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

We report analysis of sub-Alfvenic magnetohydrodynamic (MHD) perturbations in the low-beta 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 (Alfven, fast, and slow modes) to explore the properties of sub-Alfvenic perturbations and the role of compressible perturbations in solar wind heating. The MHD perturbations show a high degree of Alfvenicity in the radial-field solar wind, with the energy fraction of Alfven modes dominating (similar to 45%-83%) over those of fast modes (similar to 16%-43%) and slow modes (similar to 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-beta plasma.