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

TL;DR

This study analyzes sub-Alfvénic MHD perturbations in the low-beta radial solar wind from Parker Solar Probe data, identifying dominant Alfvénic modes and their role in solar wind heating through collisionless damping.

Contribution

It provides a detailed characterization of MHD wave modes in the near-Sun solar wind and highlights the significance of compressible modes in plasma heating processes.

Findings

Alfvén modes dominate energy content (~45%-83%)

Fast modes are primary contributors to magnetic compressibility

Collisionless damping of compressible modes may significantly heat the solar wind

Abstract

We report analysis of sub-Alfv\'enic magnetohydrodynamic (MHD) perturbations in the low-\b{eta} radial-field solar wind using the Parker Solar Probe spacecraft data from 31 October to 12 November 2018. We calculate wave vectors using the singular value decomposition method and separate the MHD perturbations into three types of linear eigenmodes (Alfv\'en, fast, and slow modes) to explore the properties of the sub-Alfv\'enic perturbations and the role of compressible perturbations in solar wind heating. The MHD perturbations there show a high degree of Alfv\'enicity in the radial-field solar wind, with the energy fraction of Alfv\'en 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 10 November 2018. 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-\b{eta} plasma.