Analysis of Magnetohydrodynamic Perturbations in the Radial-field Solar Wind from Parker Solar Probe Observations

S. Q. Zhao,Mingzhe Liu,Huirong Yan,Mijie Shi,Terry Z. Liu

doi:10.3847/1538-4357/ac2ffe

Abstract

Abstract We report analysis of sub-Alfvénic magnetohydrodynamic (MHD) perturbations in the low-β radial-field solar wind employing the Parker Solar Probe spacecraft data from 2018 October 31 to November 12. We calculate wavevectors using the singular value decomposition method and separate MHD perturbations into three eigenmodes (Alfvén, fast, and slow modes) to explore the properties of sub-Alfvénic perturbations and the role of compressible perturbations in solar wind heating. The MHD perturbations show a high degree of Alfvénicity in the radial-field solar wind, with the energy fraction of Alfvén modes dominating (∼45%–83%) over those of fast modes (∼16%–43%) and slow modes (∼1%–19%). We present a detailed analysis of a representative event on 2018 November 10. Observations show that fast modes dominate magnetic compressibility, whereas slow modes dominate density compressibility. The energy damping rate of compressible modes is comparable to the heating rate, suggesting the collisionless damping of compressible modes could be significant for solar wind heating. These results are valuable for further studies of the imbalanced turbulence near the Sun and possible heating effects of compressible modes at MHD scales in low-β plasma.

Highlights

Plasma turbulence appears ubiquitous and plays a crucial role in various astrophysical processes, such as solar wind heating and acceleration (e.g., Bandyopadhyay et al 2020), scattering of cosmic rays (e.g., Yan 2015), turbulent heating in galaxy clusters (e.g., Zhuravleva et al 2014), and star formation (e.g., Federrath 2018)
The kinetic energy fraction PKEs accounts for ∼ 5%, whereas their magnetic energy fraction PMEs is negligible
All the events show similar properties to the representative case presented above, such as a high degree of Alfvénicity, stable wavevectors, low magnetic compressibility mainly provided by fast modes, and low density compressibility primarily resulting from slow modes

Summary

Introduction

Plasma turbulence appears ubiquitous and plays a crucial role in various astrophysical processes, such as solar wind heating and acceleration (e.g., Bandyopadhyay et al 2020), scattering of cosmic rays (e.g., Yan 2015), turbulent heating in galaxy clusters (e.g., Zhuravleva et al 2014), and star formation (e.g., Federrath 2018). The solar wind, a plasma flow originating from the Sun and continuously blowing into the interplanetary space, provides an excellent laboratory for studying plasma turbulence at magnetohydrodynamic (MHD) and sub-ion-kinetic scales (e.g., Dobrowolny et al 1980; Verscharen et al 2019). Using the term mode in this study, we refer to the carriers of turbulent perturbations in wave turbulence rather than classical linear waves (e.g., Cho & Lazarian 2003; Verscharen et al 2019). The mode composition affects almost all turbulence dynamics and the mechanisms of solar wind heating (e.g., Suzuki et al 2006; Cranmer & van Ballegooijen 2012; Makwana & Yan 2020). Clarifying the mode composition and the properties of each mode can help us further understand astrophysical mysteries, e.g., corona

Results

Discussion

Conclusion