the 2D turbulence contains more energy than slab turbulence (Zank & Matthaeus 1992;Bieber et al 1996;Zank et al 2017Zank et al , 2020Zank et al , 2024Adhikari et al 2017Adhikari et al , 2021bAdhikari et al , 2022. Figure 7(d) shows the ratio between 2D and slab cascade rates corresponding to the fluctuating kinetic energy and magnetic energy density.…”

“…Several studies (MacBride et al 2008;Adhikari et al 2022;Andrés et al 2022) found that the turbulent cascade rate in a direction perpendicular to the magnetic field is larger than that in parallel direction. This difference in the heating rate in both directions is likely to related the turbulence energy being larger in the perpendicular (or 2D) direction than in the parallel (or slab) direction (Zank & Matthaeus 1993;Bieber et al 1996;Adhikari et al Adhikari et al Adhikari et al Adhikari et al 2022;Zank et al 2017Zank et al , 2018aZank et al , 2024. We compute the 2D and slab cascade rates by using a method described by Adhikari et al (2022).…”

Anisotropy of Density Fluctuations in the Solar Wind at 1 au

“…Nevertheless, these authors did suggest that solar wind density anisotropy varies with scale. On the other hand, Zank et al (2024) studied density fluctuations in sub-Alfvénic wind using mode-decomposition analysis, and found anisotropy dominated by a combination of the slablike entropy mode and the 2D-like backward propagating slow magnetosonic mode. The anisotropy of their entropy mode is consistent with our results, although their results extend over the scales = ´k 2 10 6 to -10 2 km 1 , which slightly overlaps with the smallest scales we investigate.…”

Plasma Motions and Compressive Wave Energetics in the Solar Corona and Solar Wind from Radio Wave Scattering Observations

“…Energy could be supplied to the corona and solar wind via absorption of the energy contained in ion-sound or slow magnetosonic waves (e.g., Kellogg 2020, and references therein), that are often observed in the corona (e.g., DeForest & Gurman 1998; Wang et al 2009;Gupta et al 2012) and solar wind close to the Sun (Zank et al 2024). For q λ De = 1 (where λ De is the Debye length), the spectral energy density of parallel propagating ion-sound waves W q (erg cm −3 [cm −1 ] −3 ]) is related to the spectrum of density fluctuations S(q) ([cm −1 ] −3 ]) by (see, e.g., Appendix C in Lyubchyk et al 2017)…”